Cascade Dissociations of Peptide Cation-Radicals. Part 1. Scope and Effects of Amino Acid Residues in Penta- Nona- and Decapeptides

摘要

Amino acid residue-specific backbone and side-chain dissociations of peptide >z ions in MS³ spectra were elucidated for over 40 pentapeptides with arginine C-terminated sequences of the AAXAR and AAHXR type, nonapeptides of the AAHAAXYAR and AAHAXAYAR type, and AAHAAXYAAR decapeptides. Peptide >zn ions containing amino acid residues with readily transferrable benzylic or tertiary β-hydrogen atoms (Phe, Tyr, His, Trp, Val) underwent facile backbone cleavages to form dominant >zn-2 or >zn-3 ions. These backbone cleavages are thought to be triggered by a side-chain β-hydrogen atom transfer to the >z ion Cα radical site followed by homolytic dissociation of the adjacent Cα—CO bond, forming >zn-2 + HNCO cation-radicals that spontaneously dissociate by loss of HNCO. Amino acid residues that do not have readily transferrable β-hydrogen atoms (Gly, Ala) do not undergo the >zn → >zn-2 dissociations. The backbone cleavages compete with side-chain dissociations in >z ions containing Asp and Asn residues. Side-chain dissociations are thought to be triggered by α-hydrogen atom transfers that activate the Cβ—Cγ or Cβ—heteroatom bonds for dissociations that dominate the MS³ spectra of >z ions from peptides containing Leu, Cys, Lys, Met, Ser, Arg, Glu and Gln residues. The Lys, Arg, Gln, and Glu residues also participate in γ-hydrogen atom transfers that trigger other side-chain dissociations.