title: Mechanistic investigation of phosphate ester bond cleavages of glycylphosphoserinyltryptophan radical cations under low-energy collision-induced dissociation.
abstract: 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](+)/[z(1) - 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](•+).
authors.affiliation: Department of Chemistry, University of Hong Kong, Hong Kong, China.
authors.forename: Quan
authors.initials: Q
authors.lastname: Quan
authors.forename: Qiang
authors.initials: Q
authors.lastname: Hao
authors.forename: Tao
authors.initials: T
authors.lastname: Song
authors.forename: Chi-Kit
authors.initials: CK
authors.lastname: Siu
authors.forename: Ivan K
authors.initials: IK
authors.lastname: Chu
mesh_terms: D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D005998:Glycine / Q000737:chemistry*; D013058:Mass Spectrometry; D010748:Phosphopeptides / Q000737:chemistry*; D010766:Phosphorylation; D010768:Phosphoserine / Q000737:chemistry*; D013816:Thermodynamics; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D002412:Cations; D005609:Free Radicals; D010748:Phosphopeptides; D010768:Phosphoserine; D014364:Tryptophan; D005998:Glycine