LOW-ENERGY DISSOCIATION PATHWAYS OF SMALL DEPROTONATED PEPTIDES IN THE GAS-PHASE

The unimolecular dissociation dynamics-of small deprotonated peptides generated with an external fast atom bombardment source have been inve stigated using Fourier transform ion cyclotron resonance mass spectrom etry. Because the charge site is well defined in peptides lacking stro ngly acidic side chains, deprotonated peptides present a good model sy stem for investigating the unimolecular dissociation dynamics of ''lar ge'' molecules. Off-resonance collisional activation was used to deter mine 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 part ial sequence information in favorable cases. Almost all observed pathw ays were brought about by charge-induced mechanisms. The lowest energy dissociation pathway for all peptides without acidic side chains is e limination of-the conjugate base of the C-terminus amino acid as the i onic fragment. This generally occurs in up to 100% yield with no compe tition. For peptides with acidic side chains alternate pathways are al so observed. However, in most cases through competing or sequential di ssociation processes the C-terminus amino acid could be determined. Ca lculations were carried out at the AMl level to determine the minimum energy configurations of these species. Intramolecular hydrogen bondin g 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 syste ms allows a qualitative understanding of the energetic changes associa ted with the observed dissociation processes, distinguishing in partic ular processes arising from competitive as opposed to sequential disso ciations. The bimolecular reactivity of deprotonated peptides was also investigated. Several reactions taking advantage of the nucleophilici ty of the deprotonated carboxylic group were observed.