Structure-property relationships in gas-phase protonated and metalated peptide ions

Abstract

Peptide synthesis and metal doping, combined with mass spectrometric and ion mobility spectrometric techniques, have provided a picture of the fragmentation behavior of a large field of homologous peptide ions, represented as XVGVAZG, where the X amino acid is either arginine, histidine, lysine, aspartic acid or tryptophan and the Z amino acid is proline, glycine, serine, or histidine. These homologous peptide ions have been carefully selected to probe the effects of charge site location and secondary interactions upon the fragmentation chemistry of peptides. Peptides were synthesized on solid support, doped with appropriate metal salts to attach Li+, Na+, K+, Cu+ and Ag+ , and then examined using ion mobility spectrometry, and tandem mass spectrometry, both high energy collision induced dissociation (CID) and photodissociation using 193- nm laser light. Molecular dynamics calculations enabled me to derive candidate structures for these ions that agree with the ion mobility data for the ions. The fragmentation chemistry and structure selection of the first group of peptides, those that contain a proline residue, indicate that the presence of high proton and high metal ion affinity residues at the N-terminal position of the peptide direct the fragmentation of the highly charge-solvated ions according to a charge site directed mechanism. Further examples of charge-solvated structures and charge-directed fragmentation are shown for peptides where the sixth amino acid residue has been replaced with glycine or serine, eliminating the influence of the proline residue in the sixth position. Photodissociation of the peptides indicates that the position of valine residues along the peptide backbone influences the types of abundant fragment ions observed and ai and dai ions are observed exclusively at the site of valine residues. This observation continued, even when the position of the valine residues were altered by synthesis, leading me to the conclusion that the fragmentation of these peptides. The study was expanded to include significantly more complex peptides, those containing second high proton and high metal ion affinity residues, and though the data are complex, the influence of charge solvation in those systems is strong as well, according to my analysis of the candidate structures obtained and the types of fragment ions observed.