Mechanistic examination of Cβ-Cγ bond cleavages of tryptophan residues during dissociations of molecular peptide radical cations.

In this study, we used collision-induced dissociation (CID) to examine the gas-phase fragmentations of G(n)W (n = 2-4) and GXW (X = C, S, L, F, Y, Q) species. The C(β)-C(γ) bond cleavage of a C-terminal decarboxylated tryptophan residue (M - CO(2)) can generate M - CO(2) - 116, M - CO(2) - 117, and 1H-indole (m/z 117) species as possible product ions. Competition between the formation of M - CO(2) - 116 and 1H-indole systems implies the existence of a proton-bound dimer formed between the indole ring and peptide backbone. Formation of such a proton-bound dimer is facile via a protonation of the tryptophan γ-carbon atom as suggested by density functional theory (DFT) calculations. DFT calculations also suggested the initially formed ion 2, the decarboxylated species that is active against C(β)-C(γ) bond cleavage, can efficiently isomerize to form a more stable π-radical isomer (ion 9) as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. The C(β)-C(γ) bond cleavage of a tryptophan residue also can occur directly from peptide radical cations containing a basic residue. CID of WG(n)R (n = 1-3) radical cations consistently resulted in predominant formation of M - 116 product ions. It appears that the basic arginine residue tightly sequesters the proton and allows the charge-remote C(β)-C(γ) bond cleavage to prevail over the charge-directed one. DFT calculations predicted that the barrier for the former is 6.2 kcal mol(-1) lower than that of the latter. Furthermore, the pathway involving a salt-bridge intermediate also was accessible during such a bond cleavage event.