V. V. Malev

Cluster Organization of Ion Channels Formed by the Antibiotic Syringomycin E in Bilayer Lipid Membranes

The cyclic lipodepsipeptide, syringomycin E, when incorporated into planar lipid bilayer membranes, forms two types of channels (small and large) that are different in conductance by a factor of sixfold. To discriminate between a cluster organization-type channel structure and other possible different structures for the two channel types, their ionic selectivity and pore size were determined. Pore size was assessed using water-soluble polymers. Ion selectivity was found to be essentially the same for both the small and large channels. Their reversal (zero current) potentials with the sign corresponding to anionic selectivity did not differ by more than 3 mV at a twofold electrolyte gradient across the bilayer. Reduction in the single-channel conductance induced by poly(ethylene glycol)s of different molecular weights demonstrated that the aqueous pore sizes of the small and large channels did not differ by more than 2% and were close to 1 nm. Based on their virtually identical selectivity and size, we conclude that large syringomycin E channels are clusters of small ones exhibiting synchronous opening and closing.

Syringomycin E Channel: A Lipidic Pore Stabilized by Lipopeptide?

Highly reproducible ion channels of the lipopeptide antibiotic syringomycin E demonstrate unprecedented involvement of the host bilayer lipids. We find that in addition to a pronounced influence of lipid species on the open-channel ionic conductance, the membrane lipids play a crucial role in channel gating. The effective gating charge, which characterizes sensitivity of the conformational equilibrium of the syringomycin E channels to the transmembrane voltage, is modified by the lipid charge and lipid dipolar moment. We show that the type of host lipid determines not only the absolute value but also the sign of the gating charge. With negatively charged bilayers, the gating charge sign inverts with increased salt concentration or decreased pH. We also demonstrate that the replacement of lamellar lipid by nonlamellar with the negative spontaneous curvature inhibits channel formation. These observations suggest that the asymmetric channel directly incorporates lipids. The charges and dipoles resulting from the structural inclusion of lipids are important determinants of the overall energetics that underlies channel gating. We conclude that the syringomycin E channel may serve as a biophysical model to link studies of ion channels with those of lipidic pores in membrane fusion.

Surfactin activity depends on the membrane dipole potential.

The effect of dipole modifying agents phloretin and RH 421 on the membrane conductance induced by surfactin, a lipopeptide antibiotic from Bacillus subtilis, was studied. Surfactin added on both sides of a bilayer formed from diphytanoylphosphocholine in 1 M KCl (pH 6.5) leads to the formation of voltage-independent channels of different conductance levels. The conductance of different states of SA channels varies from tens of picosiemens for small pores up to tens of nanosiemens for large ones. Small channels demonstrate pronounced cationic selectivity, whereas large ones practically lose their K(+)/Cl(-) selectivity, most probably because of their large effective radii. The addition of phloretin to the bilayer bathing solution, the agent known to decrease the membrane dipole potential, results in a decrease in the surfactin-induced membrane conductance. At the same time, increasing the membrane dipole potential because of the introduction of RH 421 leads to a rise in the steady-state conductance. Increasing dipole potential is accompanied by increases in both the number of open channels and their conductance. The observed changes in the channel-forming activity of surfactin might be caused by varying the partition coefficient of lipopeptide between the lipid and aqueous phases.

Electrochemical properties of composite films based on poly-3,4-ethylenedioxythiophene with inclusions of metallic palladium

The electrochemical behavior of composite Pd-PEDOT films is studied. These films are obtained by chemical deposition of Pd particles in the polymeric matrix of PEDOT (poly-3,4-ethylenedioxythiophene). Characteristics of the films are determined by means of cyclic voltammetry, faradaic impedance, microgravimetry, and energy-dispersive x-ray fluorescence analysis. Impedance spectra of composite Pd-PEDOT films, compared to the original PEDOT film, reveal a new response at potentials of −0.3 and −0.4 V in the form of a distinct semicircle, which results from processes in the electrochemical sorption-desorption of hydrogen. Weight gain during the chemical deposition of palladium in the polymer structure is estimated by mircrogravimetry. It is shown that the mass of palladium loaded in the film depends on the time of synthesis and the initial concentration of palladium ions in solution. The size of the actual surface and the average radius of dispersed palladium particles in the film are also estimated.

Structure of Cobalt Hexacyanoferrate Films Synthesized from a Complex Electrolyte

Films of cobalt hexacyanoferrate (CHCF) are produced under potentiodynamic conditions (E= –0.25 to 0.6 V, v= 0.05 V s–1) from a 2 mM K3Fe(CN)6+ 2 mM Na3Co(NO2)6+ 0.5 M Na2SO4solution. According to scanning electron microscopy, the thick CHCF film initially obtained during the synthesis has a heavily porous structure unusual for films of hexacyanoferrates of transition metals, the size of its crystallites is 200–600 nm. A thinner film has a more compact structure and its crystallites are smaller. The X-ray diffraction pattern for a reduced film shows rhombohedral distortion of the fcc lattice with parameters a= 1.035 nm and α = 91.43°. The patterns for partially and completely oxidized films nicely fit an fcc crystalline lattice typical for hexacyanoferrates (ais 1.006 and 0.993 nm, respectively). The assumption that both hexacyanoferrate and cobalt ions are electroactive in this particular case is confirmed by the IR and X-ray photoelectron spectroscopy techniques.

Composite electrode materials based on conducting polymers loaded with metal nanostructures

The electrochemical and chemical methods for synthesizing conducting polymer-metal nanocomposite materials are considered as well as the main factors affecting the structure and electrochemical properties of these composites. The experimental data on the catalytic activity of conducting polymer–metal electrodes are analyzed in respect to several electrochemical reactions. The approaches to theoretical description of electrochemical processes on heterogeneous conducting polymer–metal electrodes are discussed and examples of experimental testing of applicability of the proposed the theoretical models are shown. The bibliography includes 335 references.

Altering the activity of syringomycin E via the membrane dipole potential.

The membrane dipole potential is responsible for the modulation of numerous biological processes. It was previously shown (Ostroumova, O. S.; Kaulin, Y. A.; Gurnev, P. A.; Schagina, L. V. Langmuir 2007, 23, 6889-6892) that variations in the dipole potential lead to changes in the channel properties of the antifungal lipodepsipeptide syringomycin E (SRE). Here, data are presented demonstrating the effect of the membrane dipole potential on the channel-forming activity of SRE. A rise in the dipole potential is accompanied by both an increase in the minimum SRE concentration required for the detection of single channels at fixed voltage and a decrease in the steady-state number of open SRE channels at a given SRE concentration and voltage. These alterations are determined by several factors: gating charge, connected with translocations of lipid and SRE dipoles during channel formation, the bilayer-water solution partitioning of SRE, and the chemical work related to conformational changes during channel formation.

Redox transformations in electroactive polymer films derived from complexes of nickel with SalEn-type ligands: computational, EQCM, and spectroelectrochemical study

Polymer complexes of nickel with SalEn-type ligands (SalEn = N,N′-bis (salicylidene) ethylenediamine) possess a number of unique properties, such as high redox conductivity, electrochromic behavior and selective catalytic activity in heterogeneous reactions. However, the mechanism of their redox transformation is still not clear. To understand this mechanism, we have performed a combined study of electrochemical and spectral properties of polymers derived from nickel complexes with various SalEn-type ligands containing methoxy substituents in phenyl rings, and methyl substituents in imino bridges. Experimental data were correlated with the results of density functional theory (DFT) calculations for model chains consisting of one to four monomer units. We found that, in acetonitrile-based supporting electrolyte, oxidation of such complexes, regardless of ligand substituents, proceeds via two routes, leading to formation of two oxidized forms: for the first one, a good correlation between experimental and computation results was observed. It has been demonstrated that positive charge in this form is delocalized in the phenyl moieties of ligand. The second oxidized form is stable only in coordinating solvents at high electrode polarizations and is likely to have the charge localized on the central metal atom, stabilized by axial coordination of solvent molecules. The complicated electrochemical response of each of the polymers that we have studied can be explained in the scope of this model without any additional assumptions by taking into account conversion of one oxidized form into another. Understanding the solvent effect on the oxidation route of the complexes will enable controlling their catalytic properties and stability.

Gold electroless deposition into poly-3,4-ethylenedioxythiophene films

By electroless deposition of gold into poly-3,4-ethylenedioxythiophene (PEDOT) films, the composite films (PEDOT-Au) are synthesized. Their electrochemical properties are studied by cyclic voltammetric (CVA) method. It is shown that in contrast to the original PEDOT film, the CVA curves of composite PEDOT-Au films measured in the presence of chloride ions reveal additional redox peaks associated with the presence of gold particles. The loading of metal gold particles by its chemical deposition into the polymer film is quantitatively assessed using quartz crystal microbalance method. The film mass is shown to depend on the time of gold loading and its original concentration in solution. The gold particles are shown to be oxidized by a reaction of the first order with respect to chloride ions. Based on the results of voltammetric and microbalance methods, the formation of a poorly soluble gold oxidation product Au(I)Cl in chloride-containing solutions was inferred.

Effective gating charge of ion channels induced by toxin syringomycin E in lipid bilayers

To elucidate the voltage gating of syringomycin E (SRE) ion channels in lipid bilayers, the effective gating charge q was measured under different conditions. It was shown that q and its sign are dependent on membrane surface charge, dipole potential, and the outer potential (Delta phi). The q values were positive for charged bilayers and negative for uncharged bilayers bathed in the same 0.1 M NaCl solutions. Effects of dipole modifying agents on the gating properties of SRE channels were measured. In uncharged bilayers, addition of phloretin resulted in an increase of q values. For charged bilayers, the presence of RH-421 or 6-ketocholestanol leads to the reverse in the sign of q from positive to negative. The q values were potential-dependent at higher negative voltages with charged membranes bathed in solutions with high salt concentrations. It is concluded that lipid molecules participating in the SRE channel structure contribute to channel formation work due to Coulomb and dipolar interactions with the electric field applied to a membrane. The potential dependence of q is explained by interactions of charged and uncharged lipids with SRE molecules in the channels.