Abstract
The membrane-active antibiotic valinomycin has become an important model compound for studying selective ion transport in biological and synthetic membranes. This paper reports results of the first complete Raman spectroscopic study of uncomplexed valinomycin in the solid state. Splittings in the ester and amide C=O stretch regions of valinomycin samples recrystallized from n-octane, CC14, CHC13, CH3(CH2)2CI, CH3COCH3, or CH3CN indicate a structure resembling that ob- tained by x-ray crystallography. However, valinomycin recrystallized from o-dichlorobenzene and p-dioxane exhibits a consid- erably different structure. Comparison is made with the Raman spectra of model compounds in order to facilitate the identifi- cation of valinomycin vibrations. These results are extended to valinomycin solutions in an adjoining publication. The antibiotic valinomycin (VM) produced by the bacte- ria Streptomyces fulvissimus is known to complex with alkali cations selectively2 in the order Rb+ > K+ > Cs+ > Na+ > Li+ and to facilitate the transport of alkali cations across mi- tochondrial membranes with the same ~electivity.~~ Similar results have been obtained in model membrane system^.^,^ Studies of valinomycin analogues2 emphasize the importance of structural factors in complex formation, induced ionic permeability, and antimicrobial activity. Valinomycin is a 12-membered macrocyclic depsipetide in which L-valine, L-lactic acid, D-valine, and D-hydroxyisovaleric acid (HIV) are alternately joined by amide and ester linkagesaS The primary structure of VM (Figure la) suggests how it can facilitate the transport of cations across otherwise impermeable lipid barriers. There are 12 polar C=O groups, some of which can form structurally stabilizing intramolecular hydrogen bonds with the NH groups of the valine subunits, while others are free to bind a cation at the waterllipid interface via ion- dipole interactions. The nonpolar isopropyl and methyl residues can shield the hydrophilic C=O coordinated cation to facili- tate its diffusion through the hydrophobic regions of membrane interiors. This structure for the K+-VM complex has been verified by x-ray ~rystallography~ ~~ and inferred from NMR
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