title: Novel Cβ-Cγ bond cleavages of tryptophan-containing peptide radical cations.
abstract: In this study, we observed unprecedented cleavages of the C(β)-C(γ) bonds of tryptophan residue side chains in a series of hydrogen-deficient tryptophan-containing peptide radical cations (M(•+)) during low-energy collision-induced dissociation (CID). We used CID experiments and theoretical density functional theory (DFT) calculations to study the mechanism of this bond cleavage, which forms [M - 116](+) ions. The formation of an α-carbon radical intermediate at the tryptophan residue for the subsequent C(β)-C(γ) bond cleavage is analogous to that occurring at leucine residues, producing the same product ions; this hypothesis was supported by the identical product ion spectra of [LGGGH - 43](+) and [WGGGH - 116](+), obtained from the CID of [LGGGH](•+) and [WGGGH](•+), respectively. Elimination of the neutral 116-Da radical requires inevitable dehydrogenation of the indole nitrogen atom, leaving the radical centered formally on the indole nitrogen atom ([Ind](•)-2), in agreement with the CID data for [WGGGH](•+) and [W(1-CH3)GGGH](•+); replacing the tryptophan residue with a 1-methyltryptophan residue results in a change of the base peak from that arising from a neutral radical loss (116 Da) to that arising from a molecule loss (131 Da), both originating from C(β)-C(γ) bond cleavage. Hydrogen atom transfer or proton transfer to the γ-carbon atom of the tryptophan residue weakens the C(β)-C(γ) bond and, therefore, decreases the dissociation energy barrier dramatically.
authors.affiliation: Department of Chemistry, The University of Hong Kong, Hong Kong, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Song
authors.forename: Qiang
authors.initials: Q
authors.lastname: Hao
authors.forename: Chun-Hin
authors.initials: CH
authors.lastname: Law
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; D013058:Mass Spectrometry / Q000379:methods*; D010455:Peptides / Q000737:chemistry*; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D002412:Cations; D005609:Free Radicals; D010455:Peptides; D014364:Tryptophan