Terence H. Lilley

Polyphenols (tannins) in the diet not only precipitate oral proteins, producing an astringent sensation, but also interact with dietary proteins and digestive enzymes in the gut, resulting in a variety of antinutritive and toxic effects. Salivary proline-rich proteins (PRPs), which are secreted into the oral cavity, form complexes with and precipitate dietary polyphenols, and thus, they constitute the primary mammalian defense directed against ingested tannins. In order to characterize the interaction, NMR studies were performed which involved titrating a series of polyphenols into a synthetic 19-residue PRP fragment. The results show that the predominant mode of association is a hydrophobic stacking of the polyphenol ring against the pro-S face of proline and that the first proline residue of a Pro-Pro sequence is a particularly favored binding site. Measurement of dissociation constants indicates that the larger and more complex polyphenols interact more strongly with the PRP fragment; the order of binding affinity was determined as procyanidin dimer B-2 > pentagalloylglucose > trigalloylglucose >> proanthocyanidin monomer (-)-epicatechin approximately propyl gallate. Smaller polyphenols can bind with one phenolic ring stacked against each proline residue, whereas larger polyphenols occupy two or three consecutive prolines. The more complex polyphenols interact with the PRP fragment in a multidentate fashion; moreover, they self-associate or stack when bound. Thus, a model is proposed in which multiple polyphenol/polyphenol and polyphenol/PRP interactions act cooperatively to achieve precipitation.

The structures of plant polyphenols (vegetable tannins) are briefly reviewed. Their interactions with proteins, polysaccharides, and the alkaloid caffeine are discussed at the molecular level, and these fundamental properties are related to the quality of astringency that polyphenols possess. The various ways in which astringency may be modified and ultimately lost are outlined in relation to the aging of red wines, the formation of nonbiological hazes in beers and lagers, and the ripening of fruit.

Early studies of the interactions between polyphenols and proteins are reviewed. The complexation of some simple phenols and a group of biosynthetically inter-related esters of gallic acid with bovine serum albumin (BSA) is examined by equilibrium dialysis and microcalorimetry. The phenomenon is pH dependent. The results indicate that molecular size and conformational flexibility of the polyphenol substrate lead to enhanced interactions with the protein. Preliminary studies with polysaccharides indicate that the binding here is pH independent. These studies suggest that whilst the binding of polyphenols to these macromolecules is influenced by similar structural features the ability of the polysaccharide to form structures which encapsulate the polyphenol is, in this instance, a further critical feature of the complexation.