Analyzing the Selectivity and Successiveness of a Two-Electron Capture on a Multiply Disulfide-Linked Protein

Hybrid QM/MM calculations were performed on a circular macropeptide (kalata B1, PDB ID 1NB1) containing three disulfide linkages, to evaluate their respective reactivities toward (gas-phase) electron valence-attachment of one and two electron(s). The three disulfide bonds -CH2-S-S-CH2- were simultaneously described at the MP2/6-31+G**(S),6-31G*(C,H) level of theory, and the remaining of the 29 residues of kalata B1 were described by the CHARMM27 force field. The one-electron addition is favored on the linkage between cysteine residues 1 and 15, Cys(1-15), by ca. 1 eV over the two other disulfide linkages. The decomposition of the overall effect into geometrical and electrostatic contributions evidence (i) the key role of an arginine (R24) and (ii) a weaker geometrical penalty for elongating the nonstructural Cys(1-15) linkage. The addition of a second electron leads to the formation of the dithiolate Cys(1,15), favored by more than 1 eV over other adducts (either dithiolates or diradical dianionic species). This can be traced back to a structural reorganization, with a flip of R24 side chain. Its positively charged extremity points almost equidistantly toward one thiolate -CH2-S(-), hence stabilizing this dianion.