NMR STRUCTURAL INVESTIGATIONS AND CONFORMATIONAL-ANALYSIS OF CONDENSED TANNINS - A CONTINUING CHALLENGE DUE TO RESTRICTED ROTATION ABOUT THE INTERFLAVANOID LINKAGE

Inverse detection and gradient accelerated 2D NMR techniques have made possible the unambiguous elucidation of the primary structure of smal l quantities (a few tons of mg) of oligomeric tannins. In future studi es, simulation of interactions with other biomolecules such as carbohy drates rind proteins will require knowledge of the secondary (3D) stru cture of these polyphenols which tan be determined from quantitative N MR (in solution) or crystallographic (solid state) data. The existence of stable rotamers due to restricted rotation about the interflavanoi d linkage remains a major handicap in the conformational analysis of c ondensed tannins with NMR methods.