Acid-base equilibria at interface separating electrolyte solution and lipid bilayer formed from phosphatidylserine

The dependence of the interfacial tension of a lipid membrane formed from phosphatidylserine on the pH of the aqueous solution has been studied. The model described the H(+) and OH(-) ions adsorption in the bilayer lipid surface has been presented in this work. We take suitable equations to describe the dependence of interfacial tension of a lipid bilayer membrane on H(+) and OH(-) ion concentrations. A theoretical equation is derived to describe this dependence in the range of pH, i.e. from 2 to 12.     

Effect of pH on the interfacial tension of bilayer lipid membrane formed from phosphatidylcholine or phosphatidylserine

The effect of pH of an electrolyte solution on the interfacial tension of lipid membrane formed from phosphatidylcholine (PC) or phosphatidylserine (PS) was studied. The relationships were well described by an equation presented earlier based on the Gibbs isotherm but only in the proximity of the isoelectric point. Therefore, in this work models have been derived to describe the adsorption of the H(+) and OH(-) ions at lipid surfaces formed from PC or PS, which would reproduce changes in interfacial tension more correctly, particularly in the ranges distant from the isoelectric point. In one model, the surface is continuous with uniformly distributed functional groups constituting the centres of H(+) and OH(-) ion adsorption while in the other the surface is built of lipid molecules, free or with attached H(+) and OH(-) ions. In both models, the contributions of the individual lipid molecule forms to the interfacial tension of the bilayer were assumed to be additive.     

Effect of pH on the interfacial tension of lipid bilayer membrane

The dependence of the interfacial tension of a lipid bilayer on the pH of the aqueous solution has been studied. A theoretical equation is derived to describe this dependence. Interfacial tension measurements of an egg phosphatidylcholine bilayer were carried out. The experimental results agreed with those derived from the theoretical equation obtained close to the isoelectric point within a range of three pH units. A maximum corresponding to the isoelectric point appears both in the theoretical equation and in the experimental data.     

Interfacial tension of bilayer lipid membrane formed from phosphatidylethanolamine

The dependence of the interfacial tension of a lipid membrane on the pH of the aqueous solution has been studied. Interfacial tension measurements of phosphatidylethanolamine (PE) were carried out. A theoretical equation is derived to describe this dependence in the whole pH range. A maximum corresponding to the isoelectric point appears both in the theoretical equation and in the experimental data.     

pH Effect of the Sphingomyelin Membrane Interfacial Tension

The effect of pH on the interfacial tension of a sphingomyelin membrane in aqueous solution has been studied. Three models describing H⁺ and OH⁻ ion adsorption on the bilayer lipid surface are presented. In models I and II, the membrane surface is continuous, with uniformly distributed functional groups as centers of H⁺ and OH⁻ ion adsorption. In model III, the membrane surface is composed of lipid molecules, with and without adsorbed H⁺ and OH⁻ ions. The contribution of each individual lipid molecule to the overall interfacial tension of the bilayer was assumed to be additive in models I and II. In model III, the Gibbs isotherm was used to describe adsorption of H⁺ and OH⁻ ions at the bilayer surface. Theoretical equations are derived to describe the interfacial tension as a function of pH for all three models. Maximum interfacial tension was observed experimentally at the isoelectric point.     

Interfacial tension of phosphatidylcholine-phosphatidylserine system in bilayer lipid membrane

The effect of pH of electrolyte solution on the interfacial tension of lipid membrane formed of phosphatidylcholine (PC, lecithin)–phosphatidylserine (PS) system was studied. In this article, three models describing the H⁺ and OH⁻ ions adsorption in the bilayer lipid surface are presented. In Model I and Model II, the surface is continuous with uniformly distributed functional groups constituting the centres of H⁺ and OH⁻ ions adsorption while in the other the surface is built of lipid molecules, free or with attached H⁺ and OH⁻ ions. In these models contribution of the individual lipid molecule forms to interfacial tension of the bilayer were assumed to be additive. In Model III the adsorption of the H⁺ and OH⁻ ions at the PC–PS bilayer surface was described in terms of the Gibbs isotherm. Theoretical equations are derived to describe this dependence in the whole pH range.     

Effect of pH on the interfacial tension of bilayer lipid membrane formed from phosphatidylcholine or phosphatidylserine

The effect of pH of an electrolyte solution on the interfacial tension of lipid membrane formed from phosphatidylcholine (PC) or phosphatidylserine (PS) was studied. The relationships were well described by an equation presented earlier based on the Gibbs isotherm but only in the proximity of the isoelectric point. Therefore, in this work models have been derived to describe the adsorption of the H⁺ and OH⁻ ions at lipid surfaces formed from PC or PS, which would reproduce changes in interfacial tension more correctly, particularly in the ranges distant from the isoelectric point. In one model, the surface is continuous with uniformly distributed functional groups constituting the centres of H⁺ and OH⁻ ion adsorption while in the other the surface is built of lipid molecules, free or with attached H⁺ and OH⁻ ions. In both models, the contributions of the individual lipid molecule forms to the interfacial tension of the bilayer were assumed to be additive.     

Adsorption equilibria at interface separating electrolyte solution and phosphatidylcholine-stearylamine liposome membrane

The effect of the pH of an electrolyte solution on the electric surface charge of the liposome membrane was studied. The membrane of vesicles contained egg phosphatidylcholine (PC) with different proportions of stearylamine (ST). The surface charge density of the membrane was determined as a function of pH from electrophoretic mobility measurements. A six equilibria model describing the solution ions adsorption on the PC-ST liposome membrane surface was presented in this paper. The knowledge of the association constants of the -PO(-) and -N(+)CH(3)(3) groups of PC with H(+), OH(-), Na(+), Cl(-) ions: K(A(1)H), K(B(1)OH), K(A(1)Na), K(B(1)Cl), that had been presented earlier, allowed to determine the association constants of the -N(+)H(3) group of ST with OH(-) and Cl(-) ions: K(B(2)OH), K(B(2)Cl). The proposed model has been proved to be correct by comparing the resulting theoretic charge variation curves of the PC-ST liposomal membrane with the experimental data.     

The Influence of pH on Phosphatidylethanolamine Monolayer at the Air/Aqueous Solution Interface

The dependence of the interfacial tension of a phosphatidylethanolamine (PE) monolayer on the pH of the aqueous solution has been studied. A theoretical equation is derived to describe this dependence. A simple model of the influence of pH on the phosphatidylethanolamine monolayer at the air/hydrophobic chains of PE is presented. The contributions of additive phosphatidylethanolamine forms (both interfacial tension values and molecular area values) depend on pH. The interfacial tension values and the molecular area values for PEH⁺ and PEOH^? forms of phosphatidylethanolamine were calculated. The assumed model was verified experimentally. The experimental results agreed with those derived from the theoretical equation in a whole range of pH values.     

Adsorption equilibria between liposome membrane formed of phosphatidylcholine and aqueous sodium chloride solution as a function of pH

The effect has been studied of the adsorption of ions (H(+), Na(+), OH(-), Cl(-)) which are present in solution upon the electric charge of the liposome membrane formed of phosphatidylcholine (PC). The surface charge density of the membrane was determined as a function of pH and electrolyte concentration from electrophoretic mobility measurements. The measurements were carried out by the laser-Doppler microelectrophoresis method. A four-equilibria model has been proposed to describe the phenomena occurring on the membrane surface. The equilibria in which the adsorption of other ions on the liposome membrane surface was involved were assumed to exist beside the equilibria in which the H(+) and OH(-) ions were engaged. The idea was confirmed by mathematical calculations. Association constants of the liposome membrane surface with ions of solution (K(AH), K(ANa), K(BOH), K(BCl)) were determined. The proposed model has been proved to be correct by comparing the resulting theoretic charge variation curves of the lecithin membrane with the experimental data.     

pH modulation of large conductance potassium channel from adrenal chromaffin granules

We report here that large conductance K(+) selective channel in adrenal chromaffin granules is controlled by pH. We measured electrogenic influx of (86)Rb(+) into chromaffin granules prepared from bovine adrenal gland medulla. The (86)Rb(+) influx was inhibited by acidic pH. Purified chromaffin granule membranes were also fused with planar lipid bilayer. A potassium channel with conductance of 432+/-9 pS in symmetric 450 mM KCl was observed after reconstitution into lipid bilayer. The channel activity was unaffected by charybdotoxin, a blocker of the Ca(2+)-activated K(+) channel of large conductance. It was observed that acidification to pH 6.4 cis side of the membrane lowered the channel open probability and single channel conductance. Whereas only weak influence on the single channel current amplitude and open probability were observed upon lowering of the pH at the trans side. We conclude that a pH-sensitive large conductance potassium channel operates in the chromaffin granule membrane.     

Protein shape change has a major effect on the gating energy of a mechanosensitive channel

Increasing experimental evidence has shown that membrane protein functionality depends on molecular composition of cell membranes. However, the origin of this dependence is not fully understood. It is reasonable to assume that specific lipid-protein interactions are important, yet more generic effects due to mechanical properties of lipid bilayers likely play a significant role too. Previously it has been demonstrated using models for elastic properties of membranes and lateral pressure profiles of lipid bilayers that the mechanical properties of a lipid bilayer can contribute as much as ∼10 k_BT to the free energy difference associated with a change in protein conformational state. Here, we extend those previous approaches to a more realistic model for a large mechanosensitive channel (MscL). We use molecular dynamics together with the MARTINI model to simulate the open and closed states of MscL embedded in a DOPC bilayer. We introduce a procedure to calculate the mechanical energy change in the channel gating using a three-dimensional pressure distribution inside a membrane, computed from the molecular dynamics simulations. We decompose the mechanical energy to terms associated with area dilation and shape contribution. Our results highlight that the lateral pressure profile of a lipid bilayer together with the shape change in gating can induce a contribution of ∼30 k_BT on the gating energy of MscL. This contribution arises largely from the interfacial tension between hydrophobic and hydrophilic regions in a lipid bilayer.     

Study of interaction of polystirolsulphonate with polymerization degree of 8 and polyallylamin with bilayer lipids membranes

Interaction of polystirolsulphonate with polymerization degree of 8 (PSS-8) and polyallylamin PAA (molecular mass 60 kilodaltons) with viruses from bloodline of paramixo- and orthomixoviruses by the example of measles virus, parotitis and flu leads to the decreasing of infective activity. The possible mechanism of viral inhibitive action of these chemical compounds is damaging of interfacial antigenic proteins of paramixo- and orthomixoviruses. In this study it was detected the change of surface tension of bilayer lipid membrane in the presence of PSS-8 and PAA. The change of surface tension leads to disorder in viral proteins adsorption in bilayer lipid membrane. This process could lead to disorder of juncture and self-assembly of virions.     

A quantitative explanation of the effects of some alcohols on gramicidin single-channel lifetime

The effects of n-decanol, n-hexadecanol, n-octyl(oxyethylene)3 alcohol and cholesterol on gramicidin single-channel lifetime in planar lipid bilayers have been determined. The bilayers used were formed from a solution of monoolein in squalene. Measurements have also been made of the above compounds' effects on membrane thickness (as measured by electrical capacity and optical reflectance technique) and surface tension (as derived from bulk interfacial tension and bilayer-lens contact angle measurements). The reduction in single-channel lifetime caused by the n-alkanols may be accounted for quantitatively in terms of the effects of these compounds on bilayer thickness and surface tension. The n-octyl(oxyethylene)3 alcohol caused an increase in single-channel lifetime which is also consistent with the thickness/tension theory. The reduction in channel lifetime caused by cholesterol, however, was much larger than would be predicted from its effects on bilayer thickness and surface tension.     

The dependence of Fluorescein-PE fluorescence intensity on lipid bilayer state. Evaluating the interaction between the probe and lipid molecules

The degree of dependence of a lipid bilayer's surface properties on its conformational state is still an unresolved question. Surface properties are functions of molecular organization in the complex interfacial region. In the past, they were frequently measured using fluorescence spectroscopy. Since a fluorescent probe provides information on its local environment, there is a need to estimate the effect caused by the probe itself. In this paper, we address this question by calculating how lipid head-group orientation effects the fluorescence intensity of Fluorescein-PE (a probe that is sensitive to surface potential). In the theoretical model assumed the lipid bilayer state and the interactions between the charged fluorescent probe and the surrounding lipid molecules was evaluated. The results of this theoretical analysis were compared with experimentally obtained data. A lipid bilayer formed from DPPC was chosen as the experimental system, since it exhibits all the major conformational states within a narrow temperature range of 30 degrees C-45 degrees C. Fluorescein-PE fluorescence intensity depends on local pH, which in turn is sensitive to local electrostatic potential in the probe's vicinity. This local electrostatic potential is generated by lipid head-group dipole orientation. We have shown that the effect of the probe on lipid bilayer properties is limited when the lipid bilayer is in the gel phase, whereas it is more pronounced when the membrane is liquid-crystalline. This implies that Fluorescein-PE is a good reporter of local electrostatic fields when the lipid bilayer is in the gel phase, and is a poor reporter when the membrane is in the liquid-crystalline state.     

Acidic interaction of the colicin A pore-forming domain with model membranes of Escherichia coli lipids results in a large perturbation of acyl chain order and stabilization of the bilayer.

2H and 31P NMR techniques were used to study the effects on acyl chain order and lipid organization of the well-characterized pore-forming domain of colicin A (20-kDa thermolytic fragment of colicin A) upon insertion in model membrane systems derived from the Escherichia coli fatty acid auxotrophic strain K 1059, which was grown in the presence of [11,11-2H2]-labeled oleic acid. Addition of the protein to dispersions of the E. coli total lipid extract, in a 1/70 molar ratio of peptide to lipids, resulted in a large pH-dependent decrease in quadrupolar splitting of the 2H NMR spectra. The decrease of the quadrupolar splitting obtained at the various pH values was correlated with the pH dependence of the insertion of the protein in monolayer films using the same E. coli lipid extracts. The pK governing the perturbing effects on the order of the fatty acyl chains was around 5, in agreement with the values of the pH-dependent conformational changes of the pore-forming domain of colicin A required for membrane insertion as reported by van der Goot et al. [(1991) Nature 354, 408-410]. 31P NMR measurements show that the bilayer organization remains intact upon addition of the protein to dispersions of lipid extract. Surprisingly, 31P NMR measurements as a function of temperature indicate that the pore-forming domain of colicin A even stabilizes bilayer lipid structure at pH 4. Both the large effect of the protein on acyl chain order and its bilayer-stabilizing activity are indicative of a surface localization of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of the binding of the local anesthetics procaine and tetracaine to model membranes of phosphatidylethanolamine: a deuterium nuclear magnetic resonance study

The interaction of the local anesthetics tetracaine and procaine with multilamellar dispersions of phosphatidylethanolamine has been investigated by using 2H NMR of specifically deuterated anesthetics. Tetracaine was found to partition more strongly than procaine into the lipid. The 2H NMR spectra showed a quadrupole doublet and a narrow line, with the former corresponding to membrane-bound anesthetic and the latter to anesthetic free in solution. The integrated areas of the narrow line and of the doublet correspond to the concentrations of free and bound anesthetic predicted from the Kp values. There is no strong pH dependence for the quadrupole splittings of tetracaine, suggesting a similar depth of penetration into the lipid bilayer over the entire pH range. The data are consistent with a model in which tetracaine acts as a wedge to stabilize the phosphatidylethanolamine bilayer against transition to a hexagonal structure. Procaine is proposed to sit higher in the phosphatidylethanolamine bilayer than does tetracaine. The T1 values were generally shorter in the membrane than in solution, suggesting slower motions, particularly for the aromatic ring of tetracaine.     

Interfacial ionization and partitioning of membrane-bound local anesthetics.

Consideration of the interfacial protonation equilibria of membrane-associated amphiphiles indicates that the partition coefficients of the protonated and unprotonated species will differ considerably. The partition coefficients of the charged and uncharged forms of spin-labelled myristic acid in dimyristoylphosphatidylcholine bilayer dispersions have been measured by EPR spectroscopy and found to be approximately 140-fold higher for the protonated acid than for the dissociated salt form. This ratio of partition coefficients is found to be in good agreement with that predicted from the interfacial shift in pKa of the fatty acid on its partitioning into the membrane. The latter was determined from the changes in the EPR spectra of the membrane-associated fatty acid with pH and was found to be +2.1 pH units. The interfacial shifts in pKa for a series of spin-labelled analogues of tertiary amine local anaesthetics have been determined from the pH dependence of the partition coefficients in dimyristoylphosphatidylcholine bilayer dispersions and are found mostly to be in the range of approx. -1.0 to -1.5 pH units, corresponding to a 10- to 30-fold higher partition coefficient of the uncharged base compared with that of the charged ammonium form.     

Effect of membrane tension on the physical properties of DOPC lipid bilayer membrane

Molecular dynamics simulations of a dioleoylphosphocholine (DOPC) lipid bilayer were performed to explore its mechanosensitivity. Variations in the bilayer properties, such as area per lipid, volume, thickness, hydration depth (HD), hydration thickness (HT), lateral diffusion coefficient, and changes in lipid structural order were computed in the membrane tension range 0 to 15dyn/cm. We determined that an increase in membrane tension results in a decrease in the bilayer thickness and HD of ~5% and ~5.7% respectively, whereas area per lipid, volume, and HT/HD increased by 6.8%, 2.4%, and 5% respectively. The changes in lipid conformation and orientation were characterized using orientational (S(2)) and deuterium (S(CD)) order parameters. Upon increase of membrane tension both order parameters indicated an increase in lipid disorder by 10-20%, mostly in the tail end region of the hydrophobic chains. The effect of membrane tension on lipid lateral diffusion in the DOPC bilayer was analyzed on three different time scales corresponding to inertial motion, anomalous diffusion and normal diffusion. The results showed that lateral diffusion of lipid molecules is anomalous in nature due to the non-exponential distribution of waiting times. The anomalous and normal diffusion coefficients increased by 20% and 52% when the membrane tension changed from 0 to 15dyn/cm, respectively. In conclusion, our studies showed that membrane tension causes relatively significant changes in the area per lipid, volume, polarity, membrane thickness, and fluidity of the membrane suggesting multiple mechanisms by which mechanical perturbation of the membrane could trigger mechanosensitive response in cells.     

Partitional and motional properties of a spin-labeled daunomycin in lipid bilayers. An ESR study.

The partition coefficient of a spin-labeled daunomycin (DAU-SL) in dimyristoylphosphatidylcholine membrane has been determined using the electron spin resonance (ESR) method. The experiment was carried out as a function of temperature between 5 degrees C and 35 degrees C, giving partition coefficients between 2 and 6 without abrupt change at the phase transition. The thermodynamic parameters on transferring the DAU-SL from the aqueous phase to the lipid bilayer were also calculated. The calculated values are: delta H = 6.11 kcal/mol and delta S = 23 cal/K mol. The partitioning of the DAU-SL and its motion in the membrane were investigated in a wide range of pH (4-10.3). The data show that pH has no effect on partitioning of the DAU-SL which suggest that the drug exists in the uncharged form in the bilayer.