Differentiation of diasteromeric alpha-sulfanyl-beta-amino acid derivatives by electrospray ionisation tandem mass spectrometry

A set of diastereomeric alpha-sulfanyl-beta-amino acid derivatives, which are important building blocks for pharmaceuticals with potent biological activity, are studied by electrospray ionisation tandem mass spectrometry. The collision-induced dissociation (CID) spectra of [M + H](+), [M + NH4](+), [M + Na](+) and [M + Li](+) of the diastereomers were studied, among them the CID of [M + Na](+) and [M + Li](+) showed consistent differences in the relative abundance of characteristic ions that enabled distinction of the anti isomers from syn isomers. The decomposition pathways for the diagnostic ions were arrived at based on high-resolution mass spectrometry data, multiple mass spectrometry data, deuterium labelling experiments and the mass shift in accordance with the substituents located at different places. Loss of (R-1-C6H4-CH=NH) and (Cat-NH-SO2R2) from [M + Cat](+), where Cat = Na and Li and the product ions, as a results of McLafferty rearrangement involving either the >S=0 or >C=0 groups, were found to be diagnostic. The McLafferty rearrangement product ions involving the >S=0 group were more abundant in syn isomers while those involving the >C=0 group were more abundant in anti isomers. The selectivity observed in the decomposition of [M + Li](+) ions was found to be similar to that of EM [M + Na](+) ions but, in a few cases, the differences are marginal in the decomposition of [M + Li](+) ions.