Department of Chemistry, York University, Toronto, Canada.
Phys Chem Chem Phys. 2009 Sep 21;11(35):7629-39. doi: 10.1039/b905615g.
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.
甲硫氨酸、α-甲基甲硫氨酸和 S-甲基半胱氨酸自由基阳离子通过 [CuII(M)(CH3CN)2]2+ 配合物的氧化解离形成。自由基阳离子 M+ 被捕获，并呈现这些离子的 CID 谱（MS3）。甲硫氨酸和 S-甲基半胱氨酸自由基阳离子的碎裂，由 α-碳原子上的氢原子向硫原子迁移引发，导致甲硫氨酸失去水和硫代甲醇，S-甲基半胱氨酸失去硫代甲醇。氘标记实验表明，在碎裂之前，甲硫氨酸自由基阳离子中发生了相当数量的 H-D 重排和涉及 N-H 和 S-H 氢的重排。S-甲基半胱氨酸的另一个通道是β-氢迁移后氨的损失。甲硫氨酸的 α-碳原子甲基化导致了不同的自由基阳离子碎裂。α-甲基甲硫氨酸的两个中性损失，氨和甲基乙烯基硫，CH2=CH-S-CH3，是由γ-氢迁移引发的；第三个通道是 *COOH 的损失。在 B3LYP/6-311+ +G(d,p) 水平上进行的 DFT 计算用于测试自由基阳离子提出的碎裂机制的各个方面。
