Nonbonded interactions in Membrane Active cyclic biopolymers. IV. cation dependence

Binding energies of various analogues of valinomycin to the alkali metal cations Li+, Na+, K+, Rb+, and Cs+ have been evaluated. Different conformations of the ligand molecule—representing various stages in the capture and release of the ion by the ligand—were considered. In addition to the binding energy calculations, conformational potential energies of the valinomycin analogues have also been evaluated. The conformational energies lead to several predictions pertaining to the design of analogues with a desired ring configuration and conformation. The ion binding energies are smaller than corresponding hydration energies. While the valinomycin-to-ion binding energy decreases constantly when smaller ions are substituted for a larger one, the observed specific preference for potassium appears to be explainable in terms of the differences between the binding energies and corresponding hydration energies.