Nonbonded interactions in membrane active cyclic biopolymers. I. Valinomycin–potassium ion complex

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Abstract
A method for quantitative comparison of various conformers of cyclic macromolecules in terms of nonbonded interaction energy terms is proposed. The first step in the procedure is an analytical evaluation of a set of Cartesian atomic coordinates consistent with the ring closure condition and prescribed values for internal parameters. A variant of the same technique enables different conformations of the macromolecule to be generated. Nonbonded interactions of the electrostatic, polarization and dispersion type and van der Waals repulsion are calculated using the generated coordinates, empirical bond polarizability data and results of simple molecular orbital calculations. An application of the proposed techniques to the closed symmetric conformation of valinomycin indicates that the macromolecule by itself is somewhat constrained in this state but that its affinity for a potassium ion is very strong—much stronger than the hydration energy of the latter.