New acylated flavonols identified in Vitis vinifera grapes and wines

Abstract

Flavonols are a class of polyphenol compounds whose importance for wine quality has increased as their structures and properties have become better understood. Here, the acetylated and p-coumaroylated derivatives of the flavonol 3-O-glucosides of isorhamnetin, laricitrin and syringetin have been identified for the first time in Vitis vinifera grape skins and wines. First, the MS² fragmentation patterns of the new flavonol derivatives showed a main signal attributable to the expected flavonol aglycone. In the p-coumaroylated derivatives, the signal corresponding to the intermediate loss of the phenolic acid was also observed. The structures of the aglycones were confirmed by their respective MS³ experiments that matched with those obtained from authentic standards of the aglycones. In addition, the fragmentation signals corresponding to the aglycone radical ions generated through homolytic cleavage assisted identification, and could support future studies of flavonoid compounds by ESI-MS. Using an HPLC-ESI-Q-ToF system, the observed m/z values of the compounds being studied were successfully matched with the expected formula. Surprisingly, just the minority methoxylated flavonol glucosides presented acylation, suggesting a high substrate specificity of the acyltransferases implicated in their synthesis. These findings show higher diversity of grape and wine flavonols. Additional studies and isolation strategies need to be followed to further characterize these metabolites as to test their presence in other grape varieties and it wines.

Keywords: Bioactive compounds; Flavonoids; Flavonols; Grapes and wines; Heterolytic cleavage; Homolytic cleavage; Polyphenolic compounds.