Sulfonated flavanols in wine: are they markers of inappropriate storage?

The ability to conduct experiments of “untargeted metabolomics” allows the chemist to attack problems related to wine-analysis that until a few years ago were completely intractable. However, in order to be really successful, this approach requires an extension of the analytical methodologies commonly used in metabolomics (LC-UV-MS) since some compounds, present in particular in wine, are not easily attributable to clearly-defined chemical structures. As a matter of fact, even after rigorous approaches of untargeted metabolomics on real samples, the list of “unknowns” should be quite long and a little bit disappointing. Not rarely, the identification of several metabolites can escape even the powerful tools (fragmentation experiments and high resolution MS measurements) made available by the modern arsenal of advanced mass spectrometry platforms. To proceed from putative versus validated markers, today the best analytical technique able to complement MS data is certainly Nuclear Magnetic Resonance (NMR). Thanks to it, the structure of an organic compound can be resolved, at least in principle, without the need to retrieve previous data from databases or to rely on the availability of the corresponding analytical standard, once its molecular formula has been established by high resolution MS measurements. Aim of this work was to investigate the time dependent formation of sulfonated-flavanols in a) Sangiovese wines produced in three consecutive years (2008-2010) at semi-industrial scale from grapes harvested from twenty vineyards of Montalcino and b) in 55 commercial Brunello di Montalcino wines from 18 different wineries (2004-2007). In particular, the major focus was to understand how the relative amount of these by-products were affected by the storage conditions (up to 24 months) of the wines themselves, eventually becoming an useful marker of wine-ageing in not-ideal storage conditions as pointed out by our recent metabolomics investigation (UPLC-Q TOF MS) [1]. The structures of these sulfonated-flavanols were unambiguously established by an integrated approach wherein high resolution mass measurements taken on-line on the corresponding chromatographic peaks were joined to extensive NMR analyses (1D- and 2D) carried out on their synthetic analogues; the latter were obtained by bisulfite addition, in model wine media, to epicatechin and commercial apple tannins. The results coming from this biomimetic synthesis will be followed by the proposal of a reasonable reaction mechanism. Moreover, comprehensive spectroscopic details of these sulfonated derivatives will be reported as well as other intriguing novel chemical properties observed during our NMR analyses of sulfonated procyanidins.