Experimental search for combined AC and DC magnetic field effects on ion channels

The hypothesis that specific combinations of DC and low frequency AC magnetic fields at so-called cyclotron-resonance conditions could affect the transport of ions through ion channels, or alter the kinetics of ion channels (opening and closing rates), has been tested. As a model system, the ion channels formed by gramicidin A incorporated in lipid bilayer membranes were studied. No significant changes in channel conductance, average lifetime, or formation rate as a function of applied fields could be detected over a wide range of frequencies and field strengths. Experiments were carried out to measure the time-resolved single-channel events and the average conductances of many-channel events in the presence of K+ and H+ ions. The channel blocking effect of Ca++ was also studied.

The nature of ion and water barrier crossings in a simulated ion channel

Using a combination of techniques, including molecular dynamics, time-correlation analysis, stochastic dynamics, and fitting of continuum diffusion theory to electrophysiological data, a characterization is made of thermally driven sodium, water, and D2O motion within the gramicidin A channel. Since the channel contents are constrained to move in a single-file fashion, the motion that corresponds to experimentally measurable rates of permeation of the membrane is the motion of the center of mass of the channel contents. We therefore emphasize channel contents center-of-mass motion in our analysis of molecular dynamics computations. The usual free energy calculation techniques would be of questionable validity when applied to such motion. As an alternative to those techniques, we postulate a periodic sinusoidal free energy profile (related to the periodic structure of the helical channel) and deduce the fluid dynamic diffusion coefficient and the height and spacing of the free energy barriers from the form of the mean-square-deviation function, using stochastic computations. The fluid dynamic friction in each case appears similar to that for aqueous solution. However, the diffusive motions are modulated by a spatially periodic free energy profile with a periodicity characteristic of an L-D pair of amino acids in the gramicidin helix, approximately 1.7 A in the model we use. The barrier height depends on which substance is moving in the channel, but in each case is several times thermal energy. For barriers of this width and height, the motion is intermediate between the low-friction (transition-state) and high-friction (Brownian) limits. Thus, neither of these formalisms that have been used commonly to describe membrane permeation gives an accurate picture of the underlying physical process (although the Brownian description seems closer to correct). The non-Markovian Langevin equation must be solved to describe properly the statistics of the process. The "channel state of matter" characteristic of the channel contents appears to have some properties typical of the solid and some typical of the liquid state. The magnitude of the local friction and nature of the ion solvation are similar to the liquid state, but the periodicities of structure, free energy, and dynamics are somewhat solid-like. The alignment of water dipoles in the channel bears some resemblance to the orientational ordering of a nematic liquid crystal, but unlike a nematic liquid crystal, the waters have a degree of translational order as well. Thus, the "channel state" is not adequately described by analogy to either the solid or liquid states or to liquid crystals but must be dealt with as its own characteristic type of condensed matter.

Theoretical perspectives on ion-channel electrostatics: continuum and microscopic approaches

Peter Läuger introduced me (P.C.J.) to the field of ion-channel electrostatics while I was a sabbatical visitor at Konstanz in 1978–79. Läuger pointed out that the relative conductance of hydrophobic ions through phosphatidyl choline (PC) and glyceryl monooleate (GMO) membranes differed by a factor of about 100 (Hladky & Haydon, 1973), quite consistent with the difference in the water-membrane potential differences in the two systems (Pickar & Benz, 1978). However, cation conductance through gramicidin channels spanning these membranes only differs by a factor of 2–3 (Bamberg et al. 1976). Why? It is the pursuit of an answer to this question which led me into my researches in this field.

Monte Carlo studies of a model for lipid-gramicidin A bilayers

This paper presents results of Monte Carlo simulations of a full bilayer of 200 lipid chains and one gramicidin A dimer. Simulations are described for systems with lipid chains of 14, 16, and 18 carbons, respectively. Using accepted potential functions to calculate interactions between all non-hydrogen atoms a Monte Carlo configuration sampling is generated from which order parameter profiles are calculated and specific configurations are displayed. Results are compared with experimental data for lipid-gramicidin bilayers.

On the supramolecular organization of gramicidin channels. The elementary conducting unit is a dimer

The question, whether the conducting channels formed by the linear gramicidins are dimers (as is generally believed) or tetramers (as has been recently proposed [Stark G., M. Strässle, and Z. Takacz. 1986. J. Membr. Biol. 89:23-37; Strässle, M., G. Stark, M. Wilhelm, P. Daumas, F. Heitz, and R. Lazaro. 1989. Biochim. Biophys. Acta. 980:305-314]) has been addressed in single-channel experiments. The experimental approach was based on the ability of electrophysiological (single-channel) experiments to resolve the number of hybrid channel types that could form between gramicidin A or C and O-pyromellityl-gramicidin A or C (in which a pyromellitic acid residue has been esterified to the ethanolamine-OH group [Apell, H.-J., E. Bamberg, H. Alpes, and P. Läuger. 1977. J. Membr. Biol. 31:171-188]). The presence of the bulky, negatively charged pyromellityl group at the channel entrances endows the hybrid channels with characteristically different features and thus facilitates the resolution of the different hybrid channel types. Only two hybrid channel types were detected, indicating that the conducting channels are membrane-spanning dimers. There was likewise no evidence for lateral association between conducting channels and nonconducting monomers. These results can be reconciled with those of Stark et al. (op. cit.) if gramicidin channel formation involves a (slow) folding into beta 6.3-helical monomers followed by the dimerization step.

On the helix sense of gramicidin A single channels

In order to resolve whether gramicidin A channels are formed by right- or left-handed beta-helices, we synthesized an optically reversed (or mirror image) analogue of gramicidin A, called gramicidin A-, to test whether it forms channels that have the same handedness as channels formed by gramicidin M- (F. Heitz et al., Biophys. J. 40:87-89, 1982). In gramicidin M- the four tryptophan residues have been replaced with phenylalanine, and the circular dichroism (CD) spectrum therefore reflects almost exclusively contributions from the polypeptide backbone. The CD spectrum of gramicidin M- in dimyristoylphosphatidylcholine vesicles is consistent with a left-handed helical backbone folding motif (F. Heitz et al., Biophys. Chem. 24:149-160, 1986), and the CD spectra of gramicidins A and A- are essentially mirror images of each other. Based on hybrid channel experiments, gramicidin A- and M- channels are structurally equivalent, while gramicidin A and A- channels are nonequivalent, being of opposite helix sense. Gramicidin A- channels are therefore left-handed, and natural gramicidin A channels in phospholipid bilayers are right-handed beta 6.3-helical dimers.

Molecular dynamics computations and solid state nuclear magnetic resonance of the gramicidin cation channel

This paper reports on a coupled approach to determining the structure of the gramicidin A ion channel, utilizing solid state nuclear magnetic resonance (NMR) of isotopically labeled gramicidin channels aligned parallel to the magnetic field direction, and molecular dynamics (MD). MD computations using an idealized right-handed beta-helix as a starting point produce a refined molecular structure that is in excellent agreement with atomic resolution solid state NMR data. The data provided by NMR and MD are complementary to each other. When applied in a coordinated manner they provide a powerful approach to structure determination in molecular systems not readily amenable to x-ray diffraction.

Effect of salt and membrane fluidity on fluorophore motions of a gramicidin C derivative

We have used fluorescence spectroscopy to investigate the effect of salt and membrane fluidity on the rotational motion of a 5-(dimethylamino)naphthalene-1-sulfonyl (dansyl) derivative of gramicidin C (dansyl-gC) in dimyristoylphosphatidylcholine bilayers, under conditions where the peptide is a formyl-NH to formyl-NH terminal dimer, and in octyl glucoside micelles, where the peptide is an intertwined helical dimer. Energy-transfer experiments showed no changes in either conformation or dimer aggregation under the conditions explored here (15-40 degrees C, 1-350 bar, 0-0.33 M Mg2+, and 0-1 M Na+). The addition of permeable (Na+) or nonpermeable (Mg2+) ions did not affect the temperature or pressure behavior of dansyl rotation. However, fluorescence lifetime measurements indicated an increase in solvent accessibility in the presence of sodium. In bilayers, the temperature dependence of the fluorescence polarization and lifetime shows strong interactions between the dansyl residue and the peptide, and at no time did the dansyl motions become solvent controlled as has been observed for aqueous solvent peptides [Scarlata, S. F., Rholam, M., & Weber, G. (1984) Biochemistry 23, 6789]. In micelles, the change in rotational motion with temperature followed solvent expansion, showing that in this case the dansyl residue does not associate extensively with the peptide. Our results indicate that because of the extensive coupling between the dansyl residue and the rest of the peptide, membrane fluidity does not play a major role in controlling side-chain motions.

Proton conductance by the gramicidin water wire. Model for proton conductance in the F1F0 ATPases?

The gramicidin channel contains a single strand of water molecules associated through hydrogen bonds. Previous work has shown that channels of similar size are formed by association of transmembrane alpha helices of synthetic leucine-serine peptides. Both types of channels translocate protons with considerable selectivity relative to other cations, and it has been proposed that the selectivity arises by proton "hopping" along hydrogen-bonded chains of water, whereas other cations must cross by ordinary diffusion processes. It is possible that a similar mechanism underlies proton transport in the Fo subunit of the F1F0 ATP synthase. Using the gramicidin channel as a model, we have tested whether a single strand of water is kinetically competent to translocate protons at a rate sufficient to support known rates of ATP synthesis. We found that the gramicidin channel saturates at approximately 530 pS of protonic current in 4 M HCl, more than sufficient for typical ATP synthesis rates. It follows that proton diffusion to a putative channel in Fo, rather than the channel itself, may limit ATP synthesis rates.

Effective pore radius of the gramicidin channel. Electrostatic energies of ions calculated by a three-dielectric model

Electrostatic calculation of the gramicidin channel is performed on the basis of a three-dielectric model in which the peptide backbone of the channel is added as a third dielectric region to the conventional two-dielectric channel model (whose pore radius is often referred to as the effective pore radius reff). A basic principle for calculating electrostatic fields in three-dielectric models is introduced. It is shown that the gramicidin channel has no unique value of reff. The reff with respect to the "self-image energy" (i.e., the image energy in the presence of a single ion) is 2.6-2.7 A, slightly depending upon the position of the ion (the least-square value over the whole length of the pore is 2.6 A). In contrast, the reff with respect to the electric potential due to an ion (and hence the reff with respect to the interaction energy between two ions) is dependent upon the distance s of separation; it ranges from 2.6 to greater than 5 A, increasing with an increase in s. However, for the purpose of rough estimation, the reff with respect to the self-image energy can also be used in calculating the electric potential and the interaction energy, because the error introduced by this approximation is an overestimation of the order of 30% at most. It is also shown that the apparent dielectric constant for the interaction between two charges depends markedly upon the positions of the charges. In the course of this study, the dielectric constant and polarizability of the peptide backbone in the beta-sheet structure is estimated to be 10 and 8.22 A3.

Small iminium ions block gramicidin channels in lipid bilayers

Guanidinium and acetamidinium, when added to the bathing solution in concentrations of approximately 0.1M, cause brief blocks in the single channel potassium currents from channels formed in planar lipid bilayers by gramicidin A. Single channel lifetimes are not affected indicating that the channel structure is not modified by the blockers. Guanidinium block durations and interblock times are approximately exponential in distribution. Block frequencies increase with guanidinium concentration whereas block durations are unaffected. Increases in membrane potential cause an increase in block frequency as expected for a positively charged blocker but a decrease in block duration suggesting that the block is relieved when the blocker passes through the channel. At low pH, urea, formamide, and acetamide cause similar blocks suggesting that the protonated species of these molecules also block. Arginine and several amines do not block. This indicates that only iminium ions which are small enough to enter the channel can cause blocks in gramicidin channels.

The structure of gramicidin A in dimethylsulfoxide/acetone

It has been demonstrated by two-dimensional NMR cross-relaxation spectroscopy that gramicidin A exists in dimethylsulfoxide/acetone solution in random coil form. This contradicts earlier conclusions by Hawkes et al. [Hawkes, G. E., Lian, L. Y., Randall, E. W., Sales, K. D. & Curzon, E. H. (1987) Eur. J. Biochem. 166, 437-445] that were based on the interpretation of vicinal proton coupling constants.

Solid-state 13C-NMR studies of the effects of sodium ions on the gramicidin A ion channel

End-to-end helical dimers of gramicidin A form transmembrane pores in lipid bilayers, through which monovalent ions may pass. The groups within the peptide that interact with these ions have been studied by application of solid-state spectroscopic methods to a series of gramicidin A analogues synthesized with 13C in selected peptide carbonyl groups. The resonances of D-Leu10, D-Leu12 and D-Leu14 analogues were perturbed in the presence of 0.16 M sodium ions, whereas the resonances of the carbonyls of Gly2, Ala3, D-Leu4 and Val7, which are closer to the formylated N-terminal end of the peptide, were unaffected. The observed changes in chemical shift anisotropy are indicative of a change in orientation of the abovementioned leucine carbonyls.

pH-control of the miscibility properties of a binary lipid alloy and its influence on the ion transport by gramicidin

We studied the coupling of a membrane function (the transport of ions by the pore forming polypeptide gramicidin) to chemically driven phase changes in black membranes of binary lipid mixtures. In particular, we investigated the influence of the aqueous pH value on the fluid-fluid demixing effect of Ca2+ to phosphatidylcholine/phosphatidylglycerol bilayers. It is found that one can switch, under certain conditions, between a homogeneously mixed and a phase separated membrane by changing the pH. We interpret this as being caused by the change in the degree of dissociation of one of the lipid components.

The use of single-wavelength anomalous scattering to solve the crystal structure of a gramicidin A/caesium chloride complex

Single-wavelength Cu K alpha anomalous scattering has been used to determine the structure of a crystalline complex of gramicidin A and caesium chloride. The asymmetric unit in these crystals, with space group P2(1)2(1)2(1) and a = 32.118 (6), b = 52.103 (12), c = 31.174 (7) A, contains four independent monomers (two dimers) of the pentadecapeptide. This structure falls in an intermediate size range for which direct methods and multiple isomorphous replacement are generally not successful for obtaining phase information. However, using the Bijvoet differences and the partial structure of the caesium atoms which have been incorporated in the crystals, it has been possible to obtain information on this crystal form. Because the caesium atoms dominate the scattering of these crystals, inclusion of the Friedel mate information in the restrained least-squares refinement has been essential. These studies extend the utility of single-wavelength anomalous-scattering phase determination to a macromolecular structure in which the partial structure of the anomalous scatterer is large.

The intrinsic molecular potential of glyceryl monooleate layers and its effect on the conformation and orientation of an inserted molecule: example of gramicidin A

It is shown by explicit calculation that the distribution of the atomic charges in the constituent molecules of a lipid monolayer or bilayer of glyceryl monooleate creates an intrinsic potential difference between the head region and the hydrocarbon region which tends to repel positive charges towards the exterior and attract negative charges to the interior. The analogies and differences between a bilayer and a monolayer are analyzed. The possible consequences of the intrinsic potential gradient in a lipid layer on the preferred orientation and conformation of a polar neutral molecule are illustrated on the case of a gramicidin A monomer.

Calculation of deformation energies and conformations in lipid membranes containing gramicidin channels

In this paper we calculate surface conformation and deformation free energy associated with the incorporation of gramicidin channels into phospholipid bilayer membranes. Two types of membranes are considered. One is a relatively thin solvent-free membrane. The other is a thicker solvent-containing membrane. We follow the approach used for the thin membrane case by Huang (1986) in that we use smectic liquid crystal theory to evaluate the free energy associated with distorting the membrane to other than a flat configuration. Our approach is different from Huang, however, in two ways. One is that we include a term for surface tension, which Huang did not. The second is that one of our four boundary conditions for solving the fourth-order differential equation describing the free energy of the surface is different from Huang's. The details of the difference are described in the text. Our results confirm that for thin membranes Huang's neglect of surface tension is appropriate. However, the precise geometrical form that we calculate for the surface of the thin membrane in the region of the gramicidin channel is somewhat different from his. For thicker membranes that have to deform to a greater extent to accommodate the channel, we find that the contribution of surface tension to the total energy in the deformed surface is significant. Computed results for the shape of the deformed surface, the total energy in the deformed surface, and the contributions of different components to the total energy, are presented for the two types of membranes considered. These results may be significant for understanding the mechanisms of dimer formation and breakup, and the access resistance for ions entering gramicidin channels.

Deuterium NMR of 2HCO-Val1...gramicidin A and 2HCO-Val1-D-Leu2...gramicidin A in oriented DMPC bilayers

Deuterium NMR is used to study the structure and dynamics of the formyl C-2H bond in selectively deuterated gramicidin molecules. Specifically, the functionally different analogues 2HCO-Val1...gramicidin A and 2HCO-Val1-D-Leu2...gramicidin A are studied by 2H NMR so that any conformational or dynamical differences between the two analogues can be correlated with their difference in lifetime. These analogues are first synthesized, purified, and characterized and then incorporated into oriented bilayers of dimyristoylphosphatidylcholine sandwiched between glass coverslips. Phosphorous NMR line shapes obtained from these samples are consistent with the presence of the bilayer phase and indicate that the disorder exhibited by the lipid matrix is approximately of the same type and degree for both analogues. Deuterium NMR line shapes obtained from these samples indicate that the motional axis of the formyl group of gramicidin is parallel to the coverslip normal, that the distribution of motional axis orientations has a width of 7-9 degrees, and that a similar, major conformational and dynamical state exists for the formyl C-2H bond of both analogues. In this state, if the only motion present is fast axial rotation, then the experimentally derived angle between the formyl C-2H bond and the motional axis is consistent with the presence of a right-handed, single-stranded, beta 6.3 helical dimer but is not consistent with the presence of a left-handed, single-stranded, beta 6.3 helical dimer. However, if fast axial rotation is not the only motion present, then the left-handed, single-stranded, beta 6.3 helical dimer cannot be absolutely excluded as a possibility. Also, a second, minor conformational and dynamical state appears to be present in the spectrum of 2HCO-Val1-D-Leu2...gramicidin A but is not observed in the spectrum of 2HCO-Val1...gramicidin A. This minor conformational and dynamical state may reflect the presence of monomers, while the major conformational and dynamical state may reflect the presence of dimers.

A quantitative infrared determination of acyl chain conformation in gramicidin/dipalmitoylphosphatidylcholine mixtures

A quantitative infrared characterization of phospholipid acyl chain disordering in 6,6,6'6'-d4 dipalmitoylphosphatidylcholine/ Gramicidin D bilayers has been made. Three CD2 rocking modes, at 622 cm-1, 646-649 cm-1, and 651-653 cm-1, assigned to particular conformers, were used to determine disorder in the presence of peptide, as well as percentages of particular classes of conformer within the total gauche population. At 44C, the gauche percentages in 10:1 and 30:1 lipid/peptide mixtures were 15% and 17%, respectively. At 34C, the corresponding values were 9.8% and 2.6%. The percentage of (single gauche bend + kink) conformers, relative to multiple gauche forms, decreases dramatically from 78% in the 30:1 mixture to 15% in the 10:1 mixture at 44C. These data provide the first quantitative measure of the extent to which a membrane-spanning peptide disorders phospholipid gel phases and orders liquid crystal phases.

Transverse nuclear spin relaxation in phosphatidylcholine bilayers containing gramicidin

Deuterium nuclear magnetic resonance has been used to study transverse relaxation in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, perdeuterated and specifically deuterated at the alpha position of the chains, containing the polypeptide gramicidin at concentrations of 0, 1, and 4 mol%. For 4 mol% gramicidin, the bilayer is thought to undergo a continuous phase change rather than a phase transition proceeding via two phase coexistence. Information is obtained regarding lipid dynamics in the continuous phase change region of the phase diagram. In the presence of gramicidin, the transverse relaxation time measured by the quadrupole echo technique, T2e, passes through a minimum in the gel phase. The gramicidin concentration dependence of T2e suggests that the polypeptide reduces the temperature sensitivity of the correlation time responsible for the minimum. The polypeptide also increases the sensitivity of the first spectral moment, M1, to the quadrupole echo pulse separation. This behavior is attributed to a polypeptide-induced enhancement of the spread in T2e along the acyl chains. Quadrupole Carr-Purcell-Meiboom-Gill experiments are used to separate contributions to the observed behavior from fast and slow motions.

A novel method for measuring membrane conductance changes by a voltage-sensitive optical probe

This study presents a method whose principles enable using a voltage-sensitive optical probe, to quantitatively measure conductivity changes elicited in membrane vesicles and cells. The procedure is based on the fact that the amplitude of the transmembrane potential difference, established across a membrane by an external electric field, is decreased when membrane conductivity is increased upon incorporation of ionophores into the membrane. The method was applied to osmotically swollen thylakoid membranes whose membrane conductivity was changed by the addition of gramicidin or ionomycin. The electric field induced stimulated luminescence from photosystem I (electrophotoluminescence-EPL) was used as a voltage-sensitive optical probe. We calculated the induced conductance changes by using a calibrated EPL vs external electric field response curve and measuring the ionophore-mediated attenuation of the EPL signal. The calculated ionophore-unmodified conductance of the thylakoid membrane yields a value of 171 +/- 56 nS/cm. The value of the membrane conductance, modified by 10 nM gramicidin was found to be 190 +/- 56 nS/cm. The modified membrane conductance and the membrane conductance changes induced by 1 microM ionomycin in the presence of CaCl2 were found to be 186 +/- 3 nS/cm and 15 +/- 3 nS/cm, respectively.

Effect of proteins on fluorophore lifetime heterogeneity in lipid bilayers

The effect of three different membrane proteins on the fluorescence lifetime heterogeneity of 1,6-diphenyl-1,3,5-hexatriene (DPH) in phospholipid vesicle systems was investigated. For large unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) at 37 degrees C, the fluorescence decay was essentially monoexponential (8.6 and 8.2 ns, respectively) except for a minor component typical of DPH. For gramicidin D reconstituted into DMPC vesicles at a protein/lipid molar ratio of 1/7, the most appropriate analysis of the data was found to be in the form of a bimodal Lorentzian distribution. Centers of the major lifetime components were almost identical with those recovered for vesicles without proteins, while broad distributional widths of some 4.0 ns were recovered. Variation of the protein/lipid molar ratio in sonicated POPC vesicles revealed an abrupt increase in distributional width at ratios approximating 1/15-1/20, which leveled off at about 2.5 ns. For bacteriorhodopsin in DMPC vesicles and cytochrome b5 in POPC, the most appropriate analysis of the data was again found to be in the form of a bimodal Lorentzian also with broad distributional widths in the major component. Lifetime centers were decreased for these proteins due to fluorescence energy transfer to the retinal of the bacteriorhodopsin and heme of the cytochrome b5. Fluorescence energy transfer is distance dependent, and since a range of donor-acceptor distances would be expected in a membrane, lifetime distributions should therefore be recovered independently of other effects for proteins possessing acceptor chromophores.(ABSTRACT TRUNCATED AT 250 WORDS)

Two mechanisms by which fluorescent oxonols indicate membrane potential in human red blood cells

Optical potentiometric indicators have been used to monitor the transmembrane electrical potential (Em) of many cells and organelles. A better understanding of the mechanisms of dye response is needed for the design of dyes with improved responses and for unambiguous interpretation of experimental results. This paper describes the responses to delta Em of 20 impermeant oxonols in human red blood cells. Most of the oxonols interacted with valinomycin, but not with gramicidin. The fluorescence of 15 oxonols decreased with hyperpolarization, consistent with an "on-off" mechanism, whereas five oxonols unexpectedly showed potential-dependent increases in fluorescence at less than 2 microM [dye]. Binding curves were determined for two dyes (WW781, negative response and RGA451, positive response) at 1 mM [K]o (membrane hyperpolarized with gramicidin) and at 90 mM [K]o (delta Em = 0 with gramicidin). Both dyes showed potential-dependent decreases in binding. Changes in the fluorescence of cell suspensions correlated with changes in [dye]bound for WW781, in accordance with the "on-off" mechanism, but not for RGA451. Large positive fluorescence changes (greater than 30%) dependent on Em were observed between 0.1 and 1.0 microM RGA451. A model is suggested in which RGA451 moves between two states of different quantum efficiencies within the membrane.