Promotion and Stabilization of b 1 ions in Peptide Phenythiocarbamoyl Derivatives: Analogies with Condensed‐phase Chemistry

The preparation of the N-terminal phenylthiocarbamoyl (PTC) derivative is the first step in the condensed phase chemistry employed in the Edman method for peptide sequencing; subsequent treatment with anhydrous acid effects cleavage of the N-terminal peptide bond yielding a derivatized amino acid and a truncated peptide. Low-energy collisional activation of peptide PTC derivative [M+2H]2+ ions during electrospray tandem mass spectrometry results in highly favoured cleavage of the N-terminal peptide bond yielding complementary b1 and yn-1 fragments. The cleavage is evidently promoted by protonation of the peptide backbone. The apparently close mechanistic similarity between the gas-phase and condensed-phase processes may be readily understood in terms of current thinking concerning the mechanism of formation of b-type ions, which involves nucleophilic attack by an N-terminal carbonyl moiety on the carbonyl carbon of the first peptide bond. Collisionally activated decomposition of source-formed b1 ions from a peptide PTC derivative is consistent with ion rearrangement similar to the PTC–phenylthiohydantoin isomerization observed in the condensed phase.