The interactions between the B3 (catechin-4alpha,8-catechin) red wine tannin and the human salivary protein fragment IB7(14) (SPPGKPQGPPPQGG) were monitored by (1)H magic angle spinning NMR, circular dichroism, electrospray ionization mass spectrometry, and molecular modeling. It is found that the secondary structure of IB7(14) is made of a type II helix (collagen helix) and random coil. The central glycine 8 appears to act as a flexible rotula separating two helix II regions. Three tannin molecules tightly complex the peptide, without modifying its secondary structure, but seem to reduce its conformational dynamics. The binding dissociation constant is in the millimolar range. B3 tannins with a "tweezers" conformation bind to the hydrophilic side of the saliva peptide, suggesting that the principal driving forces toward association are governed by hydrogen bonding between the carbonyl functions of proline residues and both the phenol and catechol OH groups. These findings are further discussed in the frame of an astringency phenomenon.

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. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: procyanidin dimers; NMR; molecular modeling; 3D structure INTRODUCTIONTannins play a central role in red wine flavour and in mouth-feel sensations such as astringency. 1 It is now well established that astringency is a tactile sensation 2,3 that results from the binding of polyphenols with one category of salivary proteins, the Proline-Rich Proteins (PRP). 4 -6 Interestingly, this type of interaction governs also leather tanning processes. 7 It is strong enough to bring on the subsequent precipitation of the complex, thus modifying the lubricating properties of saliva. 8 The principal source of wine tannins comes from the grape, and particularly from its solids parts, skins, seeds and stems. They are transferred to wine during wine-making operations (crushing, maceration and fermentation). 9 They are polymers of flavan-3-ols, also called proanthocyanidins or condensed tannins and they are able to reach a concentration up to 3 g/l, depending on vine variety, vintage, soil and wine-making practice. 10 Procyanidins are able to self-associate in a nonspecific way, leading to colloidal and/or aggregate formation. 11,12 Understanding this physicochemical behavior in a wine-like medium is of importance because it first determines the limpidity of wine, a quality that is demanded by consumers. Secondly, it governs the tannins' bioavailability by reducing their effective concentration with regard to their astringency partner, saliva proteins. Because 3D structure can influence physicochemical behavior, we firstly focus our attention on the comparison of the three-dimensional structure of procyanidin dimers throughout five examples (Scheme 1) that differ by the nature of their constitutive units and by the presence of a galloyl group on the lower unit. In this study, two approaches were simultaneously developed: NMR with its classical battery of two-dimensional experiments, which gave access to structural data throughout, coupling constant values and NOE values, and molecular modeling calculations in order to obtain clusters of minimized structures. The structures ...