The structure and energetics of complexes obtained upon interaction between cysteine and Zn2+, Cd2+, Hg2+ and Cu2+ cations were studied using quantum chemical density functional theory calculations with the 6-311++G** orbital basis set and relativistic pseudopotentials for the cations. Different coordination sites for metal ions on several cysteine conformers were considered. In their lowest energy complexes with the amino acid, the Zn2+ and Cd2+ cations appear to be three-coordinated to carbonyl oxygen, nitrogen and sulfur atoms, whereas the Cu2+ and Hg2+ ions are coordinated to both the carbonyl oxygen and sulfur atoms of one of the zwitterion forms of the amino acid. Bonds of metal cations with the coordination sites are mainly ionic except those established with sulfur, which show a small covalent character that become most significant when Cu2+ and Hg2+ are involved. The order of metal ion affinity proposed is Cu>Zn>Hg>Cd.

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The structural, electronic, and energetic properties of cadmium ion complexes with one and two protonated and deprotonated 3-mercaptopropionic acid ligands were studied theoretically in the framework of density functional theory by using the B3LYP exchange-correlation functional coupled with 6-311++G** Gaussian orbital basis sets for C, H, O, and S atoms and LANL2DZ for the metal ion. The study was extended to the aqueous complexes introducing explicitly some water molecules in order to simulate the tetrahedral and octahedral coordination sphere of the Cd2+ ion. Results indicate that the complexes containing only one ligand have a cyclic structure in which the metal ion binds both oxygen and sulfur atoms. The tetrahedral topology is preferred in the complexes with two ligands.

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