Molecular origin of the internal dipole potential in lipid bilayers: calculation of the electrostatic potential

Nano-scale domains in the plasma membrane are like macroscopic domains in asymmetric bilayers

Unfavorable lipid-lipid pairwise interactions between HiTm and LowTm lipids drive liquid-disordered (Ld) + liquid-ordered (Lo) phase separation. Large size of phase domains is opposed by lipid dipole repulsions, which are more significant compared with the pairwise interactions for naturally abundant LowTm lipids such as palmitoyl oleoyl phosphatidylcholine. During the nano-to-macro domain size transition, no lipid phase transition occurs, and measured properties of Ld + Lo nanodomains are found to be essentially the same as those of macrodomains. Use of macrodomains in mixtures to model cell plasma membranes (PM) is helpful, enabling study by optical microscopy. Use of asymmetric giant unilamellar vesicles to model a PM reveals that ordered phase domains in one leaflet induce ordered domains in an otherwise uniform phase in the apposing leaflet that models a cytoplasmic leaflet. Because macro and nano phase properties are so similar, we conclude that a cell PM that has nano-scale Ld + Lo phase domains in the exoplasmic leaflet is likely to induce nano-scale ordered domains in the cytoplasmic leaflet.

Surface Characterization of Lipid Biomimetic Systems

Zeta potential and dipole potential measures are direct operational methodologies to determine the adsorption, insertion and penetration of ions, amphipathic and neutral compounds into the membranes of cells and model systems. From these results, the contribution of charged and dipole groups can be deduced. However, although each method may give apparent affinity or binding constants, care should be taken to interpret them in terms of physical meaning because they are not independent properties. On the base of a recent model in which the lipid bilayer is considered as composed by two interphase regions at each side of the hydrocarbon core, this review describes how dipole potential and zeta potential are correlated due to water reorganization. From this analysis, considering that in a cell the interphase region the membrane extends to the cell interior or overlaps with the interphase region of another supramolecular structure, the correlation of dipole and electrostatic forces can be taken as responsible of the propagation of perturbations between membrane and cytoplasm and vice versa. Thus, this picture gives the membrane a responsive character in addition to that of a selective permeability barrier when integrated to a complex system.

Nanoscale Probing of Cholesterol-Rich Domains in Single Bilayer Dimyristoyl-Phosphocholine Membranes Using Near-Field Spectroscopic Imaging

Cholesterol is believed to induce the formation of membrane domains, "rafts", which are implicated in a range of natural and pathologic membrane processes. Therefore, it is important to understand the role that cholesterol plays in the formation of these structures. Here, we use label-free spectroscopic imaging to investigate cholesterol fractioning in supported bilayer membranes at nanoscale. Scattering-type scanning near-field optical microscopy (s-SNOM) was used to visualize the formation of cholesterol-induced domains in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes. Our results revealed the coexistence of phase separated domains in DMPC lipids with 10 mol % cholesterol content, whereas a mostly homogeneous bilayer was found at low (5 mol %) and high (15 mol %) cholesterol content. Near-field nano-FTIR spectroscopy was used to identify the cholesterol-rich domains based on their qualitative chemical compositions. It was determined that cholesterol binds to phosphodiester and alkyl glycerol ester moieties, likely via hydrogen bonding of the alcohol to either of the ester oxygens. The results also confirm the existence of an ideal cholesterol-lipid mixture ratio (∼15:85) with a geometrically defined packing. At lower cholesterol content there is phase separation between liquid ordered and almost neat DMPC domains. Thus, the liquid ordered phase exists at an energy minimum at a given lipid-cholesterol ratio.

Structure and Dynamics of GPCRs in Lipid Membranes: Physical Principles and Experimental Approaches

Over the past decade, the vast amount of information generated through structural and biophysical studies of GPCRs has provided unprecedented mechanistic insight into the complex signalling behaviour of these receptors. With this recent information surge, it has also become increasingly apparent that in order to reproduce the various effects that lipids and membranes exert on the biological function for these allosteric receptors, in vitro studies of GPCRs need to be conducted under conditions that adequately approximate the native lipid bilayer environment. In the first part of this review, we assess some of the more general effects that a membrane environment exerts on lipid bilayer-embedded proteins such as GPCRs. This is then followed by the consideration of more specific effects, including stoichiometric interactions with specific lipid subtypes. In the final section, we survey a range of different membrane mimetics that are currently used for in vitro studies, with a focus on NMR applications.

Lipid-mediated regulation of pore-forming activity of syringomycin E by thyroid hormones and xanthene dyes

The effects of dipole modifiers, thyroid hormones (thyroxine and triiodothyronine) and xanthene dyes (Rose Bengal, phloxineB, erythrosin, eosinY and fluorescein) on the pore-forming activity of the lipopeptide syringomycin E (SRE) produced by Pseudomonas syringae were studied in a model bilayer. Thyroxine does not noticeably influence the steady-state number of open SRE channels (N_op), whereas triiodothyronine decreases it 10-fold at -50mV. Rose Bengal, phloxine B and erythrosin significantly increase N_op by 350, 100 and 70 times, respectively. Eosin Y and fluorescein do not practically affect the pore-forming activity of SRE. Recently, we showed that hormones decrease the dipole potential of lipid bilayers by approximately 60mV at 50μM, while Rose Bengal, phloxine B and erythrosin at 2.5μM reduce the membrane dipole potential by 120, 80 and 50mV, respectively. In the present study using differential scanning microcalorimetry, confocal fluorescence microscopy, the calcein release technique and measurements of membrane curvature elasticity, we show that triiodothyronine strongly affects the fluidity of model membranes: its addition leads to a significant decrease in the temperature and cooperativity of the main phase transition of DPPC, calcein leakage from DOPC vesicles, fluidization of solid domains in DOPC/DPPC liposomes, and promotion of lipid curvature stress. Thyroxine exerts a weaker effect. Xanthene dyes do not influence the phase transition of DPPC. Despite the decrease in the dipole potential, thyroid hormones modulate SRE channels predominantly via the elastic properties of the membrane, whereas the xanthene dyes Rose Bengal, phloxine B and erythrosine affect SRE channels via bilayer electrostatics.

Carbohydrate-derived amphiphilic macromolecules: a biophysical structural characterization and analysis of binding behaviors to model membranes

The design and synthesis of enhanced membrane-intercalating biomaterials for drug delivery or vascular membrane targeting is currently challenged by the lack of screening and prediction tools. The present work demonstrates the generation of a Quantitative Structural Activity Relationship model (QSAR) to make a priori predictions. Amphiphilic macromolecules (AMs) "stealth lipids" built on aldaric and uronic acids frameworks attached to poly(ethylene glycol) (PEG) polymer tails were developed to form self-assembling micelles. In the present study, a defined set of novel AM structures were investigated in terms of their binding to lipid membrane bilayers using Quartz Crystal Microbalance with Dissipation (QCM-D) experiments coupled with computational coarse-grained molecular dynamics (CG MD) and all-atom MD (AA MD) simulations. The CG MD simulations capture the insertion dynamics of the AM lipophilic backbones into the lipid bilayer with the PEGylated tail directed into bulk water. QCM-D measurements with Voigt viscoelastic model analysis enabled the quantitation of the mass gain and rate of interaction between the AM and the lipid bilayer surface. Thus, this study yielded insights about variations in the functional activity of AM materials with minute compositional or stereochemical differences based on membrane binding, which has translational potential for transplanting these materials in vivo. More broadly, it demonstrates an integrated computational-experimental approach, which can offer a promising strategy for the in silico design and screening of therapeutic candidate materials.

Analytic Model for the Dipole Potential of a Lipid Layer

The larger permeability of anions than cations through a lipid bilayer can be rationalized by the positive sign of the bilayer's dipole potential. That is, upon crossing the lipid headgroups toward the hydrocarbon chain region, the electrostatic potential increases by several hundred millivolts. We derive an analytic expression for the dipole potential of a single lipid layer using an electrostatic model that is based on an extended version of the linearized Poisson-Boltzmann theory. The model highlights the ability of the lipid headgroups to render the dipole potential positive by inducing an orientational ordering of the solvent molecules. The positive contribution of the solvent overcompensates the negative dipole potential due to the bare lipids. Our theoretical prediction compares accurately with measurements of the dipole potential that we have conducted for mixed anionic-zwitterionic lipid monolayers at the air-water interface.

Electromechanics of a membrane with spatially distributed fixed charges: flexoelectricity and elastic parameters

We investigate the electrostatic contribution to the lipid membrane mechanical parameters: tension, bending rigidity, spontaneous curvature, and flexocoefficient, using an approach where stress in the membrane is explicitly balanced. Our model includes an applied electrostatic potential as well as a charge distribution in the membrane. We apply our theory to membranes having surface charges and electric dipoles at the surface.

Surface electrostatics of lipid bilayers by EPR of a pH-sensitive spin-labeled lipid

Many biophysical processes such as insertion of proteins into membranes and membrane fusion are governed by bilayer electrostatic potential. At the time of this writing, the arsenal of biophysical methods for such measurements is limited to a few techniques. Here we describe a, to our knowledge, new spin-probe electron paramagnetic resonance (EPR) approach for assessing the electrostatic surface potential of lipid bilayers that is based on a recently synthesized EPR probe (IMTSL-PTE) containing a reversibly ionizable nitroxide tag attached to the lipids' polar headgroup. EPR spectra of the probe directly report on its ionization state and, therefore, on electrostatic potential through changes in nitroxide magnetic parameters and the degree of rotational averaging. Further, the lipid nature of the probe provides its full integration into lipid bilayers. Tethering the nitroxide moiety directly to the lipid polar headgroup defines the location of the measured potential with respect to the lipid bilayer interface. Electrostatic surface potentials measured by EPR of IMTSL-PTE show a remarkable (within ±2%) agreement with the Gouy-Chapman theory for anionic DMPG bilayers in fluid (48°C) phase at low electrolyte concentration (50 mM) and in gel (17°C) phase at 150-mM electrolyte concentration. This agreement begins to diminish for DMPG vesicles in gel phase (17°C) upon varying electrolyte concentration and fluid phase bilayers formed from DMPG/DMPC and POPG/POPC mixtures. Possible reasons for such deviations, as well as the proper choice of an electrostatically neutral reference interface, have been discussed. Described EPR method is expected to be fully applicable to more-complex models of cellular membranes.

Observing a model ion channel gating action in model cell membranes in real time in situ: membrane potential change induced alamethicin orientation change

Ion channels play crucial roles in transport and regulatory functions of living cells. Understanding the gating mechanisms of these channels is important to understanding and treating diseases that have been linked to ion channels. One potential model peptide for studying the mechanism of ion channel gating is alamethicin, which adopts a split α/3(10)-helix structure and responds to changes in electric potential. In this study, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), has been applied to characterize interactions between alamethicin (a model for larger channel proteins) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers in the presence of an electric potential across the membrane. The membrane potential difference was controlled by changing the pH of the solution in contact with the bilayer and was measured using fluorescence spectroscopy. The orientation angle of alamethicin in POPC lipid bilayers was then determined at different pH values using polarized SFG amide I spectra. Assuming that all molecules adopt the same orientation (a δ distribution), at pH = 6.7 the α-helix at the N-terminus and the 3(10)-helix at the C-terminus tilt at about 72° (θ(1)) and 50° (θ(2)) versus the surface normal, respectively. When pH increases to 11.9, θ(1) and θ(2) decrease to 56.5° and 45°, respectively. The δ distribution assumption was verified using a combination of SFG and ATR-FTIR measurements, which showed a quite narrow distribution in the angle of θ(1) for both pH conditions. This indicates that all alamethicin molecules at the surface adopt a nearly identical orientation in POPC lipid bilayers. The localized pH change in proximity to the bilayer modulates the membrane potential and thus induces a decrease in both the tilt and the bend angles of the two helices in alamethicin. This is the first reported application of SFG to the study of model ion channel gating mechanisms in model cell membranes.

Differential effect of cholesterol and its biosynthetic precursors on membrane dipole potential

Dipole potential is the potential difference within the membrane bilayer, which originates due to the nonrandom arrangement of lipid dipoles and water molecules at the membrane interface. Cholesterol, a representative sterol in higher eukaryotic membranes, is known to increase membrane dipole potential. In this work, we explored the effects of immediate (7-DHC and desmosterol) and evolutionary (ergosterol) precursors of cholesterol on membrane dipole potential, monitored by the dual wavelength ratiometric approach utilizing the probe di-8-ANEPPS. Our results show that the effect of these precursors on membrane dipole potential is very different from that observed with cholesterol, although the structural differences among them are subtle. These results assume relevance, since accumulation of cholesterol precursors due to defective cholesterol biosynthesis has been reported to result in several inherited metabolic disorders such as the Smith-Lemli-Opitz syndrome. Interestingly, cholesterol (and its precursors) has a negligible effect on dipole potential in polyunsaturated membranes. We interpret these results in terms of noncanonical orientation of cholesterol in these membranes. Our results constitute the first report on the effect of biosynthetic and evolutionary precursors of cholesterol on dipole potential, and imply that a subtle change in sterol structure can significantly alter the dipolar field at the membrane interface.

Probing amphotericin B single channel activity by membrane dipole modifiers

The effects of dipole modifiers and their structural analogs on the single channel activity of amphotericin B in sterol-containing planar phosphocholine membranes are studied. It is shown that the addition of phloretin in solutions bathing membranes containing cholesterol or ergosterol decreases the conductance of single amphotericin B channels. Quercetin decreases the channel conductance in cholesterol-containing bilayers while it does not affect the channel conductance in ergosterol-containing membranes. It is demonstrated that the insertion of styryl dyes, such as RH 421, RH 237 or RH 160, in bilayers with either cholesterol or ergosterol leads to the increase of the current amplitude of amphotericin B pores. Introduction of 5α-androstan-3β-ol into a membrane-forming solution increases the amphotericin B channel conductance in a concentration-dependent manner. All the effects are likely to be attributed to the influence of the membrane dipole potential on the conductance of single amphotericin B channels. However, specific interactions of some dipole modifiers with polyene-sterol complexes might also contribute to the activity of single amphotericin B pores. It has been shown that the channel dwell time increases with increasing sterol concentration, and it is higher for cholesterol-containing membranes than for bilayers including ergosterol, 6-ketocholestanol, 7-ketocholestanol or 5α-androstan-3β-ol. These findings suggest that the processes of association/dissociation of channel forming molecules depend on the membrane fluidity.

Effect of membrane tension on the electric field and dipole potential of lipid bilayer membrane

The dipole potential of lipid bilayer membrane controls the difference in permeability of the membrane to oppositely charged ions. We have combined molecular dynamics (MD) simulations and experimental studies to determine changes in electric field and electrostatic potential of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid bilayer in response to applied membrane tension. MD simulations based on CHARMM36 force field showed that electrostatic potential of DOPC bilayer decreases by ~45mV in the physiologically relevant range of membrane tension values (0 to 15dyn/cm). The electrostatic field exhibits a peak (~0.8×10(9)V/m) near the water/lipid interface which shifts by 0.9Å towards the bilayer center at 15dyn/cm. Maximum membrane tension of 15dyn/cm caused 6.4% increase in area per lipid, 4.7% decrease in bilayer thickness and 1.4% increase in the volume of the bilayer. Dipole-potential sensitive fluorescent probes were used to detect membrane tension induced changes in DOPC vesicles exposed to osmotic stress. Experiments confirmed that dipole potential of DOPC bilayer decreases at higher membrane tensions. These results are suggestive of a potentially new mechanosensing mechanism by which mechanically induced structural changes in the lipid bilayer membrane could modulate the function of membrane proteins by altering electrostatic interactions and energetics of protein conformational states.

Polarizable water model for the coarse-grained MARTINI force field

Coarse-grained (CG) simulations have become an essential tool to study a large variety of biomolecular processes, exploring temporal and spatial scales inaccessible to traditional models of atomistic resolution. One of the major simplifications of CG models is the representation of the solvent, which is either implicit or modeled explicitly as a van der Waals particle. The effect of polarization, and thus a proper screening of interactions depending on the local environment, is absent. Given the important role of water as a ubiquitous solvent in biological systems, its treatment is crucial to the properties derived from simulation studies. Here, we parameterize a polarizable coarse-grained water model to be used in combination with the CG MARTINI force field. Using a three-bead model to represent four water molecules, we show that the orientational polarizability of real water can be effectively accounted for. This has the consequence that the dielectric screening of bulk water is reproduced. At the same time, we parameterized our new water model such that bulk water density and oil/water partitioning data remain at the same level of accuracy as for the standard MARTINI force field. We apply the new model to two cases for which current CG force fields are inadequate. First, we address the transport of ions across a lipid membrane. The computed potential of mean force shows that the ions now naturally feel the change in dielectric medium when moving from the high dielectric aqueous phase toward the low dielectric membrane interior. In the second application we consider the electroporation process of both an oil slab and a lipid bilayer. The electrostatic field drives the formation of water filled pores in both cases, following a similar mechanism as seen with atomistically detailed models.

Many biomolecular processes involve charged species moving between regions of high polarity, such as the water phase, and regions of lower polarity, such as the lipid membrane. Due to the change in electrostatic screening between these two environments, the strength of the interactions between the moving charge and the surrounding molecules also changes. This has important consequences for the way biological activity is controlled. To help understand the forces driving the movement of biomolecules, we developed a computational model which is capable of describing these processes at near-atomic detail. To do so efficiently, we use a coarse-grained description of the molecules, in which some of the atomistic detail is averaged out. To capture the inhomogeneous nature of the dielectric response, we re-introduce some detail in the water model; the new model effectively mimics the orientational polarizability of real water molecules, and screens electrostatic interactions realistically. This enables the study of a number of important biological processes that were hitherto considered challenging for coarse-grained models, such as the permeation of ions across a lipid membrane and the rupture of membranes due to an electrostatic field, at relatively low computational cost.

Nanoscopic description of biomembrane electrostatics: results of molecular dynamics simulations and fluorescence probing

Electrostatic fields generated on and inside biological membranes are recognized to play a fundamental role in key processes of cell functioning. Their understanding requires an adequate description on the level of elementary charges and the reconstruction of electrostatic potentials by integration over all elementary interactions. Out of all the available research tools, only molecular dynamics simulations are capable of this, extending from the atomic to the mesoscopic level of description on the required time and space scale. A complementary approach is that offered by molecular probe methods, with the application of electrochromic dyes. Highly sensitive to intermolecular interactions, they generate integrated signals arising from electric fields produced by elementary charges at the sites of their location. This review is an attempt to provide a critical analysis of these two approaches and their present and potential applications. The results obtained by both methods are consistent in that they both show an extremely complex profile of the electric field in the membrane. The nanoscopic view, with two-dimensional averaging over the bilayer plane and formal separation of the electrostatic potential into surface (Psi(s)), dipole (Psi(d)) and transmembrane (Psi(t)) potentials, is constructive in the analysis of different functional properties of membranes.

Effect of headgroup on the dipole potential of phospholipid vesicles

The dipole potentials, psi (d), of phospholipid vesicles composed of pure dimyristoylphosphatidylcholine (DMPC) or vesicles in which 50 mol% of the DMPC was substituted by dimyristoylphosphatidylserine (DMPS), dimyristoylphosphatidylglycerol (DMPG), dimyristoylethanolamine (DMPE), dimyristoylphosphatidic acid (DMPA) or monomyristoylphosphatidylcholine (MMPC) were measured via a fluorescent ratiometric method utilizing the probe di-8-ANEPPS. The PS and PG headgroups were found to cause only minor changes in psi (d). PE caused an increase in psi (d) of 51 mV. This could be explained by a decrease in the dielectric constant of the glycerol backbone region as well as a movement of the P(-)-N(+) dipole of the less bulky PE headgroup to a position more parallel to the membrane surface than in PC. The negatively charged PA headgroup increases psi (d) by 215 mV relative to PC alone. This indicates that the positive pole of the dipole predominantly responsible for the dipole potential is located at a position closer to the interior of the membrane than the phosphate group. The increase in the charge of the negative pole of the dipole by the phosphate group of PA increases the electrical potential drop across the lipid headgroup region. The incorporation of the single chain lipid MMPC into the membrane causes a decrease in psi (d) of 142 mV. This can be explained by a decrease in packing density within the membrane of carbonyl dipoles from the sn-2 chain of DMPC. The results presented should contribute to a better understanding of the electrical effect of lipid headgroups on the functioning of membrane proteins.

Interaction of peptides with biomembranes assessed by potential-sensitive fluorescent probes

Peptide-membrane interaction is an important step to be evaluated in a study of the activity and mode of action of several bioactive peptides. A variety of methods are available; however, few of them satisfy the criteria of being sensitive, biocompatible, versatile, easy to perform, and allowing real-time monitoring as the use of potential-sensitive fluorescent probes. Here we review methods for detecting the effects of membrane-active peptides, even those that are not intrinsically fluorescent, on the different types of membrane potentials, with a special emphasis on studies conducted with living cells. FPE is a probe sensitive to surface potential and detects electrostatic interactions at the water-lipid interface. Di-8-ANEPPS is sensitive to dipole potential and detects membrane incorporations. Transmembrane potential changes reveal major membrane destabilizations, such as in pore formation. The combination of the information obtained from the three potential variations can lead to a more elucidative picture of the mechanisms of the interaction of relevant peptides with biomembranes.

pH modulation of transport properties of alamethicin oligomers inserted in zwitterionic-based artificial lipid membranes

Electric features of biological membranes are major determinants of the function and physiological manifestation of membrane-penetrating peptides, and such features are prone to be modulated by the properties of the surrounding aqueous medium. In this work, we demonstrate that pH plays crucial roles in modulating electric characteristics of zwitterionic-based artificial lipid membranes. The effect of pH on electrical properties of such membranes was probed by evaluating the transport properties of embedded alamethicin oligomers over a wide range of pH values (i.e., 0.65, 2.08, 2.94, 7 and 10.1). Our data strongly support the paradigm of a pH-dependent variation of the surface and membrane dipole potential which, in conjunction with possible lateral pressure effects within the lipid membrane, lead to a non-monotonic modulation of the electrical conductance of alamethicin oligomers. As expected, pH modulation of transport properties through the alamethicin oligomer is more visible for narrower pores (that is, the 1st conductive state) with slightly better cation selectivity as compared to larger oligomers.

Nanopore-facilitated, voltage-driven phosphatidylserine translocation in lipid bilayers--in cells and in silico

Nanosecond, megavolt-per-meter pulses--higher power but lower total energy than the electroporative pulses used to introduce normally excluded material into biological cells--produce large intracellular electric fields without destructively charging the plasma membrane. Nanoelectropulse perturbation of mammalian cells causes translocation of phosphatidylserine (PS) to the outer face of the cell, intracellular calcium release, and in some cell types a subsequent progression to apoptosis. Experimental observations and molecular dynamics (MD) simulations of membranes in pulsed electric fields presented here support the hypothesis that nanoelectropulse-induced PS externalization is driven by the electric potential that appears across the lipid bilayer during a pulse and is facilitated by the poration of the membrane that occurs even during pulses as brief as 3 ns. MD simulations of phospholipid bilayers in supraphysiological electric fields show a tight association between PS externalization and membrane pore formation on a nanosecond time scale that is consistent with experimental evidence for electropermeabilization and anode-directed PS translocation after nanosecond electric pulse exposure, suggesting a molecular mechanism for nanoelectroporation and nanosecond PS externalization: electrophoretic migration of the negatively charged PS head group along the surface of nanometer-diameter electropores initiated by field-driven alignment of water dipoles at the membrane interface.

Phlorizin- and 6-ketocholestanol-mediated antagonistic modulation of alamethicin activity in phospholipid planar membranes

As a result of the interfacial chemical heterogenity, membrane-penetrating peptides will experience a dramatic variation in environmental polarity manifested via electrical interactions with the surface and dipole potential of membranes prone to modulate the membrane insertion and folding of different peptides and proteins. Herein we present evidence demonstrating that roughly a 30 mV, phlorizin-induced lowering of the magnitude of the dipole potential of a phosphatidyilcholine membrane leads to a 4-fold increase in the electrical activity of embedded alamethicin. The effect is voltage-independent, implying that the dipole potential affects the barrier of alamethicin adsorption to the membrane rather than the translocation of it across the hydrophobic core. Our interpretation points to an enhanced interfacial accumulation of alamethicin monomers on the cis side of the membrane caused by a lower value of the cis dipole potential, which will promote an elevated activity of alamethicin oligomers across the membrane. As expected for a modestly selective ion channel, the enhancing effect of such dipole potential changes on the electrical conductivity is limited (80 +/- 3 pS before and 100 +/- 2 pS after phlorizin addition to the membrane, for the first conductive state of the channel). Our study emphasizes the possibility that, by manipulating at will the sign of change and the magnitude of the interfacial dipole field, it is possible to modulate the extent of the membrane penetration of ion-channel-forming peptides and thereby provide deeper insights into mechanisms of protein-lipid and protein-protein interactions within membranes.

Inner field compensation as a tool for the characterization of asymmetric membranes and Peptide-membrane interactions

Symmetric and asymmetric planar lipid bilayers prepared according to the Montal-Mueller method are a powerful tool to characterize peptide-membrane interactions. Several electrical properties of lipid bilayers such as membrane current, membrane capacitance, and the inner membrane potential differences and their changes can be deduced. The time-resolved determination of peptide-induced changes in membrane capacitance and inner membrane potential difference are of high importance for the characterization of peptide-membrane interactions. Intercalation and accumulation of peptides lead to changes in membrane capacitance, and membrane interaction of charged peptides induces changes in the charge distribution within the membrane and with that to changes in the membrane potential profile. In this study, we establish time-resolved measurements of the capacitance minimization potential DeltaPsi on various asymmetric planar lipid bilayers using the inner field compensation method. The results are compared to the respective ones of inner membrane potential differences DeltaPhi determined from ion carrier transport measurements. Finally, the time courses of membrane capacitances and of DeltaPsi have been used to characterize the interaction of cathelicidins with reconstituted lipid matrices of various Gram-negative bacteria.

Activation of phospholipase C increases intramembrane electric fields in N1E-115 neuroblastoma cells

We imaged the intramembrane potential (a combination of transmembrane, surface, and dipole potential) on N1E-115 neuroblastoma cells with a voltage-sensitive dye. After activation of the B(2) bradykinin receptor, the electric field sensed by the dye increased by an amount equivalent to a depolarization of 83 mV. The increase in intramembrane potential was blocked by the phospholipase C (PLC) inhibitors U-73122 and neomycin, and was invariably accompanied by a transient rise of [Ca(2+)](i). A depolarized inner surface potential, as the membrane loses negative charges via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis, and an increase in the dipole potential, as PIP(2) is hydrolyzed to 1,2-diacylglycerol (DAG), can each account for a small portion of the change in intramembrane potential. The primary contribution to the measured change in intramembrane potential may arise from an increased dipole potential, as DAG molecules are generated from hydrolysis of other phospholipids. We found bradykinin produced an inhibition of a M-type voltage-dependent K(+) current (I(K(M))). This inhibition was also blocked by the PLC inhibitors and had similar kinetics as the bradykinin-induced modulation of intramembrane potential. Our results suggest that the change in the local intramembrane potential induced by bradykinin may play a role in mediating the I(K(M)) inhibition.

Phospholipid flop induced by transmembrane peptides in model membranes is modulated by lipid composition

Since phospholipid synthesis is generally confined to one leaflet of a membrane, membrane growth requires phospholipid translocation (flip-flop). It is generally assumed that this process is protein-mediated; however, the mechanism of flip-flop remains elusive. Previously, we have demonstrated flop of 2-[6-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]caproyl] (C6NBD) phospholipids, induced by the presence of membrane-spanning peptides in vesicles composed of an Escherichia coli phospholipid extract, supporting the hypothesis that the presence of transmembrane stretches of proteins in the bilayer is sufficient to allow phospholipid flip-flop in the inner membrane of E. coli [Kol et al. (2001) Biochemistry 40, 10500]. Here, we investigated whether the specific phospholipid composition of E. coli is a prerequisite for transmembrane helix-induced flop of phospholipids. This was tested by determining the amount of C6NBD-phospholipid that was translocated from the inner leaflet to the outer leaflet of a model membrane in time, using a dithionite reduction assay. The transmembrane peptides GWWL(AL)8WWA (WALP23) and GKKL(AL)8KKA (KALP23) induced phospholipid flop in model membranes composed of various lipid mixtures. The rate of peptide-induced flop was found to decrease with increasing dioleoylphosphatidylethanolamine (DOPE) content of vesicles composed of DOPE and dioleoylphosphatidylcholine (DOPC), and the rate of KALP23-induced flop was shown to be stimulated by higher dioleoylphosphatidylglycerol (DOPG) content in model membranes composed of DOPG and DOPC. Furthermore, the incorporation of cholesterol had an inhibitory effect on peptide-induced flop. Finally, flop efficiency was strongly dependent on the phospholipid headgroup of the NBD-phospholipid analogue. Possible implications for transmembrane helix-induced flop in biomembranes in general are discussed.

Membrane insertion and dissociation processes of a model transmembrane helix

An important subject for elucidating membrane protein (MP) folding is how transmembrane helices (TMHs) insert into and dissociate from membranes. We investigated helix dissociation kinetics and insertion topology by means of intervesicular transfer of the fluorophore-labeled completely hydrophobic model transmembrane helix NBD-(LALAAAA)(3)-NH(2) (NBD = 7-nitro-2-1,3-benzoxadiazol-4-yl). The peptide forms a topologically stable transmembrane helix, which is in a monomer-antiparallel dimer equilibrium [Yano, Y., Takemoto, T., Kobayashi, S., Yasui, H., Sakurai, H., Ohashi, W., Niwa, M., Futaki, S., Sugiura, Y., and Matsuzaki, K. (2002) Biochemistry 41, 3073-3080]. The helix transfer kinetics, representing the helix dissociation process, was monitored by fluorescence recovery of the quenched peptide in donor vesicles containing a quencher upon its transfer to acceptor vesicles without the quencher. The transfer kinetics and vesicle concentration dependence demonstrated that the transfer was mediated by monomer in the aqueous phase. Furthermore, the activation enthalpy was estimated to be +17.7 +/- 1.3 kcal mol(-1). Helix insertion topology, detected by chemical quenching of the NBD group in the outer leaflet by dithionite ions, was found to be controlled by transmembrane electric potential-helix macro dipole interaction. On the basis of these observations, a model for the helix insertion/dissociation processes was discussed.

Bridging implicit and explicit solvent approaches for membrane electrostatics

Conformations of a zwitterionic bilayer were sampled from a molecular dynamics simulation and their electrostatic properties analyzed by solution of the Poisson equation. These traditionally implicit electrostatic calculations were performed in the presence of varying amounts of explicit solvent to assess the magnitude of error introduced by a uniform dielectric description of water surrounding the bilayer. It was observed that membrane dipole potential calculations in the presence of explicit water were significantly different than wholly implicit solvent calculations with the calculated dipole potential converging to a reasonable value when four or more hydration layers were included explicitly.

Neutral fluorescence probe with strong ratiometric response to surface charge of phospholipid membranes

We report on dramatic differences in fluorescence spectra of 4'-dimethylamino-3-hydroxyflavone (probe F) studied in phospholipid membranes of different charge (phosphatidyl glycerol, phosphatidylcholine (PC), their mixture and the mixture of PC with a cationic lipid). The effect consists in variations of relative intensities at two well-separated band maxima at 520 and 570 nm belonging to normal (N*) and tautomer (T*) excited states of flavone chromophore. Based on these studies we propose a new approach to measure electrostatic potential at the surface layer of phospholipid membranes, which is based on potential-dependent changes of bilayer hydration and involves very sensitive and convenient ratiometric measurements in fluorescence emission.

Effect of phloretin on ionophore mediated electroneutral transmembrane translocations of H(+), K(+) and Na(+) in phospholipid vesicles

Rates of M(+)/H(+) exchange (M(+)=K(+), Na(+)) across phospholipid membranes by ionophore mediated electroneutral translocations and transports through channels could either increase or decrease or change negligibly on adding the polar molecule phloretin to the membrane. The changes depend on pH, the concentration and choice of M(+) and choice of ionophore/channel. Such diverse behaviours have been inferred from studies on the decay of the pH difference across soybean phospholipid vesicular membrane (=Delta pH). The transporters used in this study are (a) the exchange ionophores: nigericin, monensin; (b) combinations of alkali metal ion carriers, valinomycin or nonactin with weak acids carbonyl cyanide m-chlorophenylhydrazone or 2,4-dinitrophenol and (c) channels formed by gramicidin A. All the diverse results can be rationally explained if we take note of the following. (i) The rate limiting steps are associated with the transmembrane translocations involving the rate limiting species identified in the literature. (ii) Phloretin in the membrane decreases the apparent M(+) dissociation constant, K(M), of the M(+) bound ionophores/channels which has the effect of increasing the concentration of these species. (iii) The concentrations of H(+) bound ionophores/channels decrease on adding phloretin. (iv) Phloretin inhibits ternary complex formation (involving valinomycin or nonactin, M(+) and an anion) by forming 1:2 complexes with valinomycin-M(+) or nonactin-M(+). (v) On adding 6-ketocholestanol to the membrane (instead of phloretin) K(M) increases. The decreases/increases in K(M) mentioned above are consistent with the consequences of a hypothesis in which phloretin decreases and 6-ketocholestanol increases the positive internal membrane dipole potential.

Influence of anions and cations on the dipole potential of phosphatidylcholine vesicles: a basis for the Hofmeister effect

Anions and cations have long been recognized to be capable of modifying the functioning of various membrane-related physiological processes. Here, a fluorescent ratio method using the styrylpyridinium dyes, RH421 and di-8-ANEPPS, was applied to determine the effect of a range of anions and cations on the intramembrane dipole potential of dimyristoylphosphatidylcholine vesicles. It was found that certain anions cause a decrease in the dipole potential. This could be explained by binding within the membrane, in support of a hypothesis originally put forward by A. L. Hodgkin and P. Horowicz [1960, J. Physiol. (Lond.) 153:404-412.] The effectiveness of the anions in reducing the dipole potential was found to be ClO4- > SCN- > I- > NO3- > Br- > Cl- > F- > SO42-. This order could be modeled by a partitioning of ions between the membrane and the aqueous phase, which is controlled predominantly by the Gibbs free energy of hydration. Cations were also found to be capable of reducing the dipole potential, although much less efficiently than can anions. The effects of the cations was found to be trivalent > divalent > monovalent. The cation effects were attributed to binding to a specific polar site on the surface of the membrane. The results presented provide a molecular basis for the interpretation of the Hofmeister effect of lyotropic anions on ion transport proteins.

Peptide-bilayer interactions: simulations of dermaseptin B, an antimicrobial peptide

Dermaseptins, a family of antimicrobial peptides, are believed to act by forming amphipathic alpha-helices which associate with the cell membrane, leading to its permeabilisation and disruption. A simple mean field method is described for simulation of the interactions of peptides with lipid bilayers which includes an approximate representation of the electrostatic effects of the head-group region of the bilayer. Starting from an atomistic model of a PC phospholipid bilayer we calculate an average electrostatic potential along the bilayer normal. By combining the interaction of the peptide with this electrostatic potential and with the hydrophobic core of the membrane we arrive at a more complete description of peptide-bilayer energetics than would be obtained using sidechain hydrophobicities alone. Using this interaction potential in MD simulations of the frog skin peptide dermaseptin B reveals that the lipid bilayer stabilises the alpha-helical conformation of the peptide. This is in agreement with FTIR data. A surface associated orientation thus appears to be the most stable arrangement of the peptide, at least at zero ionic strength and without taking account of possible peptide-peptide interactions.

Surface dipole potential at the interface between water and self-assembled monolayers of phosphatidylserine and phosphatidic acid

The nature and magnitude of the surface dipole potential chi at a membrane/water interface still remain open to discussion. By combining measurements of differential capacity C and charge density sigma at the interface between self-assembled monolayers of phosphatidylserine and phosphatidic acid supported by mercury and aqueous electrolytes of different concentration and pH, a sigmoidal dependence of chi upon sigma is revealed, with the inflection at sigma = 0. This behavior is strongly reminiscent of the surface dipole potential due to reorientation of adsorbed water molecules at electrified interfaces. The small increase in C with a decrease in the frequency of the AC signal below approximately 80 Hz, as observed with phospholipid monolayers with partially protonated polar groups, is explained either by a sluggish collective reorientation of some polar groups of the lipid or by a sluggish movement of protons across the polar head region.

Optical detection of membrane dipole potential: avoidance of fluidity and dye-induced effects

Fluorescent styrylpyridinium dyes have recently been suggested as probes of the membrane dipole potential and of the kinetics of electrogenic ion pumps. It is necessary, however, to be able to confidently attribute observed fluorescence changes to electrical effects alone and avoid interference from changes in membrane fluidity. Furthermore, the effect of the dyes themselves on the dipole potential must be investigated. The effect of membrane fluidity on the fluorescence excitation and emission spectra of the dyes RH421 and di-8-ANEPPS have been investigated in lipid vesicles by temperature scans between 15 and 60 degrees C. Both dyes show significant temperature-dependent shifts of their excitation spectra, the magnitude of which depend on the emission wavelength and on the lipid structure. In order to eliminate membrane fluidity effects, fluorescence must be detected at the red edge of the emission spectrum; in this case 670 nm. In order to avoid dye-induced shifts of the excitation spectra of membrane-bound dye, an excess molar ratio of lipid to dye of at least 200-fold is necessary. Fluorescence ratio measurements indicate qualitatively that dimyristoylphosphatidylcholine has a significantly higher dipole potential than that of dioleoylphosphatidylcholine.

Utilization of monensin for detection of microdomains in cholesterol containing membrane

The effect of cholesterol on the monensin mediated proton-cation exchange reaction was measured in the time-resolved domain. The experimental system consisted of a black lipid membrane equilibrated with monensin (Nachliel, E., Finkelstein, Y. and Gutman, M. (1996) Biochim. Biophys. Acta 1285, 131-145). The membrane separated two compartments containing electrolyte solutions and pyranine (8-hydroxypyrene 1,3,6-trisulfonate) was added on to one side of the membrane. A short laser pulse was used to cause a brief transient acidification of the pyranine-containing solution and the resulting electric signal, derived from proton-cation exchange, was measured in the microsecond time domain. Incorporation of cholesterol had a clear effect on the electric transients as measured with Na+ or K+ as transportable cations. The measured transients were subjected to rigorous analysis based on numeric integration of coupled, non-linear, differential rate equations which correspond with the perturbed multi-equilibria state between all reactants present in the system. The various kinetic parameters of the reaction and their dependence on the cholesterol content had been determined. On the basis of these observations we can draw the following conclusions: (1) Cholesterol perturbed the homogeneity of the membrane and microdomains were formed, having a composition that differed from the average value. The ionophore was found in domains which were practically depleted of phosphatidylserine. (2) The diffusivity of the protonated monensin (MoH) was not affected by the presence of cholesterol, indicating that the viscosity of the central layer of the membrane was unaltered. (3) The diffusivity of the monensin metal complexes (MoNa and MoK) was significantly increased upon addition of cholesterol. As the viscosity along the cross membranal diffusion route is unchanged, the enhanced motion of the MoNa and MoK is attributed to variations of the electrostatic potential within the domains.

The interaction of polyphenols with bilayers: conditions for increasing bilayer adhesion

Because proteins and other molecules with a high polyphenol content are commonly involved in adhesion processes, we are investigating the interactions between polyphenols and biological materials. A naturally occurring polyphenol that binds a variety of proteins and lipids is tannic acid (TA), which contains five digallic acid residues covalently linked to a central D-glucose. A previous study has shown that TA increases the adhesion between apposing phosphatidylcholine (PC) bilayers and over a very narrow concentration range collapses the interbilayer fluid space from about 15 A to 5 A. To determine the chemical requirements a polyphenolic molecule must possess to increase bilayer adhesion, we have synthesized several simpler TA analogs that vary in their size, shape, and number of gallic acid and hydroxyl groups. X-ray diffraction, absorbance, binding, and differential scanning calorimetry measurements were used to investigate the interaction of these polyphenolic molecules with egg PC (EPC) and dipalmitoyl PC (DPPC) bilayers. Of these synthetic polyphenols, only penta-O-galloyl-alpha-D-glucose (PGG) was able to completely mimic the effects of TA by collapsing the interbilayer fluid space from 15 A to 5 A, decreasing the dipole potential by about 300 mV, increasing the transition enthalpy of DPPC liposomes, and inducing an interdigitated phase in DPPC. Binding studies indicated that the fluid space was reduced to 5 A at an EPC:PGG mole ratio of 5:1. We conclude that these polyphenols collapse the fluid space of PC bilayers because they 1) are amphipathic and partition into the bilayers interfacial region, 2) are long enough to span the interbilayer space, 3) contain several gallic acids distributed so that they can partition simultaneously into apposing bilayers, and 4) have sufficient gallic acid residues to interact with all lipid headgroups and cover the bilayer surface. Under these conditions we conclude that the polyphenols from interbilayer bridges. We argue that these bridges are stabilized by increased adhesion arising from an increased van der Waals interaction between apposing bilayers, electrostatic interactions between the pi electrons in the phenol ring and the -(N+CH3)3 groups on the PC headgroups, decreased hydration repulsion between bilayers, and hydrogen bonds between the H-bond-donating moieties on the polyphenols and H-bond-accepting groups in the bilayer.

Faster voltage-dependent activation of Na+ channels in growth cones versus somata of neuroblastoma N1E-115 cells

Kinetics of voltage-gated ionic channels fundamentally reflect the response of the channels to local electric fields. In this report cell-attached patch-clamp studies reveal that the voltage-dependent activation rate of sodium channels residing in the growth cone membrane differs from that of soma sodium channels in differentiating N1E-115 neuroblastoma cells. Because other electrophysiological properties of these channels do not differ, this finding may be a reflection of the difference in intramembrane electric field in these two regions of the cell. This represents a new mechanism for channels to attain a range of activities both within and between cells.

Structure and interactive properties of highly fluorinated phospholipid bilayers

Because liposomes containing fluoroalkylated phospholipids are being developed for in vivo drug delivery, the structure and interactive properties of several fluoroalkylated glycerophosphocholines (PCs) were investigated by x-ray diffraction/osmotic stress, dipole potential, and hydrophobic ion binding measurements. The lipids included PCs with highly fluorinated tails on both alkyl chains and PCs with one hydrocarbon chain and one fluoroalkylated chain. Electron density profiles showed high electron density peaks in the center of the bilayer corresponding to the fluorine atoms. The height and width of these high density peaks varied systematically, depending on the number of fluorines and their position on the alkyl chains, and on whether the bilayer was in the gel or liquid crystalline phase. Wide-angle diffraction showed that in both gel and liquid crystalline bilayers the distance between adjacent alkyl chains was greater in fluoroalkylated PCs than in analogous hydrocarbon PCs. For interbilayer separations of less than about 8 A, pressure-distance relations for fluoroalkylated PCs were similar to those previously obtained from PC bilayers with hydrocarbon chains. However, for bilayer separations greater than 8A, the total repulsive pressure depended on whether the fluoroalkylated PC was in a gel or liquid-crystalline phase. We argue that these pressure-distance relations contain contributions from both hydration and entropic repulsive pressures. Dipole potentials ranged from -680 mV for PCs with both chains fluoroalkylated to -180 mV for PCs with one chain fluoroalkylated, compared to +415 mV for egg PC. The change in dipole potential as a function of subphase concentration of tetraphenyl-boron was much larger for egg PC than for fluorinated PC monolayers, indicating that the fluorine atoms modified the binding of this hydrophobic anion. Thus, compared to conventional liposomes, liposomes made from fluoroalkylated PCs have different binding properties, which may be relevant to their use as drug carriers.

Temperature dependence of the repulsive pressure between phosphatidylcholine bilayers

Bilayer structure and interbilayer repulsive pressure were measured from 5 to 50 degrees C by the osmotic stress/x-ray diffraction method for both gel and liquid crystalline phase lipid bilayers. For gel phase dibehenoylphosphatidylcholine (DBPC) the bilayer thickness and pressure-distance relations were nearly temperature-independent, and at full hydration the equilibrium fluid spacing increased approximately 1 A, from 10 A at 5 degrees C to 11 A at 50 degrees C. In contrast, for liquid crystalline phase egg phosphatidylcholine (EPC), the bilayer thickness, equilibrium fluid spacing, and pressure-distance relation were all markedly temperature-dependent. As the temperature was increased from 5 to 50 degrees C the EPC bilayer thickness decreased approximately 4 A, and the equilibrium fluid spacing increased from 14 to 21 A. Over this temperature range there was little change in the pressure-distance relation for fluid spacings less than approximately 10 A, but a substantial increase in the total pressure for fluid spacings greater than 10 A. These data show that for both gel and liquid crystalline bilayers there is a short-range repulsive pressure that is nearly temperature-independent, whereas for liquid crystalline bilayers there is also a longer-range pressure that increases with temperature. From analysis of the energetics of dehydration we argue that the temperature-independent short-range pressure is consistent with a hydration pressure due to polarization or electrostriction of water molecules by the phosphorylcholine moiety. For the liquid crystalline phase, the 7 A increase in equilibrium fluid spacing with increasing temperature can be predicted by an increase in the undulation pressure as a consequence of a temperature-dependent decrease in bilayer bending modulus.

Incorporation of surface tension into molecular dynamics simulation of an interface: a fluid phase lipid bilayer membrane

In this paper we report on the molecular dynamics simulation of a fluid phase hydrated dimyristoylphosphatidylcholine bilayer. The initial configuration of the lipid was the x-ray crystal structure. A distinctive feature of this simulation is that, upon heating the system, the fluid phase emerged from parameters, initial conditions, and boundary conditions determined independently of the collective properties of the fluid phase. The initial conditions did not include chain disorder characteristic of the fluid phase. The partial charges on the lipids were determined by ab initio self-consistent field calculations and required no adjustment to produce a fluid phase. The boundary conditions were constant pressure and temperature. Thus the membrane was not explicitly required to assume an area/phospholipid molecule thought to be characteristic of the fluid phase, as is the case in constant volume simulations. Normal to the membrane plane, the pressure was 1 atmosphere, corresponding to the normal laboratory situation. Parallel to the membrane plane a negative pressure of -100 atmospheres was applied, derived from the measured surface tension of a monolayer at an air-water interface. The measured features of the computed membrane are generally in close agreement with experiment. Our results confirm the concept that, for appropriately matched temperature and surface pressure, a monolayer is a close approximation to one-half of a bilayer. Our results suggest that the surface area per phospholipid molecule for fluid phosphatidylcholine bilayer membranes is smaller than has generally been assumed in computational studies at constant volume. Our results confirm that the basis of the measured dipole potential is primarily water orientations and also suggest the presence of potential barriers for the movement of positive charges across the water-headgroup interfacial region of the phospholipid.

Calculations of the electrostatic potential adjacent to model phospholipid bilayers

We used the nonlinear Poisson-Boltzmann equation to calculate electrostatic potentials in the aqueous phase adjacent to model phospholipid bilayers containing mixtures of zwitterionic lipids (phosphatidylcholine) and acidic lipids (phosphatidylserine or phosphatidylglycerol). The aqueous phase (relative permittivity, epsilon r = 80) contains 0.1 M monovalent salt. When the bilayers contain < 11% acidic lipid, the -25 mV equipotential surfaces are discrete domes centered over the negatively charged lipids and are approximately twice the value calculated using Debye-Hückel theory. When the bilayers contain > 25% acidic lipid, the -25 mV equipotential profiles are essentially flat and agree well with the values calculated using Gouy-Chapman theory. When the bilayers contain 100% acidic lipid, all of the equipotential surfaces are flat and agree with Gouy-Chapman predictions (including the -100 mV surface, which is located only 1 A from the outermost atoms). Even our model bilayers are not simple systems: the charge on each lipid is distributed over several atoms, these partial charges are non-coplanar, there is a 2 A ion-exclusion region (epsilon r = 80) adjacent to the polar headgroups, and the molecular surface is rough. We investigated the effect of these four factors using smooth (or bumpy) epsilon r = 2 slabs with embedded point charges: these factors had only minor effects on the potential in the aqueous phase.

6-ketocholestanol abolishes the effect of the most potent uncouplers of oxidative phosphorylation in mitochondria

The effect of a keto-derivative of cholesterol, namely, 6-ketocholestanol (5 alpha-cholestan-3 beta-ol-6-one; kCh) on the uncoupling of oxidation and phosphorylation by various uncouplers was studied in rat heart mitochondria. kCh was found to completely abolish the uncoupling effect (the increase in the respiration rate under the respiratory control conditions and the decrease in the membrane potential) caused of FCCP, CCCP and SF6847 and partially by TTFB at low concentrations of uncouplers. It was without effect on the uncoupling by PCP, DNP and palmitate. Carboxyatractylate, a specific inhibitor of the ADP/ATP-antiporter, was shown to almost completely abolish the uncoupling induced by palmitate and partially by low concentration of TTFB, PCP and DNP. Effects of high concentrations of all these uncouplers as well as of any concentrations of gramicidin proved to be kCh- and carboxyatractilate-insensitive. The data are discussed in terms of the hypothesis on the protein-mediated mechanism of the protonophorous uncoupling.