Dimyristoylphosphatidylserine Research Articles

Lipid lateral heterogeneity in phosphatidylcholine/phosphatidylserine/diacylglycerol vesicles and its influence on protein kinase C activation

To test the hypothesis that the activation of protein kinase C (PKC) is influenced by lateral heterogeneities of the components of the lipid bilayer, the thermotropic phase behavior of dimyristoylphosphatidylcholine (DMPC)/dimyristoylphosphatidylserine (DMPS)/dioleoylglycerol (DO) vesicles was compared with the activation of PKC by this system. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to monitor the main transition (i.e., the gel-to-fluid phase transition) as a function of mole fraction DO (chi(DO)) in DMPC/DO, DMPS/DO, and [DMPC/DMPS (1:1, mol/mol)]/DO multilamellar vesicles (MLVs). In each case, when chi(DO) < or approximately 0.3, DO significantly broadened the main transition and shifted it to lower temperatures; but when chi(DO) > approximately 0.3, the main transition became highly cooperative, i.e., narrow, again. The coexistence of overlapping narrow and broad transitions was clearly evident in DSC thermograms from chi(DO) approximately 0.1 to chi(DO) approximately 0.3, with the more cooperative transition growing at the expense of the broader one as chi(DO) increased. FTIR spectroscopy, using analogs of DMPC and DMPS with perdeuterated acyl chains, showed that the melting profiles of all three lipid components in [DMPC/DMPS (1:1, mol/mol)]/DO MLVs virtually overlay when chi(DO) = 0.33, suggesting that a new type of phase, with a phospholipid/DO mole ratio near 2:1, is formed in this system. Collectively, the results are consistent with the coexistence of DO-poor and DO-rich domains throughout the compositions chi(DO) approximately 0.1 to chi(DO) approximately 0.3, even at temperatures above the main transition. Comparison of the phase behavior of the binary mixtures with that of the ternary mixtures suggests that DMPS/DO interactions may be more favorable than DMPC/DO interactions in the ternary system, especially in the gel state. PKC activity was measured using [DMPC/DMPS (1:1, mol/mol)]/DO MLVs as the lipid activator. At 35 degrees C (a temperature above the main transition of the lipids), PKC activity increased gradually with increasing chi(DO) from chi(DO) approximately 0.1 to chi(DO) approximately 0.4, and activity remained high at higher DO contents. In contrast, at 2 degrees C (a temperature below the main transition), PKC activity exhibited a maximum between chi(DO) approximately 0.1 and chi(DO) approximately 0.3, and at higher DO contents activity was essentially constant at 20-25% of the activity at the maximum. We infer from these results that the formation of DO-rich domains is related to PKC activation, and when the lipid is in the gel state, the coexistence of DO-poor and DO-rich phases also contributes to PKC activation.

The interaction of α-tocopherol with phosphatidylserine vesicles and calcium

The interaction of α-tocopherol with dimyristoylphosphatidylserine (DMPS) has been studied in the presence and in the absence of Ca 2+ by using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and 45Ca 2+-binding. In the absence of Ca 2+, DSC showed that α-tocopherol decreases the temperature of the lamellar gel to lamellar liquid crystalline phase transition as well as it decreases ΔH of this transition. Two different peaks were detected at 10 mol% of α-tocopherol and probably one of the peaks correspond to pure or nearly pure DMPS and the other to DMPS incorporating most of the α-tocopherol. The phase transition was totally abolished at 30 mol% of α-tocopherol. In the presence of Ca 2+ this L β to L α phase transition of DMPS was even more perturbed by α-tocopherol, so that it was totally abolished by only 7 mol% of α-tocopherol, at Ca 2+ concentrations which were clearly non-saturating, like those giving DMPS/Ca 2+ molar ratio of 4:1 and 10:1. Furthermore, the transition of the DMPS/Ca 2+ complex observed at 91.6°C was perturbed by the presence of α-tocopherol, indicating a change in the structure of the crystalline complex. The FT-IR analysis of the effect of α-tocopherol on DPMS phase transition confirmed the decrease in the phase transition temperature of the phospholipid, and also that α-tocopherol increases the number of gauche isomers in the gel state but has no effect in the liquid crystalline state. The binding of 45Ca 2+ was also affected by the presence of α-tocopherol, so that the number of binding sites was decreased, and this may be interesting for situations in which phosphatidylserine and Ca 2+ are simultaneously implicated in biological functions, such as membrane fusion and enzyme activation.

Kinetics of Beryllium-Induced Aggregation of Acidic Phospholipid Vesicles

Stopped-flow kinetic measurements were carried out for the aggregation of acidic phospholipid vesicles induced by the addition of BeSO4solution. The phospholipid species examined were dimyristoylphosphatidic acid (DMPA), dimyristoylphosphatidylglycerol (DMPG), and dimyristoylphosphatidylserine (DMPS). For all of these phospholipid vesicles, BeSO4induced a rapid aggregation at extremely low concentration compared with other alkaline-earth metal salts. By increasing the solution pH progressively, the aggregation-inducing effect of BeSO4was promoted increasingly, and the profile of the initial velocity of the aggregation with respect to the salt concentration approached to that observed for the aggregation induced by La3+ion. This result demonstrates that the high activity of BeSO4to induce the aggregation of acidic phospholipid vesicles is attributed to the formation of trivalent complex cation, [Be3(OH)3]3+, due to the hydrolysis of Be2+in an aqueous solution.

Pressure tuning infrared spectroscopic study of cisplatin-induced structural changes in a phosphatidylserine model membrane.

The dynamic effect of cis-diamminedichloroplatinum(II) (DPP) and its aquated metabolite (DDP-OH) on a dimyristoylphosphatidylserine (DMPS) model membrane was investigated by pressure tuning vibrational spectroscopy. The native species (DDP-Cl) and the aquated species (DPP-OH) were both observed to bind to the carboxylate group of the serine as evidenced by a frequency shift of 1622-1620 cm-1. However, only DDP-OH was observed to bind to the phosphate group (PO(-)2). The binding of either drug to DMPS resulted in an increased pressure required to halt the reorientational fluctuations of the acyl chains, indicating that the distance between the chains were increased. The two drugs did not partition into the matrix of the hydrophobic section in the model membrane. Collectively, these data suggest that DDP-Cl and DDP-OH are capable of binding to the polar head group of DMPS, resulting in an enlargement of the area of the head and a subsequent increase in the intermolecular distance between the acyl chains.

Phospholipid Vesicles as Organized Media in Spectroluminescence Analysis

The membrane properties of pure dimyristoyl phosphatidylcholine (DMPC), dimyristoyl phosphatidylserine (DMPS), and dimyristoyl phosphatidylglycerol (DMPG) vesicles were compared using the fluorescence of the probe pyrene in an assessment of the potential utility of phospholipids as organized media in spectroluminescence analysis. The partition coefficients, excimer stability constants, vibronic band ratios, triethylamine quenching constants, and fluorescence lifetime distribution widths of pyrene were used as measures of the solute capacity, solute aggregation promotion, polarity, sequestering efficiency, and heterogeneity of the three media. The results indicate that the head group hydrophobicity and membrane fluidity required to reduce the incidence of excimer formation and to promote solute sequestering in the most hydrophobic part of the aggregate precludes efficient sequestering from cosolubilized quenchers. This implies that significant improvements in dynamic range would be expected in spectroluminescence analyses utilizing DMPG, but improvements of detection limits would be moderate compared to those observed in bile salts, cyclodextrins, or surfactant aggregates.

Kinetics of Divalent Cation-Induced and pH-Induced Aggregation of Dimyristoylphosphatidylserine Vesicles

Kinetic study was carried out on the aggregation of dimyristoylphosphatidylserine (DMPS) vesicles induced by the addition of divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) as well as by pH jump, using a stopped-flow method with turbidity detection. With the increase in cation concentration, the initial rate of aggregation, V0, increased rapidly above the threshold concentration of about 1 mM, and reached the maximum velocity, V0,max . The increase in V0 toward V0,max with the concentration was appreciably dependent on the cation species, whereas the value of V0,max was not affected significantly by the cation species. This behavior of aggregation kinetics of DMPS vesicles is well described in terms of the intervesicular interaction potential composed of van der Waals attraction and electrostatic repulsion, without introducing an extra force such as hydration force which is regarded as responsible for the stability of phosphatidylcholine vesicles. The aggregation of DMPS vesicles was also induced when the negative charge on the DMPS head group was eliminated by protonating the serine carboxyl group by mixing the vesicle preparation with acid solution. This observation is in accordance with the above interpretation that no hydration force acts between DMPS vesicles to such an extent that the aggregation kinetics are affected appreciably.

Head‐group conformations in lipid bilayer membranes

AbstractThe self‐assembly of lipid molecules into lamellar liquid‐crystalline bilayer membranes was studied using a statistical thermodynamic method elaborating a Scheutjens‐Fleer‐like approach. In the modelling much attention was paid to the two‐tail character and the complex head‐group structure of the lipids that comprise the bilayer.Zwitterionic head groups like phosphatidylcholine (PC) are, on average, oriented nearly flat onto the interface. At not too low ionic strengths, we find that the PC head group has two preferred conformations. Ionic head groups like phosphatidylserine (PS) are tilted towards the solution. These two predictions are consistent with experimental data.Both co‐ and counter‐ions penetrate the head‐group region, compensating for part of the head‐group charges. Usually the counter‐ions adsorb preferentially. However, at high salt concentrations, no positive adsorption of ions is found in dimyristoylphosphatidylserine (DMPS) bilayers. Due to the lateral pressure in the head‐group area, all ions are depleted. Since negative ions are more depleted than positive ones, charge compensation is still achieved.In order to investigate how the membrane structure can be modulated, we modified the lipids by adding a hydrophilic chain onto the end of the PC head group. There are two effects. First, the increase of the bulkiness of the head group reduces the number of lipids per unit area in the membrane so that the size of the hydrophobic core diminishes. Second, we predict that the orientation of the PC part of the head group is altered non‐monotonically. Increasing the electrostatic screening promotes the out‐of‐plane tilting of the head groups toward entropically more favourable positions.

Interaction of glycophorin with lipid bilayer studied by calorimetry, densitometry, static light scattering, and electron microscopy.

In the first part of the present work the interaction of glycophorin with dimyristoylphosphatidylcholine (DMPC) is studied by freeze fracture electron microscopy, densitometry, calorimetry, and 90 degree static light scattering. An exothermic lipid/protein interaction energy of WP = 190 kJ.mol-1 was found by application of the well known Van Laar relation for the displacement of the freezing point and the Gibbs-Duhem relationship. Secondly, the effects of Ca2+ on the lipid/protein interaction were studied. Following Ca2+ addition a remarkable decoupling of the interaction of the glycophorin head group with the bilayer surface was revealed by densitometry and gold-labeling electron microscopy. It is estimated that about 80% of lipid once disturbed by the adsorption of glycophorin head groups is decoupled after addition of Ca2+. Thirdly, the selective interaction of glycophorin with binary lipid mixtures was studied, including the mixtures of DMPC with dimyristoylphosphatidylserine (DMPS) and dilauroylphosphatidylcholine (DLPC), and the mixture of dipalmitoylphosphatidylcholine (DPPC) with DLPC.

Fourier transform infrared spectroscopy of aqueous dispersions of phosphatidylserine-cholesterol mixtures

The effect of cholesterol on vibrational spectra in the non polar and in the polar region of dimyristoyl phosphatidylserine (DMPS) and of phosphatidylserine from bovine spinal cord (PS) has been investigated. The small shifts in the methylene CH stretching frequencies after taking into account the contribution of the cholesterol spectrum were interpreted as a combined effect of cholesterol on the conformation of the chains and of the lesser contributions of the cholesterol methyl groups. Cholesterol also influences the ratio of the trans (1465 cm-1) to the lower wavelength (1457 cm-1) CH2 bending bands. No significant direct effect of cholesterol on the vibration of the polar residues was discerned. The small shift of the carboxylate band observed below the phase transition is probably due to the change in the intermolecular zwitterions when the average distance between the neighboring polar groups increases due to incorporation of cholesterol molecules.

7Li NMR and DSC investigations of lithium phospholipid interactions

The effect of lithium on several phospholipids was investigated by 7 Li nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The interaction is very small in the case of dipalmitoylphosphatidylcholine (DPPC) and mixed dipalmitoylphosphatidylglycerol (DPPG)/DPPC vesicles. The main phase transition properties of DPPC were not changed by the addition of lithium ions; the lithium NMR spectrum however reflected the phase structure of the lipid. 7 Li NMR was found to be a sensitive new method for probing structural changes of the interfacial water layer in phospholipid mesophases. In bilayers containing both DPPC and DPPG (4:1), lithium redued the transition temperature and enthalpy change by only a small amount (0.8°C and 0.7 kcal/mol). Both concentrated DPPC and DPPC/DPPG dispersions showed equal quadrupolar splittings of 4.7 kHz in the 7 Li NMR spectrum at room temperature, indicating that direct binding is not enhanced by the addition of anionic DPPG. The phase behavior of dimyristoylphosphatidylserine (DMPS) in the presence of lithium depended critically on hydration conditions and lithium concentration. A reproducible endotherm at 40.6°C (>Δ H = 7.1kcal/mol) could be obtained by hydrating the lipid in 0.05 M LiCl at 90°C. Hydration at 55–60°C produced a presumably less-hydrated lipid species which melts at 81°C (Δ H = 17.1kcal/mol). X-ray diffraction of DMPS at 20°C in either 0.05 or 0.5 M LiCl showed a sharp line at 4.2A˚, characteristic of the untilted L β phase.

Structural investigation of the covalent and electrostatic binding of yeast cytochrome c to the surface of various ultrathin lipid multilayers using x-ray diffraction

X-Ray diffraction was used to characterize the profile structures of ultrathin lipid multilayers having a bound surface layer of cytochrome c. The lipid multilayers were formed on an alkylated glass surface, using the Langmuir-Blodgett method. The ultrathin lipid multilayers of this study were: five monolayers of arachidic acid, four monolayers of arachidic acid with a surface monolayer of dimyristoyl phosphatidylserine, and four monolayers of arachidic acid acid with a surface monolayer of thioethyl stearate. Both the phosphatidylserine and the thioethyl stearate surfaces were found previously to covalently bind yeast cytochrome c, while the arachidic acid surface electrostatically binds yeast cytochrome c. Meridional x-ray diffraction data were collected from these lipid multilayer films with and without a bound yeast cytochrome c surface layer. A box refinement technique, previously shown to be effective in deriving the profile structures of ultrathin multilayer lipid films with and without electrostatically bound cytochrome c, was used to determine the multilayer electron density profiles. The surface monolayer of bound cytochrome c was readily apparent upon comparison of the multilayer electron density profiles for the various pairs of ultrathin multilayer films plus/minus cytochrome c for all cases. In addition, cytochrome c binding to the multilayer surface significantly perturbs the underlying lipid monolayers.

Human platelet P-235, a talin-like actin binding protein, binds selectively to mixed lipid bilayers

The interaction of platelet talin (P-235) with mixtures of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylserine (DMPS) as well as with pure lipids was studied in reconstituted lipid bilayers. Incorporation of platelet talin into vesicles was achieved by self-assembly during cycles of freeze-thawing of co-dispersions containing vesicles and the purified protein. The yield of protein incorporation as a function of lipid composition was determined by measuring the protein/lipid ratio using protein assay, phosphate determination and gel electrophoresis in parallel. Protein-lipid interactions are monitored by high sensitive differential scanning calorimetry (DSC) measuring (i) the shifts of transition states Δ T s * and Δ T 1 *, where T s represents the solidus line, the onset of lipid chain melting, and T 1 the liquidus line, the endpoint of chain melting, and (ii) the heats of transition. Cytoplasmic talin differs from a membrane bound form by its ability and mode of lipid interaction. The latter partially penetrates into the hydrophobic region of the bilayer, which renders a low incorporation rate even into neutral lipids. This interaction is greatly enhanced in the presence of charged lipids: a marked shift of T 1 occurs due to a selective electrostatic interaction of the protein with the membrane surface. Evidence for a selective binding is also provided by Fourier transform infrared spectroscopy (FTIR). Right-side-out oriented platelet talin can be cleaved by proteinases, which truncate the extrinsic electrostatic binding domain but not the hydrophobic. In addition, reconstituted platelet talin, like in vivo, can be cleaved by thrombin. The interaction of cytoplasmic platelet talin with lipid bilayers is purely electrostatic. Our data suggest that protein reconstitution by freeze-thawing is an equilibrium process and that the protein distribution between the membrane and water is determined by the Nernst distribution law. Consequently, the work of protein transfer from water into the bilayer can be measured as a function of charged lipids.

Evaluation of membrane phase behavior as a tool to detect extrinsic protein-induced domain formation: binding of prothrombin to phosphatidylserine/phosphatidylcholine vesicles

The temperature-composition phase diagram of mixed dimyristoylphosphatidylserine (DMPS) and dimyristoylphosphatidylcholine (DMPC) small unilamellar vesicles was determined in the presence and absence of bound bovine prothrombin by monitoring the phospholipid order-disorder phase separation using diphenylhexatriene (DPH) fluorescence anisotropy. The shape of the membrane temperature-composition diagram was essentially unaltered by the binding of prothrombin in the presence of Ca2+ although the two-phase (gel/fluid) region was slightly narrowed and shifted by 1-10 degrees C to higher temperatures. This result does not support the popular idea that extensive domains rich in negatively charged phospholipid are induced in response to prothrombin binding. Instead of implying domain formation, our results demonstrate that the observed increase in melting temperature associated with binding of prothrombin to acidic phospholipid membranes can be accounted for by the observed altered membrane order both in the fluid and in the solid lamellar phases. The membrane order in the liquid-crystalline phase increased with increased acidic lipid content, and much more so for DMPS than for dipentadecanoylphosphatidylglycerol (DC15PG). These results demonstrate that simple shifts in membrane phase behavior cannot be properly interpreted to prove the existence of charged lipid domains. In addition, we report the unexpected observation that prothrombin increased the anisotropy of DPH in DMPS/DMPC vesicles in the liquid-crystalline phase in the absence of Ca2+ as well as in its presence. This effect was seen to a lesser extent and only at a much higher charged-lipid content for DC15PG/DMPC vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibodies to liposomal phosphatidylserine and phosphatidic acid

Polyclonal antisera to phosphatidylserine or phosphatidic acid were induced in rabbits by injecting liposomes containing phosphatidylserine or phosphatidic acid and lipid A. Adsorption of antisera with liposomes containing different phospholipids revealed that some degree of reactivity with one or more phospholipids other than the immunizing phospholipid was often observed. However, cross-reactivity with other phospholipids was not a universal phenomenon, and one antiserum to phosphatidylserine failed to cross-react (i.e., was not adsorbed) with liposomes containing other phospholipids. All of the antisera were inhibited by soluble phosphorylated haptens (e.g., phosphocholine but not choline), but one of the antisera to phosphatidylserine was inhibited both by phosphoserine and by serine alone. Liposomal membrane composition influenced the activity of antiserum to phosphatidylserine. Regardless of whether unsaturated (beef brain) or saturated (dimyristoyl) phosphatidylserine was used in the immunizing liposomes, the antisera reacted more vigorously with liposomes containing unsaturated than saturated phosphatidylserine. We conclude that liposomes containing lipid A can serve as vehicles for stimulating polyclonal antisera to phosphatidylserine and phosphatidic acid. Although cross-reactivity with certain other phospholipids can be observed, sera from selected animals apparently can exhibit a high degree of specific activity to the immunizing phospholipid antigen.

Fluorescence Studies on the Phase Dependence of Interactions between a Tripeptide, Lys–Trp–Lys, and Dimyristoylphosphatidylserine

Abstract Interactions of a basic tripeptide, lysyl-tryptophyl-lysine (LTL) with dimyristoylphosphatidylserine (DMPS) were investigated by fluorescence spectroscopy. An interacting LTL–DMPS complex assumes two distinctly different conformations in the lipid bilayer depending on the thermotropic phase of lipid. At temperatures below the gel–liquid crystalline phase-transition temperature (Tc), LTL binds to the polar head groups of DMPS through electrostatic force, and the tryptophan residue is located on the surface of the membrane because of the rigidity of the head groups. Above Tc, it assumes a conformation such that the tryptophan residue slips deeply into the DMPS bilayer. The temperature dependence of the fluorescence quenching by acrylamide supported the penetration of the tryptophan upon the phase transition of DMPS. A fluorescence decay analysis based on the photophysical deactivation mechanism of tryptophan residue has revealed a peculiar interaction of LTL with DMPS in the bilayer.

Effect of phosphatidylserine on the shape of McLeod red cell acanthocytes

The rare McLeod blood group phenotype is characterized by weak Kell antigens, lack of the common Kx antigen, and acanthocytic morphology. Previous studies that did not detect membrane or cytoskeletal protein abnormalities suggested a lipid disturbance. In normal red cells, dimyristoyl phosphatidylserine (DMPS) is transported across the membrane by an enzymatic process and accumulates in the inner leaflet of the membrane bilayer causing discocyte to stomatocyte shape changes. Scanning electron microscopy of McLeod red cells shows a mixture comprised of 15% discocytes, 51% with irregular surfaces, and 34% acanthocytes. On incubation with various concentrations of DMPS at 37 degrees C for periods up to two hours, McLeod red cells transported DMPS across the membrane and caused irregularly shaped and acanthocytic McLeod red cells to attain normal discocyte shape and later to become stomatocytes. Chlorpromazine, which at 0 degrees C preferentially partitions into the inner monolayer of the membrane, had a similar effect on the shape of McLeod red cells. This suggests that in McLeod cells acanthocytosis is due to a lack of lipid in the inner leaflet of the membrane bilayer but that the imbalance is not caused by defective transport of phosphatidylserine across the membrane.

Effect of Phosphatidylserine on the Shape of McLeod Red Cell Acanthocytes

The rare McLeod blood group phenotype is characterized by weak Kell antigens, lack of the common Kx antigen, and acanthocytic morphology. Previous studies that did not detect membrane or cytoskeletal protein abnormalities suggested a lipid disturbance. In normal red cells, dimyristoyl phosphatidylserine (DMPS) is transported across the membrane by an enzymatic process and accumulates in the inner leaflet of the membrane bilayer causing discocyte to stomatocyte shape changes. Scanning electron microscopy of McLeod red cells shows a mixture comprised of 15% discocytes, 51% with irregular surfaces, and 34% acanthocytes. On incubation with various concentrations of DMPS at 37 degrees C for periods up to two hours, McLeod red cells transported DMPS across the membrane and caused irregularly shaped and acanthocytic McLeod red cells to attain normal discocyte shape and later to become stomatocytes. Chlorpromazine, which at 0 degrees C preferentially partitions into the inner monolayer of the membrane, had a similar effect on the shape of McLeod red cells. This suggests that in McLeod cells acanthocytosis is due to a lack of lipid in the inner leaflet of the membrane bilayer but that the imbalance is not caused by defective transport of phosphatidylserine across the membrane.

Conformational changes of phospholipid headgroups induced by a cationic integral membrane peptide as seen by deuterium magnetic resonance

Deuterium nuclear magnetic resonance (2H NMR) was used to study the interaction of a cationic amphiphilic peptide with pure DMPC membranes and with mixed bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS). The choline and serine headgroups were selectively deuteriated at the alpha and beta positions. The amphiphilic peptide, with 20 leucine residues in the hydrophobic core and two cationic hydrophilic lysine residues at each end, spanned the lipid bilayer. Although 2H NMR experiments using DMPC with perdeuteriated fatty acyl chains showed that the average order parameter of the hydrophobic region was not significantly modified by the incorporation of the amphiphilic peptide, for either DMPC or DMPC/DMPS (5:1) bilayers, large perturbations of the quadrupolar splittings of the choline and serine headgroups were observed. The results obtained with the DMPC headgroup suggest that the incorporation of the cationic peptide in both DMPC and DMPC/DMPS (5:1) bilayers leads to a structural perturbation directly related to the net charge on the membrane surface. The magnitude of the observed effect seems to be similar to those observed previously with other cationic molecules [Seelig, J., MacDonald, P.M., & Scherer, P.G. (1987) Biochemistry 26, 7535-7541]. Two of the three quadrupolar splittings of the PS headgroup exhibited large variations in the presence of the amphiphilic peptide, while the third one remained unchanged. Our data have led us to propose a model describing the influence of membrane surface charges on headgroup conformation. In this model, the surface charge is represented as a uniform charge distribution. The electric field due to the charges produces a torque which rotates the polar headgroups.(ABSTRACT TRUNCATED AT 250 WORDS)

2H and 31P NMR study of pentalysine interaction with headgroup deuterated phosphatidylcholine and phosphatidylserine.

The interaction of cationic pentalysine with phospholipid membranes was studied by using phosphorus and deuterium Nuclear Magnetic Resonance (NMR) of headgroup deuterated dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS). In the absence of pentalysine, some of the deuterium and phosphorus spectra of DMPC/DMPS 5:1 (m:m) membranes gave lineshapes similar to those of partially-oriented bilayers with the planes of the bilayers being parallel to the magnetic field. The deuterium NMR data show that the quadrupolar splittings of the deuterated methylenes of the DMPC headgroup are not affected by adsorption of pentalysine on the PC/PS membranes. By contrast, the pentalysine produces significant changes in the quadrupolar splittings of the negatively charged DMPS headgroup. The results are discussed in relation to previous 2H NMR investigations of phospholipid headgroup perturbations arising from bilayer interaction with cationic molecules.

A calorimetric examination of the effect of myotoxin a on the thermotropic phase behavior of model lipid membranes

The effect of myotoxin a on the thermotropic phase behavior of aqueous dispersions of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS) was examined using differential scanning calorimetry (DSC). Myotoxin a significantly altered the normal phase behvior of DMPC in a concentration dependent fashion. This effect is perturbed by Ca e+ and is sensitive to ionic strength and pH. High concentrations of toxin eliminate the characteristic pretransition associated with the polar head group of DMPC. They also increase the temperature of the main gel-to-liquid crystal transition from 23°C to 32–235°C. At low concentrations of toxin, the first visible effect is upon the pretransition which is split into two components that diminish with time. The main transition is less affected at low toxin concentrations, although the magnitude of the transition is reduced while it is simultaneously shifted to higher temperatures. The main transition is also split into multiple components. The toxin also had pH specific effects on the phase behavior of DMPS. Above physiological pH (8.5) the normal transition of DMPS at 36–38°C was split in the presence of myotoxin a and new components appeared centered at 31°C and 35°C. These observations are consistent with reports that the skeletal muscle membrane system is the major site of the myonecrotic effect of myotoxin a.