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

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Title Methionine, alpha-methylmethionine and S-methylcysteine radical cations: generations and dissociations in the gas phase
Authors/Editors* Junfang Zhao, C. M. Dominic Ng, Ivan K. Chu, K. W. Michael Siu and Alan C. Hopkinson
Where published* Phys.Chem.Chem.Phys
How published* Journal
Year* 2009
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Abstract
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.
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