Résumé
Decades of research have been devoted to polyfunctional thiols in wine, but there are still questions that remain unanswered. One intriguing aspect is the formation of precursors to 4-methyl-4-sulfanylpentan-2-one (4-MSP) in grape berry and another is the fate of thiols during winemaking and beyond. In contrast to non-volatile precursors of 3-sulfanylhexan-1-ol (3-SH), which incorporate (E)-2-hexenal formed in grape berry, the alkenal skeleton necessary for analogous formation of 4-MSP precursors, namely mesityl oxide (MO), has been scarcely reported in grape juice or wine. Indeed, the pathway to MO formation has not been determined, in contrast to the LOX/HPL route to (E)-2-hexenal from linolenic acid. When it comes to chemical fate of thiols such as 3-SH after its release from precursors during fermentation, disulfide formation or reaction with quinones have been considered, but the role of other wine electrophiles such as acetaldehyde had escaped attention.Considering the research gaps, we undertook isotope tracer studies with d10-MO applied to grape berries and leaves of potted grapevines, analysing these tissues using HPLC-MS/MS to investigate the formation of glutathione conjugates of labelled 4-MSP. The experiments involved three grape varieties and treatment with d10-MO for different durations. After solid-phase extraction of berry and leaf extracts, HPLC analyses with triple quadrupole and high resolution MS led to the tentative identification of deuterated isotopologues of the glutathione conjugate of 4-MSP, which were matched against an authentic standard, in the two grape varieties that experienced a longer application of d10-MO. The presence of deuterated cysteine conjugates of 4-MSP was also observed by high resolution MS. Separately, we investigated the formation of 1,3-oxathianes arising from reaction of 3-SH and 4-MSP with acetaldehyde, which constitute a novel type of volatile sulfur compound in wine. After rigorous compound identification, deuterated standards were prepared and stable isotope dilution analysis methods were validated, including for chiral analysis of these oxathianes. A number of studies were undertaken to explore the significance of the new oxathianes, and the relationship between thiol and oxathiane chirality was assessed.