Bilusich Daniel, Brinkworth Craig S, Bowie John H
Department of Chemistry, The University of Adelaide, South Australia, 5005 Australia.
Rapid Commun Mass Spectrom. 2004;18(5):544-52. doi: 10.1002/rcm.1360.
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.
半胱氨酸（Cys）残基引发含半胱氨酸肽的M-H阴离子特有的主链裂解。实验与理论相结合表明，这些过程是由C端CONH(-)基团（在本研究中所有肽均具有C端CONH₂基团）与半胱氨酸侧链中的SH之间的分子识别引发，形成S-H...O=C氢键。该过程放热60 kJ mol⁻¹（在HF/6-31G(d)//AM1理论水平下计算）。这种反应中间体的结构使得酰胺基团的NH(-)和半胱氨酸残基的中心CH锁定在特定位置，从而这些基团引发一个双分子亲核取代（S(N)2）过程，形成一种中间体，该中间体既可以（i）解离生成RNH(-)物种[δ离子（该过程吸热37 kJ mol⁻¹，势垒为132 kJ mol⁻¹）]，或者（ii）在中间体内发生去质子化以消除RNH₂，在一个放热40 kJ mol⁻¹、势垒为132 kJ mol⁻¹的反应中生成γ主链裂解阴离子。五种选定的含半胱氨酸肽的M-H阴离子的碰撞诱导质谱均包含γ和（γ-H₂S）阴离子。其中三种质谱还显示出较不常见的δ裂解阴离子。
