Definition
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:
– Structure: 2-phenyl-1-benzopyran-4-one.
– Examples: Apigenin, Luteolin.
2. Flavonols:
– Structure: 3-hydroxyflavone.
– Examples: Quercetin, Kaempferol.
3. Flavanones:
– Structure: 2,3-dihydro-2-phenyl-1-benzopyran-4-one.
– Examples: Naringenin, Hesperetin.
4. Flavanonols (Dihydroflavonols):
– Structure: 3-hydroxy-2,3-dihydroflavone.
– Examples: Taxifolin.
5. Flavanols (Flavan-3-ols):
– Structure: 2-phenyl-3,4-dihydro-2H-chromen-3-ol.
– Examples: Catechin, Epicatechin.
6. Anthocyanidins:
– Structure: Flavylium ion.
– Examples: Cyanidin, Pelargonidin.
7. Isoflavonoids:
– Structure: 3-phenylchromen-4-one.
– Examples: Genistein, Daidzein.
8. Chalcones:
– 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.
