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.     

A View of Hydrogen/Hydroxide Flux Across Lipid Membranes

A topic emerging roughly 30 years ago and engendering an incompletely resolved controversy is reviewed in this article: the relatively high permeability and pH independence associated with H⁺/OH⁻ passive movements across lipid membranes. We summarize the expected characteristics of simple H⁺/OH⁻ diffusion and those of a reaction between H⁺ and OH⁻ being attracted from opposite surfaces and condensing in an interfacial zone of the membrane. An interfacial H⁺/OH⁻ reaction mechanism gives the experimentally observed behavior of an H⁺/OH⁻ flux that is independent of the pH measurement range. This mechanism assumes that H⁺ and OH⁻ within the interfacial zone become electrostatically aligned on opposite sides of the hydrophobic membrane core. Electrostatic attraction and charge delocalization among a small cluster of water molecules surrounding the ions reduce the Born energy for H⁺/OH⁻ insertion into lipid. This transmembrane condensation model predicts the magnitude of the experimentally determined H⁺/OH⁻ flux, which is significantly greater than that of other monovalent ions. The consequences of an elevated H⁺/OH⁻ permeability compared to other ions and the relative pH independence of this flux have consequences for understanding the chemical evolution of life.     

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.     

Active transport,ion movements,and pH changes

The transport of substances across cell membranes may be the most fundamental activity of living things. When the substance transported is any ion there can be a change in the concentration of hydrogen ions on the two sides of the membrane. These hydrogen ion concentration changes are not caused by fluxes of hydrogen ions although fluxes of hydrogen ions may sometimes be involved. The reason for the apparent contradiction is quite simple. All aqueous systems are subject to two constraints: (1) to maintain the charge balance, the sum of the cationic charges must equal the sum of the anionic charges and (2) the product of the molar concentration of H⁺ and the molar concentration of OH⁻, established and maintained by the association and the dissociation of water, remains always at 10⁻¹⁴. As a consequence the concentrations of H⁺ and OH⁻ are determined uniquely by differences between the concentrations of the other cations and anions, with [H⁺] and [OH⁻] being dependent variables. Hydrogen ions and hydroxyl ions can be produced or consumed in local reactions whereas any strong ions such as Cl⁻, Mg²⁺, or K⁺ can be neither produced nor consumed in biological reactions. Further consequences of these truisms are outlined here in terms of the chemistry of the kinds of reactions which can lead to pH changes.     

Complex Formation Equilibria in Two-Component Bilayer Lipid Membrane: Interfacial Tension Method

We measured the interfacial tension of lipid membranes composed of ceramide-cholesterol, ceramide-sphingomyelin, and sphingomyelin-cholesterol. The membrane components formed 1:1 complexes. Interfacial tension measurements were used to determine the membrane surface concentration A ₃⁻¹, the membrane interfacial tension γ₃, and the stability constant K.     

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

The dependence of the interfacial tension of a lipid membrane formed from phosphatidylcholine on the pH of the aqueous solution has been studied. The model describing 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 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.     

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.     

The Interfacial Tension of the Lipid Membrane Formed from Lipid–Amino Acid Systems

The interfacial tension of lipid membranes composed of phosphatidylcholine (lecithin, PC)–valine (Val), phosphatidylcholine–isoleucine (Ile), phosphatidylcholine–tyrosine (Tyr), and phosphatidylcholine–phenylalanine (Phe) has been studied. The membrane components formed 1:1 complexes. The interfacial tension measurements were used to determine the membrane surface concentration A ₃⁻¹, the membrane interfacial tension γ₃, and the stability constant K.     

The effect of H<Subscript>3</Subscript>O<Superscript>+</Superscript> on the membrane morphology and hydrogen bonding of a phospholipid bilayer

At the 2017 meeting of the Australian Society for Biophysics, we presented the combined results from two recent studies showing how hydronium ions (H₃O⁺) modulate the structure and ion permeability of phospholipid bilayers. In the first study, the impact of H₃O⁺ on lipid packing had been identified using tethered bilayer lipid membranes in conjunction with electrical impedance spectroscopy and neutron reflectometry. The increased presence of H₃O⁺ (i.e. lower pH) led to a significant reduction in membrane conductivity and increased membrane thickness. A first-order explanation for the effect was assigned to alterations in the steric packing of the membrane lipids. Changes in packing were described by a critical packing parameter (CPP) related to the interfacial area and volume and shape of the membrane lipids. We proposed that increasing the concentraton of H₃O⁺ resulted in stronger hydrogen bonding between the phosphate oxygens at the water–lipid interface leading to a reduced area per lipid and slightly increased membrane thickness. At the meeting, a molecular model for these pH effects based on the result of our second study was presented. Multiple μs-long, unrestrained molecular dynamic (MD) simulations of a phosphatidylcholine lipid bilayer were carried out and showed a concentration dependent reduction in the area per lipid and an increase in bilayer thickness, in agreement with experimental data. Further, H₃O⁺ preferentially accumulated at the water–lipid interface, suggesting the localised pH at the membrane surface is much lower than the bulk bathing solution. Another significant finding was that the hydrogen bonds formed by H₃O⁺ ions with lipid headgroup oxygens are, on average, shorter in length and longer-lived than the ones formed in bulk water. In addition, the H₃O⁺ ions resided for longer periods in association with the carbonyl oxygens than with either phosphate oxygen in lipids. In summary, the MD simulations support a model where the hydrogen bonding capacity of H₃O⁺ for carbonyl and phosphate oxygens is the origin of the pH-induced changes in lipid packing in phospholipid membranes. These molecular-level studies are an important step towards a better understanding of the effect of pH on biological membranes.     

Passive H+/OH− permeability in epithelial brush border membranes

Passive H⁺/OH^– permeability across epithelial cell membranes is rapid and leads to partial dissipation of H⁺/OH^– gradients produced by H⁺ pumps and ion gradient-coupled H⁺/OH^– transporters. A heterogeneous set of H⁺/OH^– transport mechanisms exist in biological membranes: lipid solubility/diffusion, protein-mediated transport by specific proteins or by slippage through ion-coupled H⁺/OH^– transporters, and transport at the protein/lipid interface or through protein-dependent defects in the lipid structure. A variety of methods are available to study protein transport mechanisms accurately in cells and biomembrane vesicles including pH electrode recordings, pH-sensitive fluorescent and magnetic resonance probes, and potentiometric probes. In brush border vesicles from the renal proximal tubule, the characteristics of passive H⁺/OH^– permeability are quite similar to those reported for passive H⁺/OH^– permeability through pure lipid bilayers; slippage of protons through the brush border Na⁺/H⁺ antiporter or through brush border water channels is minimal. In contrast, passive H⁺/OH^– permeability in brush border vesicles from human placenta is mediated in part by a stilbene-sensitive membrane protein. To demonstrate the physiological significance of passive renal brush border H⁺/OH^– transport, proximal tubule acidification and cell pH regulation mechanisms are modeled mathematically for states of normal and altered H⁺/OH^– permeabilities.     

The Role of H<Superscript>+</Superscript>/OH<Superscript>−</Superscript> Channels in the Salt Stress Response of <Emphasis Type="Italic">Chara australis</Emphasis>

We investigate the electrophysiological salt stress response of the salt-sensitive charophyte Chara australis as a function of time in saline artificial pond water (saline APW) containing 50 mM NaCl and 0.1 mM CaCl₂. The effects are due to an increase in Na⁺ concentration rather than an increase in Cl⁻ concentration or medium osmolarity. A previous paper (Shepherd et al. Plant Cell Environ 31:1575–1591, 2008) described the rise in the background conductance and inhibition of proton pumping in saline APW in the first 60 min. Here we investigate the shift of membrane potential difference (PD) to levels above −100 mV and the change of shape of the current–voltage (I/V) profiles to upwardly concave. Arguing from thermodynamics, the I/V characteristics can be modeled by channels that conduct H⁺ or OH⁻. OH⁻ was chosen, as H⁺ required an unrealistic increase in the number/permeability of the channels at higher pH levels. Prolonged exposure to saline APW stimulated opening of more OH⁻ channels. Recovery was still possible even at a PD near −50 mV, with partial return of proton pumping and a decrease in OH⁻ current following APW wash. Upon change of pH from 7 to 9, the response was consistent with previously observed I/V characteristics of OH⁻ channels. For a pH change to 6, the response was transient before channel closure but could still be modeled. The consequences of opening of H⁺ or OH⁻ channels while the cell is under salt stress are discussed.     

The interfacial tension of the lipid membrane formed from lipid-cholesterol and lipid-lipid systems

Interfacial tension has been determined for phosphatidylcholine-cholesterol, phosphatidylcholine-phosphatidylethanolamine, and phosphatidylethanolamine-cholesterol membranes. Phosphatidylcholine (PC), phosphatidylethanolamine (PE), and cholesterol (Ch) were to be investigated, because of their presence in biological membranes. Interfacial tension values of pure components are 0.81×10⁻³ N/m, 1.67×10⁻³ N/m, and 2.36×10⁻³ N/m, respectively. The 1∶1 complexes were formed during formation of the PC-Ch, PC-PE, and PE-Ch lipid membranes. The following parameters describing the complexes were determined: A ₃ ⁻¹ , the surface concentrations of the lipid membranes formed from these complexes; γ₃, the interfacial tensions of such membranes and K, the stability constants of these complexes.     

Effect of monovalent ion adsorption on the electric charge of phosphatidylcholine - decylamine liposomal membranes

We examined the effect of adsorbed monovalent ions on the surface charge of phosphatidylcholine (PC) – decylamine (DA) liposomal membranes. Surface charge density values were determined from electrophoretic mobility measurements of lipid vesicles performed at various pH levels. The interaction between solution ions and the PC-DA liposomal surface was described by a six component equilibrium model. The previously determined association constants of the -PO^(-) and –N⁽⁺⁾(CH₃)₃ groups of PC with H⁺, OH^-, Na⁺ and Cl^- ions (K _A1H, K _B1OH, K _A1Na, K _B1C1) were used to calculate K _B2OH, and K _B2C1, the association constants of the –N⁽⁺⁾H₃ group of DA with OH^- and Cl^- ions, providing an experimental verification for the proposed model.     

Characterization of H+/OH− currents in phospholipid vesicles

In pure phospholipid vesicles, the conductivity of H⁺/OH^– ions exceeds that for other simple inorganic ions. Protons achieve electrochemical equilibrium across egg phosphatidylcholine vesicles within tens of minutes. When pH gradients are established across vesicles, transmembrane potentials develop. Conversely, the establishment of transmembrane potentials leads to the formation of pH gradients. When the phenomenological permeability of H⁺/OH^– ions in vesicles is estimated, values are obtained that are much greater (six orders of magnitude larger) than those for Na⁺ or K⁺. A wide range in the values for this permeability has been reported; however, much of the discrepancy can be attributed to differences in the vesicle systems and experimental conditions. The H⁺/OH^– current appears to be modulated by changes in membrane dielectric constant. However, the dependence of this current on the pH gradient and on the membrane voltage argues against simple diffusion mechanisms as the source of the H⁺/OH^– current. In addition, in vesicle systems the H⁺/OH^– current shows a surprising invariance to changes in the membrane dipole potential, an observation that argues against the role of simple carriers for H⁺ and OH^– ions.     

Protoplast ion fluxes: their measurement and variation with time, position and osmoticum

The ability to measure directly individual protoplast ion fluxes is a valuable addition to patch clamp and other techniques when using protoplasts to study membrane transporters. Before interpreting observations on protoplasts in terms of behaviour of intact cells and tissues, some methodological questions should be addressed. These include effects of space and time variations of transporter activities over the membrane, the osmotic dependence of specific ion transporters and the effect of the regenerating cell wall. In this study net H⁺ and Ca²⁺ fluxes were measured from individual corn (Zea mays L.) coleoptile protoplasts using a non-invasive microelectrode technique for ion flux measurements. For Ca²⁺, the flux distribution was almost symmetrical, ranging ±30 nmol · m⁻² · s⁻¹ around zero. For H⁺ it was skewed towards efflux ranging from −100 to +10 nmol · m⁻² · s⁻¹. The distribution of H⁺ fluxes through the protoplast surface was a complex mosaic which changed with time, sometimes showing oscillations. These flux variations with time and position around the surface, apparently driven by endogenous mechanisms, may be relevant to protoplast pH homeostasis. When the new cell wall was partially regenerated on the next day, the correlation between H⁺ and Ca²⁺ fluxes increased, which is consistent with the weak-acid Donnan-Manning model of cell wall ion exchange. Received: 11 June 1997 / Accepted: 10 July 1997     

How do protons cross the membrane-solution interface? Kinetic studies on bilayer membranes exposed to the protonophore S-13 (5-chloro-3-tert-butyl-2′-chloro-4′ nitrosalicylanilide)

Summary A simple carrier model describes adequately the transport of protons across lipid bilayer membranes by the weak acid S-13. We determined the adsorption coefficients of the anionic, A^–, and neutral, HA, forms of the weak acid and the rate constants for the movement of A^– and HA across the membrane by equilibrium dialysis, electrophoretic mobility, membrane potential, membrane conductance, and spectrophotometric measurements. These measurements agree with the results of voltage clamp and charge pulse kinetic experiments. We considered three mechanisms by which protons can cross the membranesolution interface. An anion adsorbed to the interface can be protonated by (i) a H⁺ ion in the aqueous phase (protolysis), (ii) a buffer molecule in the aqueous phase or (iii) water molecules (hydrolysis). We demonstrated that the first reaction cannot provide the required flux of protons: the rate at which H⁺ must combine with the adsorbed anions is greater than the rate at which diffusion-limited reactions occur in the bulk aqueous phase. We also ruled out the possibility that the buffer is the main source of protons: the rate at which buffers must combine with the adsorbed anions is greater than the diffusion-limited rate when we reduced the concentration of polyanionic buffer adjacent to the membrane-solution interface by using membranes with a negative surface charge. A simple analysis demonstrates that a hydrolysis reaction can account for the kinetic data. Experiments at acid pH demonstrate that the transfer of H⁺ from the membrane to the aqueous phase is limited by the rate at which OH combines with adsorbed HA and that the diffusion coefficient of OH^– in the water adjacent to the bilayer has a value characteristic of bulk water. Our experimental results demonstrate that protons are capable of moving rapidly across the membrane-solution interface, which argues against some mechanisms of local chemiosmosis.     

Production Kinetics and Tensioactive Characteristics of Biosurfactant from a Pseudomonas aeruginosa Mutant Grown on Waste Frying Oils

Various waste frying oils (WFOs) were evaluated as substrates for rhamnolipid production by Pseudomonas aeruginosa mutant EBN-8 in the presence or absence of rhamnolipid precursor, under single-/batch-fed conditions. Soybean WFO was the best substrate, producing 9.3 g rhamnolipid l⁻¹ with the specific product yield of 2.7 g g⁻¹ h, under batch-fed cultivation with the addition of rhamnolipid precursor. The surface tension of the cell-free culture broth (CFCB) was 29.1 mN m⁻¹ and the interfacial tension against n-hexadecane was <1 mN m⁻¹. The hydrocarbon/ CFCB systems showed the relative emulsion stability to be in the range of 89.7–92.3.     

Inactivation of plasmalemma conductance in alkaline zones of <Emphasis Type="Italic">Chara corallina</Emphasis> after generation of action potential

A microelectrode study with Chara corallina cells has shown that post-excitation changes of membrane potential and plasmalemma resistance, induced by the action potential (AP) generation, differ substantially for cell areas producing zones of high and low external pH. In cell regions producing alkaline zones, the AP generation was followed by post-excitation hyperpolarization by about 50 mV, concomitant with four- to eightfold increase in plasmalemma resistance and a considerable drop of pericellular pH. In the acidic areas the post-excitation hyperpolarization was weak or absent, and the membrane resistance showed no significant increase within 1–2 min after AP. The membrane excitation in the acidic zones was accompanied by a noticeable pH increase near the cell surface, indicative of the inhibition of plasma membrane H⁺ pump. The results suggest that the high local conductance of the plasmalemma is closely related to alkaline zone formation and the depolarized state of illuminated cell under resting conditions. Excitation-induced changes of membrane potential and pH in the cell vicinity were fully reversible, with the recovery period of ∼15 min at a photon flux density of ∼100 μE/(m² s). At shorter intervals between excitatory stimuli, differential membrane properties of nonuniform regions turned smoothed and could be overlooked. It is concluded that the origin of alkaline zones in illuminated Chara cells cannot be ascribed to hypothetical operation of H⁺/HCO₃⁻ symport or OH⁻/HCO₃⁻ antiport.     

Monovalent ion and calcium ion fluxes in sarcoplasmic reticulum

Summary The ion permeability of sarcoplasmic reticulum vesicles from skeletal and heart muscle has been characterized by radioisotope flux, osmotic and membrane potential measurements, and by incorporating vesicles into planar phospholipid bilayers. The sarcoplasmic reticulum membrane is uniquely permeable to various biologically relevant monovalent ions. At least two and possibly three separate passive permeation systems for monovalent ions have been identified: 1) a K⁺, Na⁺ channel, 2) an anion channel, and 3) a H⁺ (OH^–) permeable pathway which may or may not be synonymous with the anion channel. A possible physiological function of these monovalent ion permeation systems is to permit rapid movement of K⁺, Na⁺, H⁺ and Cl across the membrane to counter electrogenic Ca²⁺ fluxes during Ca²⁺ release and uptake by sacroplasmic reticulum.