Surprising structural lability of a cysteine-S-conjugate precursor of 4-methyl-4-sulfanylpentan-2-one, a varietal aroma in wine of Vitis vinifera L. cv. Sauvignon blanc

4-Methyl-4-sulfanylpentan-2-one (1; 4MSP) provides a characteristic aroma compound of wines made from Vitis vinifera L. cv. Sauvignon blanc. 4MSP has a strong box-tree odor with a very low perception threshold and is derived from the cysteinylated precursor S-(1,1-dimethyl-3-oxobutyl)cysteine (41; P-4MSP). P-4MSP is transformed into 4MSP during alcoholic fermentation and is an excellent marker of varietal aroma potential. An improved synthesis of P-4MSP as well as of its deuterium-labeled analogue [D-6]-P-4MSP is described. Several analytical methods (NMR, IR, LSI-MS, GC/MS, ESI-MSn) were combined to elucidate spontaneous reversible structural changes of P-4MSP at different pH values. At low pH, P-4MSP has a linear keto form. The keto-enol tautomerism was observed at neutral pH. At pH 8, the formation of N-substituted intramolecular hemiaminal was characterized by ESI-MS and ESI-MSn experiments. The hemiaminal loses H2O at high pH to produce a cycloimine, which is easily opened by acid hydrolysis. The keto-enol tautomerism explained the incorporation of only six D-atoms during the preparation of the P-4MSP deuterated standard even if [D-10]mesityl oxide was used. Derivatization conditions for GC/MS analysis strongly affected the ratio of the monosilylated intramolecular cyclic form and the disilylated linear form of P-4MSP. The structural changes of P-MSP may have a considerable impact on the development of methods of measuring varietal aroma potential.