Fragmentation of alpha-radical cations of arginine-containing peptides.

Fragmentation pathways of peptide radical cations, M(+*), with well-defined initial location of the radical site were explored using collision-induced dissociation (CID) experiments. Peptide radical cations were produced by gas-phase fragmentation of Co(III)(salen)-peptide complexes [salen = N,N'-ethylenebis (salicylideneiminato)]. Subsequent hydrogen abstraction from the beta-carbon of the side-chain followed by C(alpha)-C(beta) bond cleavage results in the loss of a neutral side chain and formation of an alpha-radical cation with the radical site localized on the alpha-carbon of the backbone. Similar CID spectra dominated by radical-driven dissociation products were obtained for a number of arginine-containing alpha-radicals, suggesting that for these systems radical migration precedes fragmentation. In contrast, proton-driven fragmentation dominates CID spectra of alpha-radicals produced via the loss of the arginine side chain. Radical-driven fragmentation of large M(+*) peptide radical cations is dominated by side-chain losses, formation of even-electron a-ions and odd-electron x-ions resulting from C(alpha)-C bond cleavages, formation of odd-electron z-ions, and loss of the N-terminal residue. In contrast, charge-driven fragmentation produces even-electron y-ions and odd-electron b-ions.

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.