Cα–Cβ Bond Cleavage Occurs at the Side Chain of the N-Terminal Phenylalanine Residue During CID of Tyrosine-Containing Peptide Radical Cations
作者单位：Department of ChemistryUniversity of Hong Kong Department of ChemistryCity University of Hong Kong
会议名称：《2018年中国质谱学术大会（CMSC2018）》
会议日期：2018年
学科分类：0710[理学-生物学] 071010[理学-生物化学与分子生物学] 081704[工学-应用化学] 07[理学] 08[工学] 0817[工学-化学工程与技术] 070302[理学-分析化学] 0703[理学-化学]
关 键 词：Radical-site migration hydrogen atom transfer proton transfer Cα–Cβ bond cleavage 91 Da loss
摘 要：Molecular radical cationic peptides(M) display rich and diverse gas-phase ion chemistry that can differ substantially from that of their protonated counterparts. Dissociation of M radical cations can provide relevant structural information for protein sequencing, often complementary to that obtained using traditional approaches1-2. In this study, low-energy collision-induced dissociation(CID) of a series of tyrosine-containing peptide molecular radical cations having an N-terminal phenylalanine residue led to new products resulting from radical-mediated C–C bond cleavage at the side chain of the phenylalanine residue. The chemical identity of the species formed through side chain loss(91 Da) occurring via N-terminal C–C bond cleavage was examined using a combination of lowenergy CID experiments, isotopic labeling mass spectrometric experiments, and density functional theory calculations. This unusual dissociation process occurs with fascinating unimolecular gas phase ion chemistry and requires a π-radical cationic tyrosine residue with significant spin density delocalized into the N-terminal amino group. The radical character at the resulting –NH moiety then induces β-scission at the side chain C–C bond of the phenylalanine residue. This unique neutral radical loss(91 Da) from the N-terminal phenylalanine residue, observed here for the first time, should provide additional sequence information for biological tandem mass spectrometry. This discovery also demonstrates the role of the NH group in facilitating C–C bond cleavage of the phenylalanine residue by the proximal tyrosine π-radical cation, probably involving a through-space electron transfer process.