Membrane dipole potentials, hydration forces, and the ordering of water at membrane surfaces

Characterization of magnetically oriented phospholipid micelles for measurement of dipolar couplings in macromolecules

Weak alignment of solute molecules with the magnetic field can be achieved in a dilute liquid crystalline medium, consisting of an aqueous mixture of dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl-phosphatidylcholine (DHPC). For a certain range of molar ratios, DMPC and DHPC can form large, disc-shaped particles, commonly referred to as bicelles (Sanders and Schwonek, 1992), which cooperatively align in the magnetic field and induce a small degree of alignment on asymmetrically shaped solute molecules. As a result, dipolar couplings between pairs of 1H, 13C or 15N nuclei are no longer averaged to zero by rotational diffusion and they can be readily measured, providing valuable structural information. The stability of these liquid crystals and the degree of alignment of the solute molecules depend strongly on experimental variables such as the DMPC:DHPC ratio and concentration, the preparation protocol of the DMPC/DHPC mixtures, as well as salt, temperature, and pH. The lower temperature limit for which the liquid crystalline phase is stable can be reduced to 20 degrees C by using a ternary mixture of DHPC, DMPC, and 1-myristoyl-2-myristoleoyl-sn-glycero-3-phosphocholine, or a binary mixture of DHPC and ditridecanoyl-phosphatidylcholine. These issues are discussed, with an emphasis on the use of the medium for obtaining weak alignment of biological macromolecules.

Molecular ordering of interfacially localized tryptophan analogs in ester- and ether-lipid bilayers studied by 2H-NMR

Perdeuterated indole-d6 and N-methylated indole-d6 were solubilized in lamellar liquid crystalline phases composed of either 1,2-diacyl-glycero-3-phosphocholine (14:0)/water or 1,2-dialkyl-glycero-3-phosphocholine(14:0/water. The molecular ordering of the tryptophan analogs was determined from deuteron quadrupole splittings observed in 2H-NMR spectra on macroscopically aligned lipid bilayers. NMR spectra were recorded with the bilayers oriented perpendicular to or parallel with the external magnetic field, and the values of the splittings differed by a factor of 2 between these distinct orientations, indicating fast rotational motion of the molecules about an axis parallel to the bilayer normal. In all cases the splittings were found to decrease with increasing temperature. Relatively large splittings were observed in all systems, demonstrating that the tryptophans partition into a highly anisotropic environment. Solubilization most likely occurs at the lipid/water interface, as indicated by 1H-NMR chemical shift studies. The 2H-NMR spectra obtained for each analog were found to be rather similar in ester and ether lipids, but with smaller splittings in the ether lipid under similar conditions. The difference was slightly less for the indole molecule. Furthermore, in both lipid systems the positions of the splittings from indole were different from those of N-methyl indole. The results suggest that 1) the tryptophan analogs are solubilized in the interfacial region of the lipid bilayer, 2) the behavior may be modulated by hydrogen bonding in the case of indole, and 3) hydrogen bonding with the lipid carbonyl groups is not likely to play a major role in the solubilization of single indole molecules in the ester lipid bilayer interface.

Changes of the membrane potential profile induced by verapamil and propranolol

The effects of the organic calcium channel blocker verapamil and the beta-receptor blocker propranolol on dipole (phi(d)) and surface (phi(s)) potentials of bilayer lipid membranes were studied. The boundary potentials (phi(b)= phi(d) + phi(s)) of black lipid membranes, monitored by conductance measurements in the presence of nonactin and by capacitive current measurements were compared with phi(s) calculated from the electrophoretic mobility of lipid vesicles. It was shown that the increase of boundary potential, induced by the adsorption of the positively charged propranolol, was caused solely by an increase in surface potential. Although phi(s) also increases due to the adsorption of verapamil, phi(b) diminishes. A sharp decrease of the dipole potential was shown to be responsible for this effect. From Langmuir adsorption isotherm the dissociation constant Kd of verapamil was estimated. The uncharged form of verapamil (Kd=(0.061+/-0.01) mM at pH 10.5) has a tenfold higher affinity to a neutral bilayer membrane than the positively charged form. The alteration of membrane dipole potential due to verapamil adsorption may have important implications for both membrane translocation and partitioning of small or hydrophobic ions and charged groups of membrane proteins.

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.

Hydration of the dienic lipid dioctadecadienoylphosphatidylcholine in the lamellar phase--an infrared linear dichroism and x-ray study on headgroup orientation, water ordering, and bilayer dimensions

In the phospholipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) in each of the fatty acid chains, a rigid diene group is inserted in a position near the polar/apolar boundary that is exceptionally sensitive for membrane stability. DODPC transforms upon gradual dehydration from the liquid-crystalline to a metastable gel state, which rearranges into two subgel phases at low and intermediate degrees of hydration. The molecular dimensions of the respective bilayers were determined by means of x-ray diffraction. Infrared linear dichroism of selected vibrations of the phosphate and trimethylammonium groups and of the nu13(OH) band of water adsorbed onto the lipid was used to study the molecular order in the polar part of the bilayers in macroscopically oriented samples. The dense packing of the tilted acyl chains in the subgel causes the in-plane orientation of the phosphatidylcholine headgroups with direct interactions between the phosphate and trimethylammonium groups, and a strong orientation of adsorbed water molecules. In the more disordered gel, the thickness of the polar part of the bilayer increases and the lateral interactions between the lipid headgroups weaken. The higher order in the headgroup region of the subgels correlates with shorter decay lengths of the repulsive forces acting between opposite membrane surfaces. This result can be understood if the work to dehydrate the lipid is determined to a certain degree by the work to break up the lipid-water interactions without compensation by adequate lipid-lipid contacts. Almost similar area compressibility moduli are found in the liquid-crystalline and solid phases. Obviously, the lipid avoids lateral stress by the structural rearrangement.

The adsorption of phloretin to lipid monolayers and bilayers cannot be explained by langmuir adsorption isotherms alone

Phloretin and its analogs adsorb to the surfaces of lipid monolayers and bilayers and decrease the dipole potential. This reduces the conductance for anions and increases that for cations on artificial and biological membranes. The relationship between the change in the dipole potential and the aqueous concentration of phloretin has been explained previously by a Langmuir adsorption isotherm and a weak and therefore negligible contribution of the dipole-dipole interactions in the lipid surface. We demonstrate here that the Langmuir adsorption isotherm alone is not able to properly describe the effects of dipole molecule binding to lipid surfaces--we found significant deviations between experimental data and the fit with the Langmuir adsorption isotherm. We present here an alternative theoretical treatment that takes into account the strong interaction between membrane (monolayer) dipole field and the dipole moment of the adsorbed molecule. This treatment provides a much better fit of the experimental results derived from the measurements of surface potentials of lipid monolayers in the presence of phloretin. Similarly, the theory provides a much better fit of the phloretin-induced changes in the dipole potential of lipid bilayers, as assessed by the transport kinetics of the lipophilic ion dipicrylamine.

Structure and dynamic properties of diunsaturated 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine lipid bilayer from molecular dynamics simulation

Unsaturated fatty acid chains are known to be an essential structural part of biomembranes, but only monounsaturated chains have been included in the molecular dynamics (MD) simulations of membrane systems. Here we present a 1-ns MD simulation for a diunsaturated 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine (PLPC; 16:0/18:2[delta9,12]) bilayer. The structural behavior of the phosphatidylcholine headgroup, the glycerol backbone, and the hydrating water were assessed and found to be consistent with the existing information about similar systems from both experimental and computational studies. Further analysis was focused on the structure of the double bond region and the effects of the diunsaturation on the bilayer interior. The behavior of the diunsaturated sn-2 chains is affected by the tilted beginning of the chain and the four main conformations of the double bond region. The double bonds of the sn-2 chains also influenced the characteristics of the saturated chains in the sn-1 position. Furthermore, extreme conformations of the sn-2 chains existed that are likely to be related to the functional role of the double bonds. The results here point out the importance of polyunsaturation for the biological interpretations deduced from the membrane MD simulations.

Effect of lipid structure on the dipole potential of phosphatidylcholine bilayers

A fluorescent ratio method utilizing styrylpyridinium dyes has recently been suggested for the measurement of the membrane dipole potential. Up to now only qualititative measurements have been possible. Here the fluorescence excitation ratio of the dye di-8-ANEPPS has been measured in lipid vesicles composed of a range of saturated and unsaturated phosphatidylcholines. It has been found that the fluorescence ratio is inversely proportional to the surface area occupied by the lipid in its fully hydrated state. This finding allows, by extra- and interpolation, the packing density to be estimated of phosphatidylcholines for which X-ray crystallographic data are not yet available. Comparison of the fluorescence data with literature data of the dipole potential from electrical measurements on monolayers and bilayers allows a calibration curve to be constructed, so that a quantitative determination of the dipole potential using di-8-ANEPPS is possible. It has been found that the value of the dipole potential decreases with increasing unsaturation and, in the case of unsaturated lipids, with increasing length of the hydrocarbon chains. This effect can be explained by the effects of chain packing on the spacing between the headgroups. In addition to the effects of lipid structure on membrane fluidity, these measurements demonstrate the possibility of a direct electrical mechanism for lipid regulation of protein function, in particular of ion transport proteins.

Transmembrane helix structure, dynamics, and interactions: multi-nanosecond molecular dynamics simulations

To probe the fundamentals of membrane/protein interactions, all-atom multi-nanosecond molecular dynamics simulations were conducted on a single transmembrane poly(32)alanine helix in a fully solvated dimyristoyphosphatidylcholine (DMPC) bilayer. The central 12 residues, which interact only with the lipid hydrocarbon chains, maintained a very stable helical structure. Helical regions extended beyond these central 12 residues, but interactions with the lipid fatty-acyl ester linkages, the lipid headgroups, and water molecules made the helix less stable in this region. The C and N termini, exposed largely to water, existed as random coils. As a whole, the helix tilted substantially, from perpendicular to the bilayer plane (0 degree) to a 30 degrees tilt. The helix experienced a bend at its middle, and the two halves of the helix at times assumed substantially different tilts. Frequent hydrogen bonding, of up to 0.7 ns in duration, occurred between peptide and lipid molecules. This resulted in correlated translational diffusion between the helix and a few lipid molecules. Because of the large variation in lipid conformation, the lipid environment of the peptide was not well defined in terms of "annular" lipids and on average consisted of 18 lipid molecules. When compared with a "neat" bilayer without peptide, no significant difference was seen in the bilayer thickness, lipid conformations or diffusion, or headgroup orientation. However, the lipid hydrocarbon chain order parameters showed a significant decrease in order, especially in those methylene groups closest to the headgroup.

Molecular dynamics simulations of a fluid bilayer of dipalmitoylphosphatidylcholine at full hydration, constant pressure, and constant temperature

Molecular dynamics simulations of 500 ps were performed on a system consisting of a bilayer of 64 molecules of the lipid dipalmitoylphosphatidylcholine and 23 water molecules per lipid at an isotropic pressure of 1 atm and 50 degrees C. Special attention was devoted to reproduce the correct density of the lipid, because this quantity is known experimentally with a precision better than 1%. For this purpose, the Lennard-Jones parameters of the hydrocarbon chains were adjusted by simulating a system consisting of 128 pentadecane molecules and varying the Lennard-Jones parameters until the experimental density and heat of vaporization were obtained. With these parameters the lipid density resulted in perfect agreement with the experimental density. The orientational order parameter of the hydrocarbon chains agreed perfectly well with the experimental values, which, because of its correlation with the area per lipid, makes it possible to give a proper estimate of the area per lipid of 0.61 +/- 0.01 nm2.

Determining ethanol distribution in phospholipid multilayers with MAS-NOESY spectra

The location of an ethanol molecule within a membrane, an issue of considerable controversy, was investigated directly by NMR with two-dimensional NOESY. Lipid and ethanol 1H NMR resonances of multilamellar liposomes were resolved by magic-angle spinning (MAS). We observed strong proton lipid-ethanol crosspeaks in dispersions of saturated dimyristoylphosphatidylcholine and monounsaturated stearoyloleoylphosphatidylcholine and in polyunsaturated stearoyldocosahexaenoylphosphatidylcholine. Crosspeak intensity has been interpreted in terms of an ethanol distribution function over the lipid bilayer. Ethanol resides with the highest probability at the lipid water interface near the lipid glycerol backbone and upper methylene segments of lipid hydrocarbon chains. Chain unsaturation has only a minor influence on the ethanol distribution function. In all cases, the ethanol concentration in the bilayer core is significantly lower. At ambient temperature all lipid-ethanol crosspeaks are positive. Crosspeak intensity decreases with increasing water content and increasing temperature most likely because of shorter correlation times of lipid and ethanol reorientation. This suggests a lifetime for specific lipid-ethanol contacts of about 1 ns. Lipid-ethanol and lipid-lipid crosspeaks reflect the high degree of motional disorder of lipids and incorporated ethanol in membranes and the rather arbitrary nature of the location of the lipid-water interface.

Permeation of phloretin across bilayer lipid membranes monitored by dipole potential and microelectrode measurements

The transmembrane diffusion of phloretin across planar bilayer lipid membranes is studied under steady-state conditions. Diffusion restrictions and adsorption related effects are measured independently. The adsorption of aligned phloretin dipoles generates a change in the intrinsic dipole potential difference between the inner and outer leaflets of the lipid bilayer. It is monitored by capacitive current measurements carried out with a direct current (dc) bias. The variation of the intramembrane electric field indicates a saturation of the binding sites at the membrane interface. In contrast, pH profile measurements undertaken in the immediate membrane vicinity show a constant membrane permeability. If phloretin binding and transmembrane diffusion are treated as two competitive events rather than subsequent steps in the transport queue the contradictory results become explainable. A mathematical model is developed where it is assumed that diffusing phloretin molecules are randomly oriented, i.e., that they do not contribute to the intrinsic membrane potential. Only the dipoles adsorbing onto the membrane are oriented. Based on these theory the membrane permeability is calculated from the capacitive current data. It is found to agree very well with the permeability deduced from the microelectrode measurements.

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 mechanism of monensin-mediated cation exchange based on real time measurements

Monensin is an ionophore that supports an electroneutral ion exchange across the lipid bilayer. Because of this, under steady-state conditions, no electric signals accompany its reactions. Using the Laser Induced Proton Pulse as a synchronizing event we selectively acidify one face of a black lipid membrane impregnated by monensin. The short perturbation temporarily upsets the acid-base equilibrium on one face of the membrane, causing a transient cycle of ion exchange. Under such conditions the molecular events could be discerned as a transient electric polarization of the membrane lasting approx. 200 microseconds. The proton-driven chemical reactions that lead to the electric signals had been reconstructed by numeric integration of differential rate equations which constitute a maximalistic description of the multi equilibria nature of the system (Gutman, M. and Nachliel, E. (1989) Electrochim. Acta 34, 1801-1806). The analysis of the reactions reveals that the ionic selectivity of the monensin (H+ > Na+ > K+) is due to more than one term. Besides the well established different affinity for the various cations, the selectivity is also derived from a large difference in the rates of cross membranal diffusivities (MoH > MoNa > MoK), which have never been detected before. (v) Quantitative analysis of the membrane's crossing rates of the three neutral complexes reveals a major role of the membranal dipolar field in regulating ion transport. The diffusion of MoH, which has no dipole moment, is hindered only by the viscose drag. On the other hand, the dipolar complexes (MoNa and MoK) are delayed by dipole-dipole interaction with the membrane. (vi) Comparison of the calculated dipoles with those estimated for the crystalline conformation of the [MoNa(H2O)2] and [MoK(H2O)2] complexes reveals that the MoNa may exist in the membrane at its crystal configuration, while the MoK definitely attains a structure having a dipole moment larger than in the crystal.

Dielectric spectroscopy as a sensor of membrane headgroup mobility and hydration

Dielectric spectroscopy is based on the response of the permanent dipoles to a driving electric field. The phospholipid membrane systems of dimyristoylphosphatidylcholine and dioleoylphosphatidylcholine can be prepared as samples of multilamellar liposomes with a well known amount of interlamellar water. For optimal resolution in dielectric spectroscopy one has to design the experimental set-up so that the direction of the permanent headgroup dipole moment is mostly parallel to the field vector of the external radio frequency (rf) electric field in this layered system. A newly developed coaxial probe technique makes it possible to sweep the measuring frequency between 1 and 1000 MHz in the temperature range 286-323 K. The response yields both the dispersion (epsilon') and the absorption part (epsilon") of the complex dielectric permittivity, which are attributed to the rotational diffusions of the zwitterionic phosphatidylcholine headgroup and the hydration water, respectively. Although the contributions of the headgroup and the hydration dipole moments to the dielectric relaxation are found to be situated close together, we succeeded in separating them. In the language of the Debye description, we propose to assign the lower frequency portion of the signal response to the relaxation contributed by the headgroups. The respective relaxation frequency is a discrete value in the range of 15-100 MHz and it shows normal temperature dependence. The contribution of the hydration water molecules exhibits a similar behavior in the range of 100-500 MHz but with the attributed relaxation frequency as the center of an asymmetric distribution of frequencies in analogy to simulation models known from the literature. Activation energies are derived for each of these relaxation processes from the Arrhenius plots of the temperature-dependent relaxation frequencies.

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.

Interactions of anesthetics with the membrane-water interface

Although the potency of conventional anesthetics correlates with lipophilicity, an affinity to water also is essential. It was recently found that compounds with very low affinities to water do not produce anesthesia regardless of their lipophilicity. This finding implies that clinical anesthesia might arise because of interactions at molecular sites near the interface of neuronal membranes with the aqueous environment and, therefore, might require increased concentrations of anesthetic molecules at membrane interfaces. As an initial test of this hypothesis, we calculated in molecular dynamics simulations the free energy profiles for the transfer of anesthetic 1,1,2-trifluoroethane and nonanesthetic perfluoroethane across water-membrane and water-hexane interfaces. Consistent with the hypothesis, it was found that trifluoroethane, but not perfluoroethane, exhibits a free energy minimum and, therefore, increased concentrations at both interfaces. The transfer of trifluoroethane from water to the nonpolar hexane or interior of the membrane is accompanied by a considerable, solvent-induced shift in the conformational equilibrium around the C-C bond.

Polyunsaturation in cell membranes and lipid bilayers and its effects on membrane proteins

The effect of variation of the degree of cis-unsaturation on cell membrane protein functioning was investigated using a model lipid bilayer system and protein kinase C (PKC). This protein is a key element of signal transduction. Furthermore it is representative of a class of extrinsic membrane proteins that show lipid dependent interactions with cell membranes. To test for dependence of activity on the phospholipid unsaturation, experiments were devised using a vesicle assay system consisting of phosphatidylcholine (PC) and phosphatidylserine (PS) in which the unsaturation was systematically varied. Highly purified PKC alpha and epsilon were obtained using the baculovirus-insect cell expression system. It was shown that increased PC unsaturation elevated the activity of PKC alpha. By contrast, increasing the unsaturation of PS decreased the activity of PKC alpha, and to a lesser extent PKC epsilon. This result immediately rules out any single lipid bilayer physical parameter, such as lipid order, underlying the effect. It is proposed that while PC unsaturation effects are explainable on the basis of a contribution to membrane surface curvature stress, the effects of PS unsaturation may be due to specific protein-lipid interactions. Overall, the results indicate that altered phospholipid unsaturation in cell membranes that occurs in certain disease states such as chronic alcoholism, or by dietary manipulations, are likely to have profound effects on signal transduction pathways involving PKC and similar proteins.

Nuclear magnetic resonance studies of lipid hydration in monomethyldioleoylphosphatidylethanolamine dispersions

Solid-state proton nuclear magnetic resonance has been used to examine surface hydration in suspensions of monomethyldioleoylphosphatidylethanolamine (MeDOPE). The magic-angle spinning (MAS) 1H spectra for aqueous suspensions of MeDOPE in the L alpha phase exhibited two resonances of roughly equal intensity that could be ascribed to water protons, but both their spin-lattice relaxation times and chemical shifts converged upon conversion to the hexagonal phase. Only a single water peak was observed for analogous samples of dioleoylphosphatidylcholine (DOPC). MAS-assisted two-dimensional nuclear Overhauser effect spectroscopy (NOESY) was conducted for multibilayers of both MeDOPE and DOPC. Through-space interactions were identified between pairs of lipid protons, as expected from their chemical structure. For lamellar suspensions of MeDOPE, positive NOESY cross-peaks were observed between the downfield-shifted water resonance (only) and both CH2N and NH2CH3+ protons of the lipid headgroup. These cross-peaks were not observed in the NOESY spectra of MeDOPE in its hexagonal or cubic phases or for lamellar DOPC reference samples. Taken together, the observation of two water peaks, spin-lattice relaxation behavior, and NOESY connectivities in MeDOPE suspensions support the interpretation that the low-field water peak corresponds to hydrogen-bonded interlamellar water interacting strongly with the lipid. Such a population of water molecules exists in association with MeDOPE in the lamellar phase but not for its inverted phases or for lamellar dispersions of DOPC.

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.

Forces and factors that contribute to the structural stability of membrane proteins

While a considerable amount of literature deals with the structural energetics of water-soluble proteins, relatively little is known about the forces that determine the stability of membrane proteins. Similarly, only a few membrane protein structures are known at atomic resolution, although new structures have recently been described. In this article, we review the current knowledge about the structural features of membrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-c oxidase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements in membrane proteins. This property manifests itself as incomplete unfolding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief general account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.

Ethanol- and acetonitrile-induced inhibition of water diffusional permeability across bovine red blood cell membrane

The effect of 0-3% (v/v) ethanol and acetonitrile on water diffusional permeability of bovine and chicken red blood cells (RBCs) was studied using a pulse 1H-T2 NMR technique. Transmembrane water diffusional exchange times, tau exch, of 9.2 +/- 0.46 ms and 18.3 +/- 1.0 ms were determined for bovine and chicken erythrocytes at 27.5 degrees C, respectively. Arrhenius activation energies Ea of water diffusion were 20.4 and 35.8 kJ mol-1. Ethanol, and acetonitrile being 2-fold more effective, markedly increased both tau exch and Ea in bovine RBC as compared to the well-known mercurial inhibitor of water channels, p-chloromercuribenzene sulfonate. Chicken RBCs that have no protein water channels, were found to be completely insensitive for either agent. It was suggested that ethanol and acetonitrile partitioning into the lipid phase of bovine RBC membrane affects the permeability of CHIP28 water channel but not the lipid confined water diffusion. The results suggest that the inhibition of transmembrane movement of water via CHIP28 channels might be involved in the anti-hemolytic action of anaesthetics such as ethanol.

Membrane hydration and structure on a subnanometer scale as seen by high resolution solid state nuclear magnetic resonance: POPC and POPC/C12EO4 model membranes

The position on a subnanometer scale and the dynamics of structurally important water in model membranes was determined using a combination of proton magic-angle spinning NMR (MAS) with two-dimensional NOESY NMR techniques. Here, we report studies on phosphocholine lipid bilayers that were then modified by the addition of a nonionic surfactant that is shown to dehydrate the lipid. These studies are supplemented by 13C magic-angle spinning NMR investigations to get information on the dynamics of segmental motions of the membrane molecules. It can be shown that the hydrophilic chain of the surfactant is positioned at least partially within the hydrophobic core of the lipid bilayer. With the above NMR approach, we are able to establish molecular contacts between water and the lipid headgroup as well as with certain groups of the hydrocarbon chains and the glycerol backbone. This is possible because high resolution proton and 13C-NMR spectra of multilamellar bilayer membranes are obtained using MAS. A phase-sensitive NOESY must also be applied to distinguish positive and negative cross-peaks in the two-dimensional plot. These studies have high potential to investigate membrane proteins hydration and structural organization in a natural lipid bilayer surrounding.

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.

Forces and factors that contribute to the structural stability of membrane proteins

While a considerable amount of literature deals with the structural energetics of water-soluble proteins, relatively little is known about the forces that determine the stability of membrane proteins. Similarly, only a few membrane protein structures are known at atomic resolution, although new structures have recently been described. In this article, we review the current knowledge about the structural features of membrane proteins. We then proceed to summarize the existing literature regarding the thermal stability of bacteriorhodopsin, cytochrome-c oxidase, the band 3 protein, Photosystem II and porins. We conclude that a fundamental difference between soluble and membrane proteins is the high thermal stability of intrabilayer secondary structure elements in membrane proteins. This property manifests itself as incomplete unfolding, and is reflected in the observed low enthalpies of denaturation of most membrane proteins. By contrast, the extramembranous parts of membrane proteins may behave much like soluble proteins. A brief general account of thermodynamics factors that contribute to the stability of water soluble and membrane proteins is presented.

The surface properties of phospholipid liposome systems and their characterisation

The field of liposome (vesicle) research has expanded considerably over the last 30 years. In physical chemical terms liposomes have many of the characteristics of colloidal particles and their stability is determined in part by the classical surface forces. It is now possible to engineer a wide range of liposomes varying in size, phospholipid composition and surface characteristics. The surfaces of liposomes can be modified by the choice of bilayer lipid as well as by the incorporation and covalent linkage of proteins (e.g. antibodies and sugar binding proteins [lectins]), glycoproteins and synthetic polymers. Much of the impetus for liposome design has come from their potential value as drug delivery systems. The development of technologies for the production of such a range of liposome systems has presented interesting problems in the characterisation of their properties. The review addresses the progress that has been made in characterising the surfaces of different types of liposomes with specific reference to their electrophoretic properties and their interpretation and the physical interactions between liposomal bilayers.

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.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.

A transmembrane potential does not affect the vertical location of charged lipid spin labels with respect to the surface of a phosphatidylcholine bilayer

The effect of a transmembrane potential on the vertical location of a charged lipid in a neutral phosphatidylcholine (PC) lipid bilayer has been investigated using negatively and positively charged spin-labeled lipids. A transmembrane potential was generated across extruded large unilamellar vesicles either by using a K+/Na+ ion gradient and a K+ ionophore or by using a pH gradient. Since a transmembrane potential could have opposing effects on lipids in the inner and outer monolayer, some of the acidic spin labels were asymmetrically located in the inner monolayer as a result of a pH gradient. No significant effect on their order parameters was observed upon applying a transmembrane potential. The internal dipole potential of the bilayer was modified by using dialkyl-PC or by incorporating 10 mol% phloretin, or 6-ketocholestanol in the PC, but a transmembrane potential still had no detectable effect on the spin labeled lipids. Therefore, it is concluded that the electrochemical potential across membranes probably does not cause a significant change in the vertical location of charged lipids with respect to the surface of a PC bilayer. This suggests that polar interactions and/or van der Waals interactions between the spin probe and the surrounding lipids stabilize the overall structure of the membranes and these interactions are not disrupted by a selective effect of the transmembrane potential on the charged lipids.

Hydration force parameters of phosphatidylcholine lipid bilayers as determined from 2H-NMR studies of deuterated water

The continuous decrease of the quadrupolar splitting of deuterated water interacting with phosphocholine lipid bilayers with growing water concentration is analyzed as a function of the water activity. From the apparent linear dependence on water activity a measure for hydration forces is obtained. The forces calculated are in the range of published data using sorption isotherms and osmotic stress technique in combination with SAXS. A simple interaction potential which includes orientational order of water adsorbed on surfaces gives a physical base for these findings. Therefore, deuterium NMR may become a powerful tool for hydration force analysis complementing well-known methods.

Dipole potential of lipid membranes

Of the individual potentials which comprise the potential profile of a membrane, the least well understood is the dipole potential. In general, the dipole potential is manifested between the hydrocarbon interior of the membrane and the first few water layers adjacent to the lipid head groups. Changes in dipole potential caused by spreading a lipid at an air- or oil-water interface can be measured directly and the dipole potential of bilayers can be estimated from the conductances of hydrophobic ions. For a typical phospholipid, like phosphatidylcholine, its measured value is approximately 400 mV in monomolecular films and approximately 280 mV in bilayer membranes, with the hydrocarbon region being positive relative to the aqueous phase. The difference between dipole potentials measured in monolayers and bilayer membranes appears to arise from the use of the lipid-free air- or oil-water interface as the reference point for monolayer measurements and can be corrected for. The species-specific correction term is a lipid concentration-independent potential, the existence of which suggests the ability of lipid headgroups to globally reorganize water structure at the interface. The dipole potential arises from the functional group dipoles of the terminal methyl groups of aliphatic chains, the glycerol-ester region of the lipids and the hydrated polar head groups. Classical methods for obtaining partial dipole moments for each of the three contributing regions are all based on questionable assumptions and give conflicting results. More sophisticated mean-field models of dipole potential origin recognize the important role of interfacial water in determining its value but still cannot adequately describe the microscopic nature of the interactions from which it arises. In part this is because the dipole potential develops in a region over which the dielectric constant of the medium is changing from 2 to 80. Despite of our limited understanding of the dipole potential, it is an important regulator of membrane structure and function. Membrane-membrane and membrane-ligand interactions are regulated by the hydration force, the value of which can be related to the dipole potential of the membrane. For thermotropically phase-separated or multicomponent membranes the size and shape of lipid domains is controlled by the balance between the line tension at the domain borders and the difference in dipole density between the domains. Line tension tends to make the domains compact and circular whereas dipole repulsion promotes transitions to complex domain shapes with larger perimeters.(ABSTRACT TRUNCATED AT 400 WORDS)

An NMR study of pyridine associated with DMPC liposomes and magnetically ordered DMPC-surfactant mixed micelles

With molecular dynamics simulations of phospholipid membranes becoming a reality, there is a growing need for experiments that provide the molecular details necessary to test these computational results. Pyridine is used here to explore the interaction of planar aromatic groups with the water-lipid interface of membranes. It is shown by magic angle spinning 13C nuclear magnetic resonance (NMR) to bind between the glycerol and choline groups of dimyristoylphosphatidylcholine (DMPC) liposomes. The axial pattern for the 31P NMR spectrum of DMPC liposomes is preserved even with more than half of the interfacial sites occupied, indicating that pyridine does not disrupt the lamellar phase of this lipid. 2H NMR experiments of liposomes in deuterium oxide demonstrate that pyridine might promote greater penetration of water into restricted regions in the interface. Magnetically oriented DMPC/surfactant micelles were investigated as a means for improving resolution and sensitivity in NMR studies of species bound to bilayers. The quadrupolar splittings in the 2H NMR spectra of d5-pyridine in DMPC liposomes and magnetically oriented DMPC/Trixon X-100 micelles indicate a common bound state for the two bilayer systems. The well resolved quadrupolar splittings of d5-pyridine in oriented micelles were used to establish the tilt of the pyridine ring relative to the bilayer plane.

Increased adhesion between neutral lipid bilayers: interbilayer bridges formed by tannic acid

Tannic acid (TA) is a naturally occurring polyphenolic compound that aggregates membranes and neutral phosolipid vesicles and precipitates many proteins. This study analyzes TA binding to lipid membranes and the ensuing aggregation. The optical density of dispersions of phosphatidylcholine (PC) vesicles increased upon the addition of TA and electron micrographs showed that TA caused the vesicles to aggregate and form stacks of tightly packed disks. Solution calorimetry showed that TA bound to PC bilayers with a molar binding enthalpy of -8.3 kcal/mol and zeta potential measurements revealed that TA imparted a small negative charge to PC vesicles. Monolayer studies showed that TA bound to PC with a dissociation constant of 1.5 microM and reduced the dipole potential by up to 250 mV. Both the increase in optical density and decrease in dipole potential produced by TA could be reversed by the addition of polyvinylpyrrolidone, a compound that chelates TA by providing H-bond acceptor groups. NMR, micropipette aspiration, and x-ray diffraction experiments showed that TA incorporated into liquid crystalline PC membranes, increasing the area per lipid molecule and decreasing the bilayer thickness by 2 to 4%. 2H-NMR quadrupole splitting measurements also showed that TA associated with a PC molecule for times much less than 10(-4) s. In gel phase bilayers, TA caused the hydrocarbon chains from apposing monolayers to fully interdigitate. X-ray diffraction measurements of both gel and liquid crystalline dispersions showed that TA, at a critical concentration of about 1 mM, reduced the fluid spacing between adjacent bilayers by 8-10 A. These data place severe constraints on how TA can pack between adjacent bilayers and cause vesicles to adhere. We conclude that TA promotes vesicle aggregation by reducing the fluid spacing between bilayers by the formation of transient interbilayer bridges by inserting its digallic acid residues into the interfacial regions of adjacent bilayers and spanning the interbilayer space.