Negative ion mass spectra of Cys-containing peptides. The characteristic Cys gamma 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 S-H...O=C 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 delta 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 gamma 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 M-H anions of five selected Cys-containing peptides all contain gamma and (gamma-H(2)S) anions. Three of these spectra also show the less favoured delta cleavage anions.