PROMOTION AND STABILIZATION OF B(1) IONS IN PEPTIDE PHENYLTHIOCARBAMOYL 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 Ed man method for peptide sequencing; subsequent treatment with anhydrous acid effects cleavage of the N-terminal peptide bond yielding a deriv atized amino acid and a truncated peptide. Low-energy collisional acti vation of peptide PTC derivative [M+2H](2+) ions during electrospray t andem mass spectrometry results in highly favoured cleavage of the N-t erminal peptide bond yielding complementary b(1) and y(n-1) fragments. The cleavage is evidently promoted by protonation of the peptide back bone. The apparently close mechanistic similarity between the gas-phas e 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 o n the carbonyl carbon of the first peptide bond. Collisionally activat ed decomposition of source-formed b(1) ions from a peptide PTC derivat ive is consistent with ion rearrangement similar to the PTC-phenylthio hydantoin isomerization observed in the condensed phase. (C) 1997 by J ohn Wiley & Sons, Ltd.