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larger alkyl groups, and other substituents, X. It could be extended to di-, tri-, and polysubstituted alkanes if induced dipoles or van der Waals forces are includedLLJ5 Following Benson and Luna4 it could be extended to radicals and alkenes. Once it has been refined, it could be made accessible in molecular mechanics programs, where it might reduce the number of input parameters required or provide better understanding of the forces involved.With the inclusion of appropriate induced dipoles, it could be used to calculate molecular dipole moments and intermolecular forces.As it stands, the method does provide a simple, quantitative explanation for the interesting effects of methyl groups on molecular stability in Table 11; an explanation which has been lacking for more than 20 years.' Acknowledgment. The authors wish to thank Y.-R. Luo, T.

23) Snyder, R. G.; Conti, G.; Straws, H. L. To be published. (24) Lifshitz, I. hi.; Slyozov, V. V.The ion-selective properties of the cyclic depsipeptide molecule wlinomycin in methanol are studied by free energy perturbation molecular dynamics simulations. The dependence of the alkali ation selectivity on the dipole moment of the bckbone carbonyl groups that ligate the ion is examined and found to behave in a systematic way. Since available force fields have not been parametrized for this type of ion-carbonyl interaction, the results are used to determine carbonyl partial charges that reproduce the observed selectivity quantitatively. The optimal dipole moment is somewhat higher than that normally used in molecular mechanics force fields and indicates that there is a significant polarization of the carbonyl groups by the field from the ion. The simulations also suggest that the unloading of ions mainly would OCCUT through the lactic acid face of the molecule since it does not provide as effective a shield against attack from the solvent as the more hydrophobic isohydroxyvaleric acid face.

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