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

The molecular conformations of uncomplexed valinomycin in CCl4, CS2, CH2Cl2, CHCl3, CH3OH, C2H5OH, C3H7Cl, p-dioxane (C4H8O2), and dioxane/D2O 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.