Neighboring group versus cis-elimination mechanisms for side chain loss from protonated methionine, methionine sulfoxide and their peptides

The gas-phase fragmentation reactions of protonated methionine and methionine containing dipeptides have been compared with their methionine sulfoxide oxidized forms using tandem mass spectrometry (MS/MS). A structurally diagnostic fragment ion for methionine sulfoxide involves side chain loss of neutral CH3SOH. The mechanism for this loss has been probed using a combination of labelling experiments (both deuterium isotope and structural) as well as MS/MS/MS experiments. Charge-directed neighboring group processes compete with or dominate over charge-remote cis eliminations in all cases.