Abstract:The molecular conformations of uncomplexed valinomycin in CC14, CS2, CHzCI2, CHC13, CH3OH, C~H S O H , C3H,CI, p-dioxane (C4HsO2), and dioxane/D20 have been studied using laser Raman spectroscopy. The stretching frequencies of the ester and amide carbonyl groups are found to be affected by both the polarity of the solvent and its ability to form hydrogen bonds. Results in nonpolar solvents are consistent with the presence of hydrogen bonding ester carbonyl groups, reopening the question of whether the conformation found in valinomycin recrystallized from n-octane can exist in nonpolar solution. In polar solvents, a conformation is detected that contains fewer hydrogen bonds. As the dielectric constant of the solvent increases, the stretching frequency of the amide carbonyl groups increases (perhaps reflecting a reduction of intramolecular hydrogen bonding), while the stretching frequency of the ester carbonyl groups decreases.The macrocyclic dodecadepsipeptide valinomycin (hereafter abbreviated VM; Figure la) was among the first ionspecific antibiotics to be used as a model of ionic transport in biological m e m b r a n e~l -~ Although the three-dimensional structure of crystals can often be obtained with x-ray or neutron diffraction, these techniques cannot provide information about whether conformations found in the solid state persist in solution. In contrast, Raman spectroscopy can be applied to both solids and liquids, permitting one to compare structural characteristics of molecules in a variety of environments. Raman spectroscopic investigations of the molecular conformations of VM in the solid state have recently been r e p~r t e d ,~ and in this work we report studies of VM in a variety of solvents. These studies, which include deuteration of the N H groups, reveal new information about the dependence of VM conformation on environment.X-ray methods have been used to reveal6,' the complete structure of one form of uncomplexed VM; all six N H groups are intramolecularly hydrogen bonded, four to amide C=O groups and two to ester C=O groups (conformation D, Figure 1 b). Structural similarities between crystalline uncomplexed VM and the VM-K+ complex have led to the suggestion6 that this form may be involved in ion complexation at the membrane-water interface. However, nuclear magnetic resonance (NMR), infrared absorption (IR), and optical rotatory dispersion (ORD) studies4J-I I reveal no evidence of conformation * Address correspondence to this author at the Department of Physics, Boston University, I I I Cummington St., Boston, Mass. 02215. D in solution. One question we address is whether or not there is Raman spectroscopic evidence that conformation D persists in solution.A mixture of several VM conformations exists in soluti~n.~.*-' I It is believed that the predominant conformation of VM in nonpolar solvents contains six hydrogen-bonded amide C=O groups and six unbonded ester C=O groups (conformation A, Figure IC), while the predominant conformation in polar solvents is believed to contain on...

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