Structure Of Valinomycin Research Articles

Influence of the lipid environment on valinomycin structure and cation complex formation

Carrier-type molecular ionophores, such as the cyclic dodecadepsipeptide valinomycin, often must undergo structural changes during the binding and transport of a cation across the lipid membrane. Observing the structural fluctuations that occur during this process experimentally has proven extremely difficult due to the complexities of spectroscopic analysis of protein structure/dynamics in native lipid bilayer environments. Currently, our understanding of how valinomycin selectively transports ions across membranes is derived from atomic structures solved of the cyclic macromolecule solvated in various organic solvents and complimentary in silico dynamics experiments. We have shown recently that deep-UV excited resonance Raman spectroscopy (DUVRR) has a unique ability to characterize secondary structure content and simultaneously provide information about the relative solvation of the probed peptide backbone C.M. Halsey, J. Xiong, O. Oshokoya, J.A. Johnson, S. Shinde, J.T. Beatty, G. Ghirlanda, R.D. JiJi, J.W. Cooley, Simultaneous observation of peptide backbone lipid solvation and a-helical structure by deep-UV resonance Raman spectroscopy, ChemBioChem 12 (2011) 2125–2128, [16]. Interpretation of DUVRR spectra of valinomycin in swelled lipid and unilamellar lipid bilayer environments indicate that the uncomplexed valinomycin molecule dynamically samples both the open and closed conformations as described for the structures derived from polar and non-polar organic solvents, respectively. Upon introduction of potassium, the structure of valinomycin in swelled lipid environments resembles more closely that of the open conformation. The shift in structure upon complexation is accompanied by a significant decrease in the valinomycin DUVRR spectral amide I intensity, indicating that the open conformation is more water solubilized and is seemingly “trapped” or predominantly located close to the lipid–water interface. The trapping of the valinomycin in the act of complex of potassium at the bilayer-solvent interface and its analysis by DUVRR represents the first spectroscopic description of this state. Conversely, an opposite trend is observed in the amide I intensity upon potassium complexation in unilamellar (or extruded) vesicles, implying the predominant conformation upon potassium binding in native bilayers is one where the peptide backbone of valinomycin is desolvated as would be expected if the molecule were more readily able to traverse a bilayer interior. Interpretation of the DUVRR spectral features is also consistent with the loss or formation of hydrogen bonds observed in the open and closed structures, respectively. Valinomycin must then sample several conformations in the absence of appropriate ions depending upon its locale in the lipid bilayer until potassium causes a greater degree of closure of the open conformer and an increased residency within the more non-polar interior. The potassium induced decreased solubility enables diffusion across the membrane where potassium release can occur by equilibration at the opposite lipid water interface.

Valinomycin and its interaction with ions in organic solvents, detergents, and lipids studied by Fourier transform IR spectroscopy

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 beta-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+ 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.(ABSTRACT TRUNCATED AT 250 WORDS)

ChemInform Abstract: RAMAN SPECTROSCOPY OF UNCOMPLEXED VALINOMYCIN. I. THE SOLID STATE

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

Raman spectroscopy of uncomplexed valinomycin. I. The solid state.

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