Phenolic compounds are important secondary metabolites found widely in medicinal plants. They include phenolic acids, flavonoids, tannins, stilbenes, lignans, coumarins, and related compounds. These compounds have attracted considerable research interest because of their diverse biological properties. Their chemical structures influence their solubility, stability, antioxidant capacity, and biological activity. Medicinal plants provide a rich and natural source of these compounds. This review discusses the major classes of phenolic compounds found in medicinal plants. It also describes their basic chemical characteristics, distribution, and important plant sources. Special attention is given to their pharmacological activities. These include antioxidant, anti-inflammatory, antimicrobial, antiviral, antidiabetic, hepatoprotective, cardioprotective, neuroprotective, and anticancer effects. The possible mechanisms responsible for these activities are also discussed. Phenolic compounds can influence oxidative stress, inflammatory mediators, cellular signaling pathways, and other biological processes. The review also highlights the importance of phenolic compounds in human health and their potential applications in pharmaceutical, nutraceutical, and functional food products. Their bioavailability, metabolism, safety, and current research challenges are also discussed. A better understanding of these compounds may contribute to the development of plant-based therapeutic agents and health-promoting products.
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Keywords
Antioxidant activity
Flavonoids
Health benefits
Medicinal plants
Pharmacological activities
Phenolic acids
Phenolic compounds
Tannins
INTRODUCTION
Medicinal plants have been used as sources of food and medicine for centuries. Different parts of these plants contain numerous biologically active compounds. Leaves, flowers, fruits, seeds, stems, bark, and roots may contain phenolic compounds and other secondary metabolites. These compounds contribute to the biological properties of many medicinal plants (Sun and Shahrajabian, 2023; Kumar et al., 2021).
Phenolic compounds are a large group of plant secondary metabolites. They contain one or more hydroxyl groups attached to an aromatic ring. Their structures range from simple phenolic molecules to complex polymers. More than 8,000 naturally occurring phenolic compounds have been described. This structural diversity contributes to their different biological functions (Tsitsin et al., 2023; Zhang et al., 2022).
Phenolic compounds are broadly divided into flavonoid and non-flavonoid groups. Important classes include phenolic acids, flavonoids, tannins, stilbenes, lignans, and coumarins. Phenolic acids are mainly represented by hydroxybenzoic and hydroxycinnamic acids. Flavonoids include flavones, flavonols, flavanones, flavan-3-ols, isoflavones, and anthocyanins (Sun and Shahrajabian, 2023; de la Rosa et al., 2022).
The formation of phenolic compounds is closely related to plant secondary metabolism. The phenylpropanoid pathway plays a central role in their biosynthesis. Phenylalanine is converted into important intermediates through the action of enzymes such as phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and 4-coumarate-CoA ligase. These intermediates contribute to the formation of phenolic acids, flavonoids, lignin, and other phenolic metabolites (Gligor et al., 2019).
Phenolic compounds have attracted considerable interest because of their diverse pharmacological activities. Studies have reported antioxidant, anti-inflammatory, antimicrobial, antiviral, antidiabetic, cardioprotective, neuroprotective, and anticancer effects. These activities are associated with their ability to influence oxidative stress, inflammatory responses, cellular signaling, and other biological processes (Sun and Shahrajabian, 2023; Rana et al., 2022). The antioxidant properties of phenolic compounds are particularly important. Some phenolics can directly interact with reactive oxygen species. Others may support endogenous antioxidant systems. These effects may help protect cells against oxidative damage. Such mechanisms have been investigated in relation to cardiovascular, neurological, metabolic, and other chronic disorders (Santos et al., 2021).
Phenolic compounds are also important from a nutritional perspective. They occur in many fruits, vegetables, cereals, herbs, spices, beverages, and medicinal plants. Their biological effects, however, cannot be judged only by their concentration in foods or plant extracts. Absorption, metabolism, intestinal microbiota, and chemical structure can strongly influence their activity in the human body (Del Bo' et al., 2019; Fraga et al., 2019).
Despite extensive research, several challenges remain. Many biological effects have been demonstrated in cell and animal models. Their translation to human health requires further clinical evidence. Low bioavailability is another important limitation for several phenolic compounds. Individual differences in metabolism and gut microbiota may also influence their biological effects (Fraga et al., 2019; Mutha et al., 2022).
Phenolic Compounds in Medicinal Plants
Phenolic compounds are a large and diverse group of secondary metabolites found in plants. They generally contain an aromatic ring with one or more hydroxyl groups. Their structures vary from simple phenolic molecules to complex polymeric compounds. More than 8,000 naturally occurring phenolic compounds have been described. They are widely distributed in medicinal plants and contribute to several biological and physiological functions (Tungmunnithum et al., 2018; Sun and Shahrajabian, 2023).
Phenolic compounds occur in different plant tissues. They have been reported in leaves, flowers, fruits, seeds, stems, bark, and roots. Their concentration can vary between plant species and plant parts. Environmental conditions can also influence their production and accumulation. Light, temperature, ultraviolet radiation, water availability, and other stress factors may alter the phenolic profile of plants (Tsitsin et al., 2023).
These compounds perform several important functions in plants. They can contribute to plant defense against pathogens and herbivores. Some phenolics also participate in plant growth and development. They may protect plant tissues against ultraviolet radiation and oxidative stress. Their antioxidant properties are also important for maintaining cellular protection under stressful conditions (Sun and Shahrajabian, 2023; Tsitsin et al., 2023).
Phenolic compounds are chemically diverse. Major groups include phenolic acids, flavonoids, tannins, coumarins, lignans, stilbenes, quinones, curcuminoids, and related compounds. Phenolic acids are mainly divided into hydroxybenzoic acids and hydroxycinnamic acids. Gallic acid, vanillic acid, protocatechuic acid, caffeic acid, ferulic acid, and p-coumaric acid are common examples (Kumar and Goel, 2019; Sun and Shahrajabian, 2023).
Flavonoids represent one of the largest groups of plant phenolics. They include flavones, flavonols, flavanones, flavan-3-ols, isoflavones, and anthocyanins. Their structural differences are mainly related to the substitution pattern and oxidation state of their carbon rings. These structural variations influence their biological properties and distribution in plants (Tungmunnithum et al., 2018; Sun and Shahrajabian, 2023).
Medicinal plants are important sources of phenolic compounds. Many traditional medicinal plants contain several phenolic classes at the same time. The chemical composition may differ according to species, geographical origin, plant part, harvesting stage, and environmental conditions. Therefore, the selection and authentication of plant material are important when studying phenolic-rich medicinal plants (Tungmunnithum et al., 2018; Kisiriko et al., 2021).
The biological importance of phenolic compounds has increased interest in medicinal plant research. Phenolic-rich extracts and individual compounds have been investigated for antioxidant, anti-inflammatory, antimicrobial, antiviral, cardioprotective, neuroprotective, antidiabetic, and anticancer activities. However, the biological response depends on the chemical structure and concentration of the compound. Absorption and metabolism can also influence its activity in the human body (Mutha et al., 2021; Sun and Shahrajabian, 2023).
The diversity of phenolic compounds makes them valuable for pharmaceutical and health-related research. Their presence in medicinal plants provides a basis for investigating new natural products. However, the reported biological activity of a plant extract should not be attributed to a single phenolic compound without adequate chemical and biological evidence. Standardized extraction, accurate identification, and suitable biological models are therefore important for reliable research.
Definition and General Characteristics
Phenolic compounds are a large and diverse group of plant secondary metabolites. They are characterized by the presence of at least one hydroxyl group attached directly to an aromatic ring. Their structures range from simple phenolic acids to complex compounds such as tannins and other polymers. More than 8,000 phenolic structures have been reported in plants (Albuquerque et al., 2021; Al Mamari, 2021).
Phenolic compounds are widely distributed in the plant kingdom. They occur in leaves, flowers, fruits, seeds, stems, bark, and roots. They may be present as free compounds or in bound forms. Many phenolics also occur as glycosides. The distribution and concentration of these compounds vary among plant species and plant tissues (Kisiriko et al., 2021). The chemical diversity of phenolic compounds is considerable. It results from differences in the number of aromatic rings, hydroxyl groups, substituents, and linkages between structural units. Important groups include phenolic acids flavonoids, tannins, coumarins, lignans, stilbenes, quinones, and curcuminoids., . This structural diversity is closely related to their biological properties (Sun and Shahrajabian, 2023; Tsitsin et al., 2023).
Phenolic compounds have important functions in plants. They participate in plant defense and protection against pathogens and herbivores. They can also protect plant tissues from ultraviolet radiation and oxidative stress. Some phenolics act as pigments and contribute to the colour of flowers and fruits. Others participate in plant growth, development, seed germination, and adaptation to environmental stress (Kisiriko et al., 2021).
The production of phenolic compounds is influenced by several factors. These include plant species, genetic characteristics, geographical location, maturity, temperature, light, water availability, and exposure to biotic or abiotic stress. Such factors can change both the amount and composition of phenolic compounds in medicinal plants (Tsitsin et al., 2023).
Phenolic compounds are also important because of their potential biological activities. Many phenolics can act as antioxidants by donating hydrogen or electrons to reactive species. Some compounds can also influence cellular antioxidant systems. Their biological effects are therefore not limited to direct radical scavenging. They may also involve regulation of cellular signaling and endogenous defense mechanisms (Albuquerque et al., 2021; Sun and Shahrajabian, 2023).
In medicinal plants, phenolic compounds may occur as individual molecules or as complex mixtures within plant extracts. Their biological effects may result from a single compound or from interactions among several constituents. Therefore, chemical characterization is important when evaluating the pharmacological potential of phenolic-rich medicinal plants. Extraction method, plant material, and analytical technique can also influence the detected phenolic profile ( (Table 1)) (Kisiriko et al., 2021; Alara et al., 2021).
Classification of Phenolic Compounds
Phenolic compounds show considerable structural diversity. Their classification is mainly based on their carbon skeleton, number of aromatic rings, and the groups connecting these rings. They are broadly divided into flavonoids and non-flavonoid phenolics. The major groups include phenolic acids, flavonoids, tannins, coumarins, lignans, stilbenes, quinones, xanthones, and curcuminoids (Kisiriko et al., 2021; Sun and Shahrajabian, 2023).
Phenolic Acids
contain a phenolic ring and a carboxylic acid group. They are mainly divided into hydroxybenzoic acids and hydroxycinnamic acids. Hydroxybenzoic acids have a C6-C1 skeleton. Gallic acid, vanillic acid, protocatechuic acid, syringic acid, and p-hydroxybenzoic acid are common examples. Hydroxycinnamic acids have a C6-C3 skeleton. Caffeic acid, ferulic acid, p-coumaric acid, and sinapic acid belong to this group (Sun and Shahrajabian, 2023).
Flavonoids
form one of the largest groups of plant phenolics. They generally possess a C6-C3-C6 skeleton. The basic structure contains two aromatic rings connected through a three-carbon bridge. Major subclasses include flavones, flavonols, flavanones, flavan-3-ols, isoflavones, anthocyanins, and related compounds. Small structural changes can produce major differences in their biological properties (Zhang et al., 2022; Sun and Shahrajabian, 2023).
Tannins
are high-molecular-weight phenolic compounds. They are known for their ability to bind proteins and other macromolecules. Tannins are commonly divided into hydrolysable tannins and condensed tannins. Hydrolysable tannins contain phenolic acids linked to a sugar or related core. Condensed tannins are also known as proanthocyanidins. Their structures are formed mainly from flavonoid units (Kisiriko et al., 2021).
Stilbenes
contain two aromatic rings connected by a two-carbon bridge. Resveratrol is one of the best-known members of this group. Stilbenes occur in several medicinal plants and are produced as part of plant defense responses. biological activities have generated considerable interest in pharmaceutical research (Teka et al., 2022).
Lignans and Neolignans
Lignans are formed from two phenylpropanoid units. The units are generally linked through their β-carbons. Neolignans have related structures but differ in the pattern of linkage between their phenylpropanoid units. These compounds occur in several medicinal plants and have been investigated for antioxidant, anti-inflammatory, antimicrobial, and other biological activities (Kisiriko et al., 2021).
Coumarins
are benzopyrone derivatives. They may occur as free compounds or as glycosides. Important groups include simple coumarins, furanocoumarins, pyranocoumarins, and related derivatives. Coumarins have been identified in several medicinal plant families. Their structural diversity is associated with a broad range of biological activities (Kisiriko et al., 2021).
Other Phenolic Compounds
Other important phenolic groups include quinones, xanthones, curcuminoids, biflavonoids, and lignins. These compounds differ considerably in their structures and biological properties. Curcuminoids, for example, include curcumin and related compounds. Quinones include several naturally occurring aromatic compounds with important biological functions. Lignin is a complex phenolic polymer that contributes to plant cell-wall structure ( (Table 2)) (Sun and Shahrajabian, 2023).
Chemical Structure and Properties
The chemical structure of phenolic compounds varies considerably. This variation is mainly related to the number and position of aromatic rings and hydroxyl groups. Other structural groups, such as methoxy, carbonyl, carboxyl, and glycosyl groups, also influence their properties. These structural differences contribute to the wide biological diversity of plant phenolics (Kisiriko et al., 2021; Tsitsin et al., 2023).
The basic structural feature of a phenolic compound is an aromatic ring containing at least one hydroxyl group. Simple phenolics may contain only one aromatic ring. More complex compounds contain several aromatic rings. Flavonoids generally have a C6-C3-C6 carbon skeleton. Phenolic acids commonly possess C6-C1 or C6-C3 skeletons. Tannins may contain multiple phenolic units linked together (Kisiriko et al., 2021).
The number and position of hydroxyl groups can affect the chemical behaviour of phenolic compounds. Hydroxyl groups can donate hydrogen atoms or electrons to reactive species. Their position within the molecule can also influence radical stabilization. In flavonoids and phenolic acids, hydroxylation patterns are therefore important determinants of antioxidant activity (Rice-Evans et al., 1996; Heim et al., 2002).
Methoxy groups can modify the physical and biological properties of phenolic compounds. They can increase lipophilicity and alter membrane partitioning. Glycosylation is another important structural modification. Attachment of a sugar moiety can change solubility, stability, absorption, and metabolism. Therefore, the aglycone and glycoside forms of the same phenolic compound may show different biological behaviour (Heim et al., 2002).
Phenolic compounds can occur in free, esterified, glycosylated, or polymeric forms. Phenolic acids may be linked to cell-wall components or other molecules. Flavonoids commonly occur as glycosides in plants. Tannins contain several phenolic units and can form complexes with proteins and other macromolecules. These different forms influence extraction, stability, bioavailability, and biological activity (Kisiriko et al., 2021). The conjugated structures of many phenolic compounds also influence their optical and redox properties. Extended conjugation can facilitate electron delocalization. This property is important for the stabilization of phenoxy radicals. It also contributes to the colour of several plant phenolics, particularly anthocyanins and related pigments (Tsitsin et al., 2023).
Solubility is another important property. Phenolic compounds with several hydroxyl groups are generally more hydrophilic. Methoxylation and other structural modifications can increase lipophilicity. Glycosylation may improve water solubility in some compounds. These differences influence extraction efficiency and the behaviour of phenolics in biological systems (Heim et al., 2002; Albuquerque et al., 2021).
The chemical structure also affects the stability of phenolic compounds. Factors such as pH, temperature, oxygen, light, and metal ions may influence their stability. The response differs between individual phenolic classes. Therefore, processing and storage conditions can change the phenolic profile of medicinal plant extracts ( (Table 3) and (Figure 1)) (Tsitsin et al., 2023).
Distribution and Major Plant Sources
Phenolic compounds are widely distributed in medicinal plants. They can occur in leaves, flowers, fruits, seeds, stems, bark, and roots. However, their concentration is not uniform throughout the plant. Different tissues may contain different phenolic profiles. This variation is related to the physiological role of each plant part and its exposure to environmental conditions (Kisiriko et al., 2021).
Leaves are important sources of flavonoids and phenolic acids. Flowers may contain high levels of flavonoids and anthocyanins. Fruits commonly contain phenolic acids, flavonoids, and tannins. Roots and bark can contain tannins, lignans, coumarins, and other phenolic constituents. The distribution also varies between plant species and even between different tissues of the same species.
Phenolic compounds may occur in both free and bound forms.
Free phenolics can be extracted relatively easily from plant tissues. Bound phenolics are associated with cell-wall components or other macromolecules. They may require additional treatment for their release. This difference is important when comparing the total phenolic content of medicinal plant materials (Wang et al., 2020).
The phenolic profile of a medicinal plant is affected by several factors. These include genetic characteristics, geographical origin, soil conditions, plant age, maturity, harvesting time, and plant tissue. Environmental factors are also important. Temperature, light, water availability, salinity, and other stresses can modify secondary metabolite production (Pant et al., 2021).
Geographical variation can produce considerable differences in phenolic content. Plants of the same species grown in different regions may show different chemical profiles. Differences in rainfall, temperature, soil composition, and altitude can contribute to this variation. Therefore, geographical origin should be considered during the quality assessment of phenolic-rich medicinal plants.
Plant stress can also influence phenolic accumulation. Phenolic compounds participate in plant protection against several environmental challenges. Recent research indicates that changes in temperature, drought, carbon dioxide, and other climate-related factors can modify secondary metabolite production in medicinal plants. Such changes may affect both the quantity and composition of bioactive compounds (Jangpangi et al., 2025).
Environmental contamination can further alter phenolic metabolism. Exposure to heavy metals may affect enzymes involved in phenolic biosynthesis. Changes have been reported in enzymes such as phenylalanine ammonia-lyase and chalcone synthase. Therefore, cultivation and harvesting conditions are important for maintaining the chemical quality of medicinal plant materials (Vinogradova et al., 2023).
The distribution of phenolic compounds is therefore not a fixed characteristic. It represents the combined effect of plant genetics, tissue type, developmental stage, environment, and cultivation conditions. This variation has important implications for the standardization of medicinal plants and herbal preparations ( (Table 4) and (Figure 2)).
Biosynthesis and Formation of Phenolic Compounds
The biosynthesis of phenolic compounds is closely linked to primary plant metabolism. Two important pathways are involved. These are the shikimate pathway and the phenylpropanoid pathway.
The shikimate pathway provides aromatic amino acids.
Phenylalanine is particularly important because it acts as a major precursor for many phenolic compounds (Maeda and Dudareva, 2012; Tohge and Fernie, 2017). The shikimate pathway converts phosphoenolpyruvate and erythrose-4-phosphate into chorismate. Chorismate is an important precursor for aromatic amino acids. These include phenylalanine, tyrosine, and tryptophan. The pathway therefore provides essential building blocks for several plant secondary metabolites (Maeda and Dudareva, 2012).
Phenylalanine enters the phenylpropanoid pathway through the action of Phenylalanine Ammonia-Lyase (PAL). PAL converts phenylalanine into cinnamic acid. This reaction represents an important entry point into phenolic metabolism. Cinnamic acid is then hydroxylated by Cinnamate 4-Hydroxylase (C4H) to form p-coumaric acid. The enzyme 4-Coumarate-Coa Ligase (4CL) subsequently converts p-coumaric acid into p-coumaroyl-CoA (Vogt, 2010; Tohge and Fernie, 2017).
p-Coumaroyl-CoA is a central intermediate in phenolic biosynthesis.
p-Coumaroyl-CoA is a central intermediate in phenolic biosynthesis. It can enter several branches of the pathway. These branches produce flavonoids, lignin precursors, stilbenes, coumarins, phenolic acids, and other phenylpropanoid derivatives. This branching pattern contributes greatly to the chemical diversity of plant phenolics (Vogt, 2010).
Flavonoid biosynthesis represents one of the major branches.
Chalcone Synthase (CHS) combines p-coumaroyl-CoA with malonyl-CoA to form chalcone. Chalcone is then converted into different flavonoid precursors. Further enzymatic reactions produce flavones, flavonols, flavanones, anthocyanins, and other flavonoids (Chen et al., 2023).
The pathway also contributes to the formation of lignin and related phenolic polymers. Enzymes such as cinnamoyl-CoA reductase and cinnamyl alcohol dehydrogenase participate in the formation of monolignols. These compounds are important precursors of lignin. Other branches produce stilbenes, coumarins, and additional specialized phenolic metabolites (Vogt, 2010).
Phenolic biosynthesis is strongly regulated. Genetic factors influence the expression of biosynthetic enzymes. Transcription factors also regulate important pathway genes. Environmental conditions can further modify this process. Light, ultraviolet radiation, temperature, drought, salinity, and pathogen attack may change phenolic accumulation in plant tissues (Tsitsin et al., 2023).
Recent advances in transcriptomics, metabolomics, and other omics technologies have improved the understanding of phenolic biosynthesis. These approaches help identify genes, enzymes, and metabolites involved in specific pathways. They may also support metabolic engineering for increased production of valuable phenolic compounds in medicinal plants (Zhang et al., 2023).
The biosynthetic pathway therefore provides a useful link between plant metabolism and the chemical diversity of medicinal plants.
Understanding this pathway is important for studying the accumulation of phenolic compounds and for improving their production through cultivation, elicitation, and biotechnological approaches ( (Figure 3)).
Extraction, Identification and Characterization
Extraction is an important step in the study of phenolic compounds from medicinal plants. The extraction method can affect the yield, composition, and biological activity of the final extract. No single extraction method is suitable for all plant materials. Phenolic compounds differ in polarity, stability, molecular size, and location within plant tissues. Therefore, the extraction procedure should be selected according to the target compounds and plant material (Alara et al., 2021; Zhang et al., 2018).
Conventional Extraction Methods
Maceration is one of the simplest methods used for plant extraction. The powdered plant material is mixed with a suitable solvent for a defined period. The mixture is usually stirred or shaken during extraction. The method requires simple equipment and is suitable for heat-sensitive compounds. However, it may require a long extraction time and relatively large amounts of solvent.
Percolation is another traditional method. In this process, the solvent passes slowly through the powdered plant material. It provides continuous contact between the solvent and plant matrix. Soxhlet extraction uses repeated cycles of hot solvent. It can provide efficient extraction but may expose phenolic compounds to prolonged heating. This can be a limitation for thermolabile compounds (Alara et al., 2021).
The choice of solvent is particularly important. Water, ethanol, methanol, acetone, and their mixtures are commonly used for phenolic extraction. Ethanol-water mixtures are frequently preferred because they can extract a broad range of phenolic compounds and are more suitable for food and pharmaceutical applications than some toxic organic solvents (Mokrani and Madani, 2016; Do et al., 2014).
Ultrasound-Assisted Extraction
(UAE) uses ultrasonic energy to improve the transfer of compounds from plant tissues into the solvent. Cavitation produced by ultrasound can disrupt plant cells and improve solvent penetration. UAE can reduce extraction time and solvent consumption. It is therefore considered a useful alternative to conventional extraction methods (Ameer et al., 2017).
The efficiency of UAE depends on several factors. These include ultrasound frequency, extraction temperature, extraction time, solvent composition, and solid-to-liquid ratio. Excessive ultrasound intensity or prolonged treatment may cause degradation of some sensitive compounds. Optimization is therefore necessary for individual plant materials.
Microwave-Assisted Extraction
(MAE) uses microwave energy to heat the solvent and plant matrix. Rapid heating can improve the release of phenolic compounds from plant tissues. MAE generally requires less time and solvent than several conventional methods. However, excessive heating may affect thermolabile compounds. Careful control of microwave power and extraction temperature is therefore important (Alara et al., 2021).
Pressurized Liquid Extraction
(PLE) uses elevated temperature and pressure to improve solvent penetration and solute transfer.
The method can reduce extraction time and solvent use. It has been investigated for the recovery of phenolic compounds and other bioactive constituents from food and herbal materials. Solvent composition, temperature, pressure, and extraction cycles influence the final recovery (Mustafa and Turner, 2011).
Supercritical Fluid Extraction
(SFE) commonly uses supercritical carbon dioxide. The method provides good selectivity and uses relatively mild temperatures. It is particularly useful for compounds that are compatible with carbon dioxide. However, the low polarity of carbon dioxide can limit the extraction of highly polar phenolic compounds. Co-solvents may therefore be required for some applications (Herrero et al., 2010).
Enzyme-Assisted Extraction
(EAE) uses enzymes to break down components of the plant cell wall. This can improve the release of phenolic compounds that are trapped within the plant matrix. Cellulases, pectinases, and related enzymes may be used. The method can operate under relatively mild conditions. However, enzyme cost and optimization requirements can limit its wider application.
Deep Eutectic Solvents
(DES) have gained attention as alternative solvents for phenolic extraction. They can be prepared from suitable hydrogen-bond donors and acceptors. DES may offer advantages such as low volatility, tunable polarity, and potential biodegradability. Their combination with UAE or MAE can plant matrix (Siddiqui et al., 2025). 5 and (Figure 4)). further improve phenolic recovery (Serna-Vázquez et al., 2021; Aktaş and Kurek, 2024).
However, DES-based extraction also has limitations. High viscosity can reduce mass transfer. Solvent composition, water content, temperature, extraction time, and solvent-to-sample ratio require optimization. The suitability of a DES should therefore be assessed for the target phenolic compounds and
Green Extraction Approaches
Green extraction aims to reduce solvent consumption, energy use, waste generation, and environmental impact. UAE, MAE, PLE, SFE, pressurized hot water extraction, and DES-based approaches are among the important techniques investigated for polyphenol recovery. These methods can provide shorter extraction times and improved recovery compared with some conventional procedures.
The selection of an extraction method should therefore consider both extraction efficiency and the intended application. Pharmaceutical preparations may require high purity and reproducibility. Food applications may place greater emphasis on solvent safety. Large-scale production also requires consideration of energy consumption, equipment cost, solvent recovery, and process scalability ( (Table 5) and (Figure 4)). process scalability (Table
Chromatographic Techniques
are important for the separation, identification, and quantification of phenolic compounds in medicinal plants. These techniques provide better information than general colorimetric assays. They can help distinguish individual phenolic compounds present in a complex plant extract (Zhang et al., 2022; Al Mamari, 2021).
High-Performance Liquid Chromatography
(HPLC) is widely used for the analysis of phenolic compounds. It can separate compounds according to their interaction with the stationary and mobile phases. Reverse-phase HPLC is commonly used for phenolic acids and flavonoids. Detection is often performed using ultraviolet or diode-array detection. HPLC can provide both qualitative and quantitative information (Zhang et al., 2022).
HPLC is useful for determining individual compounds in medicinal plant extracts. Commonly analyzed compounds include gallic acid, caffeic acid, ferulic acid, chlorogenic acid, quercetin, kaempferol, and rutin. The method can also be used for the development of chemical fingerprints. Such fingerprints are useful for comparing plant materials and supporting quality control.
High-Performance Thin-Layer Chromatography
(HPTLC) is another useful technique for phenolic analysis. It allows simultaneous separation of several compounds on a chromatographic plate. Different phenolic constituents produce characteristic bands at specific positions. Densitometric measurement can be used for quantitative analysis.
HPTLC is particularly useful for herbal materials because several samples can be analyzed under the same conditions. It can also provide a characteristic fingerprint of a medicinal plant extract. This makes the technique useful for authentication and quality assessment.
Liquid Chromatography-Mass Spectrometry
(LC-MS) combines chromatographic separation with mass-based detection. It provides information about the molecular mass of compounds and their fragmentation patterns. This improves the identification of phenolic constituents in complex extracts.
LC-MS/MS provides additional structural information through fragmentation analysis. It is therefore useful for identifying minor phenolic compounds that may not be easily characterized by conventional HPLC detection. The technique has become an important tool in the detailed analysis of plant phenolics and metabolite profiling (Zhang et al., 2022).
Gas Chromatography
(Gas Chromatography ( GC) can be used for the analysis of phenolic compounds that are sufficiently volatile or can be converted into suitable derivatives. Derivatization may improve volatility and chromatographic behaviour. GC coupled with mass spectrometry can provide additional information for compound identification.
However, many phenolic compounds are non-volatile and thermally unstable. Therefore, GC is less suitable for direct analysis of several high-molecular-weight or highly polar phenolics. Liquid chromatographic techniques are generally more appropriate for such compounds.
Chromatographic Fingerprinting
provides an overall chemical profile of a plant extract. It is useful when the therapeutic activity is associated with several constituents rather than a single marker compound. HPLC and HPTLC fingerprints can help compare different batches of medicinal plant material.
Fingerprint analysis can also support authentication and detection of adulteration. Differences in chromatographic profiles may indicate variations in plant species, geographical origin, harvesting conditions, processing, or storage. Therefore, chromatographic fingerprinting has an important role in the standardization of medicinal plant preparations ( (Table 6) and (Figure 5)).
Spectroscopic and Spectrometric Techniques
provide important information about the chemical characteristics of phenolic compounds. They are useful for preliminary screening, functional-group identification, structural elucidation, and confirmation of individual compounds. These techniques are often used together with chromatographic methods for reliable characterization of complex medicinal plant extracts (Al Mamari, 2021; Zhang et al., 2022).
Their reported
UV-visible Spectroscopy
is a simple technique for the preliminary analysis of phenolic compounds. Many phenolics contain conjugated aromatic systems. These structures show characteristic absorption in the ultraviolet or visible region. The absorption pattern can provide useful information about the presence of phenolic constituents.
UV-visible spectroscopy is also widely used in total phenolic content assays. The Folin-Ciocalteu method is commonly applied for this purpose. The result is generally expressed as gallic acid equivalents. However, the assay is not completely specific for phenolics. Other reducing substances may also contribute to the measured response (Zhang et al., 2022).
Fourier Transform Infrared Spectroscopy
Fourier Transform Infrared (FTIR) spectroscopy provides information about functional groups present in plant extracts. Phenolic compounds commonly show absorption associated with hydroxyl, aromatic, carbonyl, and C-O groups. The resulting spectrum can therefore provide a characteristic chemical profile.
FTIR is useful for rapid screening and comparison of plant extracts. It can also support authentication and quality assessment. However, FTIR alone may not always allow identification of individual phenolic compounds in a complex extract.
Nuclear Magnetic Resonance Spectroscopy
Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique for structural characterization. It provides information about the chemical environment of hydrogen and carbon atoms. Therefore, 1 H-NMR and13 C-NMR can provide detailed structural information about phenolic compounds.
NMR can help distinguish closely related compounds and identify structural modifications. It is particularly valuable when combined with chromatographic and mass spectrometric data. Its major limitation is the relatively high amount of purified sample required for detailed analysis.
Mass Spectrometry
(Mass Spectrometry ( MS) provides molecular mass information and is particularly useful for the identification of phenolic compounds. It can detect compounds at relatively low concentrations. Fragmentation patterns can provide additional structural information.
molecular masses. This approach is therefore valuable for the characterization of both known and previously unreported (phenolic constituents ( Zhang et al., 2022).
LC-MS and LC-MS/MS are especially useful for complex medicinal plant extracts. Chromatographic separation reduces sample complexity before mass-based detection. MS/MS fragmentation can help differentiate compounds with similar
Combined Analytical Approaches
No single analytical technique can provide all information required for complete characterization. UV-visible spectroscopy is useful for preliminary assessment. FTIR provides functional-group information. NMR provides detailed structural information. MS provides molecular mass and fragmentation data.
Combining these techniques with HPLC or HPTLC improves the reliability of phenolic identification. A combined approach is particularly important for medicinal plants because their extracts contain numerous compounds with similar chemical properties ( (Table 7) and (Figure 6)).
Pharmacological Activities of Phenolic Compounds
Anti-inflammatory Activity
Phenolic compounds have demonstrated anti-inflammatory activity in several experimental studies. Their effects are associated with the regulation of oxidative stress and inflammatory signalling. Many phenolics can reduce inflammatory mediators such as Tumour Necrosis Factor-alpha (TNF-α), Interleukin-6 (IL-6), and Nitric Oxide (NO). They may also modulate pathways such as Nuclear Factor-Kappa B (NF-κB) and enzymes involved in inflammation (Sun and Shahrajabian, 2023; Rana et al., 2022). Flavonoids, phenolic acids, tannins, and other phenolic compounds have shown anti-inflammatory effects in cell and animal models. These effects may help reduce oxidative and inflammatory tissue damage. However, the response depends on the chemical structure, concentration, bioavailability, and composition of the plant extract ( (Table 9)) (Oluwole et al., 2022; Sun and Shahrajabian, 2023).
Antimicrobial and Antiviral Activities
Phenolic compounds have shown antimicrobial activity against a wide range of microorganisms. Their effects may involve disruption of microbial membranes, alteration of cellular proteins, and interference with microbial enzymes. Flavonoids, phenolic acids, tannins, and related compounds have been investigated for these activities (Rana et al., 2022; Sun and Shahrajabian, 2023).
Phenolics may also show antiviral effects. Proposed mechanisms include interference with viral attachment, entry, replication, and viral enzyme activity. However, the activity varies among individual compounds and viral species. The chemical structure and concentration of the phenolic compound are important factors.
Tannins and flavonoids are particularly interesting because they can interact with proteins and other biological targets. Phenolic-rich plant extracts have therefore been studied as potential sources of antimicrobial and antiviral agents. However, much of the current evidence is based on laboratory and experimental models. Further studies are required to establish their clinical relevance ( (Table 10)).
Antidiabetic Activity
Phenolic compounds have shown promising antidiabetic effects in experimental studies. Their activity may involve regulation of glucose metabolism, improvement of insulin sensitivity, and reduction of oxidative stress. Some phenolics can also influence carbohydrate-digesting enzymes such as α-amylase and α-glucosidase (Rana et al., 2022; Sun and Shahrajabian, 2023).
Flavonoids and phenolic acids are among the major compounds investigated for these effects. They may help regulate glucose uptake and protect pancreatic β-cells from oxidative damage. Some compounds may also influence pathways involved in lipid metabolism and insulin signalling.
Phenolic-rich medicinal plant extracts have therefore attracted interest as potential supportive agents for diabetes management. However, most evidence comes from in vitro and animal studies. Differences in absorption, metabolism, dose, and bioavailability may affect their activity in humans. Clinical studies are required to confirm their antidiabetic potential ( (Table 11)).
Hepatoprotective Activity
Phenolic compounds have shown promising hepatoprotective effects in experimental studies. Their activity is mainly associated with the reduction of oxidative stress and inflammation. Phenolics can scavenge reactive oxygen species, reduce lipid peroxidation, and support endogenous antioxidant defenses. These actions help protect hepatocytes from chemical and metabolic injury (Salehi et al., 2019; Sun and Shahrajabian, 2023).
Flavonoids, phenolic acids, tannins, and other polyphenols may also regulate inflammatory pathways involved in liver injury. Some compounds can reduce the release of inflammatory mediators and help maintain hepatocyte membrane integrity.
Phenolic-rich extracts have therefore been investigated in models of drug-induced, alcohol-induced, and metabolic liver injury. The hepatoprotective effects reported for medicinal plants are generally reflected by reduced serum alanine Aminotransferase (ALT), aspartate Aminotransferase (AST), Alkaline Phosphatase (ALP), and bilirubin levels, together with improved liver histology. However, most evidence remains preclinical. Differences in phenolic composition, dose, metabolism, and bioavailability may influence the observed effects. Further well-designed clinical studies are needed to confirm their therapeutic potential ( (Table 12)).
Cardioprotective Activity
Phenolic compounds may provide cardioprotective effects through antioxidant, anti-inflammatory, and lipid-regulating actions. They can reduce oxidative stress and lipid peroxidation, which are important factors in cardiovascular damage. Flavonoids and phenolic acids have also been associated with improved endothelial function and vascular protection (Rana et al., 2022; Sun and Shahrajabian, 2023).
Some phenolic compounds may help regulate blood lipid levels and inhibit oxidation of Low-Density Lipoprotein (LDL). They may also reduce inflammatory signalling involved in atherosclerosis.
These effects suggest a possible role of phenolic-rich medicinal plants in maintaining cardiovascular health.
However, the available evidence is largely based on experimental and observational studies. Differences in compound structure, dose, metabolism, and bioavailability can affect the response. More clinical studies are required to establish the cardiovascular benefits of individual phenolic compounds ( (Table 13)).
Neuroprotective Activity
Phenolic compounds have attracted interest for their potential neuroprotective effects. Their antioxidant activity may help reduce oxidative stress in neuronal cells. Some phenolics can also regulate inflammatory responses and protect neurons from oxidative and mitochondrial damage (Rana et al., 2022; Sun and Shahrajabian, 2023).
Flavonoids and phenolic acids have been investigated in experimental models of neurodegenerative disorders. Their reported actions include modulation of inflammatory signalling, protection against neuronal apoptosis, and maintenance of cellular antioxidant defenses. Some phenolics may also influence pathways related to amyloid processing and neuronal signalling.
However, the therapeutic potential of phenolic compounds in neurological disorders is influenced by their absorption, metabolism, and ability to cross the blood-brain barrier. Most available evidence is still based on in vitro and animal studies. Clinical studies are therefore needed to confirm their neuroprotective effects in humans ( (Table 14)).
Anticancer Activity
Phenolic compounds have been widely studied for their potential anticancer effects. Their activity may involve antioxidant and anti-inflammatory actions, as well as direct effects on cancer cell growth. Flavonoids, phenolic acids, tannins, and other polyphenols have shown inhibitory effects on cancer cells in experimental models (Oluwole et al., 2022; Sun and Shahrajabian, 2023).
Several phenolics can influence important cellular processes, including cell-cycle regulation, apoptosis, and intracellular signalling. They may also reduce angiogenesis and cancer cell migration. These effects suggest that phenolic compounds may act on multiple targets involved in cancer development and progression.
However, anticancer activity observed in laboratory studies cannot be directly translated into clinical efficacy. Factors such as dose, metabolism, bioavailability, and tissue distribution can strongly influence their effects. Further in vivo and clinical studies are required to determine their therapeutic value ( (Table 15)).
Other Pharmacological Activities
In addition to the major activities discussed above, phenolic compounds have been investigated for several other pharmacological effects. These include wound healing, gastrointestinal protection, antimicrobial protection, and effects on metabolic health. Tannins and flavonoids are of particular interest because of their astringent, antioxidant, and anti-inflammatory properties.
Some phenolic compounds may support tissue repair by reducing oxidative damage and inflammation. Tannins can also interact with proteins and form protective layers over damaged tissues. These properties have contributed to their traditional use in conditions such as diarrhoea, minor wounds, and inflammatory disorders.
Phenolic compounds have also been investigated for antiparasitic, antiviral, and antibacterial effects. However, the strength of evidence varies considerably between individual compounds and plant extracts. Most reported activities are based on experimental studies and should not be considered proof of clinical efficacy ( (Table 16) and (Figure 8)).
Mechanisms of Action
The pharmacological effects of phenolic compounds are mediated through several interconnected biological mechanisms. Their activity is not limited to direct antioxidant effects. Phenolics can regulate oxidative stress, inflammatory pathways, cellular signalling, and programmed cell death. The exact mechanism depends on the chemical structure, concentration, metabolism, and biological target of the compound (Del Bo' et al., 2019; Fraga et al., 2019).
Regulation of Oxidative Stress
Phenolic compounds can regulate oxidative stress through direct and indirect mechanisms. Their hydroxyl groups allow them to donate hydrogen atoms or electrons to Reactive Oxygen Species (ROS). This can reduce the formation of highly reactive free radicals and limit oxidative damage to cellular components (Rice-Evans et al., 1996; Heim et al., 2002).
Phenolics may also support endogenous antioxidant defenses. They can influence cellular systems involved in maintaining redox balance. These effects may increase the activity or expression of antioxidant enzymes and improve the cellular response to oxidative stress. Therefore, their antioxidant action involves both direct radical scavenging and regulation of cellular defense mechanisms ( (Table 17)) (Albuquerque et al., 2021; Sun and Shahrajabian, 2023).
Modulation of Inflammatory Pathways
Phenolic compounds can influence several signalling pathways involved in inflammation. They may reduce the activation of inflammatory mediators and transcription factors. The Nuclear Factor-Kappa B (NF-κB) pathway is one important target associated with the regulation of inflammatory responses. Phenolics may also influence cytokines such as Tumour Necrosis Factor-alpha (TNF-α) and Interleukin-6 (IL-6).
The anti-inflammatory effects of phenolics are closely related to their antioxidant properties. Reduction of oxidative stress may decrease the activation of redox-sensitive inflammatory pathways. Thus, antioxidant and anti-inflammatory actions can occur together and contribute to cellular protection (Del Bo' et al., 2019; Sun and Shahrajabian, 2023).
Regulation of Cellular Signaling and Apoptosis
Phenolic compounds can affect cellular signalling pathways that control cell survival, proliferation, differentiation, and apoptosis. Flavonoids and other phenolics have been reported to interact with signalling networks involved in cellular stress responses. These effects may contribute to their antioxidant, anti-inflammatory, and anticancer activities (Chen et al., 2023; Teka et al., 2022).
Phenolics may also influence apoptosis, a controlled process involved in the removal of damaged or abnormal cells. Depending on the compound and cellular condition, they may promote apoptosis in cancer cells while protecting normal cells from excessive cell death. Such effects are associated with changes in pro-apoptotic and anti-apoptotic signalling pathways.
Phenolic Compounds and Human Health
Phenolic compounds are important dietary and plant-derived bioactive compounds with potential benefits for human health. Their reported effects include antioxidant, anti-inflammatory, cardiovascular, metabolic, and neuroprotective actions. These effects are related to their ability to influence oxidative stress, inflammatory responses, and cellular processes (Del Bo' et al., 2019; Fraga et al., 2019).
Regular intake of phenolic-rich foods and plant products has been associated with several positive health outcomes. Flavonoids, phenolic acids, stilbenes, and other phenolics may contribute to cardiovascular and metabolic health. However, the effects can vary according to the type and number of phenolic compounds consumed (Del Bo' et al., 2019).
The health effects of phenolics are not determined only by their content in a plant or food. Chemical structure, absorption, metabolism, and interaction with the gut microbiota can influence their biological activity. Therefore, high phenolic content does not always indicate greater health benefits (Fraga et al., 2019).
Phenolic compounds may therefore contribute to a healthy diet and support the development of plant-based health products. However, most evidence does not justify considering phenolic compounds as substitutes for established medical treatments. More human studies are required to determine effective doses, long-term effects, and clinically relevant benefits ( (Table 18)).
Bioavailability, Metabolism and Safety
The biological effects of phenolic compounds depend not only on their concentration but also on their absorption and metabolism in the human body. Many phenolics have limited bioavailability because of poor absorption, rapid metabolism, and extensive transformation in the gastrointestinal tract. Their chemical structure and glycosylation can also influence their absorption and biological activity (Del Bo' et al., 2019; Fraga et al., 2019).
After ingestion, phenolic compounds may undergo several metabolic processes. These include hydrolysis, conjugation, and transformation by intestinal microorganisms. The resulting metabolites can differ from the original compounds in their biological activity. Gut microbiota therefore plays an important role in determining the fate and effects of dietary phenolics (Fraga et al., 2019).
Safety is another important consideration. Phenolic compounds are generally associated with beneficial biological effects, but their activity can depend on dose and duration of exposure. High concentrations of some phenolics may produce undesirable effects or interfere with nutrient absorption. Therefore, safety evaluation is essential before their use in concentrated pharmaceutical or nutraceutical preparations.
The bioavailability and safety of phenolic compounds remain important challenges for their therapeutic development.
Improving absorption, identifying active metabolites, and establishing appropriate doses may help translate experimental findings into practical health applications ( (Figure 9)).
Pharmaceutical, Nutraceutical and Functional Food Applications
Phenolic compounds have considerable potential in pharmaceutical, nutraceutical, and functional food products. Their diverse biological activities make them attractive for developing plant-based products with antioxidant, anti-inflammatory, cardioprotective, metabolic, and other health-related benefits. However, their practical use requires proper characterization, standardization, stability assessment, and safety evaluation (Kumar et al., 2021; Albuquerque et al., 2021).
Pharmaceutical applications
Phenolic-rich extracts and purified phenolic compounds can be investigated as active or supportive ingredients in pharmaceutical formulations. Their potential applications include products targeting oxidative stress, inflammation, metabolic disorders, and other chronic conditions. Standardized extracts are particularly important because the phenolic profile can vary with plant species, plant part, cultivation conditions, and extraction method.
Nutraceutical applications
Phenolics are widely considered for nutraceutical formulations such as capsules, tablets, powders, and concentrated plant extracts. Such products may provide a convenient source of selected phenolic compounds. However, low bioavailability remains a major limitation. Formulation approaches that improve solubility, stability, and absorption may increase their practical value.
Functional food applications
Phenolic compounds can also be incorporated into functional foods and beverages. Fruits, vegetables, cereals, herbs, spices, teas, and other plant materials naturally provide phenolic compounds. Their incorporation into food products may enhance the functional value of the product. Food applications, however, require attention to taste, stability, processing conditions, and consumer acceptability ( (Table 19)).
Table 1: General characteristics of phenolic compounds.
|
Characteristic |
Description |
|---|---|
|
Chemical nature |
Plant secondary metabolites |
|
Basic structural feature |
Aromatic ring with one or more hydroxyl groups |
|
Major forms |
Free, bound, glycosylated, monomeric, and polymeric forms |
|
Major classes |
Phenolic acids, flavonoids, tannins, coumarins, lignans, stilbenes, quinones, and curcuminoids |
|
Plant distribution |
Leaves, flowers, fruits, seeds, stems, bark, and roots |
|
Major plant functions |
Defense, UV protection, antioxidant protection, pigmentation, and adaptation to stress |
|
Important biological properties |
Antioxidant, anti-inflammatory, antimicrobial, antiviral, and other pharmacological activities |
Table 2: Major classes of phenolic compounds found in medicinal plants.
|
Phenolic class |
Main structural feature |
Representative compounds |
Examples of medicinal plant sources |
|---|---|---|---|
|
Phenolic acids |
C6-C1 or C6-C3 skeleton |
Gallic acid, caffeic acid, ferulic acid |
Syzygium aromaticum, Salvia officinalis, Zingiber officinale |
|
Flavonoids |
C6-C3-C6 skeleton |
Quercetin, kaempferol, catechin, apigenin |
Camellia sinensis, Ginkgo biloba, Ocimum basilicum |
|
Tannins |
Polyphenolic structures |
Tannic acid, proanthocyanidins |
Terminalia spp., Acacia spp. |
|
Stilbenes |
C6-C2-C6 skeleton |
Resveratrol, pterostilbene |
Vitis vinifera, Pinus spp. |
|
Lignans |
Two phenylpropanoid units |
Sesamin, podophyllotoxin |
Sesamum indicum, Podophyllum spp. |
|
Coumarins |
Benzopyrone nucleus |
Coumarin, esculetin, scopoletin |
Cinnamomum spp., Angelica spp. |
|
Quinones |
Quinone aromatic system |
Juglone, lawsone |
Juglans spp., Lawsonia inermis |
|
Curcuminoids |
Diarylheptanoid-type structure |
Curcumin, demethoxycurcumin |
Curcuma longa |
|
Xanthones |
Xanthone nucleus |
Mangiferin, α-mangostin |
Mangifera indica, Garcinia spp. |
Table 3: Important structural features of phenolic compounds and their effects.
|
Structural feature |
Examples |
Possible influence |
|---|---|---|
|
Hydroxyl groups |
Phenolic acids, flavonoids |
Hydrogen donation and radical stabilization |
|
Number of aromatic rings |
Flavonoids, tannins |
Influences molecular size and biological interactions |
|
Methoxy groups |
Ferulic acid, methoxylated flavonoids |
Can increase lipophilicity and alter reactivity |
|
Glycosyl groups |
Rutin, quercetin glycosides |
Can affect solubility, stability, absorption, and metabolism |
|
Carboxyl group |
Gallic, caffeic, ferulic acids |
Influences polarity and chemical reactivity |
|
Conjugated system |
Flavonoids, anthocyanins, stilbenes |
Supports electron delocalization and colour formation |
|
Polymerization |
Tannins, proanthocyanidins |
Increases molecular size and affects protein binding |
Table 4: Distribution of major phenolic groups in medicinal plant parts.
|
Plant part |
Common phenolic groups |
Representative examples |
|---|---|---|
|
Leaves |
Flavonoids, phenolic acids |
Quercetin, rutin, caffeic acid |
|
Flowers |
Flavonoids, anthocyanins, phenolic acids |
Kaempferol, anthocyanins, gallic acid |
|
Fruits |
Phenolic acids, flavonoids, tannins |
Chlorogenic acid, quercetin, catechins |
|
Seeds |
Phenolic acids, flavonoids, lignans |
Ferulic acid, catechins, lignans |
|
Bark |
Tannins, phenolic acids, flavonoids |
Tannins, gallic acid, catechins |
|
Roots |
Phenolic acids, flavonoids, lignans, coumarins |
Caffeic acid derivatives, lignans, coumarins |
|
Stems |
Phenolic acids, flavonoids, tannins |
Ferulic acid, flavonoids, condensed tannins |
Table 5: Major extraction methods used for phenolic compounds from medicinal plants.
|
Extraction method |
Main principle |
Major advantages |
Important limitations |
|---|---|---|---|
|
Maceration |
Soaking plant material in solvent |
Simple and inexpensive |
Long extraction time |
|
Percolation |
Continuous passage of solvent through plant material |
Better solvent contact |
Time and solvent requirements |
|
Soxhlet extraction |
Repeated hot-solvent extraction |
Exhaustive extraction |
Prolonged heating |
|
UAE |
Ultrasonic cavitation improves mass transfer |
Rapid and lower solvent use |
Requires process optimization |
|
MAE |
Microwave energy provides rapid heating |
Short extraction time |
Possible thermal degradation |
|
PLE |
High pressure and temperature improve extraction |
Efficient and reproducible |
Higher equipment cost |
|
SFE |
Supercritical fluid extracts target compounds |
Selective and low solvent residue |
Less suitable for highly polar compounds |
|
EAE |
Enzymes disrupt plant cell-wall components |
Mild conditions and improved release |
Enzyme cost |
|
DES extraction |
Tunable alternative solvent systems |
Potentially greener and selective |
High viscosity and optimization requirements |
Table 6: Major chromatographic techniques used for phenolic compounds.
|
Technique |
Major application |
Information obtained |
|---|---|---|
|
HPLC |
Separation and quantification |
Individual phenolic compounds |
|
HPTLC |
Separation and fingerprinting |
Characteristic bands and quantitative data |
|
LC-MS |
Identification of phenolics |
Molecular mass and chromatographic profile |
|
LC-MS/MS |
Detailed identification |
Molecular mass and fragmentation pattern |
|
GC-MS |
Analysis of volatile or derivatized phenolics |
Retention behaviour and mass spectral information |
|
Chromatographic fingerprinting |
Quality control and authentication |
Overall chemical profile |
Table 7: Spectroscopic and spectrometric techniques used for phenolic characterization.
|
Technique |
Major application |
Information obtained |
|---|---|---|
|
UV-visible spectroscopy |
Preliminary screening and total phenolic estimation |
Absorption characteristics |
|
FTIR |
Functional-group analysis |
Characteristic chemical bonds |
|
^1H-NMR |
Structural analysis |
Hydrogen environment |
|
^13C-NMR |
Structural analysis |
Carbon environment |
|
MS |
Compound identification |
Molecular mass |
|
MS/MS |
Structural confirmation |
Fragmentation pattern |
|
HPLC-DAD |
Separation and detection |
Retention time and UV spectrum |
|
LC-MS/MS |
Detailed characterization |
Retention time, molecular mass, and fragments |
Table 8: Major phenolic compounds with reported antioxidant activity.
|
Phenolic compound/group |
Major antioxidant mechanism |
Representative source |
|---|---|---|
|
Gallic acid |
Radical scavenging and electron donation |
Medicinal plants and fruits |
|
Caffeic acid |
Radical scavenging and metal interaction |
Leaves, fruits and herbs |
|
Ferulic acid |
Radical scavenging and oxidative protection |
Cereals and medicinal plants |
|
Quercetin |
Radical scavenging and metal chelation |
Leaves, fruits and medicinal plants |
|
Kaempferol |
Radical scavenging and cellular antioxidant effects |
Leaves and flowers |
|
Catechins |
Radical scavenging and protection against lipid oxidation |
Tea and medicinal plants |
|
Tannins |
Radical scavenging and metal chelation |
Bark, leaves and fruits |
|
Resveratrol |
Antioxidant and cellular protective effects |
Selected medicinal plants |
Table 9: Major anti-inflammatory mechanisms of phenolic compounds.
|
Mechanism |
Major effect |
|---|---|
|
NF-κB modulation |
Reduced inflammatory signalling |
|
Cytokine regulation |
Reduced TNF-α and IL-6 |
|
iNOS/NO modulation |
Reduced excessive NO production |
|
COX/LOX modulation |
Reduced inflammatory mediators |
|
Antioxidant action |
Reduced oxidative stress |
Table 10: Major antimicrobial and antiviral mechanisms of phenolic compounds.
|
Activity |
Proposed mechanism |
|---|---|
|
Antibacterial |
Membrane disruption and enzyme inhibition |
|
Antifungal |
Alteration of fungal membranes and cellular functions |
|
Antiviral |
Interference with viral attachment and replication |
|
Antioxidant-mediated protection |
Reduction of oxidative damage associated with infection |
Table 11: Major antidiabetic actions of phenolic compounds.
|
Action |
Proposed effect |
|---|---|
|
α-Amylase inhibition |
Reduced carbohydrate digestion |
|
α-Glucosidase inhibition |
Delayed glucose release |
|
Improved insulin signalling |
Better glucose utilization |
|
Antioxidant action |
Protection of pancreatic β-cells |
Table 12: Major hepatoprotective actions of phenolic compounds.
|
Mechanism |
Protective effect |
|---|---|
|
Antioxidant activity |
Reduces oxidative damage |
|
Inhibition of lipid peroxidation |
Preserves hepatocyte membranes |
|
Anti-inflammatory action |
Reduces inflammatory signalling |
|
Enhancement of endogenous antioxidants |
Supports cellular defense |
|
Antifibrotic activity |
May limit excessive collagen deposition |
Table 13: Major cardioprotective actions of phenolic compounds.
|
Mechanism |
Potential benefit |
|---|---|
|
Antioxidant activity |
Reduces oxidative vascular damage |
|
Anti-inflammatory action |
Limits vascular inflammation |
|
LDL oxidation inhibition |
May reduce atherosclerotic risk |
|
Lipid regulation |
Supports healthy lipid levels |
|
Endothelial protection |
Supports vascular function |
Table 14: Major neuroprotective actions of phenolic compounds.
|
Mechanism |
Potential benefit |
|---|---|
|
Antioxidant activity |
Reduces neuronal oxidative stress |
|
Anti-inflammatory action |
Limits neuroinflammation |
|
Anti-apoptotic effects |
Supports neuronal survival |
|
Mitochondrial protection |
Maintains neuronal energy function |
|
Modulation of neuronal signalling |
Supports cognitive and neuronal functions |
Table 15: Major anticancer actions of phenolic compounds.
|
Mechanism |
Potential effect |
|---|---|
|
Cell-cycle regulation |
Inhibits uncontrolled cell proliferation |
|
Apoptosis induction |
Promotes cancer cell death |
|
Anti-inflammatory activity |
Reduces tumour-promoting inflammation |
|
Anti-angiogenic activity |
May limit tumour blood supply |
|
Inhibition of cell migration |
May reduce metastatic potential |
Table 16: Selected other pharmacological activities of phenolic compounds.
|
Activity |
Major proposed action |
|---|---|
|
Wound healing |
Antioxidant and tissue-protective effects |
|
Gastroprotective |
Reduction of oxidative and inflammatory damage |
|
Antidiarrheal |
Astringent and antimicrobial effects |
|
Antiparasitic |
Interaction with microbial or parasitic targets |
|
Antiviral |
Interference with viral activity |
Table 17: Major mechanisms involved in the regulation of oxidative stress.
|
Mechanism |
Biological effect |
|---|---|
|
Free-radical scavenging |
Reduces reactive species |
|
Hydrogen/electron donation |
Stabilizes free radicals |
|
Metal chelation |
Limits radical-generating reactions |
|
Lipid peroxidation inhibition |
Protects cellular membranes |
|
Antioxidant defense regulation |
Supports endogenous cellular protection |
Table 18: Major health benefits associated with phenolic compounds.
|
Health area |
Potential benefit |
|---|---|
|
Cardiovascular health |
Supports vascular and lipid health |
|
Metabolic health |
May support glucose regulation |
|
Brain health |
May protect against oxidative and inflammatory damage |
|
Cellular protection |
Reduces oxidative stress |
|
Inflammation |
Helps regulate inflammatory responses |
Table 19: Potential applications of phenolic compounds.
|
Application |
Examples |
Major consideration |
|---|---|---|
|
Pharmaceutical |
Standardized extracts, phenolic-based formulations |
Purity, dose and safety |
|
Nutraceutical |
Capsules, tablets, powders, extracts |
Bioavailability and stability |
|
Functional food |
Beverages, cereals, fortified foods |
Sensory quality and processing stability |
Figure 1: Relationship between phenolic structure and major physicochemical properties.
Figure 2: Distribution of major phenolic classes across different medicinal plant parts.
Figure 3: Biosynthetic pathway of major phenolic compounds in medicinal plants.
Figure 4: Selection of extraction methods for phenolic compounds from medicinal plants.
Figure 5: Application of chromatographic techniques for the separation, identification, quantification, and quality assessment of phenolic compounds from medicinal plants.
Figure 6: Spectroscopic and spectrometric approaches for the characterization of phenolic compounds from medicinal plants.
Figure 7: Antioxidant mechanisms of phenolic compounds.
Figure 8: Overview of the Major Pharmacological Activities of Phenolic Compounds in Medicinal Plants.
Figure 9: Journey of Phenolic Compounds in the Human Body: Absorption, Metabolism, Bioavailability and Health Effects.
CURRENT CHALLENGES AND FUTURE PERSPECTIVES
Despite the growing interest in phenolic compounds, several challenges limit their translation into pharmaceutical and health applications. The major concerns include variation in phenolic composition, low bioavailability, limited clinical evidence, difficulties in standardization, and differences in extraction and processing methods. Plant species, geographical origin, harvesting stage, and environmental conditions can also influence phenolic content.
Another important challenge is the gap between experimental and clinical evidence. Many reported pharmacological effects are based on in vitro and animal studies. Human studies are still limited for many individual phenolic compounds. Differences in absorption, metabolism, gut microbiota, and effective dose further complicate the interpretation of clinical outcomes.
Future research should focus on standardized extracts, improved delivery systems, pharmacokinetic studies, and well-designed clinical trials. Advanced analytical approaches can help identify active compounds and establish reliable quality markers. Sustainable extraction technologies may also support large-scale production while reducing solvent use and environmental impact.
Integration of metabolomics, genomics, and other omics approaches may improve understanding of phenolic biosynthesis and biological activity. Nanotechnology and advanced formulation strategies may further improve solubility, stability, and bioavailability. These approaches can support the development of more consistent and evidence-based phenolic products ( (Table 20)).
Table 20: Major Challenges and Future Directions for Phenolic Compounds.
|
Current challenge |
Future direction |
|---|---|
|
Variable phenolic composition |
Standardized plant materials and extracts |
|
Low bioavailability |
Advanced delivery and formulation systems |
|
Limited clinical evidence |
Well-designed clinical trials |
|
Difficult standardization |
Validated analytical markers |
|
Variation in extraction methods |
Optimized and sustainable extraction |
|
Limited pharmacokinetic data |
Detailed ADME studies |
|
Complex metabolism |
Metabolomics and gut microbiota studies |
|
Safety uncertainties |
Long-term toxicological evaluation |
CONCLUSION
Phenolic compounds are an important group of bioactive constituents found widely in medicinal plants. Their structural diversity contributes to a broad range of biological activities, including antioxidant, anti-inflammatory, antimicrobial, antidiabetic, hepatoprotective, cardioprotective, neuroprotective, and anticancer effects. Their activities are associated with regulation of oxidative stress, inflammatory pathways, cellular signalling, and apoptosis.
Advances in extraction and analytical techniques have improved the identification, characterization, and standardization of plant phenolics. However, low bioavailability, complex metabolism, variation in plant composition, and limited clinical evidence remain important challenges. Future research should therefore focus on standardized extracts, improved delivery systems, sustainable extraction, detailed pharmacokinetic studies, and well-designed clinical trials.
Overall, phenolic compounds have significant potential for pharmaceutical, nutraceutical, and functional food applications. Better understanding of their chemistry, biological mechanisms,
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Conflict of Interest
The authors declare that there is no conflict of interest.