A chromatographic and mass-spectrometric approach for the analysis of lipase-produced thioester derivatives

The synthesis of thioethyl 2-methylpropanoate, butanoate, 3-methylbutanoate, hexanoate, and of thiobutyl propanoate, butanoate, and pentanoate was achieved via esterification of ethanethiol or butanethiol with short-chain acids using an immobilised lipase from Rhizomucor miehei. High acid conversions were obtained (47% for thiobutyl pentanoate). The analysis of educts and products was carried out by reverse-phase liquid chromatography (HPLC), gas chromatography (GC), GC/mass spectrometry, and GC/olfactometry. All of the thioesters imparted a similar, onion-like smell, which remained unchanged on dilution. The thresholds of thioesters derived from the same thiol were loosely correlated with the size of the acid moiety: the larger the molecular mass the higher the threshold. Surprisingly, higher odour thresholds were obtained for the branched-chain thioesters than for their linear analogues.