Mechanistic investigation of phosphate ester bond cleavages of glycylphosphoserinyltryptophan radical cations under low-energy collision-induced dissociation

Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G _p SW] ^•+) fragments differently from its tautomer with the radical initially generated on the α-carbon atom of the glycine residue ([G ^• _p SW]⁺). The dissociation of [G ^• _p SW]⁺ is dominated by the neutral loss of H₃PO₄ (98 Da), with backbone cleavage forming the [b₂ - H]^•+/y₁ ⁺ pair as the minor products. In contrast, for [G _p SW]^•+, competitive cleavages along the peptide backbone, such as the formation of [G _p SW - CO₂]^•+ and the [c₂ + 2H]⁺/[z₁ - H]^•+ pair, significantly suppress the loss of neutral H₃PO₄. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G^• _p SW]⁺ and [G _p SW]^•+ and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. We also propose that the loss of H₃PO₄ from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G^• _p SW]⁺ results in its charge being more mobile, thereby favoring charge-driven loss of H₃PO₄; in contrast, radical-driven pathways are more competitive during the CID of [G _p SW]^•+. [Figure not available: see fulltext.] © 2013 American Society for Mass Spectrometry.