Interactions of saturated diacylglycerols with phosphatidylcholine bilayers: A 2H NMR study.

The interactions of a series of saturated diacylglycerols (DAGs) with fatty acid side chain lengths of 6-14 carbons with multilamellar phospholipid bilayers consisting either of dipalmitoylphosphatidylcholine (DPPC) or of a mixture of DPPC and bovine liver phosphatidylcholine (BL-PC) extracts were studied by 2H NMR spectrometry. We found that the perturbation induced by the DAGs into the bilayer structure strongly depends on the length of the DAG fatty acid side chain. Shorter chain 1,2-sn-dihexanoylglycerol and, to a larger degree, 1,2-sn-dioctanoylglycerol (diC8) induce transverse perturbation of the bilayer structure: the order parameters of the phospholipid side chains are increased by the intercalating DAG molecules in the region adjacent to the phospholipid headgroups and decreased toward the terminal methyls, corresponding to the bilayer interior. The longer chain DAGs (C greater than or equal to 12) studied in this and previous [De Boeck & Zidovetzki (1989) Biochemistry 28, 7439] work induce lateral phase separation of the lipids into DAG-enriched gellike domains and relatively DAG-free regions in the liquid-crystalline phase. Each of the DAGs studied induces a decrease in the area per phospholipid molecule, and a corresponding increase in the lateral surface pressure of the bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of cholesterol on the structure and dynamic properties of unsaturated phospholipid bilayers

Molecular dynamics (MD) computer simulations of five different hydrated unsaturated phosphatidylcholine lipid bilayers built up by 18:0/18:1(n-9)cis PC, 18:0/18:2(n-6)cis PC, 18:0/18:3(n-3)cis PC, 18:0/20:4(n-6)cis PC, and 18:0/22:6(n-3)cis PC molecules with 40 mol% cholesterol, and the same five pure phosphatidylcholine bilayers have been performed at 303 K. The simulation box of a lipid bilayer contained 96 phosphatidylcholines, 64 cholesterols, and 3840 water molecules (48 phosphatidylcholine molecules and 32 cholesterols per layer and 24 water molecules per phospholipid or cholesterol in each case). The lateral self-diffusion coefficients of the lipids in these systems and mass density profiles with respect to the bilayer normal have been analyzed. It has been found that the lateral diffusion coefficients of phosphatidylcholine molecules increase with increasing number of double bonds in one of the lipid chains, both in pure bilayers and in bilayers with cholesterol. It has been found as well that the lateral diffusion coefficient of phosphatidylcholine molecules of a lipid bilayer with 40 mol% cholesterol is smaller than that for the corresponding pure phosphatidylcholine bilayer.     

NMR study of the interaction of retinoids with phospholipid bilayers

The interaction of three vitamin A derivatives or retinoids: all-trans-retinoic acid, 13-cis-retinoic acid and retinol with multilamellar phospholipid bilayers was studied using a combination of 2H- and 31P-NMR measurements. The following model membrane systems were used: (1) dipalmitoylphosphatidylcholine (DPPC) bilayers; (2) bilayers composed of a mixture of DPPC and bovine heart phosphatidylcholine (PC); (3) mixed PC/phosphatidylethanolamine (PE) bilayers. Only a weak interaction was observed between 13-cis-retinoic acid and DPPC membranes. Addition of all-trans-retinoic acid at a molar ratio of 1:2 to the lipid causes a small decrease (5 C degrees) in the gel to liquid crystalline phase-transition temperature of DPPC, a small increase in the order parameters of the lipid side-chains of single component bilayers and no measurable effect in the other lipid systems studied. Considerably larger perturbation in the lipid bilayer structure is introduced by addition of retinol which, at a molar ratio of 1:2 to the lipid, lowered the gel to liquid crystalline phase-transition temperature of DPPC by 21 C degrees and caused a decrease of order parameters of the lipid side-chains in all three lipid bilayer systems. These effects are consistent with intercalation of retinol molecules into the bilayer interior. The results for the mixed PC/PE bilayers indicate that the presence of retinol caused lateral separation of PE- and retinol-enriched regions.     

Cholesterol modifies the short-range repulsive interactions between phosphatidylcholine membranes

Pressure versus distance relationships have been obtained for egg phosphatidylcholine bilayers containing a range of cholesterol concentrations. Water was removed from between adjacent bilayers by the application of osmotic pressures in the range of 0.4-2600 atm (4 x 10(5)-2.6 x 10(9) dyn/cm2), and the distance between adjacent bilayers was obtained by Fourier analysis of X-ray diffraction data. For applied pressures up to about 50 atm and bilayer surface separations of 15-5 A, the incorporation of up to equimolar cholesterol has little influence on plots of pressure versus bilayer separation. However, for the higher applied pressures, cholesterol reduces the interbilayer separation distance by an amount that depends on the cholesterol concentration in the bilayer. For example, the incorporation of equimolar cholesterol reduces the distance between bilayers by as much as 6 A at an applied pressure of 2600 atm. At this applied pressure, electron density profiles show that the high-density head-group peaks from apposing bilayers have merged. This indicates that equimolar concentrations of cholesterol spread the lipid molecules apart in the plane of the bilayer enough to allow the phosphatidylcholine head groups from apposing bilayers to interpenetrate as the bilayers are squeezed together. All of these X-ray and pressure-distance data indicate that, by reducing the volume fraction of phospholipid head groups, cholesterol markedly reduces the steric repulsion between apposing bilayers but has a much smaller effect on the sum of the longer ranged repulsive hydration and fluctuation pressures. Increasing concentrations of cholesterol monotonically increase the dipole potential of egg phosphatidylcholine monolayers, from 415 mV with no cholesterol to 493 mV with equimolar cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of a carane derivative local anesthetic on a phospholipid bilayer studied by molecular dynamics simulation

Molecular dynamics (MD) simulations of two hydrated palmitoyloleoylphosphatidylcholine (POPC) bilayers each containing eight carane derivative (KP-23) local anesthetic (LA) molecules in neutral (POPC-LA) or protonated (POPC-LAH) forms were carried out to investigate the effect of KP-23 and its protonation on the bilayer. 3-ns trajectories were used for analyses. A pure POPC bilayer was employed as a reference system. In both POPC-LA and POPC-LAH systems a few KP-23 molecules intercalated into the bilayer and moved near the bilayer/water interface. They were located on the hydrophobic core side of the interface in the POPC-LA bilayer, but on the water phase side in the POPC-LAH bilayer. The order of the POPC chains was higher in the POPC-LA bilayer than in the pure POPC bilayer and was lower in the POPC-LAH bilayer. Interactions between polar groups of KP-23 and POPC or water were responsible for a lower hydration of POPC headgroups in POPC bilayers containing KP-23 than in the pure POPC bilayer. KP-23 molecules were found to form aggregates both in POPC-LA and POPC-LAH bilayers. Due to higher amphiphilicity of LAH, the LAH aggregate was more micelle-like and larger than the LA one. The results demonstrate the rapid timescales of the initial processes that take place at and near the bilayer interface as well as details of the atomic level interactions between local anesthetic and the lipid matrix of a cell membrane.     

Properties of unsaturated phospholipid bilayers: Effect of cholesterol

Properties of hydrated unsaturated phosphatidylcholine (PC) lipid bilayers containing 40 mol % cholesterol and of pure PC bilayers have been studied. Various methods were applied, including molecular dynamics simulations, self-consistent field calculations, and the pulsed field gradient nuclear magnetic resonance technique. Lipid bilayers were composed of 18:0/18:1(n-9)cis PC, 18:0/18:2(n-6)cis PC, 18:0/18:3(n-3)cis PC, 18:0/20:4(n-6)cis PC, and 18:0/22:6(n-3)cis PC molecules. Lateral self-diffusion coefficients of the lipids in all these bilayers, mass density distributions of atoms and atom groups with respect to the bilayer normal, the C-H and C-C bond order parameter profiles of each phospholipid hydrocarbon chain with respect to the bilayer normal were calculated. It was shown that the lateral self-diffusion coefficient of PC molecules of the lipid bilayer containing 40 mol % cholesterol is smaller than that for a corresponding pure PC bilayer; the diffusion coefficients increase with increasing the degree of unsaturation of one of the PC chains in bilayers of both types (i.e., in pure bilayers or in bilayers with cholesterol). The presence of cholesterol in a bilayer promoted the extension of saturated and polyunsaturated lipid chains. The condensing effect of cholesterol on the order parameters was more pronounced for the double C=C bonds of polyunsaturated chains than for single C-C bonds of saturated chains.     

The effect of dimethyl sulphoxide on the structure and phase behaviour of palmitoleoylphosphatidylethanolamine

The thermotropic phase behaviour and structure of a nonbilayer-forming lipid, 1-palmitoyl-2-oleoyl-phosphatidylethanolamine, dispersed in water and in aqueous solutions of up to 50 wt% dimethyl sulphoxide (DMSO) have been characterised using synchrotron X-ray diffraction methods. It was found that the presence of DMSO in the solvent induced an increase in the temperature of lamellar-gel to lamellar-liquid-crystal phase transition and a decrease in the temperature of the lamellar-liquid-crystal to inverted-hexagonal phase transition of the phospholipid. The presence of DMSO also caused a decrease in the X-ray repeat spacings of all the phases studied. Electron density profiles of the phospholipid dispersed in water and 50 wt% DMSO in the bilayer gel state were calculated. The presence of 50 wt% DMSO caused the apparent disappearance of the solvent layer separating phospholipid bilayers in the gel state. The results suggest that DMSO contributes to the bilayer electron density profile and that the amphiphilic solvent molecules partition into the interfacial region.     

Origin of membrane dipole potential: contribution of the phospholipid fatty acid chains

The large intrinsic membrane dipole potential, phi(d), is important for protein insertion and functioning as well as for ion transport across natural and model membranes. However, the origin of phi(d) is controversial. From experiments carried out with lipid monolayers, a significant dependence on the fatty acid chain length is suggested, whereas in experiments with lipid bilayers, the contribution of additional -CH(2)-groups seems negligibly small compared with that of the phospholipid carbonyl groups and lipid-bound water molecules. To compare the impact of the -CH(2)-groups of dipalmitoylphosphatidylcholine (DPPC) near and far from the glycerol backbone, we have varied the structure of DPPC by incorporation of sulfur atoms in place of methylene groups in different positions of the fatty acid chain. The phi(d) of symmetric lipid bilayers containing one heteroatom was obtained from the charge relaxation of oppositely charged hydrophobic ions. We have found that the substitution for a S-atom of a -CH(2)-group decreases phi(d). The effect (deltaphi(d) = -22.6 mV) is most pronounced for S-atoms near the lipid head group while a S-atom substitution in the C(13)- or C(14)-position of the hydrocarbon chain does not effect the bilayer dipole potential. Most probably deltaphi(d) does not originate from an altered dipole potential of the acyl chain containing an heteroatom but is mediated by the disruption of chain packing, leading to a decreased density of lipid dipoles in the membrane.     

Different oxidized phospholipid molecules unequally affect bilayer packing

The aim of this study was to gain more detailed knowledge about the effect of the presence of defined oxidized phospholipid molecules in phospholipid bilayers. After chromatographic and mass spectrometry analysis, the previously used product of the Fenton reaction with unsaturated lecithins proved to consist of a plethora of oxidatively modified lecithins, unuseful either for the detailed study of the effects brought about in the bilayer or as the source of defined oxidized phospholipid molecules. The latter, particularly 2-(ω-carboxyacyl)- and 2-(n-hydroperoxyacyl)-lecithins, can be more conveniently prepared by chemical or enzymatic synthesis rather than by chemical or physical oxidation. The effect of those molecules and of commercially available 12-hydroxy-stearic and dodecanedioic acid was studied in planar supported phospholipid bilayers (SPBs) by use of EPR spectrometry. The SPBs also contained 2-(5-doxylstearoyl)-lecithin as the spin probe, and the EPR spectral anisotropy loss, indicative of bilayer disordering, was measured as a function of the molar percentage of oxidized lipid. Most oxidized lipid molecules examined in this study were able to induce bilayer disordering, while hydroperoxyl group-bearing acyl chains appeared to be much less effective. It is concluded that the effects of different oxidized phospholipids on phospholipid bilayer structure cannot be generalized, as happens with batch-oxidized phospholipids, and that the use of defined oxidized phospholipid molecular species for membrane oxidative stress guarantees a more reliable and detailed response.     

Different oxidized phospholipid molecules unequally affect bilayer packing

The aim of this study was to gain more detailed knowledge about the effect of the presence of defined oxidized phospholipid molecules in phospholipid bilayers. After chromatographic and mass spectrometry analysis, the previously used product of the Fenton reaction with unsaturated lecithins proved to consist of a plethora of oxidatively modified lecithins, useless either for the detailed study of the effects brought about in the bilayer or as the source of defined oxidized phospholipid molecules. The latter, particularly 2-(omega-carboxyacyl)- and 2-(n-hydroperoxyacyl)-lecithins, can be more conveniently prepared by chemical or enzymatic synthesis rather than by chemical or physical oxidation. The effect of those molecules and of commercially available 12-hydroxy-stearic and dodecanedioic acid was studied in planar supported phospholipid bilayers (SPBs) by use of EPR spectrometry. The SPBs also contained 2-(5-doxylstearoyl)-lecithin as the spin probe, and the EPR spectral anisotropy loss, indicative of bilayer disordering, was measured as a function of the molar percentage of oxidized lipid. Most oxidized lipid molecules examined in this study were able to induce bilayer disordering, while hydroperoxyl group-bearing acyl chains appeared to be much less effective. It is concluded that the effects of different oxidized phospholipids on phospholipid bilayer structure cannot be generalized, as happens with batch-oxidized phospholipids, and that the use of defined oxidized phospholipid molecular species for membrane oxidative stress guarantees a more reliable and detailed response.     

Depolarization of dehydroergosterol in phospholipid bilayers

The behavior in phospholipid bilayers of low concentrations of dehydroergosterol, a fluorescent cholesterol mimic, has been examined by fluorometry and calorimetry. In contrast to many fluorescent membrane probes, dehydroergosterol shows a decrease in fluorescence anisotropy when the matrix phospholipid goes from the liquid-crystalline to the gel state. This was observed in three systems in which the matrix lipid was either dipalmitoyl- or dimyristoylphosphatidylcholine or dilauroylphosphatidylethanolamine. The decrease in anisotropy is the result of a large increase in the fluorescence life time of dehydroergosterol in these bilayer systems which is probably the result of thermal quenching of dehydroergosterol by neighboring molecules. The rotation of dehydroergosterol in these bilayers can be described in terms of the thermal coefficient of frictional resistance offered by the environment (Weber et al. (1984) Biochemistry 23, 6785-6788). The thermal coefficients are observed to change abruptly at the onset and completion temperatures of the gel to liquid-crystalline phase transition temperatures of the three matrix phospholipids. These changes are, however, much smaller than are the corresponding changes in the thermal coefficient observed for the fluorescent probe diphenylhexatriene in dilauroylphosphatidylethanolamine bilayers. The difference in behavior of the two fluorescent probes may be the result of lateral phase separation of dehydroergosterol similar to that reported for cholesterol in similar systems.     

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayer was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12–16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.     

Study of the interaction of an alpha-helical transmembrane peptide with phosphatidylcholine bilayer membranes by means of densimetry and ultrasound velocimetry

We applied precise densimetry and ultrasound velocimetry methods to study the interaction of a synthetic alpha-helical transmembrane peptide, acetyl-K(2)-L(24)-K(2)-amide (L(24)), with model bilayer lipid membranes. The large unilamellar vesicles (LUVs) utilized were composed of a homologous series of n-saturated diacylphosphatidylcholines (PCs). PCs whose hydrocarbon chains contained from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us to analyze how the difference between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer influences the thermodynamical and mechanical properties of the membranes. We showed that the incorporation of L(24) decreases the temperature and cooperativity of the main phase transition of all LUVs studied. The presence of L(24) in the bilayer also caused an increase of the specific volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide concentration (mole ratio L(24):PC=1:50). The overall volume compressibility of the peptide-containing lipid bilayers in the liquid-crystalline state was in general higher in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs and large multilamellar vesicles (MLVs) with and without L(24). As expected, a higher cooperativity of the changes of the thermodynamical and mechanical parameters took place for MLVs in comparison with LUVs. These results are in agreement with previously reported DSC and (2)H NMR spectroscopy study of the interaction of the L(24) and structurally related peptides with phosphatidylcholine bilayers. An apparent discrepancy between (2)H NMR spectroscopy and compressibility data in the liquid crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L(24) suggest that around each peptide a distorted region exists that involves at least 2 layers of lipid molecules.     

Binding of macrophages and phospholipid flip-flop in supported lipid bilayers

Subclass-specific antibody-dependent interactions (binding and triggering) between macrophages and supported lipid bilayers have been studied. Percentages of mouse macrophage binding (J774 cell line) to the lipid bilayers were dependent on mouse monoclonal IgG subclasses. The efficiencies were as follows: IgG1 = IgG2a greater than IgG2b greater than IgG3. Furthermore, macrophage triggering (spreading) was more efficient on IgG2a- or IgG1-coated lipid bilayers than on IgG2a, IgG3, or non-specific rabbit IgG. The present experiments show also that phospholipid molecules are able to flip-flop from one side of a supported planar bilayer membrane to the other with a half-life of 10 h-1 day at 25 degrees C.     

A spin label study of perturbation effects of N-(1-methyldodecyl)-N, N, N-trimethylammonium bromide and N-(1-methyldodecyl)-N, N-dimethylamine oxide on model membranes prepared from Escherichia coli-isolated lipids

Interaction of bactericidal surfactants N-(1-methyldodecyl)-N, N, N-trimethylammonium bromide (2-ATDBr) and N-(1-methyldodecyl)-N, N-dimethylamine oxide (2-ATDNO) with phospholipid membranes prepared from Escherichia coli -- isolated lipids was studied by ESR spectroscopy using m-doxyl stearic acid (m-DSA, m = 5, 12, 16) and N-cetyl-N, N-dimethyl-N-tempoylammonium bromide spin labels located in different membrane depths. 2-ATDBr was found to be a more potent membrane perturbant than 2-ATDNO both at equal membrane and sample concentrations; this is in compliance with the respective antimicrobial activities of these agents. Using the statistical model of hydrocarbon chains in lipid bilayers, the probabilities of the formation of gauche conformations and the effective energy differences between the trans and gauche conformations were calculated from m-DSA order parameters for two different bilayer regions. Based on these parameters, a molecular model of the location of surfactant molecules in bilayer has been formulated. It has been suggested that at low concentrations the surfactant molecules are located in structural defects between lipid clusters in the bilayer. After filling up these defects, the surfactant molecules penetrate into the clusters between lipid molecules, expand the bilayer laterally and increase the amount of gauche conformations in the hydrocarbon chains in the hydrophobic core of the bilayer.     

Structure in the polar head region of phospholipid bilayers: A 31P [1H] nuclear Overhauser effect study.

The structure of the head-group region of some phospholipid bilayers in vesicle form has been studied and an intermolecular association of the N-methyl protons of phosphatidylcholine (PC) with the phosphate of phosphatidylethanolamine (PE) in mixed vesicles has been identified. Observation of a 31P[1H] nuclear Overhauser effect (NOE) in the phosphorus nuclear magnetic resonances of both PC and PE in mixed vesicles demonstrates an intimate dipolar interaction between some protons and the phosphorus nuclei. Substitution of deuterium for the N-methyl protons of PC eliminated the majority of the effect and necessitated the construction of a model of the bilayer surface in which the N-methyl protons of PC could interact closely with the phosphates of neighboring PE molecules. The predominant orientation of the head group must then be parallel to the bilayer surface. The amino protons of PE do not contribute significantly to the observed NOE. A corollary of these results is that there is little if any tendency for either PC or PE in the mixed vesicles to segregate into separate domains. A decrease in NOE in sphingomyelin vesicles on going from H2O to D2O suggests that an exchangeable proton contributes to the NOE. In addition the low value of the NOE observed in D2O suggests that the head-group conformation of sphingomyelin differs from that of PC.     

Modulation of the interbilayer hydration pressure by the addition of dipoles at the hydrocarbon/water interface.

The effects of the cholesterol analog 5 alpha-cholestan-3 beta-ol-6-one (6-ketocholestanol) on bilayer structure, bilayer cohesive properties, and interbilayer repulsive pressures have been studied by a combination of x-ray diffraction, pipette aspiration, and dipole potential experiments. It is found that 6-ketocholestanol, which has a similar structure to cholesterol except with a keto moiety at the 6 position of the B ring, has quite different effects than cholesterol on bilayer organization and cohesive properties. Unlike cholesterol, 6-ketocholestanol does not appreciably modify the thickness of liquid-crystalline egg phosphatidylcholine (EPC) bilayers, and causes a much smaller increase in bilayer compressibility modulus than does cholesterol. These data imply that 6-ketocholestanol has both its hydroxyl and keto moieties situated near the water-hydrocarbon interface, thus making its orientation in the bilayer different from cholesterol's. The addition of equimolar 6-ketocholestanol into EPC bilayers increases the magnitude, but not the decay length, of the exponentially decaying repulsive hydration pressure between adjacent bilayers. Incorporation of equimolar 6-ketocholestanol into EPC monolayers increases the dipole potential by approximately 300 mV. These data are consistent with our previous observation that the magnitude of the hydration pressure is proportional to the square of the dipole potential. These results mean that 6-ketocholestanol, despite its location in the bilayer hydrocarbon region, approximately 10 A from the physical edge of the bilayer, modifies the organization of interlamellar water. We argue that the incorporation of 6-ketocholestanol into EPC bilayers increases the hydration pressure, at least in part, by increasing the electric field strength in the polar head group region.     

Supported planar bilayer formation by vesicle fusion: the interaction of phospholipid vesicles with surfaces and the effect of gramicidin on bilayer properties using atomic force microscopy

We have used magnetic alternating current mode atomic force microscopy (MAC-AFM) to investigate the formation of supported phospholipid bilayers (SPB) by the method of vesicle fusion. The systems studied were dioleoylphosphatidylcholine (DOPC) on mica and mica modified with 3-aminopropyl-triethoxy-silane (APTES), and DOPC vesicles with gramicidin incorporated on mica and APTES-modified mica. The AFM images reveal three stages of bilayer formation: localized disklike features that are single bilayer footprints of the vesicles, partial continuous coverage, and finally complete bilayer formation. The mechanism of supported phospholipid bilayers formation is the fusion of proximal vesicles, rather than surface disk migration. This mechanism does not appear to be affected by incorporation of gramicidin or by surface modification. Once formed, the bilayer develops circular defects one bilayer deep. These defects grow in size and number until a dynamic equilibrium is reached.     

Interaction of substance P with phospholipid bilayers: A neutron diffraction study.

Neutron diffraction has been used to study the membrane-bound structure of substance P (SP), a member of the tachykinin family of neuropeptides. The depth of penetration of its C-terminus in zwitterionic and anionic phospholipid bilayers was probed by specific deuteration of leucine 10, the penultimate amino acid residue. The results show that the interaction of SP with bilayers, composed of either dioleoylphosphatidylcholine (DOPC), or a 50:50 mixture of DOPC and the anionic phospholipid dioleoylphosphatidylglycerol (DOPG), takes place at two locations. One requires insertion of the peptide into the hydrophobic region of the bilayer, the other is much more peripheral. The penetration of the peptide into the hydrophobic region of the bilayer is reflected in a marked difference in the water distribution profiles. SP is seen to insert into DOPC bilayers, but a larger proportion of the peptide is found at the surface when compared to the anionic bilayers. The positions of the two label populations show only minor differences between the two types of bilayer.     

Membranotropic action of cardiotonic agents and ubiquinone

Interaction of cardiotonic drugs (strophantidine acetate, suphan, para-oxybenzoic acid) and ubiquinone with phospholipid bilayers has been studied. Exothermic effect of the reaction followed by an increase in microviscosity and hydrophobicity of the bilayer from cardiolipin, but by a decrease of the microviscosity of the bilayer from lecithin has been estimated. A correlation is observed between changes in the lecithin bilayer fluidity and the heat effect of the interaction at the initial period of time after mixing of reagents.