Low-Energy Dissociation Pathways of Small Deprotonated Peptides in the Gas Phase

Abstract

The unimolecular dissociation dynamics of small deprotonated peptides generated with an external fast atom bombardment source have been investigated using Fourier transform ion cyclotron resonance mass spectrometry. Because the charge site is well defined in peptides lacking strongly acidic side chains, deprotonated peptides present a good model system for investigating the unimolecular dissociation dynamics of “large” molecules. Off-resonance collisional activation was used to determine the low-energy fragmentation pathways available to the peptides, which greatly contrast those of higher-energy dissociation techniques. Dissociation is governed by the site of deprotonation and yields partial sequence information in favorable cases. Almost all observed pathways were brought about by charge-induced mechanisms. The lowest energy dissociation pathway for all peptides without acidic side chains is elimination of the conjugate base of the C-terminus amino acid as the ionic fragment. This generally occurs in up to 100% yield with no competition. For peptides with acidic side chains alternate pathways are also observed. However, in most cases through competing or sequential dissociation processes the C-terminus amino acid could be determined. Calculations were carried out at the AM1 level to determine the minimum energy configurations of these species. Intramolecular hydrogen bonding to solvate and stabilize the charge is observed to be prevalent. The calculations provide further support for the dissociation mechanisms presented. Application of statistical RRKM calculations to these systems allows a qualitative understanding of the energetic changes associated with the observed dissociation processes, distinguishing in particular processes arising from competitive as opposed to sequential dissociations. The bimolecular reactivity of deprotonated peptides was also investigated. Several reactions taking advantage of the nucleophilicity of the deprotonated carboxylic group were observed.