Effects of disulfide linkages on gas-phase reactions of small multiply charged peptide ions

Abstract

Multiply protonated ions of disulfide-intact and -reduced peptides were generated by electrospray ionization and studied by Fourier transform ion cyclotron resonance mass spectrometry. The effects of disulfide bonds on gas-phase deprotonation reactions and hydrogen/deuterium (H/D) exchange were investigated. Insight into conformations was gained from molecular dynamics calculations. For ions from three small peptides containing 9–14 amino acid residues, H/D exchange is more sensitive to changes in conformation than deprotonation. However, with both gas-phase reactions the more diffuse forms of the peptides (as determined by molecular modeling) react more readily. The effects of disulfide cleavage on the conformations and on the reactions were found to depend upon the sequence of the peptide. For [M + 3H] 3+ of TGF-α (34–43), reduction of the disulfide linkage leads to a greatly extended structure and a dramatic increase in the rate and extent of H/D exchange. In contrast, [M + 2H] 2+ of Arg 8 -vasopressin becomes slightly more compact upon cleavage of the disulfide bond; these reduced ions are slower to react. For [M + 3H] 3+ of somatostatin-14, reduction of the disulfide bond has little effect on conformation or gas-phase reactivity. Overall, these results indicate that there is no general rule on how cleavage of a disulfide bond will effect a peptide ion’s gas-phase reactivity.