What are Catechins?
Catechins are a class of polyphenolic compounds known as flavanols, which are part of the larger flavonoid family. They are secondary plant metabolites and possess a fundamental structure consisting of two benzene rings (A and B) and a dihydropyran heterocycle (C ring) with a hydroxyl group at carbon-3. The name itself is derived from the tannic juices extracted from the Mimosa catechu plant.
These compounds are widely recognized for their antioxidant properties and are found in many foods and beverages, including green tea, cocoa, red wine, and various fruits. Their health-promoting effects are highly dependent on their specific chemical structure, which is the basis for their classification.
The Structural Basis for Catechin Classification
According to their molecular structure, catechins are classified primarily by two main chemical features: the arrangement of hydroxyl groups and the presence or absence of a gallate ester. This results in several types, with the most common being catechin, epicatechin, epigallocatechin, and their corresponding gallate esters.
Stereoisomerism
Catechins contain two chiral centers at carbons 2 and 3 of the C-ring, which means they can exist in different spatial arrangements known as stereoisomers. This forms two main groups:
- Trans-isomers: Known as catechins, where the hydroxyl group on C-3 is in a trans configuration relative to the B-ring substituent at C-2. The most prevalent form is (+)-catechin.
- Cis-isomers: Known as epicatechins, where the hydroxyl group on C-3 is in a cis configuration relative to the B-ring substituent. The most common isomer is (–)-epicatechin.
Gallate Esterification
A key chemical modification in catechins is the esterification with gallic acid, which attaches a gallate group to the hydroxyl group at the C-3 position. This modification significantly impacts the bioavailability, antioxidant potency, and bitterness of the compound. This leads to the classification of catechins into non-esterified and esterified types.
Major Classes of Catechins
The combination of stereoisomerism and gallate esterification defines the main types of catechins found in nature. The most prominent examples are found abundantly in green tea and are often grouped together.
Non-Esterified Catechins:
- Epicatechin (EC): A cis-isomer known for its presence in cocoa and various fruits.
- Epigallocatechin (EGC): A cis-isomer with an additional hydroxyl group on its B-ring, which increases its antioxidant capacity.
Esterified Catechins:
- Epicatechin Gallate (ECG): An ester of epicatechin with a gallate group. It has a more astringent and bitter taste compared to its non-esterified counterpart.
- Epigallocatechin Gallate (EGCG): A gallate ester of epigallocatechin and the most abundant catechin in green tea. It is known for its strong antioxidant activity and potent health effects.
Comparison of Major Catechins
| Characteristic | (+)-Catechin (C) | (–)-Epicatechin (EC) | (–)-Epigallocatechin (EGC) | (–)-Epigallocatechin Gallate (EGCG) |
|---|---|---|---|---|
| Stereochemistry | Trans | Cis | Cis | Cis |
| Gallate Group | No | No | No | Yes |
| Primary Source | Green tea, cocoa | Green tea, cocoa, apples | Green tea | Green tea |
| Taste Profile | Mildly astringent | Slightly astringent | Bitter, mildly sweet | Astringent and bitter |
| Relative Abundance | Low in green tea | Moderate in green tea | High in green tea | Highest in green tea |
| Antioxidant Potency | Lower than EGCG | Lower than EGCG | High, but lower than EGCG | Very high |
Monomers, Oligomers, and Polymers
Catechins can also be classified by their degree of polymerization. The base compounds like catechin and epicatechin are monomers. These monomers can join together to form larger molecules:
- Oligomers: Chains of two to ten catechin units linked together. The most common are procyanidins, which are dimers of catechin or epicatechin and contribute to the antioxidant properties of foods like cocoa and grape seeds.
- Polymers: Longer, more complex chains of catechin units, also known as condensed tannins or proanthocyanidins. These large molecules are responsible for the distinctive astringent taste in certain plant-based foods.
Conclusion
The classification of catechins is fundamentally based on their chemical structure, specifically their stereochemistry and whether they possess a gallate ester. From the basic monomers like catechin and epicatechin, more complex and potent molecules such as the gallate esters (ECG and EGCG) are formed. This structural variation directly influences their physiological effects, from their flavor profile to their antioxidant and anti-inflammatory properties, making this classification crucial for both nutritional science and product development.
