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 ...

The colloidal behavior of eight synthetic procyanidins (three monomers, four dimers, and a trimer) has been investigated in water or in a winelike medium using DOSY NMR spectroscopy and molecular dynamics simulations. Different behavior was observed for monomers and oligomers. Monomers self-associate with a high affinity constant (37-53 M(-1)) to form micelles at low cmc (critical micelle concentration) values (1-5 g.L(-1)). These micelles undergo a time-dependent coalescence process to form hazes and precipitates. As for dimers and the trimer, self-association also occurs but with a lower affinity (approximately 6 M(-1)) and at higher cmc values (10-20 g.L(-1)) to form small micelles (<5 nm) that remain stable throughout the experiment. The presence of 10% ethanol does not significantly affect the self-association constant for monomers and oligomers but increases their cmc values by approximately 50% and decreases the micelle size by a factor 2. However, the presence of 20 mM NaCl appears to negate the effect of ethanol. This study helps to clarify the role of procyanidin monomers versus oligomers in wine turbidity and demonstrates that procyanidin oligomers are fully available to interact with saliva proteins.