New acylated flavonols identified in the grape skin of Vitis vinifera cv. Tannat and their wines

Abstract

Flavonols are a class of flavonoid compounds derived from plant secondary metabolism. There they play different roles like antioxidants, internal regulators and UV screenings. In red wines, flavonols have increasingly received consideration by part of scientific and winemakers according their properties began to arise known. Among these stand out wine colour stabilization and their value as bioactive compounds. In this work the complete series of the acetylated and p-coumaroylated derivatives of the 3-O-glycosides of methoxylated flavonols, namely isorhamnetin, laricitrin and syringetin, have been identified in grapes and their respective wines from Vitis vinifera cv. Tannat. The assignments were based on their UV-vis and MSn spectral data. Firstly, the MS2 fragmentation pattern of each tentatively identified flavonol glycoside derivative showed a main signal attributable to the expected flavonol aglycone, together with a weak signal corresponding to the intermediate loss of the acyl moiety. The structures of the flavonol aglycones were confirmed by their respective MS3 experiments that matched with those obtained from authentic standards of the three aglycones. In addition, the DAD on-line UV-vis spectra of the suggested flavonol-3-O-(p-coumaroyl)-glucosides closely matched the sum of the respective spectra of the flavonol-3-O-glucoside and that of p-coumaric acid. Interestingly, the presence of these new flavonol derivatives was limited to the minority flavonols. Being acylation a final step in the flavonoid synthesis, our findings suggest a very high specificity of the acyltransferases implied for the flavonol glycoside substrate that in this case would be related to the presence of methoxyl groups in the B ring of the flavonol. The fact that these compounds have been found in Tannat, a Vitis vinifera with scarce cultivation around the world, may indicate that the implied acyltransferases could be little spread among grape varieties. Nevertheless it could be associated to other factors like growth environmental conditions, or to the sensitivity of the analytical technique employed, or others factors, which should be further considered and studied. These results give evidences of new flavonol derivatives in grapes and wines, and expose a case of substrate specificity of the enzymes implied in their synthesis.