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 “cys2−”, respectively) are examined with the Becke three-parameter Lee–Yang–Parr (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.
Dedicated to Professor Shigeru Nagase on the occasion of his 65th birthday and published as part of the Nagase Festschrift Issue.
The online version of this article (doi: 10.1007/s00214-011-0975-z) contains supplementary material, which is available to authorized users.
This work was supported by Grants-in-Aid No. 19550004 for Scientific Research from JSPS and by Scientific Research on Priority Areas “Molecular Theory for Real Systems”, No. 20038005 from MEXT. The generous allotment of computation time from the Research Center for Computational Science (RCCS), the National Institutes of Natural Sciences, Japan, is also gratefully acknowledged.