Phenyl thiocarbamoyl and related derivatives of peptides: Edman chemistry in the gas phase

Abstract

Formation of the b 1 ion during the low energy collision-activated dissociation of the N-terminal phenyl thiocarbamoyl (PTC) derivatives of protonated peptides is analogous to the condensed-phase cleavage step of the Edman degradation. Previous studies confined to the analysis of tryptic peptides are here extended to probe the influence of peptide structure and extent of protonation on the prevalence of this fragmentation. The data are consistent with a requirement for protonation of the peptide backbone at the N-terminal amide linkage. Generally, PTC derivatives of peptides that incorporate as many or more basic amino acid residues than charges fail to undergo favored cleavage of the N-terminal amide bond, reflecting the absence of a proton resident on the peptide backbone to promote such fragmentation. Exceptions to this rule may be explained in terms of proton bridging between basic sites to release an ionizing proton for residence on the peptide backbone. Replacement of the PTC derivative by the pentafluoro-PTC analog results in similar fragmentation chemistry but with preferential loss of the derivatized N-terminal residue as a neutral fragment. Thus, judicious choice of derivatization procedure enables not only the direction of fragmentation but also of charge retention.