Tannin Structural Elucidation and Quantitative 31P NMR Analysis. 1. Model Compounds

Tannins and flavonoids are secondary metabolites of plants that display a wide array of biol. activities. This peculiarity is related to the inhibition of extracellular enzymes that occurs through the complexation of peptides by tannins. Not only the nature of these interactions, but more fundamentally also the structure of these heterogeneous polyphenolic mols. are not completely clear. This first paper describes the development of a new anal. method for the structural characterization of tannins on the basis of tannin model compds. employing an in situ labeling of all labile H groups (aliph. OH, phenolic OH, and carboxylic acids) with a phosphorus reagent. The 31P NMR anal. of 31P-​labeled samples allowed the unprecedented quant. and qual. structural characterization of hydrolyzable tannins, proanthocyanidins, and catechin tannin model compds., forming the foundations for the quant. structural elucidation of a variety of actual tannin samples described in part 2 of this series.