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



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Eur. J. Mass Spectrom. 5, 325 - 334 (1999)

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

Richard A.J. OHair*
School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia. E-mail:
Gavin E. Reid
School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia, and Joint Protein Structure Laboratory, The Ludwig Institute for Cancer Research and The Walter and Eliza Hall Institute of Medical Research. PO 2008 Royal Melbourne Hospital, Parkville, Victoria 3050, Australia

ABSTRACT:
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.

Keywords: vinyl alcohol, photoionization, appearance energy, heat of formation

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