Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids.
The name of the catechin chemical family derives from catechu, which is the tannic juice or boiled extract of Mimosa catechu (Acacia catechu L.f).
Chemistry
Catechin possesses two benzene rings (called the A and B rings) and a dihydropyran heterocycle (the C ring) with a hydroxyl group on carbon 3. The A ring is similar to a resorcinol moiety while the B ring is similar to a catechol moiety. There are two chiral centers on the molecule on carbons 2 and 3. Therefore, it has four diastereoisomers. Two of the isomers are in trans configuration and are called catechin and the other two are in cis configuration and are called epicatechin.
The most common catechin isomer is (+)-catechin. The other stereoisomer is (−)-catechin or ent-catechin. The most common epicatechin isomer is (−)-epicatechin (also known under the names L-epicatechin, epicatechol, (−)-epicatechol, L-acacatechin, L-epicatechol, epicatechin, 2,3-cis-epicatechin or (2R,3R)-(−)-epicatechin).
The different epimers can be separated using chiral column chromatography.
Making reference to no particular isomer, the molecule can just be called catechin. Mixtures of the different enantiomers can be called (±)-catechin or DL-catechin and (±)-epicatechin or DL-epicatechin.
Catechin and epicatechin are the building blocks of the proanthocyanidins, a type of condensed tannin.
Moreover, the flexibility of the C-ring allows for two conformation isomers, putting the B-ring either in a pseudoequatorial position (E conformer) or in a pseudoaxial position (A conformer). Studies confirmed that (+)-catechin adopts a mixture of A- and E-conformers in aqueous solution and their conformational equilibrium has been evaluated to be 33:67.
As flavonoids, catechins can act as antioxidants when in high concentration in vitro, but compared with other flavonoids, their antioxidant potential is low. The ability to quench singlet oxygen seems to be in relation with the chemical structure of catechin, with the presence of the catechol moiety on ring B and the presence of a hydroxyl group activating the double bond on ring C.
Oxidation
Electrochemical experiments show that (+)-catechin oxidation mechanism proceeds in sequential steps, related with the catechol and resorcinol groups and the oxidation is pH-dependent. The oxidation of the catechol 3′,4′-dihydroxyl electron-donating groups occurs first, at very low positive potentials, and is a reversible reaction. The hydroxyl groups of the resorcinol moiety oxidised afterwards were shown to undergo an irreversible oxidation reaction.
The laccase/ABTS system oxidizes (+)-catechin to oligomeric products of which proanthocyanidin A2 is a dimer.
Spectral data
Natural occurrences
(+)-Catechin and (−)-epicatechin as well as their gallic acid conjugates are ubiquitous constituents of vascular plants, and frequent components of traditional herbal remedies, such as Uncaria rhynchophylla. The two isomers are mostly found as cacao and tea constituents, as well as in Vitis vinifera grapes.
In food
The main dietary sources of catechins in Europe and the United States are tea and pome fruits.
Catechins and epicatechins are found in cocoa, which, according to one database, has the highest content (108 mg/100 g) of catechins among foods analyzed, followed by prune juice (25 mg/100 ml) and broad bean pod (16 mg/100 g). Açaí oil, obtained from the fruit of the açaí palm (Euterpe oleracea), contains (+)-catechins (67 mg/kg).
Catechins are diverse among foods, from peaches to green tea and vinegar. Catechins are found in barley grain where they are the main phenolic compound responsible for dough discoloration. The taste associated with monomeric (+)-catechin or (−)-epicatechin is described as slightly astringent, but not bitter.