Methionine, alpha-methylmethionine and S-methylcysteine radical cations: generations and dissociations in the gas phase.
Methionine, alpha-methylmethionine and S-methylcysteine radical cations have been formed by oxidative dissociations of [CuII(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 alpha-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 beta-hydrogen migration. Methylation at the alpha-carbon of methionine results in a radical cation that fragments differently. Two neutral losses from alpha-methylmethionine, NH3 and methyl vinyl sulfide, CH2=CH-S-CH3, are initiated by gamma-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.
Zhao
Junfang
J
Department of Chemistry, York University, Toronto, Canada.
Ng
C M Dominic
CM
Chu
Ivan K
IK
Siu
K W Michael
KW
Hopkinson
Alan C
AC
D002412:Cations / Q000737:chemistry; D003545:Cysteine / Q000031:analogs & derivatives* / Q000737:chemistry; D005740:Gases / Q000737:chemistry; D008715:Methionine / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular
D002412:Cations; D005740:Gases; C108103:alpha-methylmethionine; C008425:S-methylcysteine; D008715:Methionine; D003545:Cysteine