First identification and quantification of S-3-(hexan-1-ol)-γ-glutamyl-cysteine in grape must as a potential thiol precursor, using UPLC-MS/MS analysis and stable isotope dilution assay

•Multi-step synthesis of natural and deuterated dipeptide S-conjugates.•A fast and reliable UPLC-MS/MS method for S-conjugates in must.•Formal identification of γGlutamyl-cysteine S-conjugate to 3-mercaptohexanol in must.•S-conjugates analysed in more than 200 must samples. Varietal thiols are key aroma compounds in wine issued from multiple and complex origins. Several precursor families have been identified in grapes and must and have been widely studied. But a large part of thiol origin still remains unknown. Thus, we only have an incomplete picture of thiol precursors and there is a lack of knowledge on pre-fermentative mechanisms that can impact their levels. Our study focused on the formal identification and the quantification of new varietal thiol precursors in must. First of all, we synthesized natural and labeled standards using an original multi-step strategy, then we developed and validated a UPLC-MS/MS method that allowed us to identify and quantify for the first time a dipeptide S-conjugate to 3MH, the γGluCys-3MH, in Sauvignon B. We observed the S-4-mercapto-4-methylpentan-2-one-l-cysteinyl-glycine (CysGly-4MMP) and S-4-mercapto-4-methylpentan-2-one-N-(l-γ-glutamyl)-l-cysteine (γGluCys-4MMP) but at too low concentration to be quantified.