"Effects of Disulfide Linkages on Gas-phase Reactions of Small Multiply Charged Peptide Ions," J. Wang and C.J. Cassady, Int. J. Mass Spec. Ion Proc. 182/183, 233-241 (1999).

Multiply protonated ions of disulfide-intact and -reduced peptides were generated by electrospray ionization (ESI) and studied by Fourier transform ion cyclotron resonance (FT-ICR) 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 to 14 amino acid residues, H/D exchange was 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) reacted 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- (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 Arg8-vasopressin become slightly more compact upon cleavage of the disulfide bond; these reduced ions were slower to react. For [M+3H]³⁺ of somatostatin-14, reduction of the disulfide bond had 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 reactions.