Flavonoids are a diverse group of polyphenolic compounds found in plants, characterized by a common structure consisting of two aromatic rings (A and B) joined by a three-carbon bridge that usually forms a closed pyran ring (C). They are widely recognized for their beneficial effects on health due to their antioxidant, anti-inflammatory, and anti-carcinogenic properties.
Classification of Flavonoids
Flavonoids are classified based on the oxidation state and substitution pattern of the central pyran ring. The main classes include:
1. Flavones
Flavones are a class of flavonoids, which are polyphenolic compounds found in plants. They have a C₆-C₃-C₆ backbone (two benzene rings connected by a three-carbon bridge) and are known for their antioxidant, anti-inflammatory, anticancer, and antimicrobial properties.
Structure: 2-phenyl-1-benzopyran-4-one.
Examples: Apigenin, Luteolin.
2. Flavonols
Flavonols are a subclass of flavonoids, which are polyphenolic compounds found in plants. They are structurally similar to flavones but contain a hydroxyl (-OH) group at the 3-position of the C-ring. Flavonols are widely known for their antioxidant, anti-inflammatory, anticancer, and cardioprotective effects.
Structure: 3-hydroxyflavone.
Examples: Quercetin, Kaempferol.
3. Flavanones
Flavanones are a subclass of flavonoids, characterized by their saturated C-ring (unlike flavones and flavonols, which have a double bond in the C-ring). They are widely found in citrus fruits and are known for their antioxidant, anti-inflammatory, anticancer, and cardioprotective properties.
Structure: 2,3-dihydro-2-phenyl-1-benzopyran-4-one.
Examples: Naringenin, Hesperetin.
4. Flavanonols (Dihydroflavonols)
Flavanonols, also known as dihydroflavonols, are a subclass of flavonoids characterized by a saturated C-ring (like flavanones) and an additional hydroxyl (-OH) group at the 3-position (similar to flavonols). They play a crucial role in plant metabolism and exhibit antioxidant, anti-inflammatory, cardioprotective, and anticancer properties.
Structure: 3-hydroxy-2,3-dihydroflavone.
Examples: Taxifolin.
5. Flavanols (Flavan-3-ols)
Flavanols, also known as Flavan-3-ols, are a subclass of flavonoids that contain a saturated C-ring (like flavanones) and a hydroxyl (-OH) group at the 3-position (like flavonols). They are known for their antioxidant, cardioprotective, neuroprotective, and anti-inflammatory properties and are widely found in tea, cocoa, and fruits.
Structure: 2-phenyl-3,4-dihydro-2H-chromen-3-ol.
Examples: Catechin, Epicatechin.
6. Anthocyanidins
Anthocyanidins are a subclass of flavonoids responsible for the red, purple, and blue pigments in many fruits, vegetables, and flowers. They are the aglycone (sugar-free) form of anthocyanins and exhibit strong antioxidant, anti-inflammatory, cardioprotective, and neuroprotective properties.
Structure: Flavylium ion.
Examples: Cyanidin, Pelargonidin.
7. Isoflavonoids
Isoflavonoids are a subclass of flavonoids that differ from other flavonoids by having their B-ring attached at the C3 position of the benzopyran (instead of C2). They are primarily found in leguminous plants (Fabaceae family) and are known for their phytoestrogenic, antioxidant, anti-inflammatory, and anticancer properties.
Structure: 3-phenylchromen-4-one.
Examples: Genistein, Daidzein.
8. Chalcones
Chalcones are open-chain flavonoids that serve as precursors to flavonoids and isoflavonoids. They are characterized by their 1,3-diphenylprop-2-en-1-one (chalcone) core structure and are known for their antioxidant, anti-inflammatory, antimicrobial, anticancer, and antidiabetic properties.
Structure: 1,3-diphenyl-2-propen-1-one.
Examples: Phloretin.
Properties of Flavonoids
Flavonoids exhibit a range of physical and chemical properties that are influenced by their structure and functional groups:
1. Solubility: Flavonoids are generally soluble in organic solvents such as ethanol, methanol, and dimethyl sulfoxide (DMSO). Their solubility in water varies, with glycosylated flavonoids being more water-soluble than their aglycone counterparts.
2. Color: Many flavonoids exhibit distinct colors, particularly anthocyanidins, which can range from red to blue depending on pH.
3. Stability: Flavonoids are relatively stable compounds but can be degraded by light, heat, and enzymatic oxidation.
4. Antioxidant Activity: Flavonoids are known for their strong antioxidant properties, which enable them to scavenge free radicals and reduce oxidative stress.
5. UV Absorption: Flavonoids have characteristic UV absorption spectra, which can be used for their identification and quantification.
Tests for Identification
Various chemical and chromatographic tests can be used to identify flavonoids:
1. Shinoda Test (Magnesium-Hydrochloric Acid Reduction Test):
Procedure: Mix the flavonoid extract with a few magnesium turnings and add concentrated hydrochloric acid.
Positive Result: Appearance of red, pink, or orange color indicates the presence of flavonoids.
2. Ferric Chloride Test:
Procedure: Add a few drops of ferric chloride solution to the flavonoid extract.
Positive Result: Formation of a green, blue, or black color indicates the presence of phenolic hydroxyl groups.
3. Alkaline Reagent Test:
Procedure: Add sodium hydroxide solution to the flavonoid extract.
Positive Result: Development of a yellow color that becomes colorless upon addition of dilute acid indicates the presence of flavonoids.
4. Lead Acetate Test:
Procedure: Add lead acetate solution to the flavonoid extract.
Positive Result: Formation of a yellow precipitate indicates the presence of flavonoids.
5. Chromatographic Techniques:
Thin-Layer Chromatography (TLC): Flavonoids can be separated and identified based on their Rf values and characteristic colors when sprayed with specific reagents such as AlCl₃, which fluoresces under UV light.
High-Performance Liquid Chromatography (HPLC): Allows precise separation, identification, and quantification of flavonoids based on their retention times and UV-visible spectra.
6. UV-Visible Spectroscopy:
Flavonoids have characteristic absorption maxima in the UV-visible range. By comparing the UV-visible spectra with standard flavonoids, identification can be confirmed.
7. Wilstätter “Cyanidin” Test:
Procedure: Treat the flavonoid extract with hydrochloric acid and heat.
Positive Result: Formation of a red or magenta color indicates the presence of flavonoids, particularly anthocyanins.
These tests provide a combination of chemical reactions and instrumental analyses to identify and confirm the presence of flavonoids in plant extracts and other samples.