l/f noise in black lipid membranes induced by ionic channels formed by chemically dimerized gramicidin A

Fullerenol C60(OH)24 increases ion permeability of lipid membranes in a pH-dependent manner

Fullerenols are water-soluble analogs of fullerene exhibiting both antioxidant and prooxidant activities in vitro and in vivo. Here we report, for the first time, that fullerenol C60(OH)24 can induce ion permeability of a planar lipid bilayer membrane via the formation of ion pores or conductive defects with a preference for cations over anions. The fullerenol-mediated electrical current displayed non-linear concentration dependence and was reversibly enhanced by alkalinization. Calcium and magnesium ions decreased the fullerenol-induced potassium ion permeability. Voltage dependence of the current was sensitive to membrane composition, with the conductance being well pronounced in fully saturated diphytanoylphosphatidylcholine. Fullerenol did not induce carboxyfluorescein leakage from liposomes, suggesting a small size of fullerenol-induced pores. In contrast to ion permeability, the binding of C60(OH)24 to liposomes increased at acidic pH, as measured by fluorescence quenching of pyrene-labeled lipid. In line with this, the photodynamic action of fullerenol on the peptide gramicidin A also increased at low pH. It is hypothesized that aggregates of fullerenol may stabilize transient conductive lipid defects or pores formed under a variety of stress conditions.

Pink noise of ionic conductance through single artificial nanopores revisited

We report voltage-clamp measurements through single conical nanopore obtained by chemical etching of a single ion track in polyimide film. Special attention is paid to the pink noise of the ionic current (i.e., 1/f noise) measured with different filling liquids. The relative pink-noise amplitude is almost independent of concentration and pH for KCl solutions, but varies strongly using ionic liquids. In particular, we show that depending on the ionic liquid, the transport of charge carriers is strongly facilitated (low noise and higher conductivity than in the bulk) or jammed. These results show that the origin of the pink noise can be ascribed neither to fluctuations of the pore geometry nor to the pore wall charges, but rather to a cooperative effect on ions motion in confined geometry.

Fluctuations of ionic current through lipid bilayers at the onset of peptide attacks and pore formation

Voltage-clamp measurements on lipid bilayers at the onset of peptide attacks before pore formation are reported. With four different peptides [alamethicin, melittin, and two synthetic peptides of the leucine (L)-lysine(K) copolymers (LK series)], correlations of conductivity fluctuations slowly decay over four decades in time. This slow dynamics is interpreted as being due to fluctuations of peptide concentration at the crowded surface of the bilayer and found to be compatible with the t(-1/2) relaxation of the RSA model.

Mechanism of C2-toxin Inhibition by Fluphenazine and Related Compounds: Investigation of their Binding Kinetics to the C2II-channel using the Current Noise Analysis

The binding component C2II of the binary actin ADP-ribosylating C2-toxin from Clostridium botulinum is essential for intoxication of target cells. Activation by a protease leads to channel formation and this is presumably required for the transport of the toxic C2I component into cells. The C2II-channel is cation selective and contains a binding site for fluphenazine and structurally related compounds. Ion transport through C2II and in vivo intoxication is blocked when the sites are occupied by the ligands. C2II was reconstituted into artificial lipid bilayer membranes and formed ion permeable channels. The binding constant of chloroquine, primaquine, quinacrine, chloropromazine and fluphenazine to the C2II-channel was determined using titration experiments, which resulted in its block. The ligand-induced current noise of the C2II-channels was investigated using fast Fourier transformation. The noise of the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. Upon addition of ligands to the aqueous phase the current through C2II decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically and its frequency dependence was of Lorentzian type, which was caused by the on and off-reactions of the ligand-mediated channel block. The ligand-induced current noise of C2II was used for the evaluation of the binding kinetics for different ligands to the channel. The on-rate constant of ligand binding was between 10(7) and 10(9) M(-1) s(-1) and was dependent on the ionic strength of the aqueous phase. The off-rate varied between about 10 s(-1) and 3900 s(-1) and depended on the structure of the ligand. The role of structural requirements for the effective block of C2II by the different ligands is discussed.

Noise analysis of ion current through the open and the sugar-induced closed state of the LamB channel of Escherichia coli outer membrane: evaluation of the sugar binding kinetics to the channel interior

LamB, a sugar-specific channel of Escherichia coli outer membrane was reconstituted into lipid bilayer membranes and the current noise was investigated using fast Fourier transformation. The current noise through the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. The spectral density of the noise of the open LamB channels was a quadratic function of the applied voltage. Its magnitude was not correlated to the number of channels in the lipid bilayer membrane. Upon addition of sugars to the aqueous phase the current decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically, which indicated interaction of the sugars with the binding site inside the channel. The frequency dependence of the spectral density was of Lorentzian type, although the power of its frequency dependence was not identical to -2. Analysis of the power density spectra using a previously proposed simple model (Benz, R., A. Schmid, and G. H. Vos-Scheperkeuter. 1987. J. Membr. Biol. 100: 12-29), allowed the evaluation of the on- and the off-rate constants for the maltopentaose binding to the binding site inside the LamB channels. This means also that the maltopentaose flux through the LamB channel could be estimated by assuming a simple one-site, two-barrier model for the sugar transport from the results of the noise analysis.

Brief closures of gramicidin A channels in lipid bilayer membranes

Brief closures, so called flickers, gramicidin A channels were observed for glycerol monooleate/n-decane membranes for cesium chloride and hydrochloric acid solutions. The flickers, similar in nature to the flickers observed for physiological channels, were of the order of 1 ms and the interval between flickers was of the order of 50 ms. The flicker-duration and interval between flickers both decrease with voltage. The field dependence of the flickers is consistent with the hypothesis that the membrane forms a dimple when accomodating a dimer in the membrane and that the monomers, on breaking up, are associated over displacements of the order of 2 nm. For similar measurements for glycerol monoleate/hexadecane membranes only rare occurrences of flickers were observed. It is suggested that the flicker phenomenon is governed by the physical and chemical properties of the membrane and the influence of membrane thickness and interfacial free energy is emphasized.

Temperature-jump and voltage-jump experiments at planar lipid membranes support an aggregational (micellar) model of the gramicidin A ion channel

The kinetics of formation and dissociation of channels formed by gramicidin A and two analogues in planar lipid membranes was studied using a laser temperature-jump technique developed earlier [Brock, W., Stark, G., Jordan, P.C. (1981), Biophys. Chem. 13:329-348]. The time course of the electric current was found to agree with a single exponential term plus a linear drift. In case of gramicidin A the relaxation time was identical to that reported for V-jump experiments [Bamberg, E., Läuger, P. (1973), J. Membrane Biol. 11:177-194], which were interpreted on the basis of a dimerization reaction. The same results were obtained for gramicidin A and for chemically dimerized malonyl-bis-desformylgramicidin. It is therefore suggested that the dimerization represents a parallel association of two dimers to a tetramer. There is evidence that the tetramer, contrary to the presently favored dimer hypothesis, is the smallest conductance unit of an active gramicidin channel. An additional V-jump-induced relaxation process of considerably larger time constant is interpreted as a further aggregation of gramicidin dimers.

Open channel noise. I. Noise in acetylcholine receptor currents suggests conformational fluctuations

The random passage of ions through an open channel is expected to result in shot noise fluctuations in the channel current. The patch-clamp technique now allows fluctuations of this size to be observed in single-channel currents. In the experiments reported here the acetylcholine-induced currents in cultured rat muscle cells were analyzed; fluctuations were found that were considerably larger than expected for shot noise. A low-frequency component, which was fitted with a Lorentzian, was examined in detail; it appears to arise from fluctuations in channel conductance of approximately 3% on a time scale of 1 ms. The characteristic relaxation time is voltage dependent and temperature dependent (Q10 approximately equal to 3) suggesting that the fluctuations arise from conformational fluctuations in the channel protein.

Interpretation of 1/f fluctuations in ion conducting membranes

The main objective of this work is to resolve some uncertainties associated with the analysis of conductance fluctuations that exhibit 1/f spectral density. To this end, we derive mathematical conditions under which a discrete summation of Lorentzian functions best approximates a strictly 1/f density over a given frequency range. The intrinsic errors associated with spectral density estimates are considered and used as a constraint to determine the smallest number of optimally chosen Lorentzians required to fit a 1/f-like spectrum in a statistically acceptable manner. The results provide criteria concerning the extent to which mechanisms generating a strictly 1/f spectra may be distinguished from those generating sums of Lorentzian spectra. It is found, in particular, that 1/f-like fluctuation spectra observed in a variety of biological and model membranes may well arise from the summation of a few Lorentzian components having appropriate amplitudes and corner frequencies. Consideration of physically realistic models of ion conductive channels indicates that 1/f-like conductance fluctuation spectra could originate naturally as a direct consequence of thermodynamic constraints upon the coefficients of Lorentzian components.

Dicarboxylic acid analogs of gramicidin A: dimerization kinetics and single channel properties

According to the model of Urry, the cation-permeable gramicidin channel is a dimeric helix formed by association of two peptide monomers linked at their amino ends. In this paper the channel properties of gramicidin analogs are described which have been obtained by chemical modification at the coupling site of the two half-channels. In these analogs the amino terminal -CHO group is replaced by -CO(CH2)nCCOH (n = 2, 3, 4, 5, 6). All analogs form conducting channels in black lipid membranes with the same general properties as found for gramicidin A. The observation that the channel-forming activity decreases with increasing pH is consistent with the notion that the half-channels are linked at the amino terminus. The channel lifetime of the different analogs varies between 2 msec and greater than of equal to 50 sec, the longest lifetime being found for the compound with n = 3. The single-channel conductance : formula : (see text) is always smaller than that of gramicidin A, but the reduction of : formula : (see text) depends on the nature of the permeable ion. Ion specificity was studied at 1 M electrolyte by measuring the conductance : formula : (see text) for different permeable ions (Na+, K+, Cs+). The conductance ration : formula : (see text) (Cs+)/ : formula : (see text) (Na+) was found to vary between 2 and 10.5 for the different analogs.

Formation of ionic channels in black lipid membranes by succinic derivatives of gramicidin A

Different succinyl derivatives of Gramicidin A were synthesized and their activity was investigated with different methods on lipid bilayer membranes. The succinyl derivatives of Gramicidin A can be classified as three different types, the O-succinyl derivative, the N-succinyl derivative and the N-O-succinyl derivative of Gramicidin A. An O-pyromellityl-N-succinyl gramicidin was synthesized which can be attributed to the latter class. It was found that O-succinyl gramicidin behaves like the unmodified Gramicidin A despite a charge effect on single-channel conductance, arising from the negative charge of the succinic residue at the mouth of the channel. The activity of N-succinyl and N-O-succinyl gramacidin and of O-pyromellityl-N-succinyl-gramicidin depends strongly on the pH of the electrolyte solution. It is demonstrated that at low pH (less than or equal to 5) the N-succinyl derivatives show high activity, whereas at high pH (greater than or equal to 7) the activity is sharply reduced or disappears totally. From these experiments it can be concluded that, for the formation of a dimeric gramicidin channel, the hydrogen of the formyl group can be replaced by a protonated carboxylic group of a succinic residue. Further results, obtained by measurement of the single-channel conductance and of the reaction rate constants for the channel formation, are discussed in terms of the structural basis of the single stranded model for the gramicidin channel. On this basis the double stranded helix can be excluded and an interesting head-to-head single stranded beta(pi L.D.) helical channel is described which contains carboxyl groups at the head-to-head junction.

Conductance noise of monazomycin-doped bilayer membranes

The conductance noise of the monazomycin pore has been studied by autocorrelation analysis in multi-pore systems. The autocorrelation function could be described by a superposition of two single exponential functions of different time- and voltage-dependence. The slow voltage-dependent correlation time in the range of seconds is assigned to the formation of nonconducting pore precursors. The fast voltage-independent correlation time in the msec range is related to fluctuations in the number of open pores whereby each pore adopts only two conducting states (open and closed). The corresponding correlation amplitude depends on monazomycin concentration and could be related to the single pore conductance. With increasing voltage, a slight increase of the single pore conductance was obtained which is explained on the basis of an electrostatic barrier within the pore. The pore was found to be virtually unselective for different alkali ions (Li, K, Cs).