Interaction studies of cysteine with Li+, Na+, K+, Be2+, mg2+, and Ca2+ metal cation complexes

The coordination geometries, electronic features, metal ion affinities, entropies, and the energetics of Li+, Na+, K+, Be2+, Mg2+, and Ca2+ metal cations with different possible conformations of cysteine complexes were studied. The complexes were optimized using density functional theory (B3LYP) and second order Moller-Plesset Perturbation (MP2) theory methods using 6-311 + +G** basis set. The interactions of the metal cations at different nucleophilic sites of cysteine conformations were considered after a careful selection among several binding sites. All the metal cations coordinate with cysteine in a tridentate manner and also the most preferred position for the interaction. It is found that, the overall structural parameters of cysteine are not altered by metal ion substitution, but, the metal ion-binding site has undergone a noticeable change. All the complexes were characterized by an electrostatic interaction between ligand and metal ions that appears slightly more pronounced for lithium and beryllium metal complexes. The metal ion affinity (MIA) and basis set superposition error (BSSE) corrected interaction energy were also computed for all the complexes. The effect of metal cations on the infrared (IR) stretching vibrational modes of amino N-H bond, side chain thiol group S-H bond, hydroxyl O-H bond, and Carbonyl C=O bond in cysteine molecules have also been studied. The nature of the metal ion-ligand bond and the coordination properties were examined using natural bond order (NBO) at bond critical point (electron density and their Laplacian of electron density) through Atoms in Molecules (AIM) analyses. Copyright (C) 2010 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this paper.