Raman spectroscopy of uncomplexed valinomycin. 2. Nonpolar and polar solution.

The molecular conformations of uncomplexed valinomycin in CC14, CS2, CHzCI2, CHC13, CH3OH, C~HSOH, C3H,CI, p-dioxane (C4HsO2), and dioxane/D20 have been studied using laser Raman spectroscopy. The stretching frequen- cies 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, re- opening the question of whether the conformation found in valinomycin recrystallized from n-octane can exist in nonpolar solu- tion. 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.