National Research Council of Canada. NRC Steacie Institute for Molecular Sciences
The structure of valinomycin in a range of organic solvents of varying polarity and in detergent and lipid dispersions has been studied by Fourier transform ir Spectroscopy. In solvents of low polarity such as chloroform, ir spectra of valinomycin are fully consistent with the bracelet structure proposed on the basis of nmr Spectroscopy, showing a single narrow amide I component attributable to the presence of β-turns and a single band arising from nonhydrogen-bonded ester C=O groups. K⁺ complexation results in a downward shift in the amide I band frequency, indicating an increase in the strength of the amide hydrogen bonds, along with a shift to lower frequencies of the ester C=O absorption due to a reduction in electron density in these bonds upon complexation. Identical results were obtained with NH₄⁺, a finding not previously reported. In solvents of both medium (CHCl₃/DMSO 3 : 1) and high (pure DMSO) polarity, we find evidence of significant disruption of the internal hydrogen-bonding network of the peptide and the appearance of a band suggesting the presence of free amide C=O groups. In such solvents, complexation with K⁺ and NH₄⁺ image was not observed. The structure of valinomycin in detergent micelles resembles that in nonpolar organic solvents. However, changes were found in the amide I and ester carbonyl maxima as ²H₂O penetrated the micelle which suggest significant interaction between the solvent and peptide. Complexation with K⁺ was reduced in cationic detergent micelles as a result of a decrease in the effective K⁺ concentration due to charge repulsion at the micelle surface. In lipid bilayers the structure again appears identical to that found in chloroform. As in detergent micelles, the amide I and ester carbonyl bands exhibit shifts that indicate interactions with solvent. Complexation with both K⁺ and NH₄⁺ is efficient, producing spectral changes similar to those seen in organic solvents.