A COMPREHENSIVE ENERGY COMPONENT ANALYSIS OF THE INTERACTION OF HARD AND SOFT DICATIONS WITH BIOLOGICAL LIGANDS

The interactions of the ions Zn2+, Mg2+, Ca2+, and Cd2+ with biological ligand models are analyzed with an energy decomposition method giving the contributions of electrostatics, exchange repulsion, ligand polarization, ligand-to-cation charge transfer, and the electronic correlation effect. The last two energy terms best correlate with the selectivity of ligands between group IIA and IIB ions and are therefore associated with chemical hardness and softness. The charge transfer is inhibited in a complete ligand shell and so the correlation effect, primarily a ligand-cation dispersion-like interaction, may be a more important factor in solution selectivity. The components associated with ionic binding are very similar for group IIA and IIB ions of similar size, suggesting no special stabilization for hard-hard ligand-cation pairs. Soft ligands are more selective for soft cations and hard ligands are simply non-selective with water being the least selective ligand. Other selectivity effects are discussed and the charge transfer is also analyzed in terms of physical atomic partial charges derived from dipole gradients.