The influence of charge on phosphatidic acid bilayer membranes

Interactions between the Cell Membrane Repair Protein S100A10 and Phospholipid Monolayers and Bilayers

Protein S100A10 participates in different cellular mechanisms and has different functions, especially at the membrane. Among those, it forms a ternary complex with annexin A2 and the C-terminal of AHNAK and then joins the dysferlin membrane repair complex. Together, they act as a platform enabling membrane repair. Both AHNAK and annexin A2 have been shown to have membrane binding properties. However, the membrane binding abilities of S100A10 are not clear. In this paper, we aimed to study the membrane binding of S100A10 in order to better understand its role in the cell membrane repair process. S100A10 was overexpressed by E. coli and purified by affinity chromatography. Using a Langmuir monolayer as a model membrane, the binding parameters and ellipsometric angles of the purified S100A10 were measured using surface tensiometry and ellipsometry, respectively. Phosphorus-31 solid-state nuclear magnetic resonance spectroscopy was also used to study the interaction of S100A10 with lipid bilayers. In the presence of a lipid monolayer, S100A10 preferentially interacts with unsaturated phospholipids. In addition, its behavior in the presence of a bilayer model suggests that S100A10 interacts more with the negatively charged polar head groups than the zwitterionic ones. This work offers new insights on the binding of S100A10 to different phospholipids and advances our understanding of the parameters influencing its membrane behavior.

AHNAK C-Terminal Peptide Membrane Binding-Interactions between the Residues 5654-5673 of AHNAK and Phospholipid Monolayers and Bilayers

The dysferlin membrane repair complex contains a small complex, S100A10-annexin A2, which initiates membrane repair by recruiting the protein AHNAK to the membrane, where it interacts via binding sites in the C-terminal region. However, no molecular data are available for the membrane binding of the various proteins involved in this complex. Therefore, the present study investigated the membrane binding of AHNAK to elucidate its role in the cell membrane repair process. A chemically synthesized peptide (pAHNAK), comprising the 20 amino acids in the C-terminal domain of AHNAK, was applied to Langmuir monolayer models, and the binding parameters and insertion angles were measured with surface tensiometry and ellipsometry. The interaction of pAHNAK with lipid bilayers was studied using ³¹P solid-state nuclear magnetic resonance. pAHNAK preferentially and strongly interacted with phospholipids that comprised negatively charged polar head groups with unsaturated lipids. This finding provides a better understanding of AHNAK membrane behavior and the parameters that influence its function in membrane repair.

Ionization constants pKa of cardiolipin

Cardiolipin is a phospholipid found in the inner mitochondrial membrane and in bacteria, and it is associated with many physiological functions. Cardiolipin has a dimeric structure consisting of two phosphatidyl residues connected by a glycerol bridge and four acyl chains, and therefore it can carry two negative charges. The pKa values of the phosphate groups have previously been reported to differ widely with pKa1 = 2.8 and pKa2 = 7.5-9.5. Still, there are several examples of experimental observations from cardiolipin-containing systems that do not fit with this dissociation behavior. Therefore, we have carried out pH-titration and titration calorimetric experiments on two synthetic cardiolipins, 1,1',2,2'-tetradecanoyl cardiolipin, CL (C14:0), and 1,1',2,2'-tetraoctadecenoyl cardiolipin, CL (C18:1). Our results show that both behave as strong dibasic acids with pKa1 about the same as the first pKa of phosphoric acid, 2.15, and pKa2 about one unit larger. The characterization of the acidic properties of cardiolipin is crucial for the understanding of the molecular organization in self-assembled systems that contain cardiolipin, and for their biological function.

Structure of ceramide-1-phosphate at the air-water solution interface in the absence and presence of Ca2+

Ceramide-1-phosphate, the phosphorylated form of ceramide, gained attention recently due to its diverse intracellular roles, in particular in inflammation mediated by cPLA(2)alpha. However, surprisingly little is known about the physical chemical properties of this lipid and its potential impact on physiological function. For example, the presence of Ca(2+) is indispensable for the interaction of Cer-1-P with the C2 domain of cPLA(2)alpha. We report on the structure and morphology of Cer-1-P in monomolecular layers at the air/water solution interface in the absence and presence of Ca(2+) using diverse biophysical techniques, including synchrotron x-ray reflectivity and grazing angle diffraction, to gain insight into the role and function of Cer-1-P in biomembranes. We show that relatively small changes in pH and the presence of monovalent cations dramatically affect the behavior of Cer-1-P. On pure water Cer-1-P forms a solid monolayer despite the negative charge of the phosphomonoester headgroup. In contrast, pH 7.2 buffer yields a considerably less solid-like monolayer, indicating that charge-charge repulsion becomes important at higher pH. Calcium was found to bind strongly to the headgroup of Cer-1-P even in the presence of a 100-fold larger Na(+) concentration. Analysis of the x-ray reflectivity data allowed us to estimate how much Ca(2+) is bound to the headgroup, approximately 0.5 Ca(2+) and approximately 1.0 Ca(2+) ions per Cer-1-P molecule for the water and buffer subphase respectively. These results can be qualitatively understood based on the molecular structure of Cer-1-P and the electrostatic/hydrogen-bond interactions of its phosphomonoester headgroup. Biological implications of our results are also discussed.

Solution pH alters mechanical and electrical properties of phosphatidylcholine membranes: relation between interfacial electrostatics, intramembrane potential, and bending elasticity

Solution pH affects numerous biological processes and some biological membranes are exposed to extreme pH environments. We utilized micropipette aspiration of giant unilamellar vesicles composed of 1-stearoyl-2-oleoyl-phosphatidylcholine to characterize the effect of solution pH (2-9) on membrane mechanical properties. The elastic area compressibility modulus was unaffected between pH 3 and 9 but was reduced by approximately 30% at pH 2. Fluorescence experiments utilizing the phase-sensitive probe Laurdan confirmed gel-phase characteristics at pH 2, explaining the reduction of membrane elasticity. The membrane bending stiffness, kc, increased by approximately 40% at pH 4 and pH 9 over the control value at pH 6.5. Electrophoretic mobility measurements indicate that these changes are qualitatively consistent with theoretical models that predict the effect of membrane surface charge density and Debye length on kc, substantiating a coupling between the mechanical and interfacial electrical properties of the membrane. The effect of pH on intramembrane electrical properties was examined by studying the spectral shifts of the potentiometric probe di-8 ANEPPS. The intramembrane (dipole) potential (Psid) increased linearly as the solution pH decreased in a manner consistent with the partitioning of hydroxide ions into the membrane. However, changes in Psid did not correlate with changes in kc. These mechanical and electrical studies lead to the conclusion that the effect of pH on membrane bending stiffness results from alterations in interfacial, as opposed to intramembrane, electrostatics.

Synthesis and characterization of a dianionic carbohydrate-based phospholipid

A novel carbohydrate-based phospholipid containing a phosphatidic acid head group, bis-(2,3-lauroyl)-1-methoxy-5-ribo-phosphatidic acid (DLRPA), was synthesized and characterized by 1H NMR, 13C NMR, and 31P NMR. This molecule is an analog of dilauroyl phosphatidic acid (DLPA). The T(m) of DLRPA decreases with increasing pH in a similar pattern to DLPA, as determined by MDSC. From this thermotropic behavior, the apparent pK(a)s of DLRPA are estimated to be 4 and 8.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

The lipid charge density at the bilayer surface modulates the effects of melittin on membranes

The influence of melittin on two DMPA membrane systems at pH 4.2 and 8.2 has been investigated by solid-state 31P and 2H NMR, as a function of temperature and peptide concentration. Melittin promotes greater morphological changes for both systems in the fluid phase, the effect being larger at pH 4.2. Close inspection of fatty acyl chain dynamics suggests that some parallels can be drawn between the DMPA/melittin at pH 8.2 and PC/melittin systems. In addition, at pH 8.2 a direct neutralization at the interface of one of the lipid negative charges by a positive charge of the peptide occurs, as can be monitored by 31P NMR at the molecular level. For the system at pH 4.2 and at high temperature, a lipid-to-peptide molar ratio of 30 is sufficient to transform the whole system into an isotropic phase, proposed to be inverted micelles. When the system is cooled down towards the gel phase one observes an intermediate hexagonal phase in a narrow range of temperature.

Nonideal mixing and phase separation in phosphatidylcholine-phosphatidic acid mixtures as a function of acyl chain length and pH

The miscibilities of phosphatidic acids (PAs) and phosphatidylcholines (PCs) with different chain lengths (n = 14, 16) at pH 4, pH 7, and pH 12 were examined by differential scanning calorimetry. Simulation of heat capacity curves was performed using a new approach that incorporates changes of cooperativity of the transition in addition to nonideal mixing in the gel and the liquid-crystalline phase as a function of composition. From the simulations of the heat capacity curves, first estimates for the nonideality parameters for nonideal mixing as a function of composition were obtained, and phase diagrams were constructed using temperatures for onset and end of melting, which were corrected for the broadening effect caused by a decrease in cooperativity. In all cases the composition dependence of the nonideality parameters indicated nonsymmetrical mixing behavior. The phase diagrams were therefore further refined by simulations of the coexistence curves using a four-parameter approximation to account for nonideal and nonsymmetrical mixing in the gel and the liquid-crystalline phase. The mixing behavior was studied at three different pH values to investigate how changes in headgroup charge of the PA influences the miscibility. The experiments showed that at pH 7, where the PA component is negatively charged, the nonideality parameters are in most cases negative, indicating that electrostatic effects favor a mixing of the two components. Partial protonation of the PA component at pH 4 leads to strong changes in miscibility; the nonideality parameters for the liquid-crystalline phase are now in most cases positive, indicating clustering of like molecules. The phase diagram for 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid:1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine mixtures at pH 4 indicates that a fluid-fluid immiscibility is likely. The results show that a decrease in ionization of PAs can induce large changes in mixing behavior. This occurs because of a reduction in electrostatic repulsion between PA headgroups and a concomitant increase in attractive hydrogen bonding interactions.

Electrostatics of lipid bilayer bending

The electrostatic contribution to spontaneous membrane curvature is calculated within Poisson-Boltzmann theory under a variety of assumptions and emphasizing parameters in the physiological range. Asymmetrical surface charges can be fixed with respect to bilayer midplane area or with respect to the lipid-water area, but induce curvatures of opposite signs. Unequal screening layers on the two sides of a vesicle (e.g., multivalent cationic proteins on one side and monovalent salt on the other) also induce bending. For reasonable parameters, tubules formed by electrostatically induced bending can have radii in the 50-100-nm range, often seen in many intracellular organelles. Thus membrane associated proteins may induce curvature and subsequent budding, without themselves being intrinsically curved. Furthermore, we derive the previously unexplored effects of respecting the strict conservation of charge within the interior of a vesicle. The electrostatic component of the bending modulus is small under most of our conditions and is left as an experimental parameter. The large parameter space of conditions is surveyed in an array of graphs.

The interfacial structure of phospholipid bilayers: differential scanning calorimetry and Fourier transform infrared spectroscopic studies of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine and its dialkyl and acyl-alkyl analogs

The thermotropic phase behavior of aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) and its 1,2-dialkyl, 1-acyl 2-alkyl and 1-alkyl 2-acyl analogs was examined by differential scanning calorimetry, and the organization of these molecules in those hydrated bilayers was studied by Fourier transform infrared spectroscopy. The calorimetric data indicate that substitution of either or both of the acyl chains of DPPC with the corresponding ether-linked hydrocarbon chain results in relatively small increases in the temperature (< 4 degrees C) and enthalpy (< 1 kcal/mol) of the lipid chain-melting phase transition. The spectroscopic data reveal that replacement of one or both of the ester-linked hydrocarbon chains of DPPC with its ether-linked analog causes structural changes in the bilayer assembly, which result in an increase in the polarity of the local environments of the phosphate headgroups and of the ester carbonyl groups at the bilayer polar/apolar interface. The latter observation is unexpected, given that ester linkages are considered to be intrinsically more polar that ether linkages. This finding cannot be satisfactorily rationalized unless the conformation of the glycerol backbones of the analogs containing ether-linked hydrocarbon chains differs significantly from that of diacyl glycerolipids such as DPPC. A comparison of the alpha-methylene scissoring bands and the methylene wagging band progressions of these lipids with the corresponding absorption bands of specifically chain-perdeuterated analogs of DPPC also supports the conclusion that replacement of the ester-linked hydrocarbon chains of DPPC with the corresponding ether-linked analog induces conformational changes in the lipid glycerol backbone. The suggestion that the conformation of glycerol backbones in the alkyl-acyl and dialkyl derivatives of DPPC differs from that of the naturally occurring 1,2-diacyl glycerolipid suggests that mono- and di-alkyl glycerolipids may not be good models of their diacyl analogs. These results, and previously published evidence that DPPC analogs with ether-linked hydrocarbon chains spontaneously form chain-interdigitated gel phases at low temperatures, clearly indicate that the properties of lipid bilayers can be substantially altered by small changes in the chemical structures of their polar/polar interfaces, and highlight the critical role of the interfacial region as a determinant of the structure and organization of lipid assemblies.

Critical dependence of the solubilization of lipid vesicles by the detergent CHAPS on the lipid composition. Functional reconstitution of the nicotinic acetylcholine receptor into preformed vesicles above the critical micellization concentration

The critical concentration of free detergent [Dw]c, which is necessary for lipid vesicle solubilization, is widely believed to be near to the critical micellization concentration (CMC) of the detergent. Here it is shown that [Dw]c and the critical detergent/lipid ratio Rc(M) in the mixed micelles strongly depend on the lipid composition. In agreement with the concept of packing constraints, phospholipids with a large head-group were solubilized by the detergent CHAPS at low CHAPS concentrations, for example [Dw]c = 0.27 mM for phosphatidyl-inositol and [Dw]c = 2.3 mM for phosphatidylcholine, T = 293 K (20 degrees C). In contrast, phospholipids (PL) with a small head-group required larger [CHAPS] values for (mixed) micelle formation: [Dw]c = 3.2 mM for phosphatic acid (PA) and [Dw]c = 5.2 mM for a mixture of palmitoyloleoylphosphatidylcholine (POPC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), POPC:PE:PG = 30:60:10 (mol.-%). Values obtained for the partition coefficient K = Rc(M)/[Dw]c ranged from K = 0.12 mM-1 for dimyristoylphosphatidylcholine to K = 1.1 mM-1 for (soy) phosphatidylcholine:phosphatidylglycerol (50:50). After addition of 30 mol.-% cholesterol (Ch) or with dipalmitoylphosphatidylcholine below T = 308 K (35 degrees C), [Dw]c is > 10 mM, which is far above the CMC = 4 mM. This indicates that CHAPS micelles are formed before vesicle solubilization begins. The analysis of vesicle solubilization was a prerequisite for the controlled reconstitution of protein membrane proteins. AChR proteins reconstituted into preformed PL/Ch-vesicles at [Dw] > CMC had a 4-5 time higher value of Li-influx compared to reconstitutions from completely solubilized lipid and proteins indicating a higher efficiency of right-side out AChR incorporation.

Effects of pH and cholesterol on DMPA membranes: a solid state 2H- and 31P-NMR study

The effect of pH and cholesterol on the dimyristoylphosphatidic acid (DMPA) model membrane system has been investigated by solid state 2H- and 31P-NMR. It has been shown that each of the three protonation states of the DMPA molecule corresponds to a 31P-NMR powder pattern with characteristic delta sigma values; this implies additionally that the proton exchange on the membrane surface is slow on the NMR time scale (millisecond range). Under these conditions, the 2H-labeled lipid chains sense only one magnetic environment, indicating that the three spectra detected by 31P-NMR are related to charge-dependent local dynamics or orientations of the phosphate headgroup or both. Chain ordering in the fluid phase is also found to depend weakly on the charge at the interface. In addition, it has also been found that the first pK of the DMPA membrane is modified by changes in the lipid lateral packing (gel or fluid phases or in the presence of cholesterol) in contrast to the second pK. The incorporation of 30 mol% cholesterol affects the phosphatidic acid bilayer in a way similar to what has been reported for phosphatidylcholine/cholesterol membranes, but to an extent comparable to 10-20 mol % sterol in phosphatidylcholines. However, the orientation and molecular order parameter of cholesterol in DMPA are similar to those found in dimyristoylphosphatidylcholine.

A comparative study of diffusive and osmotic water permeation across bilayers composed of phospholipids with different head groups and fatty acyl chains

Osmotic and diffusive water permeability coefficients Pf and Pd were measured for lipid vesicles of 100-250 nm diameter composed of a variety of phospholipids with different head groups and fatty acyl chains. Two different methods were applied: the H2O/D2O exchange technique for diffusive water flow, and the osmotic technique for water flux driven by an osmotic gradient. For phosphatidylcholines in the liquid-crystalline state at 70 degrees C, permeability constants Pd between 3.0 and 5.2.10(-4) cm/s and ratios Pf/Pd 7 and 23 were observed. The observation of a permeability maximum in the phase transition region and the fact that osmotically driven water flux is higher than diffusive water exchange suggest that water is diffusing through small transient pores arising from density fluctuations in the bilayers. The Pd values depend on the nature of the head group, on the chemical structure of the chains, and on the type of chain linkage. In the case of charged lipids, the ionic strength of the solution has a strong influence. For phosphatidylethanolamines, phosphatidic acids, and ether phosphatidylcholines, permeability constants Pd were considerably lower (2-4.10(-6) cm/s at 70 degrees C). For liquid-crystalline phosphatidylcholines, a strong reduction of Pd after addition of ethanol was observed (2-4.10(-6) cm/s at 70 degrees C). The experimental values are discussed in connection with different permeation models.

Influence of vesicle curvature on fluorescence relaxation kinetics of fluorophores

The effect of membrane curvature on the fluorescence decay of 2-p-toluidinyl-naphthalene-6-sulfonic acid (TNS), 2-(9-anthroyloxy) stearic acid (2-AS) and 12-(9-anthroyloxy)-stearic acid (12-AS) was investigated for egg lecithin vesicles of average diameter dm = 22 nm and 250 nm. The biexponential fluorescence decay of TNS at the red edge of the emission spectrum was analysed according to the model of Gonzalo and Montoro [1]. Over the entire temperature range (1-40 degrees C) the small TNS labelled vesicles showed significantly shorter solvent relaxation times tau(r) than their larger counterparts (e.g. 1.3 ns compared with 2.1 ns at 5 degrees C), indicating a higher mobility of the hydrated headgroups in the highly curved, small vesicles. The fluorescence decay of both AS derivatives is also biexponential. While the shorter decay times (1-3 ns) are practically identical for small and large vesicles, the longer decay times (5-14 ns) are identical only for 12-AS but not for 2-AS. This indicates that the microenvironment is similar in small and large vesicles deep in the membrane in spite of the differences in curvature.

Effect of phospholipid headgroup composition on the transfer of fluorescent long-chain free fatty acids between membranes

The transfer of long-chain anthroyloxy-labeled-free fatty acids (AOffa) between small unilamellar vesicles (SUV) was studied using a fluorescence energy transfer assay. Donor SUV were labeled with AOffa, and acceptor SUV contained the nonexchangeable quencher NBD-phosphatidylethanolamine. Donor and acceptor membranes were mixed using a stopped-flow apparatus, and intermembrane transfer was monitored by the decrease in AO fluorescence with time. The effect of donor membrane phospholipid headgroup composition on AOffa transfer was examined by incorporating phosphatidylethanolamine (PE), phosphatidic acid (PA), or phosphatidylglycerol (PG) into donor SUV otherwise composed of phosphatidylcholine (PC). Addition of 25 mol% of either of the negatively charged phospholipids (PA or PG) resulted in an increase in the rate of AOffa transfer, whereas addition of zwitterionic PE had no effect on transfer rate. The transfer kinetics were in all cases best described by a biexponential process, and it was found that the addition of PA caused an increase in the fraction of AOffa which transfer at the fast rate. This was likely due in large part to the asymmetric distribution of AOffa in these vesicles, with more fatty acid in the outer hemileaflet. This in turn may be due to the asymmetric distribution of PA between the inner and outer hemileaflets. Thus the increased AOffa transfer rate from negatively charged vesicles may be caused by charge repulsion between ffa and negatively charged headgroups. This increase in transfer rate was maximized at pH 9 as compared to pH 7, further suggesting that the increased rate of intermembrane transfer may arise because of charge repulsion. Finally, it was shown that decreasing the membrane surface potential by increasing the ionic strength caused the rate of AOffa transfer from PA-containing vesicles and PC vesicles to become identical. The results demonstrate that the ionic character of the donor membrane bilayer is an important determinant of the transfer rate of long-chain fatty acids between membranes.

Participation of Macrogolstearate 400 lamellar phases in hydrophilic creams and vesicles

In binary Macrogolstearate 400 (MS 400)/water systems, lamellar surfactant arrangements can be detected by polarized light and transmission electron microscopy. As demonstrated by X-ray diffraction and differential scanning calorimetry, the alkyl chains of the emulsifier are in the crystalline state. Ternary systems with liquid paraffin represent optically isotropic, homogeneous o/w creams for a wide composition range. Incorporation of up to 50 mol% cholesterol into the MS 400 lamellar structures leads to a gel-liquid crystalline phase separation within the bilayer, thus enabling the formation of spherical nonionic vesicles. The transition enthalpy of the samples decreases linearly with increasing cholesterol concentrations. The Macrogolstearate 400/cholesterol vesicles proved to be stable in hydrophilic cream systems. Cationic vesicles can be prepared using cetyltrimethylammonium bromide (CTAB) as a charge inducer. Low-CTAB portions are inhomogeneously distributed within the bilayer, as detected by DSC. The results also indicate a perturbation of the alkyl chains packing for the positively charged vesicles.

Monolayer characteristics and thermal behaviour of phosphatidic acids

The monolayer and thermal behaviour of different phosphatidic acids are presented. At neutral pH and 22 degrees C dilauroylphosphatidic acid and unsaturated phosphatidic acids form liquid-expanded monolayers, while dipalmitoyl- and distearoylphosphatidic acid form condensed monolayers. Dimyristoylphosphatidic acid undergoes a transition from the liquid-expanded to the condensed state. With long-chain saturated and unsaturated phosphatidic acids little change in molecular area is observed between pH 2 and 7. In contrast, the short chain saturated phosphatidic acids, dilauroyl- and dimyristoylphosphatidic acids, undergo a condensation in the pH range 2 to 7. This is so in spite of the fact that the phosphoric acid group dissociates and the phosphatidic acid molecule attains one negative charge over this pH range. This finding is interpreted to indicate that the electrostatic repulsion between negatively charged phosphatidic acid molecules is compensated for or even outweighed by other intermolecular forces. Hydrogen bonding at the lipid/water interface is supposed to play a major role. All phosphatidates studied exhibit a significant expansion in the pH range 7 to 12. The second apparent pK of the primary phosphate group of phosphatidic acids is 8.6 and the expansion observed in this pH range is therefore due to electrostatic repulsion. At neutral pH the ether analogues of saturated phosphatidic acids have monolayer properties similar to those of the ester compounds. Considering the total pH range of 2 to 12 studied the force-area curves of the ether analogues are more condensed compared to the ester compounds. Synthetic phosphatidates and their ether analogues give reversible sharp crystal(gel)-to-liquid crystal transitions while the naturally occurring egg phosphatidate gives a broad, asymmetric one. The transition temperature Tm of saturated phosphatidates increases with increasing hydrocarbon chain length and at a given chain length Tm decreases markedly with unsaturation. The Tm values of the ether analogues are about 10 degrees C higher and the delta H values are 10-15% lower than those of the corresponding esters.

Raman spectroscopic studies of dimyristoylphosphatidic acid and its interactions with ferricytochrome c in cationic binary and ternary lipid-protein complexes

The vibrational Raman spectra of both pure 1-alpha-dimyristoylphosphatidic acid (DMPA) liposomes and DMPA multilayers reconstituted with ferricytochrome c at pH 7 and pH 4, with either sodium or calcium as the cation, are reported as a function of temperature. Multilayers composed of a 1:1 mol ratio DMPA and dimyristoylphosphatidylcholine with perdeuterated acyl chains (DMPC-d54) have also been reconstituted with approximately 10(-4) M ferricytochrome c for Raman spectroscopic observation. Total integrated band intensities and relative peak height intensity ratios, two spectral Raman scattering parameters used to characterize bilayer properties, are sensitive to the presence of both ferricytochrome c and the cation in the reconstituted liposomes. Temperature profiles, derived from the various Raman intensity parameters for the 3,100-2,800 cm-1 lipid acyl chain C-H stretching mode region specifically reflect bilayer perturbations due to the interactions of ferricytochrome c. At pH 4 the calcium DMPA multilamellar gel to liquid crystalline phase transition temperatures Tm, defined by either the C-H stretching mode I2850/I2880 and I2935/I2880 peak height intensity ratios, are 58.5 +/- 0.5 degrees C and 60.0 +/- 0.3 degrees C, respectively. This difference in Tm's resolves the phase transition process into first an expansion of the lipid lattice and then a melting of the lipid acyl chains. At pH 7 the calcium DMPA liposomes show no distinct phase transition characteristics below 75 degrees C. For sodium DMPA liposomes reconstituted with ferricytochrome c at either pH 4.0 or pH 7.0, spontaneous Raman spectra show altered lipid structures at temperatures above 40 degrees C. Resonance Raman spectra indicate that ferricytochrome c reconstituted in either calcium or sodium DMPA liposomes changes irreversibly above Tm. For either the binary lipid or ternary lipid-protein systems reconstituted with DMPC-d54, linewidth parameters of the DMPC-d54 acyl chain CD2 symmetric stretching modes at 2,103 cm-1 provide a sensitive measure of the conformational and dynamic properties of the perdeuterated lipid component, while the 3,000 cm-1 C-H spectral region reflects the bilayer characteristics of the DMPA species in the complex. Although calcium clearly induces a lateral phase separation in the DMPA/DMPC-d54 system at pH 7.5 (Kouaouci, R., J.R. Silvius, I. Grah, and M. Pezolet. 1985. Biochemistry. 24:7132-7140), no distinct lateral segregation of the lipid components is observed in the mixed DMPA/DMPC-d54 lipid system in the presence of either ferricytochrome c or the sodium and calcium cations at pH 4.0. However, domain formation, consisting of regions rich in DMPA and DMPC-d54, respectively, is suggested for the calcium binary lipid mixture at pH 4.0 by the different values for Tm and AT characterizing the DMPA and DMPC-d54 species.Spectral evidence strongly suggests that ferricytochrome c also induces domain formation in the ternary lipid-protein mixtures at pH 7.0, but only for the sodium cation.

Effect of inorganic cations on phase transitions

The effect of protons and cations on the crystal (gel)-to-liquid crystal transition temperature Tm of isoelectric and negatively charged phospholipids are summarized. The general trends emerging are as follows: Tm depends on the state of ionization of the phospholipid in that Tm-vs-pH-curves parallel the titration curve of the phospholipid. Protonation of phospholipids causes Tm to increase, deprotonation or ionization has the opposite effect. The effects of cations on the Tm of phospholipids may be grouped into non-specific and specific effects. Unspecific effects of cations such as the screening of negative charges of the phospholipid polar group are qualitatively similar to protonation: Tm increases, in the order monovalent less than divalent less than trivalent cations and the effects on negatively charged phospholipids are larger than those on isoelectric phospholipids. Unspecific, electrostatic effects on Tm are reasonably well accounted for by the Gouy-Chapman theory. If, however, specific binding comes into play and/or electrostatic effects are accompanied by changes in phospholipid structure, simple, electrostatic theories fail to explain the observed changes in Tm. The crystal (gel)-to-liquid crystal transition is also a function of the degree of hydration: Tm generally decreases with increasing hydration reaching a plateau in excess H2O. In addition to screening of electric charges, ions may exert yet another non-specific effect: ions may affect Tm indirectly by competing with the phospholipid polar group for water of hydration. This indirect effect plays a role at high ionic strength and/or at low hydration of the phospholipid. Specific binding of cations to negatively charged phospholipids can lead to tight associations of the metal ion with the lipid polar group. Isothermal crystallization of the phospholipid bilayer is induced that is accompanied by a total or partial loss of water of hydration resulting in a marked increase in Tm. For instance, in crystalline Ca2(+)-phosphatidylserine complexes Tm is increased by more than 100 degrees C.

Phase transitions of polymerizable phospholipids

Polymerizable lipids have received considerable attention in the last ten years as polymerization of lipids in vesicle systems is a possibility to increase the stability of lipid bilayers. Lipids with various polymerizable groups have been synthesized in the last years. This paper is focussed on those lipids which are closely related to natural phospholipids, i.e. molecules which have two hydrophobic chains and a head group containing a phosphate moiety. The phase behaviour of polymerizable phospholipids as lipid monomers and in the polymerized state is reviewed and discussed.

Isothermal lipid phase transitions

In liotropic lipid systems phase transitions can be induced isothermally by changing the solvent concentration or composition; alternatively, lipid composition can be modified by (bio)chemical means. The probability for isothermal phase transitions increases with the decreasing transition entropy; it is proportional to the magnitude of the transition temperature shift caused by transformation-inducing system variation. Manipulations causing large thermodynamic effects, such as lipid (de)hydration, binding of protons or divalent ions and macromolecular adsorption, but also close bilayer approach are, therefore, likely to cause structural lipid change(s) at a constant temperature. Net lipid charges enhance the membrane susceptibility to salt-induced isothermal phase transitions; a large proportion of this effect is due to the bilayer dehydration, however, rather than being a consequence of the decreased Coulombic electrostatic interactions. Membrane propensity for isothermal phase transitions, consequently, always increases with the hydrophilicity of the lipid heads, as well as with the desaturation and shortening of the lipid chains. Upon a phase change at a constant temperature, some of the interfacially bound solutes (e.g. protons or calcium) are released in the solution. Membrane permeability and fusogenicity simultaneously increase. In mixed systems, isothermal phase transitions, moreover, may result in lateral phase separation. All this opens up ways for the involvement of isothermal phase transitions in the regulation of biological processes.

Studies on peroxidation processes of model membranes: role of pyrophosphate

Phosphate-containing compounds of both a lipophilic (dipalmitoylphosphatidic acid, lysophosphatidic acid and dicetylphosphate) and hydrophilic nature (orthophosphate, 3-phosphoglyceric acid, 2,3-diphosphoglyceric acid and pyrophosphate in particular) have been found to inhibit to varying degrees the lipoperoxidation of liposomal arachidonic acid. Lipophilic compounds seem to act exclusively at the lipid component level of the membrane, giving rise to polyanionic complexes with free arachidonic acid or its radical derivatives that could bind the Fe2+ strongly (thereby inhibiting the iron pro-oxidation activity) or to minimize the lateral mobility of the lipid radicals (thereby reducing the propagation of lipid peroxidation). The high antiperoxidative power of hydrophilic compounds, and in particular of pyrophosphate, must, on the contrary, be primarily attributed to their ability to form very stable complexes with the Fe present in the solution surrounding the liposomal membranes. The possible contribution of these physiological compounds to the in vivo defense mechanism against radical-induced damage is discussed.

Membrane electrostatics

In conclusion, charged membrane together with their adjacent electrolyte solution form a thermodynamic and physico-chemical entity. Their surfaces represent an exceptionally complicated interfacial system owing to intrinsic membrane complexity, as well as to the polarity and often large thickness of the interfacial region. Despite this, charged membranes can be described reasonably accurately within the framework of available theoretical models, provided that the latter are chosen on the basis of suitable criteria, which are briefly discussed in Section A. Interion correlations are likely to be important for the regular and/or rigid, thin membrane-solution interfaces. Lateral distribution of the structural membrane charge is seldom and charge distribution perpendicular to the membranes is nearly always electrostatically important. So is the interfacial hydration, which to a large extent determines the properties of the innermost part of the interfacial region, with a thickness of 2-3 nm. Fine structure of the ion double-layer and the interfacial smearing of the structural membrane charge decrease whilst the surface hydration increases the calculated value of the electrostatic membrane potential relative to the result of common Gouy-Chapman approximation. In some cases these effects partly cancel-out; simple electrostatic models are then fairly accurate. Notwithstanding this, it is at present difficult to draw detailed molecular conclusions from a large part of the published data, mainly owing to the lack of really stringent controls or calibrations. Ion binding to the membrane surface is a complicated process which involves charge-charge as well as charge-solvent interactions. Its efficiency normally increases with the ion valency and with the membrane charge density, but it is also strongly dependent on the physico-chemical and thermodynamic state of the membrane. Except in the case of the stereospecific ion binding to a membrane, the relatively easily accessible phosphate and carboxylic groups on lipids and integral membrane proteins are the main cation binding sites. Anions bind preferentially to the amine groups, even on zwitterionic molecules. Membrane structure is apt to change upon ion binding but not always in the same direction: membranes with bound ions can either expand or become more condensed, depending on the final hydrophilicity (polarity) of the membrane surface. The more polar membranes, as a rule, are less tightly packed and more fluid. Diffusive ion flow across a membrane depends on the transmembrane potential and concentration gradients, but also on the coulombic and hydration potentials at the membrane surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclopentanoid analogs of dipalmitoyl phosphatidic acid: effect of backbone geometry on thermotropic properties

Seven geometrical or positional isomers of dipalmitoyl cyclopentanophosphoric acid (DPCPA) have been synthesized and studied: 1,3/2-1P (I); 1,2/3-1P (II); 1,2/3-3P (III), 1,2,3/0-1P (IV); 1,2,3/0-2P (V); 1,3/2-2P (VI); 1,2/3-2P (VII). When dispersed in 0.1 M Tris-HCl at pH 7.4, I-VII gave thermal transitions (Tc) of 60.0 degrees, 59.0 degrees, 56.8 degrees, 55.3 degrees, 38.3 degrees, 36.8 degrees and 34.0 degrees C, respectively, as measured by differential scanning calorimetry (DSC). When the lipids were dispersed at pH 9.5 in 0.1 M borate, Tc of I-IV decreased, whereas Tc of V-VII increased. In contrast, at pH 1.5 in 0.1 M HCl/KCl, Tc of I-IV decreased slightly, but Tc of V-VII rose markedly. To determine the effect of head group geometry and substitution pattern on acyl chain motion, EPR spectra of 1-palmitoyl, 2-[16-doxylstearoyl]-glycero-3-phosphoric acid in bilayers of DPCPA isomers were acquired. Abrupt spectral changes occurred at temperatures closely correlating with transition temperatures observed by DSC. These results have led to the conclusions that: (i) isomers I-IV containing vicinal acyl chains form bilayers that exhibit structural transitions at temperatures higher than those at which transitions are exhibited by isomers V-VII which have a polar phosphate group interposed between the two chains; (ii) the effects of differences in backbone structure are transmitted down the entire length of the acyl chains; (iii) the orientation of the cyclopentane ring in the isomers I-IV is significantly different from that in isomers V-VII at pH values where the phosphate group is doubly negatively charged.

Phosphatidic acid affects structural organization of phosphatidylcholine liposomes. A study of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) fluorescence decay using distributional analysis

The fluorescence decay of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) was used to study micro-heterogeneity of 1,2-dimyristoyl-3-sn-phosphatidylcholine (DMPC) liposomes and to characterize the effect of phosphatidic acid on the correlation between fluorescence microheterogeneity and membrane permeability. The fluorescence decay, measured using multifrequency phase fluorometry, has been analyzed either by using a model of discrete exponential components or a model of continuous distribution of lifetime values. Both analyses have shown that TMA-DPH decay is characterized by two components: a long one of about 9 ns and a short one of about 5 ns. In the gel phase, at variance with previous DPH studies, the short component was associated with a large fractional intensity. The distributional analysis showed changes of lifetime values and width in correspondence to the calorimetric transitions. The presence of egg phosphatidic acid increased both long lifetime values and distributional width. The use of TMA-DPH as a probe to evaluate membrane heterogeneity using the distributional width is discussed. The effect of phosphatidic acid on the membrane surface and in the hydrophobic core has been related to its structural properties and to its role in water penetration.

Influence of alkali metal cations on the transport of calcium by phosphatidate in multi-phase systems

The effects of alkali metal cations on the rates at which Ca2+ and phosphatidic acid were cotransported from aqueous to hydrocarbon medium were examined. The alkali metal cations remained in the aqueous phase yet specifically influenced the transport of Ca2+ into the hydrocarbon solvent. For the physiological cations, Na+ and K+, there were critical concentration ranges in which small changes in concentration effected sharp changes in transport rates. The maximal rate observed with Na+ was an order of magnitude greater than that with K+; however, unlike Na+, K+ promoted low levels of transport below the critical concentration range. Li+ effected only low levels of transport even at high concentrations, whereas Rb+ and Cs+ induced transport at rates proportional to their concentrations. These results are discussed in terms of a classical ionophore model for the complex composed of a neutral phosphatidic acid dimer bridged by Ca2+.

Studies on peroxidation processes of model membranes and synaptosomes: role of phosphatidic acid

Dipalmitoylphosphatidic acid (DPPA) was found to exert a strong inhibitory effect on Fe-induced peroxidation of arachidonic acid inserted into liposomal dipalmitoylphosphatidylcholine (DPPC) vesicles. This inhibition was quite effective both below and above the phase transition temperature of the liposomes. Moreover, we demonstrated the antiperoxidative activity of phosphatidic acid (PA) in synaptosomal membranes. PA enriched synaptosomes were prepared by the stimulation of the endogenous phospholipase D activity or by the incubation of the synaptosomes with Streptomyces chromofuscus phospholipase D. The possible contribution of PA to the in vivo defense mechanism against free radical-induced damage is discussed.

Relationship between isoelectric point of native and chemically modified insulin and liposomal fusion

Native insulin causes fusion of negatively charged liposomes in the pH range from 3.0 to 5.5. In marked contrast, insulin with all three amino groups succinylated did not show fusion ability at any pH. On the other hand, insulin amidated with glycine methyl ester with all six carboxyl groups blocked shifted its activity to higher pH, showing a pH range of activity from 3.0 to 7.4. When the carboxyl groups were recovered by hydrolysis of methoxyl groups from glycine methyl ester-treated insulin, the protein obtained (glycyl-insulin with six free carboxyl groups) behaved as native insulin. A good correlation between the isoelectric point values of insulin and its derivatives and their fusion properties was found.