Methionine, α-methylmethionine and S-methylcysteine radical cations: generations and dissociations in the gas phase

Abstract

Methionine, α-methylmethionine and S-methylcysteine radical cations have been formed by oxidative dissociations of [Cu^II(M)(CH₃CN)₂]˙²⁺ complexes. The radical cations M˙⁺ were trapped, and CID spectra (MS³) of these ions are presented. Fragmentations of the methionine and S-methylcysteine radical cations, initiated by migration of the α-carbon hydrogen atom to the sulfur, trigger the losses of water and thiomethanol from methionine and thiomethanol from S-methylcysteine. Deuterium labeling experiments show that considerable H–D scrambling and rearrangements involving N–H and S–H hydrogens occur in the methionine radical cation prior to fragmentation. An additional channel for S-methylcysteine is the loss of ammonia following β-hydrogen migration. Methylation at the α-carbon of methionine results in a radical cation that fragments differently. Two neutral losses from α-methylmethionine, NH₃ and methyl vinyl sulfide, CH₂CH–S–CH₃, are initiated by γ-hydrogen migration; a third channel is the loss of ^˙COOH. DFT computations at the B3LYP/6-311++G(d,p) level have been used to test aspects of the proposed fragmentation mechanisms of the radical cations.