Effect of pH and monovalent cations on the ionization state of phosphatidylglycerol in monolayers. An experimental (surface potential) and theoretical (Gouy-Chapman) approach

The membrane activity of the antimicrobial peptide caerin 1.1 is pH dependent

Antimicrobial peptides are an important class of membrane-active peptides that can provide alternatives or complements to classic antibiotics. Among the many classes of AMPs, the histidine-rich family is of particular interest since they may induce pH-sensitive interactions with cell membranes. The AMP caerin 1.1 (Cae-1), from Australian tree frogs, has three histidine residues, and thus we studied the pH dependence of its interactions with model cell membranes. Using NMR spectroscopy and molecular dynamics simulations, we showed that Cae-1 induced greater perturbation of the lipid dynamics and water penetrations within the membrane interior in an acidic environment compared with physiological conditions. Using 31P solid-state NMR, the packing, chemical environment, and dynamics of the lipid headgroup were monitored. 2H solid-state NMR showed that Cae-1 ordered the acyl chains of the hydrophobic core of the bilayer. These results supported the molecular dynamics data, which showed that Cae-1 was mainly inserted within the lipid bilayer for both neutral and negatively charged membranes, with the charged residues pulling the water and phosphate groups inward. This could be an early step in the mechanism of membrane disruption by histidine-rich antimicrobial peptides and indicated that Cae-1 acts via a transmembrane mechanism in bilayers of neutral and anionic phospholipid membranes, especially in acidic conditions.

On the Coupling between Mechanical Properties and Electrostatics in Biological Membranes

Cell membrane structure is proposed as a lipid matrix with embedded proteins, and thus, their emerging mechanical and electrostatic properties are commanded by lipid behavior and their interconnection with the included and absorbed proteins, cytoskeleton, extracellular matrix and ionic media. Structures formed by lipids are soft, dynamic and viscoelastic, and their properties depend on the lipid composition and on the general conditions, such as temperature, pH, ionic strength and electrostatic potentials. The dielectric constant of the apolar region of the lipid bilayer contrasts with that of the polar region, which also differs from the aqueous milieu, and these changes happen in the nanometer scale. Besides, an important percentage of the lipids are anionic, and the rest are dipoles or higher multipoles, and the polar regions are highly hydrated, with these water molecules forming an active part of the membrane. Therefore, electric fields (both, internal and external) affects membrane thickness, density, tension and curvature, and conversely, mechanical deformations modify membrane electrostatics. As a consequence, interfacial electrostatics appears as a highly important parameter, affecting the membrane properties in general and mechanical features in particular. In this review we focus on the electromechanical behavior of lipid and cell membranes, the physicochemical origin and the biological implications, with emphasis in signal propagation in nerve cells.

Evaluation of EGCG Loading Capacity in DMPC Membranes

Catechins are molecules with potential use in different pathologies such as diabetes and cancer, but their pharmaceutical applications are often hindered by their instability in the bloodstream. This issue can be circumvented using liposomes as their nanocarriers for in vivo delivery. In this work, we studied the molecular details of (-)-epigallocatechin-3-gallate (EGCG) interacting with 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) monolayer/bilayer systems to understand the catechin loading ability and liposome stability, using experimental and computational techniques. The molecular dynamics simulations show the EGCG molecules deep inside the lipid bilayer, positioned below the lipid ester groups, generating a concentration-dependent lipid condensation. This effect was also inferred from the surface pressure isotherms of DMPC monolayers. In the polarization-modulated infrared reflection absorption spectra assays, the predominant effect at higher concentrations of EGCG (e.g., 20 mol %) was an increase in lipid tail disorder. The steady-state fluorescence data confirmed this disordered state, indicating that the catechin-induced liposome aggregation outweighs the condensation effects. Therefore, by adding more than 10 mol % EGCG to the liposomes, a destabilization of the vesicles occurs with the ensuing release of entrapped catechins. The loading capacity for DMPC seems to be limited by its disordered lipid arrangements, typical of a fluid phase. To further increase the clinical usefulness of liposomes, lipid bilayers with more stable and organized assemblies should be employed to avoid aggregation at large concentrations of catechin.

Interactions of linear dicationic molecules with lipid A: structural requisites for optimal binding affinity

The structural determinants of the binding affinity of linear dicationic molecules toward lipid A have been examined with respect to the distance between the terminal cationic functions, the basicity, and the type of cationic moieties using a series of spermidine derivatives and pentamidine analogs by fluorescence spectroscopic methods. The presence of two terminal cationic groups corresponds to enhanced affinity. A distinct sigmoidal relationship between the intercationic distance and affinity was observed with a sharp increase at 11 Å, levelling off at about 13 Å. The basicity (pK) and nature of the cationic functions are poor correlates of binding potency, since molecules bearing primary amino, imidazolino, or guanido termini are equipotent. The interaction of pentamidine, a bisamidine drug, with lipid A, characterized in considerable detail employing the putative intermolecular excimerization of the drug, suggests a stoichiometry of 1:1 in the resultant complex. The binding is driven almost exclusively by electrostatic forces, and is dependent on the ionization states of both lipid A and the drug. Under conditions when lipid A is highly disaggregated, pentamidine binds specifically to bis-phosphoryl- but not to monophosphoryl-lipid A indicating that both phosphate groups of lipid A are necessary for electrostatic interactions by the terminal amidininium groups of the drug. Based on these data, a structural model is proposed for the pentamidine-lipid A complex, which may be of value in designing endotoxin antagonists from first principles.

Molecular structures of fluid phase phosphatidylglycerol bilayers as determined by small angle neutron and X-ray scattering

We have determined the molecular structures of commonly used phosphatidylglycerols (PGs) in the commonly accepted biologically relevant fluid phase. This was done by simultaneously analyzing small angle neutron and X-ray scattering data, with the constraint of measured lipid volumes. We report the temperature dependence of bilayer parameters obtained using the one-dimensional scattering density profile model - which was derived from molecular dynamics simulations - including the area per lipid, the overall bilayer thickness, as well as other intrabilayer parameters (e.g., hydrocarbon thickness). Lipid areas are found to be larger than their phosphatidylcholine (PC) counterparts, a result likely due to repulsive electrostatic interactions taking place between the charged PG headgroups even in the presence of sodium counterions. In general, PG and PC bilayers show a similar response to changes in temperature and chain length, but differ in their response to chain unsaturation. For example, compared to PC bilayers, the inclusion of a first double bond in PG lipids results in a smaller incremental change to the area per lipid and bilayer thickness. However, the extrapolated lipid area of saturated PG lipids to infinite chain length is found to be similar to that of PCs, an indication of the glycerol-carbonyl backbone's pivotal role in influencing the lipid-water interface.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: a statistical model

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayer- or lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types--each one associated, respectively, with the polar-headgroup and the acyl-chain states--which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Electron Transfer in the Photosynthetic Membrane: Influence of PH and Surface Potential on the P-680 Reduction Kinetics

The primary electron donor P-680 of the Photosystem-II reaction center was photoxidized by a short flash given after dark adaptation of photosynthetic membranes in which oxygen evolution was inhibited. The P-680(+) reduction rate was measured under different conditions of pH and salt concentration by following the recovery of the absorption change at 820 nm. As previously reported for Tris-washed chloroplasts (Conjeaud, H., and P. Mathis, 1980, Biochim. Biophys. Acta, 590:353-359) a fast phase of P-680(+) reduction slows down as the bulk pH decreases. When salt concentration increases, this fast phase becomes faster for pH above 4.5-5 and slower below. A quantitative interpretation is proposed in which the P-680(+) reduction kinetics by the secondary electron donor Z are controlled by the local pH. This pH, at the membrane level, can be calculated using the Gouy-Chapman theory. A good fit of the results requires to assume that the surface charge density of the inside of the membrane, near the Photosystem-II reaction center, is positive at low pH values and becomes negative as the pH increases, with a local isoelectric point approximately 4.8. These results lead us to propose a functional scheme in which a pH-dependent proton release is coupled to the electron transfer between secondary and primary donors of Photosystem-II. The H(+)/e ratio varies from 1 at low pH to 0 at high pH, with a real pK approximately 6.5 for the protonatable species.

Helical conformation of the SEVI precursor peptide PAP248-286, a dramatic enhancer of HIV infectivity, promotes lipid aggregation and fusion

In previous in vivo studies, amyloid fibers formed from a peptide ubiquitous in human seminal fluid (semen-derived enhancer of viral infection (SEVI)) were found to dramatically enhance the infectivity of the HIV virus (3-5 orders of magnitude by some measures). To complement those studies, we performed in vitro assays of PAP(248-286), the most active precursor to SEVI, and other polycationic polymers to investigate the physical mechanisms by which the PAP(248-286) promotes the interaction with lipid bilayers. At acidic (but not at neutral) pH, freshly dissolved PAP(248-286) catalyzes the formation of large lipid flocculates in a variety of membrane compositions, which may be linked to the promotion of convective transport in the vaginal environment rather than transport by a random Brownian motion. Furthermore, PAP(248-286) is itself fusiogenic and weakens the integrity of the membrane in such a way that may promote fusion by the HIV gp41 protein. An alpha-helical conformation of PAP(248-286), lying parallel to the membrane surface, is implicated in promoting bridging interactions between membranes by the screening of the electrostatic repulsion that occurs when two membranes are brought into close contact. This suggests that nonspecific binding of monomeric or small oligomeric forms of SEVI in a helical conformation to lipid membranes may be an additional mechanism by which SEVI enhances the infectivity of the HIV virus.

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

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

Interaction of the Antimicrobial Peptide Dicynthaurin with Membrane Phospholipids at the Air−Liquid Interface

This paper reports the first study on the interaction of the antimicrobial peptide dicynthaurin with 1,2-dipalmitoyl-glycerophosphatidyl-glycerol investigated in monolayers at the air-liquid interface. The influence of the peptide on the two-dimensional phase behavior of the negatively charged lipid was elucidated by means of pressure-area isotherm measurements, fluorescence microscopy, and grazing incidence X-ray diffraction measurements. The pure peptide forms a stable monolayer at the air-liquid interface up to 30 mN/m as shown for both the monomeric and the dimeric cynthaurins. The peptide lipid interaction was monitored in isotherm measurements showing a strong adsorption of the peptide and stabilization at the interface promoted by the lipid monolayer. The X-ray diffraction measurements in agreement with fluorescence microscopy studies showed that the peptide destabilizes the condensed chain lattice, leading to a complete fluidization of the condensed lipid phase on physiological buffer. The adsorption of the peptide to the negatively charged lipid monolayer and the fluidization of the condensed chain lattice suggest a direct link to the peptides' ability to expand the bacterial membrane that would be relevant for the in vivo mode of action.

Interfacial assembly of a phospholipid Langmuir monolayer by electrodeposited silver colloids

Two-dimensional nanostructured silver films were electrodeposited at the surface of a silver nitrate subphase coated by a negatively charged dimyristoylphosphatidylglycerol (DMPG) Langmuir monolayer. The modifications of the phospholipid interfacial organization generated by the growing colloidal silver film were investigated using surface pressure-time isotherms and grazing incidence X-ray diffraction experiments (GIXD). A decrease in the initial surface pressure of the DMPG monolayer is observed outside of the growing silver film, followed by a stabilization of the surface pressure when the radius of the metallic layer reaches its plateau value. This behavior is attributed to the compression of the DMPG molecules above the silver film and to the correlated relaxation and expansion of those outside the silver film area as recorded by a Wilhelmy pressure sensor. GIXD experiments further evidenced the contraction of the phospholipid monolayer above the electrochemically growing films. Indeed, the diffraction spectra show a shift in the peak position toward higher values of the in-plane component of the wave-vector transfer, indicating a closer packing of the DMPG alkyl chains. This is also in agreement with the observed loss of the chain tilt angle, suggesting that the colloidal silver film induces interfacial structuring of the DMPG monolayer.

Ionization state and structure of l-1,2-dipalmitoylphosphatidylglycerol monolayers at the liquid/air interface

Phosphatidylglycerols are components of biological membranes. The phase behavior of these phospholipids was extensively investigated. However, there is still no definite picture about the dependence of the ionization state and monolayer structure on subphase composition. The major problem of previous investigations is that none of the methods used allow obtaining the ionization degree directly. In the present work we apply techniques developed in the past decades for Langmuir monolayers: infrared reflection absorption spectroscopy (IRRAS) as well as X-ray diffraction and reflectivity techniques, which provide straightforward information about structure and ionization state of a L-1,2-dipalmitoylphosphatidylglycerol (DPPG) monolayer. The Gouy-Chapman model is applied to evaluate the intrinsic pKa. Therewith, the ionization degree can be determined even at low pH values. The experimental titration curves are in good agreement with theoretical curves based on the Gouy-Chapman model. The obtained instrinic pKa amounts to 1. The ionization degree of a DPPG monolayer is independent of the monovalent cation size. In contrast, the structure of a DPPG monolayer is strongly affected by the type of divalent cations.

Adsorption of Amyloid β (1-40) Peptide at Phospholipid Monolayers

The folding of amyloid beta (1-40) peptide into beta-sheet-containing fibrils is thought to play a causative role in Alzheimer's disease. Because of its amphiphilic character, the peptide can interact with phospholipid membranes. Langmuir monolayers of negatively charged DPPS, DPPG, and DMPG, and also of zwitterionic DPPC and DMPC, have been used to study the influence of the peptide on the lipid packing and, vice versa, the influence of phospholipid monolayers on the peptide secondary structure by infrared reflection absorption spectroscopy and grazing incidence X-ray diffraction. The peptide adsorbs at the air/water (buffer) interface, and also inserts into uncompressed phospholipid monolayers. When adsorbed at the interface, the peptide adopts a beta-sheet conformation, with the long axis of these beta-sheets oriented almost parallel to the surface. If the lipid exhibits a condensed monolayer phase, then compression of the complex monolayer with the inserted peptide leads to the squeezing out of the peptide at higher surface pressures (above 30 mN m(-1)). The peptide desorbs completely from zwitterionic monolayers and negatively charged DPPG and DPPS monolayers on buffer, but remains adsorbed in the beta-sheet conformation at negatively charged monolayers on water. This can be explained in terms of electrostatic interactions with the lipid head groups. It also remains adsorbed at, or penetrating into, disordered anionic monolayers on buffer. Additionally, the peptide does not influence the condensed monolayer structure at physiological pH and modest ionic strength.

1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) monolayers: influence of temperature, pH, ionic strength and binding of alkaline earth cations

Ion binding and lipid ionization of the acidic phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) in monolayers was studied by measuring the lateral pressure Pi as a function of the molecular area A at the air/water interface at different temperatures. The pH of the subphase (pH 2 and 7) and the ionic strength (NaCl) was varied. In addition, different divalent cations (1mM MgCl2, CaCl2 and SrCl2, pH 7) were added. DMPG is partly protonated on pure water at pH 7. An increase in the NaCl concentration in the subphase leads to film expansion. This effect is caused by an ionization of the headgroup of DMPG, i.e. a shift of the apparent pK. More condensed films are obtained on pure water at pH 2, due to the reduction of electrostatic repulsion by headgroup protonation and the possibility for the formation of a hydrogen bonding network. The divalent cations Mg2+, Ca2+ and Sr2+ interact differently with a DMPG monolayer in pure water at pH 7. In the presence of 1mM CaCl2 a condensation of the DMPG film is induced, whereas an expansion of the monolayer is observed in the presence of Mg2+ and Sr2+. Two counteracting effects are operative: (a) ionization of the headgroup due to electrostatic screening leads to film expansion and (b) binding of the divalent cations to the lipid headgroups leads to condensation. The latter effect is more pronounced in the case of Ca2+, whereas the binding of Mg2+ and Sr2+ to DMPG is weaker. Site-specific cation binding has to be assumed in addition to electrostatic effects.

Enhanced stabilization of aerosol-OT surfactant monolayer upon interaction with small amounts of bovine serum albumin at the air–water interface

An investigation is made of the influence from small amounts of the protein bovine serum albumin (BSA) on the lateral organization of low molecular weight surfactant sodium bis-2-ethylhexyl sulfosuccinate (AOT) at the air-water interface. Surface pressure (pi - A), surface potential (deltaV - A) and Brewster angle microscopy (BAM) experiments were carried out, with particular emphasis on the monolayer stability under successive compression-expansion cycles. AOT monolayer is not stable at the air-water interface, which means that the majority of AOT molecules go into the aqueous subphase as monomers and/or normal micelles. When a waiting time elapses between spreading and compression, the surfactant monolayer tends to reorganize partially at the air-water interface, with a monolayer expansion being observed for waiting times as large as 12 h. The incorporation of very small amount of BSA (10(-9)M) at the interface, also inferred from BAM, increases the monolayer stability as revealed by pi - A and deltaV - A results. For a waiting time of circa 3 h, the mixed monolayer reaches its maximum stability. This must be related to protein (and/or protein-surfactant complexes) adsorbed onto the AOT monolayer, thus altering the BSA conformation to accommodate its hydrophobic/hydrophilic residues. Furthermore, the effects from such small amounts of BSA in the monolayer formation and stabilization mean that the AOT monolayer responds cooperatively to BSA.

The peculiar thermo-structural behavior of the anionic lipid DMPG

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG), around 100 mM ionic strength, are known to exhibit a thermal behavior similar to that of the largely studied lipid dimyristoyl phosphatidylcholine (DMPC), which undergoes a gel to liquid crystalline phase transition at 23 degrees C, well characterized by differential scanning calorimetry (DSC), and other methods. However, at low ionic strength, DMPG has been shown to present a large gel-fluid transition region, ranging from 18 to 35 degrees C. This intermediate phase is optically transparent and characterized by a continuous change in membrane packing. Structural properties of the DMPG gel-fluid transition region will be discussed, based on results obtained by several techniques: electron spin resonance (ESR) of spin labels at the membrane surface and intercalated at different depths in the bilayer; light scattering; DSC; small angle X-ray scattering (SAXS); and fluorescence spectroscopy of probes in the bilayer.

The influence of plasma membrane electrostatic properties on the stability of cell ionic composition

An electro-osmotic model is developed to examine the influence of plasma membrane superficial charges on the regulation of cell ionic composition. Assuming membrane osmotic equilibrium, the ion distribution predicted by Gouy-Chapman-Grahame (GCG) theory is introduced into ion transport equations, which include a kinetic model of the Na/K-ATPase based on the stimulation of this ion pump by internal Na(+) ions. The algebro-differential equation system describing dynamics of the cell model has a unique resting state, stable with respect to finite-sized perturbations of various types. Negative charges on the membrane are found to greatly enhance relaxation toward steady state following these perturbations. We show that this heightened stability stems from electrostatic interactions at the inner membrane side that shift resting state coordinates along the sigmoidal activation curve of the sodium pump, thereby increasing the pump sensitivity to internal Na(+) fluctuations. The accuracy of electrostatic potential description with GCG theory is proved using an alternate formalism, based on irreversible thermodynamics, which shows that pressure contribution to ion potential energy is negligible in electrostatic double layers formed at the surfaces of biological membranes. We discuss implications of the results regarding a reliable operation of ionic process coupled to the transmembrane electrochemical gradient of Na(+) ions.

Cooperativity of phospholipid reorganization upon interaction of dipyridamole with surface monolayers on water

Results from various surface sensitive characterization techniques suggest a model for the interaction of the piperidinopyrimidine dipyridamole (DIP)--known as a vasodilator and inhibitor of P-glycoprotein associated multidrug resistance of tumor cells--with phospholipid monolayers in which the drug is peripherally associated with the membrane, binding (up to) five phospholipids at a time. These multiple interactions are responsible for a very strong association of the drug with the lipid monolayer even at exceedingly low concentrations (approximately 0.2 mol%). Electrostatic interactions and hydrogen bonding are likely involved in the binding of DIP to DPPC. Cooperative effects among the lipids are invoked to explain the macroscopically measurable changes of lipid monolayer properties even when only one out of 100 DPPC molecules is directly associated with a DIP molecule. A reversal of the observed changes upon drug association with the membrane as the DIP concentration surpasses a threshold concentration (c(crit)approximately 0.5 mol%) may be explained by cooperativity in a different context, the self-aggregation of drug molecules. With its implications for the interaction of DIP with phospholipid films, this work provides a first approach to the explanation of the high sensitivity of cell membranes to piperidinopyrimidine drugs on a molecular level.

A Revised Stern Equation for Ion Exchange at Charged Membranes

We revised the classical Stern equation for ion exchange at charged membranes using the statistical mechanics concept of chemical potential and ion activity in nonhomogeneous systems. Our proposed equation does not require an iterative procedure to obtain the self-regulated surface charge at the membranes. We predict significant differences with the classical results. When compared with experimental results, our approach not only gives a better fit but also predicts dissociation constants that are physically more reasonable than those from the classical approach. Copyright 2001 Academic Press.

Adsorption of pulmonary surfactant protein D to phospholipid monolayers at the air-water interface

The intrinsic surface activity of recombinant rat surfactant-associated protein D (SP-D) expressed in CHO-K1 cells has been determined from measurements of surface tension of its aqueous solutions. The interactions of recombinant SP-D with monolayers of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylinositol (PI) spread at the air-water interface have been characterized. Injection of SP-D beneath preformed lipid monolayers at surface pressures less than 30 mN/m produced an increase in surface pressure, consistent with SP-D penetrating the lipid films. The adsorption of SP-D to the lipid monolayers did not display significant head group dependency, suggesting that the changes in surface pressure produced by the protein were likely due primarily to hydrophobic interactions with the lipid layers. In the presence of calcium ions in the subphase, SP-D displayed lower surface activity by itself and a reduced ability to generate surface pressure changes during adsorption to lipid monolayers compared to these properties of the protein in the absence of 2 mM Ca2+. Circular dichroism measurements on SP-D solutions with or without Ca2+ suggested that the cations altered the conformation of the protein and this possibly led to the calcium dependency of the surface activity of the protein in the presence or absence of lipid monolayers. Compressional isotherms of surface pressure versus area for SP-D/(DPPC-PI) and SP-D/(DPPC-PG) films formed by adsorption of the protein to preformed lipid monolayers were consistent with incorporation of some or all of the SP-D molecules into the lipid layers. The isotherms obtained on compression of the SP-D/lipid films to a maximum surface pressure of about 70 mN/m were consistent with the interpretation that any SP-D which was incorporated by adsorption was apparently not squeezed out, nor was lipid removed by the protein. The work suggests that when soluble recombinant SP-D is allowed to interact with phospholipid monolayers under the selected conditions of this experiment, it does so to a limited extent driven by primarily hydrophobic forces, and apparently without a high selectivity for phospholipid head groups.

Calcium ions and interactions of pulmonary surfactant proteins SP-B and SP-C with phospholipids in spread monolayers at the air/water interface

Spread monolayers containing hydrophobic pulmonary surfactant protein, SP-B or SP-C, or SP-B/SP-C (2:1, w/w), alone or mixed with dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG), were formed on saline subphases containing calcium ions. Surface pressure-area characteristics of the films of the proteins were not affected by the presence of Ca2+ in the subphase. Calcium ions did not alter the surface properties of the binary and ternary films of DPPC plus either SP-B, or SP-C, or SP-B/SP-C (2:1, w/w). Surface pressure-area isotherms for the spread films of DPPG plus hydrophobic surfactant protein were Ca(2+)-dependent. The exclusion pressures of SP-B, SP-C and SP-B/SP-C (2:1, w/w) from protein-DPPG films in the presence of calcium were lower than the exclusion pressures in the absence of Ca2+. The divalent cation appeared to suppress the ability of SP-C and SP-B/SP-C (2:1, w/w) to remove phospholipid during squeeze-out from their mixed films with DPPG. The effects of Ca2+ on the monolayers of DPPG plus hydrophobic surfactant proteins were consistent with calcium producing diminished lipid-protein interactions, possibly resulting from Ca(2+)-induced changes in the ionization state and molecular packing of DPPG.

On the surface activity of surfactant-associated protein C (SP-C): effects of palmitoylation and pH

The effect of palmitoylation on the surface activity of bovine surfactant-associated protein C (SP-C) in lipid mixtures was investigated. Native and chemically depalmitoylated SP-C were reconstituted with dipalmitoylphosphatidylcholine/egg phosphatidylglycerol (7:3) using two different procedures, one of which included lyophilization and sonication. When tested using a pulsating bubble surfactometer, no significant changes in the surface activity of these mixtures were observed upon the hydrolysis of the palmitates. Since the purification and deacylation procedures of SP-C included the use of acid and alkali, the effect of pH was examined. The surface activity of the mixtures was found to vary with pH. At low pH values (approx. 2.5), surface tensions between 3 and 10 mN/m at minimum bubble radius were reached within 5 pulsations, while at neutral and slightly alkaline pH, surface tension reduction was much slower and near zero (< 5 mN/m) values at minimum bubble radius were not reached by the fiftieth pulsation. Protein-free lipid samples that were exposed to acid exhibited enhanced surface activity over similar non-treated samples. It is therefore concluded that low surface tension measurements recorded for acidic samples are secondary to a pH effect and do not reflect the surface activity at physiological conditions.

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.

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.

The intrinsic pKa values for phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine in monolayers deposited on mercury electrodes

The intrinsic pKa values of the phosphate groups of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) and of the phosphate and carboxyl groups of phosphatidylserine (PS) in self-organized monolayers deposited on a hanging mercury drop electrode were determined by a novel procedure based on measurements of the differential capacity C of this lipid-coated electrode. In view of the Gouy-Chapman theory, plots of 1/C at constant bulk pH and variable KCl concentration against the reciprocal of the calculated diffuse-layer capacity Cd,0 at zero charge exhibit slopes that decrease from an almost unit value to vanishingly low values as the absolute value of the charge density on the lipid increases from zero to approximately 2 microC cm-2. The intrinsic pKa values so determined are 0.5 for PE and 0.8 for PC. The plots of 1/C against 1/Cd,0 for pure PS exhibit slopes that pass from zero to a maximum value and then back to zero as pH is varied from 7.5 to 3, indicating that the charge density of the lipid film passes from slight negative to slight positive values over this pH range. An explanation for this anomalous behavior, which is ascribed to the phosphate group of PS, is provided. Interdispersion of PS and PC molecules in the film decreases the "formal" pKa value of the latter group by about three orders of magnitude.

Pulmonary surfactant proteins SP-B and SP-C in spread monolayers at the air-water interface: I. Monolayers of pulmonary surfactant protein SP-B and phospholipids

The effects of pulmonary surfactant protein SP-B on the properties of monolayers of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG), and a mixture of DPPC:DPPG (7:3, mol:mol) were studied using spread films at the air-water interface. The addition of SP-B to the phospholipid monolayers gave positive deviations from additivity of the mean areas in the films. At low protein concentrations (less than 45% amino acid residues which corresponds to 0.5 mol% or 10 weight% SP-B) monolayers of SP-B/DPPC, SP-B/DPPG and SP-B/(DPPC:DPPG) collapsed at surface pressures of about 70 mN.m-1, comparable to those of the lipids alone. At higher concentrations of SP-B in the protein-lipid monolayers, kink points appeared in the isotherms at about 40-45 mN.m-1, implying possible exclusion of material from the films, hence, changes in the original monolayer compositions. Calculated analyses of the monolayer compositions as a function of surface pressure indicated that nearly pure SP-B, associated with small amounts of phospholipid (2-3 lipid molecules per SP-B dimer), was lost from SP-B/DPPC, SP-B/DPPG, and SP-B/(DPPC:DPPG) films at surface pressures higher than 40-45 mN.m-1. The results are consistent with a low effectiveness of SP-B in removing saturated phospholipids, DPPC or DPPG, from the spread SP-B/phospholipid films.

Some problems of zeta potential determination in electrophoretic measurements on lipid membranes

Electrophoretic mobilities were measured for cardiolipin, phosphatidylinositol and phosphatidylserine liposomes in solutions of potassium chloride. The zeta potential as a function of ionic strength deviates significantly from the predictions of the double-layer theory in the 10(-3)-5.10(-2) M range. This might be due to changes in pH in the course of the experiment and/or to the inapplicability of the Smoluchowski equation at low ionic strengths. Taking into account the relaxation effects results in theoretical curves, calculated for various particle sizes at low ionic strength, which are in better agreement with the experiment. However, for quantitative comparison with experimental data, further investigations are necessary.

Interactions between charged, uncharged, and zwitterionic bilayers containing phosphatidylglycerol

Pressure vs. distance relationships have been obtained for phosphatidylglycerol bilayers, in both charged and uncharged states. Water was removed from the lipid multilayers by the application of osmotic pressures in the range of 0-2.7 x 10(9) dyn/cm2, and the distance between adjacent bilayers was obtained from Fourier analysis of lamellar x-ray diffraction data. For phosphatidylglycerol bilayers made electrically neutral either by lowering the pH or by adding equimolar concentrations of the positively charged lipid stearylamine, the pressure-distance data could be fit with a single exponential. The measured decay lengths were 1.1 A at low pH and 1.5 A with stearylamine, which are similar to decay lengths of the hydration pressure found for gel phases of other neutral bilayers. In addition, the magnitude of this repulsive pressure was proportional to the square of the Volta potential (equivalent to the dipole potential for electrically neutral bilayers) measured in monolayers in equilibrium with bilayers, in agreement with results previously found for the hydration pressure between phosphatidylcholine bilayers. For charged phosphatidylglycerol bilayers, the pressure-distance relation had two distinct regions. For bilayer separations greater than 10 A, the pressure-distance data had an exponential decay length (11 A) and a magnitude consistent with that expected for electrostatic repulsion from double-layer theory. For bilayer separations of 2-10 A, the pressure decayed much more rapidly with increasing bilayer separation (decay length less than 1 A). We interpret these data at low bilayer separations in terms of a combination of hydration repulsion and steric hindrance between the lipid head groups and the sodium ions trapped between apposing bilayers.

Surface characteristics of phosphatidylglycerol phosphate from the extreme halophile Halobacterium cutirubrum compared with those of its deoxy analogue, at the air/water interface

The relationship between area per molecule and surface pressure of monolayers of phosphatidylglycerol phosphate from extreme halophile Halobacterium cutrirubrum and its deoxy analogue, deoxyphosphatidylglycerol phosphate, spread at an air/water interface was examined. The effect of ionization of the primary and secondary acidic functions of the phosphate groups of the two lipids on surface characteristics of compression isotherms was determined by spreading monolayers on subphases with pH values ranging from below the apparent pKa of the primary ionization (pH 0) to greater than that of secondary ionization (pH 10.9). The limiting molecular area increases with decreasing pH below 2. Ionization of the primary phosphate functions of both phospholipids (with bulk pK1 values close to 4) is associated with a marked expansion of the films, as judged by values of limiting molecular area. Ionization of the secondary phosphate functions causes further expansion of the films, with the apparent pK2 of deoxyphosphatidylglycerol phosphate slightly less than that indicated for phosphatidylglycerol phosphate. Values of surface-compressibility modulus calculated from the surface characteristics of the phosphatidylglcerol phosphate monolayers showed that films spread on subphases with a pH of about the apparent pK1 of the primary phosphate functions were the least compressible. Increasing or decreasing subphase pH caused an increase in compressibility; this effect on compressibility was much less with monolayers of deoxyphosphatidylglycerol phosphate at high pH. The effect of inorganic counter-ions on monolayer characteristics of phosphatidylglycerol phosphate was examined by using subphases of NaCl concentrations varying from 0.01 to 1 M. The limiting molecular area was found to increase exponentially with respect to the subphase NaCl concentration.

Comparison of the effects of NaCl on the thermotropic behaviour of sn-1' and sn-3' stereoisomers of 1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol

The phase behaviour of liposomes of 1,2-dimyristoyl-sn-glycero-3-phosphatidyl-sn-1'-glycerol (1'-DMPG) and the corresponding sn-3' stereoisomer (3'-DMPG) were studied by DSC as a function of NaCl concentration. The melting of the metastable gel phase to the liquid-crystalline phase was similar for both lipids. However, in the presence of salt and at 6 degrees C (T less than Tp) the gel phase of both stereoisomers of DMPG was shown to be metastable and a new phase nominated here as the highly crystalline phase was formed as the stable state. However, significant differences in the formation and melting of the highly crystalline phase were evident between the two polar headgroup stereoisomers. For 3'-DMPG in the presence of 300 mM NaCl the melting enthalpy of this phase is approx. 82 kJ/mol and the transition temperature about 11 degrees higher (at 33.6 degrees C) than for the gel to liquid-crystalline phase transition (25 kJ/mol at 23.0 degrees C). In the presence of 0.15-1.2 M NaCl at 6 to 10 degrees C the formation of the highly crystalline phase of 3'-DMPG is complete within 2 to 5 days, increasing [NaCl] facilitates the rate. For a 1:1 mixture of 1'- and 3'-DMPG the formation of the highly crystalline phase requires several weeks and melts at about 20 degrees higher than the gel phase (at approx. 40 degrees C). For 1'-DMPG partial conversion into the highly crystalline phase requires several months. For 3'-DMPG several intermediate phases appeared as endothermic peaks between the main phase transition temperature and the melting temperature of the highly crystalline phase. In contrast, for 1'-DMPG and the 1:1 mixture the subgel phase appears to be the only metastable intermediate phase. Different monovalent cations differ in their effect on the metastable behaviour.

Adsorption of the cationic antitumoral drug celiptium to phosphatidylglycerol in membrane model systems. Effect on membrane electrical properties

The binding of the cationic antitumoral drug Celiptium to the anionic phospholipid phosphatidylglycerol was studied by measuring surface potentials and surface pressures in monolayers, and by determination of electrophoretic mobility on liposomes. Surface potential and zeta potential data were interpreted in terms of the Gouy-Chapman-Stern theory of the diffuse electrical double layer. A unique drug-to-lipid adsorption constant KaD, could not be calculated. KaD was observed to increase rapidly from 10(4) M-1 to 10(6) M-1 with an increase in drug concentration from 5 x 10(-7) M to 7 x 10(-6) M. This was accompanied by a marked decrease (in absolute value) in the corresponding electrophoretic mobilities which, from negative at low drug concentrations, became positive at drug concentrations of 10(-5) M and above. This indicates that the drug-to-lipid binding cannot be accounted for by a simple Langmuir adsorption isotherm, but corresponds to a more complex process, probably of a cooperative nature. Comparison of delta V and zeta potential data shows that adsorption of Celiptium to phosphatidylglycerol not only lowers the electrical surface potential, psi 0 (in absolute value) but also markedly reduces the polarization potential, delta Vp. These observations suggest that Celiptium destabilizes the electrical properties of cell plasma membranes.

Fourier transform infrared study of fully hydrated dimyristoylphosphatidylglycerol. Effects of Na+ on the sn-1' and sn-3' headgroup stereoisomers

Molecular packing and the thermotropic phase behavior of fully hydrated ammonium salts of 1,2-dimyristoyl-sn-glycero-3-phosphatidyl-sn-1'-glycerol (1'-DMPG) and the corresponding 3' stereoisomer (3'-DMPG) as well as the effects of 300 mM NaCl on these lipids were studied by Fourier transform infrared (FTIR) spectroscopy. The ammonium salts of both stereoisomer show similar thermotropic phase behavior and have an order-disorder phase transition at approximately 21 degrees C. While complexing with Na+, however, an incubation of liposomes at +6 degrees C for 3 days results in significant structural differences between liposomes of 1'-DMPG and 3'-DMPG. In the presence of 300 mM NaCl the infrared spectra for 3'-DMPG reveal the appearance of a more solidified lipid nominated here as the highly crystalline phase with a transition into the liquid-crystalline state at a significantly higher temperature (approximately at 33 degrees C) than that for 1'-DMPG (approximately at 23 degrees C). Crystal field splitting resulting from interchain vibrational coupling is observed in the CH2 scissoring mode of the 3'-DMPG(Na+) complex in the highly crystalline phase (T less than 33 degrees C); i.e., the acyl chains are packed in a rigid orthorhombic- or monoclinic-like crystal lattice. At temperatures above the transition at 33 degrees C the acyl chains of 3'-DMPG(Na+) give rise to infrared spectra indicative of hexagonal packing. The latter type of hydrocarbon chain packing is also found for the ammonium salts of 1'-DMPG and 3'-DMPG without Na+ as well as for 1'-DMPG with Na+. In addition, the binding of Na+ to 3'-DMPG causes narrowing of the bands associated with the interfacial and polar headgroup regions of 3'-DMPG and thus reveals reduced motional freedom. This demonstrates that Na+ binds tightly to 3'-DMPG, leading to the immobilization of the entire phospholipid polar headgroup. Such effects by Na+ are not observed for 1'-DMPG.

Use of a fluorescein derivative of phosphatidylethanolamine as a pH probe at water/lipid interfaces

A fluorescent indicator for the determination of pH in the vicinity of water/lipid interfaces was produced by the covalent linkage of fluoresceinisothiocyanate to Escherichia coli phosphatidylethanolamine. When embedded in monolayers spread on an air/water interface, its apparent pK was shown to be only slightly affected by the nature of the polar headgroups or the packing density of the host phospholipids. Its properties were not affected by the ion content of the subphase. For small unilamellar vesicles, its properties were only affected when localized in the inner layer. This probe could therefore be of value in the study of proton fluxes along biological membranes.

Self-sustained oscillations of electric potential in a model membrane

A model membrane constructed from a Millipore filter, whose pores are filled with dioleyl phosphate molecules, exhibits a self-oscillation of the electric potential with a period of about a few seconds in the presence of a salt-concentration difference, pressure difference and/or electric current across the filter. In this paper, the effects of chemicals such as KCl, CaCl2, pH and sucrose on the self-oscillation are investigated experimentally. These chemical substances are shown to alter the characteristic properties as the frequency of oscillation. Theoretical consideration of electrochemical interaction between these substances and DOPH molecules gives a fairly good explanation of the observed results.