A 3D structural and conformational study of procyanidin dimers in water and hydro-alcoholic media as viewed by NMR and molecular modeling.

The three-dimensional structures of 5 procyanidin dimers have been determined in a hydro-alcoholic medium and in water using 2D NMR and molecular mechanics. They are made from monomers of catechin (CAT) and epicatechin (EPI)-B1: EPI-CAT, B2: EPI-EPI, B3: CAT-CAT, B4: CAT-EPI and B2g: EPI-EPI-3-O-gallate. These tannins exist in two conformations that are in slow exchange in the NMR timescale (s), one is compact and the other extended. The compact form is found to dominate (76-98%) when the dimer is made of at least one CAT monomer (B1, B3, B4). Both forms are found in even proportions only in the case of procyanidin B2. The latter tannin can be converted into a dominant compact form when the lower EPI unit is galloylated. The finding of a predominant compact form for procyanidin dimers is discussed in relation with tannin-saliva protein interactions that are of importance for the wine-tasting/making processes.