Negative ion mass spectra of Cys‐containing peptides. The characteristic Cys γ backbone cleavage: a joint experimental and theoretical study

The Cys residue initiates characteristic backbone cleavages of [M−H] − anions of Cys‐containing peptides. A combination of experiment and theory suggests that these processes are initiated by molecular recognition between the C‐terminal CONH − group (in this study all peptides have C‐terminal CONH 2 groups) and the SH in the Cys side chain to form an SH ··· OC hydrogen bond. This process is exothermic by 60 kJ mol −1 (calculations at the HF/6‐31G(d)//AM1 level of theory). The structure of this reactive intermediate has the NH − of the amide group and the central CH of the Cys residue locked into position such that these groups effect an S N 2 process to form an intermediate which can either (i) dissociate to give an RNH − species [the δ ion (process endothermic by 37 kJ mol −1 with a barrier of 132 kJ mol −1 )], or (ii) effect deprotonation within the intermediate to eliminate RNH 2 to give the γ backbone cleavage anion in a reaction exothermic by 40 kJ mol −1 with a barrier of 132 kJ mol −1 . Collision‐induced mass spectra of the [MH] − anions of five selected Cys‐containing peptides all contain γ and ( γ ‐H 2 S) anions. Three of these spectra also show the less favoured δ cleavage anions. Copyright © 2004 John Wiley & Sons, Ltd.