Lingjun Ma, Andrew Waterhouse,* Aude Watrelot, and Bennett Addison 
*Department of Viticulture and Enology, University of California,
Davis, CA 95616 (alwaterhouse@ucdavis.edu)

Tannins are large molecules that change during winemaking and bottle aging. Tannins trigger a tactile sensation of harshness in young red wines, while they appear to soften during aging. The changes in astringency are correlated with tannin structure alteration, such that large, condensed tannins (high DP proanthocyanidins) are degraded to shorter tannins. This reduces their binding strength with salivary proteins and is presumably the reason for reduced astringency. Under acid conditions, the interflavan bond of proanthocyanidins is labile, leading to bond cleavage and releasing a C-4 carbocation. The expected reaction is a rearrangement where the released C-4 carbocation reacts with a different flavan-3-ol subunit on the A-ring, restoring an interflavan bond. However, the electrophilic intermediate could be trapped by other nucleophiles such as thiols, which is the basis of the “thiolysis” method for analyzing the components of proanthocyanidin. Sulfur dioxide, widely used in winemaking, is also a nucleophile. However, to date, no products of SO₂ additions to proanthocyanidins have been reported in wine. In this study, flavan-3-ol sulfonates were detected in wine by LC-MS. Two of the major ones were isolated and investigated using Nuclear Magnetic Resonance Spectroscopy (NMR). They were identified as catechin-(4