A study of the conformations of valinomycin in solution phase

Vibrational absorption and vibrational circular dichroism (VCD) spectra of valinomycin are measured, in different solvents, in the ester and amide carbonyl stretching regions. The influence of cations, namely Li+, Na+, K+, and Cs+, in methanol-d4 solvent is also investigated. Ab initio quantum mechanical calculations using density functional theory and 6-31G* basis set are used to predict the absorption and VCD spectra. A bracelet-type structure for valinomycin that reproduces the experimental absorption and VCD spectra in inert solvents is identified. For the structure of valinomycin in polar solvents, a propeller-type structure was optimized, but further investigations are required to confirm this structure. A symmetric octahedral environment for the ester carbonyl groups in the valinomycin-K+ complex is supported by the experimental VCD spectra. The results obtained in the present study demonstrate that even for large macrocyclic peptides, such as valinomycin, VCD can be used as an independent structural tool for the study of conformations in solution.