Quantum chemical studies on the role of water microsolvation in interactions between group 12 metal species (Hg2+, Cd2+, and Zn2+) and neutral and deprotonated cysteines

Interactions of group 12 metal(II) species (Hg²⁺, Cd²⁺, Zn²⁺, Hg(H₂O) _n ²⁺ , Cd(H₂O) _n ²⁺ , and Zn(H₂O) _n ²⁺ (n?=?1, 2) with neutral (RSH), deprotonated (RS^?), and doubly deprotonated cysteine species (abbreviated as ??H₂cys??, ??Hcys^???, and ??cys^2???, respectively) are examined with the Becke three-parameter Lee?CYang?CParr (B3LYP) hybrid functional after preliminary screening in a conformation analysis with the Parameterized Model number 3 (PM3) semiempirical method. Effects of water on aqueous solution are evaluated by microsolvation and polarized continuum model (PCM) approaches. In the most stable conformations of M(H₂cys)²⁺ and M(Hcys)⁺ complexes (M?=?Hg²⁺, Cd²⁺, and Zn²⁺), the SH group of the cysteine moiety is already deprotonated and undergoes strong binding with the metal ion. Among Hg(H₂cys)²⁺ complexes, cysteine complexes of Hg²⁺ without deprotonation of the SH group and mercury(II) carboxylato-type structures are at least 83 and 117?kJ/mol less stable in energy than the most stable complex (B3LYP/6-311++G(d,p)-SDD+d+f//B3LYP/6-31G(d)-SDD+d). Although Zn²⁺ binds more strongly than Hg²⁺ to a H₂cys molecule at the high-level CCSD(T)/6-311++G(d,p)-SDD+d+f//B3LYP/6-311++G(d,p)-SDD+d+f level, Hg(H₂O)₂]²⁺ is stronger than Zn(H₂O)₂]²⁺ because the deformation of Zn(H₂O)₂]²⁺ required to bind to cys is much more than in Hg(H₂O)₂]²⁺. Complexes with a deprotonated cysteine, M(Hcys)⁺ and M(cys), prefer a multidentate structure.