Mass Spectrometry of 2-Alkylthio-2-methylpropanoic Acids and Their Esters and Amides. Structural and Steric Effects on the McLafferty Rearrangement

Abstract

The electron ionization mass spectra (MS) of S-methylated derivatives of N-(2-mercapto-2-methylpropanoyl)-L-cysteine and 2-alkylthio-2-methylpropanoic acids, as well as their esters and amides, were examined. Use of the deuterium labeling technique and accurate mass measurement supported the proposed fragmentation pathways. Extensive loss of CH₂S from a molecular ion by the McLafferty rearrangement of a primary hydrogen is important in the MS of S-methyl compounds of amide derivatives. It was demonstrated that the intensity of the rearrangement ion decreases in the order of amide, ester, and acid, and in the case of amides the rearrangement is suppressed by the nonbonded interaction between methyl groups on the α carbon and the amide nitrogen.