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 ([GpSW]•+) fragments differently from its tautomer with the radical initially generated on the α-carbon atom of the glycine residue ([G•pSW]+). The dissociation of [G•pSW]+ 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 [GpSW]•+, competitive cleavages along the peptide backbone, such as the formation of [GpSW – 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•pSW]+ and [GpSW]•+ 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•pSW]+ 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 [GpSW]•+.