Mass spectrometric determination of N-terminal tryptophan and N-terminal histidine in peptides

Abstract

The successful utilization of Schiff base peptide ester derivatives in sequence elucidation of peptides by mass spectrometry led to the study of N-terminal tryptophyl and histidyl peptides. We had earlier reported the formation of cyclization products when N-prolyl peptide esters had reacted with Schiff base forming reagents. We observed that the reaction of aldehydes with N-terminal tryptophyl peptide esters gave cyclization and subsequent dehydrogenation products which were found to be substituted β-carbolines (9H-pyrido(3,4-b)-indoles) formed by Mannich type condensation. The molecular ion and many of the expected sequence identifying peaks were prominent in the mass spectra. The facile pyrolysis products of these peptide derivatives also indicated the presence of β-carbolines and substituted β-carbolines. In a similar fashion peptides with N-terminal histidine gave 1H-pyrido(4,5-c)imidazoles.