Flavanols: Catechins and proanthocyanidins

Abstract

Flavanols are a wide group of polyphenols that include flavan-3-ols (e.g., catechin and proanthocyanidins), flavan-4-ols, and flavan-3,4-diols. They arise from plant secondary metabolism through condensation of phenylalanine derived from the shikimate pathway with malonyl-CoA obtained from citrate that is produced by the tricarboxylic acid cycle, leading to the formation of the key precursor in the flavonoids biosynthesis: the naringenin chalcone. The exact nature of the molecular species that undergo polymerization and the mechanism of assembly in proanthocyanidins are still unknown. From a structural point of view, flavanols comprise a C15 (C6-C3-C6) general structure composed by a benzopyran moiety (A and C rings) with an additional aromatic ring (B ring) linked to carbon C-2 of C ring. Flavanols are present in nature in monomeric, oligomeric, and polymeric forms and differ from each other essentially in the configuration of carbon C-2, the hydroxylation/methoxylation pattern of the rings, the type of linkage between each unit, and the degree of galloylation. Flavanols in foods are described to present several beneficial effects such as antioxidant and anticarcinogenic properties and also contribute to the sensory properties of some food products, such as astringency and color. Some of these aspects are discussed herein.