Differential miscibility properties of various phosphatidylcholine/lysophosphatidylcholine mixtures

Stability of Liposomal Membrane of Hemoglobin-Vesicles (Artificial Red Cells) for Over Years of Storage Evaluated Using Liquid Chromatography-Mass Spectrometry

Hemoglobin-Vesicles (Hb-V) are artificial oxygen carriers encapsulating a purified and concentrated Hb solution in liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), cholesterol, 1,5-O-dihexadecyl-N-succinyl-l-glutamate (DHSG), and 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine-N-poly(ethylene glycol) (PEG5000) (DSPE-PEG). The safety and efficacy of Hb-V have been studied extensively by both preclinical and clinical test methods. Deoxygenation of Hb-V prevents autoxidation of Hb and can extend its shelf life to 2 years at room temperature. However, the lipid components raise concerns about hydrolysis because Hb-V is dispersed in saline. For this study, we attempted to estimate the lipid degradation of long-term stored Hb-V using liquid chromatography-mass spectrometry. Analyses of lipid components extracted from the stored Hb-V showed that the degradation increased depending on the storage temperature. The calculated % remaining of intact lipids of Hb-V were 98.1% after 4 years and 90.4% after 7.2 years at 4 °C, 95.8% after 1 year and 86.7% after 2 years at 25 °C, and 85.6% after 6 months at 40 °C. The main degradation products were lyso-PC and palmitic acid which are hydrolyzed at the ester bond of DPPC. A few hydrolyzed products of DHSG and DSPE-PEG were also detected in Hb-V, but almost no degradation or oxidation products derived from cholesterol could be identified. A shear test of Hb-V at 1500 s-1 showed no significant increase in Hb leakage after storage of 2 years at 25 °C and 6 months at 40 °C. Lipid degradation products including free fatty acids would decrease the pH of the Hb-V dispersion and synergistically facilitate degradation, but it maintained pH 6.5 during 6 years at 4 °C, 2 years at 25 °C, and 3 months at 40 °C because of its high buffering capacity. These results indicate that the storage conditions for Hb-V are appropriate to minimize lipid degradation in the long term.

DODAB vesicles containing lysophosphatidylcholines: The relevance of acyl chain saturation on the membrane structure and thermal properties

The saturated LPC18:0 and unsaturated LPC18:1 lysophosphatidylcholines have important roles in inflammation and immunity and are interesting targets for immunotherapy. The synthetic cationic lipid DODAB has been successfully employed in delivery systems, and would be a suitable carrier for those lysophosphatidylcholines. Here, assemblies of DODAB and LPC18:0 or LPC18:1 were characterized by Differential Scanning Calorimetry (DSC) and Electron Paramagnetic Resonance (EPR) spectroscopy. LPC18:0 increased the DODAB gel-fluid transition enthalpy and rigidified both phases. In contrast, LPC18:1 caused a decrease in the DODAB gel-fluid transition temperature and cooperativity, associated with two populations with distinct rigidities in the gel phase. In the fluid phase, LPC18:1 increased the surface order but, differently from LPC18:0, did not affect viscosity at the membrane core. The impact of the different acyl chains of LPC18:0 and 18:1 on structure and thermotropic behavior should be considered when developing applications using mixed DODAB membranes.

Hierarchy of interactions dictating the thermodynamics of real cell membranes: Following the insulin secretory granules paradigm up to fifteen-components vesicles

A fifteen-components model membrane that reflected the 80 % of phospholipids present in Insulin Secretory Granules was obtained and thermodynamic exploitation was performed, through micro-DSC, in order to assess the synergic contributions to the stability of a mixed complex system very close to real membranes. Simpler systems were also stepwise investigated, to complete a previous preliminary study and to highlight a hierarchy of interactions that can be now summarized as phospholipid tail unsaturation > phospholipid tail length > phospholipid headgroup > membrane curvature. In particular, Small Unilamellar Vesicles (SUVs) that consisted in phospholipids with different headgroups (choline, ethanolamine and serine), was step by step considered, following inclusion of sphingomyelins and lysophosphatidylcholines together with a more complete fatty acids distribution characterizing the phospholipid bilayer of the Insulin Secretory Granules. The inclusion of cholesterol was finally considered and the influence of three FFAs (stearic, oleic and elaidic acids) was investigated in comparison with simpler systems, highlighting the magnitude of the effects on such a detailed membrane in the frame of Type 2 Diabetes Mellitus alterations.

Mechanical properties of lipid bilayers and regulation of mechanosensitive function: from biological to biomimetic channels

Material properties of lipid bilayers, including thickness, intrinsic curvature and compressibility regulate the function of mechanosensitive (MS) channels. This regulation is dependent on phospholipid composition, lateral packing and organization within the membrane. Therefore, a more complete framework to understand the functioning of MS channels requires insights into bilayer structure, thermodynamics and phospholipid structure, as well as lipid-protein interactions. Phospholipids and MS channels interact with each other mainly through electrostatic forces and hydrophobic matching, which are also crucial for antimicrobial peptides. They are excellent models for studying the formation and stabilization of membrane pores. Importantly, they perform equivalent responses as MS channels: (1) tilting in response to tension and (2) dissipation of osmotic gradients. Lessons learned from pore forming peptides could enrich our knowledge of mechanisms of action and evolution of these channels. Here, the current state of the art is presented and general principles of membrane regulation of mechanosensitive function are discussed.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Physical (in) stability of liposomes upon chemical hydrolysis: the role of lysophospholipids and fatty acids

As a consequence of chemical hydrolysis of liposomal phospholipids the organization of the lipid assembly can change from a lamellar into a micellar system. Different approaches provided evidence for this conversion: 31P-NMR analysis, turbidity measurements and ultracentrifugation experiments. Two conditions have to be met before this conversion can take place: (1) the liposomes must pass through a gel-to-liquid crystalline phase-transition during a heating or cooling run, and (2) the degree of chemical hydrolysis must exceed a critical hydrolysis percentage (or the phospholipid bilayer must contain critical amounts of lysophospholipid and fatty acid). As monitored by turbidity measurements, this critical level of hydrolysis and the relative change depended on the chain length and on the head group of the liposomal phospholipids. It does not depend on concentration, pH, storage temperature or on size of the liposomes within the experimental range. Addition of cholesterol to bilayers composed of dipalmitoylphosphatidylcholine prevents the lamellar to micellar transformation. Fluorescence anisotropy measurements of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene in 0.18-microns dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylglycerol (10:1)-liposomes indicated that behavior of the probe below and above the phase-transition temperature was not affected by chemical hydrolysis, or even by formation of micelles. However, the phase-transition temperature range broadened and shifted towards higher temperatures upon hydrolysis.

Chemical hydrolysis of phospholipids

Hydrolysis kinetics of phospholipids in liposomes composed of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine/cholesterol (DPPC/CHOL) 10/4 (molar ratio) and egg phosphatidylcholine (EPC) at pH 4.0 and different temperatures could be described by Arrhenius curves without breaks. However, the Arrhenius curves for the hydrolysis of liposomal DPPC and distearoylphosphatidylcholine (DSPC) under the same conditions were biphasic. A break was observed in the curves extending over a broad range before and after the known Tm of each of these phospholipids in liposomes (42 and 56 degrees C, respectively). The activation energy (Ea) for the hydrolysis of liposomal DPPC and DSPC below the Tm was substantially larger than the Ea for liposomal DMPC, DPPC/CHOL 10/4, and EPC and decreased when DPPC was mixed with CHOL in a 10/4 molar ratio. Hardly any influence of the presence of alpha-tocopherol, cryoprotectants (glucose, trehalose, sucrose, and propylene glycol), and the major hydrolysis products lysophospholipids and fatty acids or of the absence of sodium chloride on the hydrolysis kinetics of DPPC at pH 4.0 and 30 degrees C was observed. Changes in fatty acid chains and size did not influence the hydrolysis rate constant (kobs) of liposomal phospholipids at pH 4.0 and 30 degrees C either. The only effects of uncharged compounds on the kobs of liposomal DPPC at pH 4.0 and 30 degrees C were found upon mixing with a high concentration of the detergent Triton X-100 or palmitic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermotropic behaviour of gamma-irradiated diacylphosphatidylcholine multibilayer vesicles: role of single radiolysis products

The concentrations of two isomeric lysophosphatidylcholines (LPC), free palmitic acid (FPA) and glycerophosphorycholine (GPC), have been evaluated in multibilayer vesicles (MBV) of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) at different absorbed doses, by 1H and 31P nmr. No evidence of cross-linking of lipid molecules has been found. The presence of radiolysis products of DPPC is by itself capable of justifying the thermotropic behaviour of irradiated MBV, as it appears with differential scanning calorimetry analysis. In fact, on increasing LPC concentration, the onset temperature of the main transition (MT) is proportionally shifted towards lower temperatures, without alteration of its width. A similar effect is also produced by LPC on the lower transition (LT). On the contrary, on increasing the FPA concentration, the MT width increases proportionally. Simultaneously, the LT is shifted towards higher temperatures and disappears above a certain FPA concentration. The overall effect of LPC and FPA on MT appears as a linear combination of the two effects. As far as LT is concerned, LPC and FPA compete with one another in determining whether it will be present or not and, if present, in determining its onset temperature. No GPC effect on the phase transitions could be detected within the limits of the composition of our irradiated MBV.

Sterilization of liposomes by heat treatment

Autoclaving of liposomes composed of egg phospholipids or saturated phospholipids, the latter sometimes combined with cholesterol, was performed in an isotonic acetate buffer (pH 4.0) or Hepes buffer (pH 7.4). After a standard autoclaving cycle (15 min, 121 degrees C), no change could be observed in pH, size, and extent of oxidation. Dependent on the experimental conditions, a minor or substantial increase in the fraction of hydrolyzed phospholipids was found. After a sterilization cycle, pronounced leakage was found for a water-soluble, encapsulated compound (calcein) and for an amphiphilic compound (doxorubicin). Lipophilic, liposome bilayer-associated compounds [N-trifluoroacetyldoxorubicin-14-valerate (AD-32) and alpha-tocopherol] remained in the liposomes after autoclaving. However, substantial degradation of AD-32 was observed. Under proper conditions liposomes without or with thermostable, lipophilic drugs can be sterilized by autoclaving. However, the hydrolysis of phospholipids can pose a problem, as hydrolysis kinetics depend on the pH used. In the chosen circumstances the autoclaving cycle caused massive loss of hydrophilic, nonbilayer interacting compounds; under those conditions "free" drug removal or drug encapsulation should be performed after the autoclaving step.

Effects of platelet-activating factor and related lipids on dielaidoylphosphatidylethanolamine by DSC, FTIR and NMR

The effect of platelet-activating factor (1-O-hexadecyl-2-acetyl-sn- glycero-3-phosphocholine, PAF) and two related molecules, 1-O-hexadecyl-sn-glycero-3-phosphocholine (LPAF) and 1-palmitoyl-sn-glycero-3- phosphocholine (LPC) on dielaidoylphosphatidylethanolamine (DEPE) lipid structure and polymorphism has been studied by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and 31P nuclear magnetic resonance (31P-NMR) spectroscopies. From the interaction of these molecules with DEPE it is concluded that all of them stabilize the lamellar phase with respect to the hexagonal HII phase and this effect is clear even at concentrations of these compounds as low as 1 mol%. It is also shown that, although they perturb the gel to liquid-crystalline phase transition of DEPE up to a similar extent, fluidizing the membrane, PAF but not LPAF or LPC, induces the presence of more than one peak in the calorimetric profile. Moreover, FTIR data indicate that lateral phase separations formed by PAF-rich phases are taking place. Remarkably, delta H of the main transition decreases at concentrations lower than 2 mol% but remains nearly constant up to 30 mol%. 31P-NMR measurements showed that all these molecules were capable of inducing isotropic signals in the spectra produced by molecules associated to membranes before micellization of the vesicles.

Effects of acyl chain length on the conformation of myelin basic protein bound to lysolipid micelles

The interactions of myelin basic protein with micelles of lysophosphatidylcholine detergents of different acyl chain lengths were investigated by circular dichroism (CD), small-angle X-ray scattering, Fourier transform infrared spectroscopy (FT-IR), and 1H, 13C and 31P nuclear magnetic resonance spectroscopy (NMR). Circular dichroic, FT-IR, and 1H NMR measurements indicated that the conformational changes induced in the protein molecules by association with micelles depended on the acyl chain length of the detergents. Size is one of the physical properties of micelles which is a function of the length of the acyl chains. The radii of gyration of detergent micelles in complexes with the protein measured by small-angle X-ray scattering indicated that the average size of the micelles was a quadratic function of the acyl chain length. The dependence of the protein conformational changes on micelle size was used to ascertain the order in which different protein segments associate with the detergents. Several procedures were employed to change the fluidity of micelles formed with detergents of given acyl chain lengths. The conformational changes observed on the MBP molecule by varying the micelle properties without changing the length of the chain, suggested that the changes depended on the size and fluidity of the micelles.

Spontaneous domain formation of phospholipase A2 at interfaces: fluorescence microscopy of the interaction of phospholipase A2 with mixed monolayers of lecithin, lysolecithin and fatty acid

Fluorescence microscopy has recently been proven to be an ideal tool to investigate the specific interaction of phospholipase A2 with oriented substrate monolayers. Using a dual labeling technique, it could be shown that phospholipase A2 can specifically attack and hydrolyze solid analogous L-alpha-DPPC domains. After a critical extent of monolayer hydrolysis the enzyme itself starts to aggregate forming regular shaped protein domains (Grainger et al. (1990) Biochim. Biophys. Acta 1023, 365-379). In order to confirm that the existence of hydrolysis products in the monolayer is necessary for the observed aggregation of phospholipase A2, mixed monolayers of D- and L-alpha-DPPC, L-alpha-lysoPPC and palmitic acid in different ratios were examined. The phase behavior and the interaction of these films with phospholipase A2 were directly visualized with an epifluorescence microscope. Above a certain critical concentration of lysolecithin and palmitic acid in the monolayer, compression of these mixed films leads to phase separation and formation of mixed domains of unknown composition. Their high negative charge density is evidenced by preferential binding of a cationic dye to these phase-separated areas. Introduction of fluorescence-labeled phospholipase A2 underneath these mixed domains results in rapid binding of the protein to the domains without visible hydrolytic activity, regardless of whether the L-form or the D-form of the DPPC were used. In binary mixtures, only those with DPPC/palmitic acid show formation of phase-separated areas which can be specifically targeted by phospholipase A2 leading to a rapid formation (within 2 min) of protein domains. Experiments with pyrenedecanoic acid containing monolayers give the first direct evidence that acid is located above the enzyme domains. These results show that a locally high negative charge density of the phase-separated domains is one of the prerequisites for the binding of phospholipase A2. In addition, however, small amounts of D- or L-alpha-DPPC headgroups within the domains of the monolayer seem to be necessary for recognition followed by fast binding of the protein to the domains. This is confirmed by experiments with mixed monolayers of diacetylene carboxylic acid and D-alpha-DPPC. The acid--immiscible with lecithin--forms well defined pure acid domains in the monolayer. While the cationic dye can be docked rapidly to these phase separated areas, no preferential enzyme binding and thus no protein domain formation below these acid domains can be induced.

Magnetic alignment and orientational order of dipalmitoylphosphatidylcholine bilayers containing palmitoyllysophosphatidylcholine

Mixed bilayers of 1-palmitoyl-sn-glycero-3-phosphocholine (palmitoyllysophosphatidylcholine; PaLPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (dipalmitoyl phosphatidylcholine; DPPC) have been investigated by 2H-NMR and 31P-NMR spectroscopy. Binary phospholipid mixtures were studied in which the acyl chains of one or the other component were perdeuterated. At temperatures below the main order-disorder phase transition, the mixed PaLPC/DPPC bilayers appear to coexist with PaLPC micelles. The micelles disappear at temperatures above the phase transition, where mixed bilayers in the liquid-crystalline state are formed. The orientational order of the alkyl chains of the PaLPC component is essentially identical to that of the DPPC component in the mixed bilayers, both in the low temperature and liquid-crystalline phases. However, the presence of PaLPC perturbs the segmental ordering of DPPC as compared to the pure system. The order is increased in the low-temperature phase, where effective diffusion of the chains about their long axes occurs, but is decreased in the liquid-crystalline phase compared to pure DPPC bilayers. The mixed liquid-crystalline bilayers orient preferentially with their director axes perpendicular to the magnetic field. This alignment is easily observed in 31P- and 2H-NMR spectra, where the intensity of the perpendicular edges of the lineshapes is pronounced. One possible explanation of the magnetic alignment involves alteration of the curvature free energy of the DPPC bilayer due to incorporation of PaLPC in the mixed membranes.

Phosphatidylcholine-cholesterol interactions: bilayers of heteroacid lipids containing linoleate lose calorimetric transitions at low cholesterol concentration

Model membranes composed of cholesterol plus one of two phosphatidylcholines (PC), each containing a saturated and a dienoic acyl chain, have been studied by differential scanning calorimetry. The gel to liquid-crystalline phase transition temperature of 1-palmitoyl-2-linoleoyl PC was -19.5 degrees C and that of 1-stearoyl-2-linoleoyl PC was -13.7 degrees C. The addition of cholesterol to the phosphatidylcholines in aqueous dispersion resulted in the progressive removal of the phase transition as observed by differential scanning calorimetry. Per mole of sterol in the membrane, cholesterol was more effective at reducing the enthalpy change of the phase transitions of these bilayers containing dienoic phosphatidylcholines than it is in eliminating the transition of membranes made with other phospholipids that contain more saturated chains. No transitions in membranes made with palmitoyl-linoleoyl PC or stearoyl-linoleoyl PC could be detected calorimetrically when 17 mol% cholesterol was present.

The influence of unsaturation on the phase transition temperatures of a series of heteroacid phosphatidylcholines containing twenty-carbon chains

A series of heteroacid sn-1,2 diacyl phosphatidylcholines (PC) with twenty-carbon fatty acyl chains has been synthesized. Each PC contained eicosanoate (20:0) in the sn-1 position and one of a group of eicosaenoic acids with increasing numbers of cis double bonds in the sn-2 position. The double bonds were at positions delta 11 (20:1), delta 11,14 (20:2), delta 11,14,17 (20:3), or delta 5,8,11,14 (20:4). The disaturated PC containing two eicosanoate chains was also studied. Aqueous dispersions of these PC were analyzed by differential scanning calorimetry, and data for the gel to liquid-crystalline transitions (given as PC: Tc (degrees C), Tmax (degrees C), delta H (kcal/mol)) were as follows - 20:0-20:0 PC: 66.8, 68.4, 15; 20:0-20:1 PC: 19.8, 22.2, 8; 20:0-20:2 PC: -4.3, 1.8, 5; 20:0-20:3 PC: 1.2, 4.4, 7; 20:0-20:4 PC: -10.7, -6.8, 3. Double bonds in excess of two per chain did not substantially change the transition temperatures of these heteroacid PC. There was a small effect of the location of the multiple double bonds on the transition temperature. The data is consistent with the model that the transition temperatures are determined by a balance between a decrease in the packing density in the gel and a decrease in the rotational freedom of the chains in the liquid crystal, both caused by the double bonds ((1983) Biochemistry 22, 1466-1473).

Effect of ethylene glycol on the phase transition kinetics of gluco- and galactocerebrosides

The effect of different concentrations of ethylene glycol in water on the phase transition (metastable----stable state) of Gaucher's glucocerebroside, of bovine brain cerebroside type II (non hydroxy acyl chains only) and of N-palmitoylgalactocerebroside has been investigated. The phase transition and its kinetics were inferred from the thermograms at different heating and cooling rates and confirmed by FTIR spectra of the cerebrosides in the different states. The significance of the conformational differences of the glucose and of the galactose residues with respect to their solvation, and the subsequent effect on the intermolecular interactions and the phase transition is discussed.

Comparative 2H- and 31P-NMR study on the properties of palmitoyllysophosphatidylcholine in bilayers with gramicidin, cholesterol and dipalmitoylphosphatidylcholine

The stoichiometric palmitoyllysophosphatidylcholine (lysoPC)/gramicidin (4:1, mol/mol) lamellar complex (Killian, J.A., De Kruijff, B., Van Echteld, C.J.A., Verkleij, A.J., Leunissen-Bijvelt, J. and De Gier, J. (1983) Biochim. Biophys. Acta 728, 141-144) is a useful model system to investigate the various aspects of lipid protein interactions. To study the effect of gramicidin on local order and motion of 1-palmitoyl-sn-glycero-3-phosphocholine (lysoPC) we employed 31P and 2H nuclear magnetic resonance (NMR) using selectively deuterated lysoPC's and we compared the results to those obtained for lysoPC in bilayers with cholesterol (1:1, mol/mol) and dipalmitoylphosphatidylcholine (DPPC) (1:4, mol/mol). 2H-NMR experiments on acyl chain deuterated lysoPC showed similar quadrupole splittings in the liquid crystalline state for the lysoPC/DPPC and the lysoPC/gramicidin samples. In the lysoPC/cholesterol sample an increase of the quadrupole splitting was found. T1 measurements showed that gramicidin decreases the lysoPC acyl chain motion, especially at the C12 position. In the lysoPC/cholesterol sample an increase of motion was observed as compared to lysoPC in fluid bilayers of DPPC. 31P-NMR and 2-H-NMR measurements of lysoPC, deuterated at the alpha- and beta-position of the choline moiety, indicated an increase in headgroup flexibility in all samples as compared to the parent compound DPPC. In addition, a change in headgroup conformation was observed. The alpha- and beta-segments in all samples exhibited concerted motion. It was found that also in the polar headgroup gramicidin induces a decrease of the rate of motion.

Mode of organization of lipid aggregates: a conformational analysis

A computational approach is used to predict the mode of organization of lipid molecules. The calculated aggregate states are fully compatible with the lipid phase preference: bilayers for dipalmitoylphosphatidylcholine, micelles for palmitoyllysophosphatidylcholine, and inverted structures for protonated dioleoylphosphatidic acid. The minimal conformational energy of the isolated molecule is calculated as the sum of the contributions resulting from the Van der Waals interactions, the torsional potentials, the electrostatic interactions, and the transfer energy. Assembly of these structures in monolayers yields the most probable modes of organization of lipids. The described method may represent a useful tool to predict the phase preference of lipids and to give a molecular description of their mode of association.

Activation of phospholipase A2 by freshly added lysophospholipids

Reaction progress curves for the hydrolysis of dimyristoylphosphatidylcholine by pig pancreatic phospholipase A2 exhibits a latency phase. Addition of 1-palmitoyllysophosphatidylcholine to the preformed vesicles reduces the latency phase and enhances the binding of phospholipase A2 to the vesicles. In contrast, the binary codispersions prepared from diacylphospholipids premixed with lysophosphatidylcholine do not exhibit such enhanced susceptibility to the phospholipase. This effect appears to be due to organizational defects created by asymmetrical incorporation of lysophospholipid molecules into the outer monolayer of the vesicles, and the action of phospholipase is not observed when the additive is equilibrated in both the monolayers of the vesicles.

Lipid oxidation and gel to liquid-crystalline transition temperatures of synthetic polyunsaturated mixed-acid phosphatidylcholines

Synthetic preparations of the polyunsaturated phosphatidylcholines 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) and 1-stearoyl-2-alpha-linolenoyl-sn-glycero-3-phosphocholine (SLnPC) were observed to undergo autooxidation sometimes during synthesis and also on storage. Oxidation was also induced by treatment of unoxidized SLPC with ultraviolet irradiation. Oxidation was estimated by thin layer chromatographic, fatty acid and ultraviolet spectral analyses. With limited oxidation, the gel to liquid-crystalline transition temperatures of aqueous dispersions of these lipids were seen to increase. Extensive oxidation led to a reduction in the enthalpies of the transitions. The increases in transition temperatures were consistent with the presence of conjugated double bonds, as shown by increased absorption at 230 nm, and trans double bonds, in the oxidized lipids leading to the creation of more rigid domains within the bilayer. Some of the changes in the transitions, especially the decreasing enthalpy after extensive oxidation, may have occurred because of the presence of small amounts of lysophosphatidylcholine and other oxidation intermediates or breakdown products seen by thin layer analysis. Thermograms of mixtures of unoxidized SLPC with amounts of lysostearoylPC found in the oxidized samples showed, however, that lysoPC likely did not contribute significantly to the increase in transition temperatures. Thin layer analysis suggested that the presence of cross-linked products could have contributed to the observed thermotropic properties.

Gel to liquid-crystalline phase transitions of aqueous dispersions of polyunsaturated mixed-acid phosphatidylcholines

The thermotropic properties of aqueous dispersions of synthetic mixed-acid polyunsaturated 1,2-diacyl-3-sn-phosphatidylcholines (PC) have been studied by differential scanning calorimetry. The gel to liquid-crystalline phase transition temperature (Tc) of 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) was -16.2 +/- 1.6 degrees C (means +/- SD, nine dispersions, three separate synthetic preparations); that for a preparation of 1-stearoyl-2-linolenoyl-PC (SLnPC) was -13 degrees C and for a preparation of 1-stearoyl-2-arachidonoyl-PC (SAPC) was -12.6 +/- 1.0 degrees C (three dispersions, one preparation). The temperatures of maximum heat flow (Tmax) for the lipid transitions were -14.4 +/- 1.3, -11, and -10.7 +/- 0.9 degrees C and the enthalpies were 3.3 +/- 1.0, 6.6, and 5.3 +/- 1.8 kcal X mol-1 for SLPC, SLnPC, and SAPC, respectively. The transition temperatures and enthalpies are rationalized on the basis of existing data on the effect of double-bond position on Tc and are interpreted with the use of a statistical mechanical model. The trend of the transition temperatures with the introduction of multiple double bonds reflects opposing effects due to increased molecular area and decreasing degrees of freedom of rotation about carbon-carbon bonds.

Phospholipid miscibility in ternary mixtures

The gel to liquid-crystalline phase transition of aqueous dispersions of phospholipid mixtures was investigated by means of the repartition of the spin label 2,2,6,6-tetramethylpiperidine-I-oxyl between aqueous space and lipid hydrocarbon region. The dimyristoylphosphatidylcholine (DMPC)/dibehenoylphosphatidylcholine (DBPC) and dipalmitoylphosphatidylcholine (DPPC)/DBPC phase diagrams indicate gel phase immiscibility, whereas the distearoylphosphatidylcholine (DSPC)/DBPC phase diagram indicates non-ideal gel phase miscibility at low DBPC molar fractions. Aqueous dispersions of DMPC/DPPC/DBPC ternary mixtures show two distinct phase transitions, the first associated with the melting of a DMPC/DPPC phase and the second with the melting of a DBPC phase. Aqueous dispersions of DMPC/DSPC/DBPC ternary mixtures show to phase transitions at low DSPC molar fractions; the first is probably associated with the melting of a DMPC/DSPC phase, and the second with the melting of a DBPC/DSPC phase. At high DSPC molar fractions, only one phase transition is observed; this suggests that all the lipids are mixed in gel state membranes.

Thermal and 13C-NMR study of the dynamic structure of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-glycero-3-phosphocholine in aqueous dispersions

Mixed-acid monounsaturated phosphatidylcholines containing palmitate and oleate chains have been synthesized by phospholipase A2 digestion of the appropriate single-acid phosphatidylcholine, followed by reacylation of the lysophosphatidylcholine with the desired fatty acid anhydride. The positional isomers 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-phosphocholine have been thus obtained. The thermotropic behavior of these lipids dispersed in excess water has been studied by differential scanning calorimetry. Positional isomers of mixed-acid monounsaturated phosphatidylcholines are found to have different gel to liquid-crystalline transition temperatures and enthalpies. It was found that mixtures of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine or 1,2-distearoyl-sn-glycero-3-phosphocholine exhibited inmiscibility in the phosphatidylcholine gel state. The dynamic structure of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and 1-oleyl-2-palmitoyl-sn-glycero-3-phosphocholine bilayers has been investigated by measuring the 13C nuclear spin-lattice relaxation times of sonicated aqueous dispersions. No difference was found between the two systems, suggesting that above the thermal transition the presence of the unsaturated acyl group in the 1 or 2 position does not affect significantly the dynamic structure of the bilayer.

The use of differential scanning calorimetry and differential thermal analysis in studies of model and biological membranes

Differential scanning calorimetry (DSC), and to a lesser extent differential thermal analysis (DTA), are powerful yet relatively rapid and inexpensive thermodynamic techniques for studying the thermotropic phase behavior of lipids in model and biological membranes, without the introduction of exogenous probe molecules. In this review the principles as well as the scope and limitations of DSC and DTA are discussed first. The application of these techniques to the study of the thermotropic phase behavior of aqueous dispersions of various single synthetic phospholipids are then summarized, and the effects of cholesterol, free fatty acids, lysophospholipids, drugs, anesthetics and proteins on the gel to liquid-crystalline phase transitions exhibited by these model systems are discussed. The phase mixing properties of model membranes consisting of mixtures of two or more synthetic or natural phospholipids are considered next. Finally, the thermotropic phase behavior of prokaryotic plasma membranes and of the plasma, microsomal and mitochondrial membranes of eukaryotic cells are reviewed, and the applications of DSC and DTA to study the thermal behavior of specific membrane proteins, as well as the physical properties of the membrane lipid phase, are summarized.

Effects of lysophosphatidylcholines on phosphatidylcholine and phosphatidylcholine/cholesterol liposome systems as revealed by 31P-NMR, electron microscopy and permeability studies

(1) The effect of incorporation of different lysophosphatidylcholine species on the structure, barrier properties and dynamics of bilayers made of various phosphatidylcholines both the presence and absence of cholesterol have been investigated by 31P-NMR, freeze-fracture electron microscopy and K+-permeability measurements. (2) In a dispersion of lysophosphatidylcholine : cholesterol (1 : 1) the lipids are organized in extended bilayers. Upon cooling a micellar solution of 1-palmitoyllysophosphatidylcholine below the chain-melting temperature a transition to a lamellar, most likely interdigitating organization is observed. 31P-NMR shows in both situations a marked decrease in effective chemical shift anisotropy. (3) 1-Palmitoyllysophosphatidylcholine can be incorporated up to 30 mol% into liquid crystalline bilayers of dipalmitoylphosphatidylcholine and up to 35 mol% into dioleoylphosphatidylcholine bilayers. Above this concentration micellization of the bilayers occurs. In the gel state, bilayer structure is maintained up to 60 mol% of the lysocompound. (4) 1-Oleoyllysophosphatidylcholine can be incorporated to higher concentrations into liquid crystalline phosphatidylcholine bilayers than the palmitoyl analogue, which can be explained by the more cylindrical shape of the 1-oleoyllysophosphatidylcholine. (5) In marked contrast, incorporation of only 1 mol% of 1-oleoyllysophosphatidylcholine into gel state dipalmitoylphosphatidylcholine already destabilizes bilayer structure and makes the membranes completely permeable for K+. These results are discussed with respect to the mixing properties of the various lysophosphatidylcholines. (6) In general these effects are accompanied by a loss of K+-permeability barrier, which however occurs at lower lysophosphatidylcholine concentrations than needed for the start of micellization. (7) Cholesterol incorporation counteracts the bilayer destabilizing role of lysophosphatidylcholines. (8) 31P-NMR demonstrates with increasing lysophosphatidylcholine concentrations in the bilayers of phosphatidylcholines a decrease in the effective chemical shift anisotropy. As the rigid lattice spectra of lysophosphatidylcholine and phosphatidylcholine are identical, this reflects a change in the conformational and/or motional properties of the phospholipid head groups. This phenomenon might play a role in the observed permeability changes.

Gel to liquid-crystalline transition temperatures of water dispersions of two pairs of positional isomers of unsaturated mixed-acid phosphatidylcholines

The gel to liquid-crystalline phase transition temperatures of dispersions of mixed-acid sn-1,2-lecithins which contain one unsaturated and one saturated fatty acid have been studied by differential scanning calorimetry. The temperature for 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (containing no reversed isomer) was -9.3 degrees C while that for 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (containing 8% of the reversed isomer) was -2.6 degrees C. The temperature for 2-oleoyl-1-stearoyl-sn-glycero-3-phosphocholine (containing 6% of the reversed isomer) was 6.3 degrees C while that for 1-oleoyl-2-stearoyl-sn-glycero-3-phosphocholine (containing 18% of the reversed isomer) was 8.6 degrees C. The differences in transition temperatures for the isomers of a pair containing the same two acids were consistent with those observed for positional isomers of saturated mixed-acid lecithins in that the isomer of the pair which had the longer fatty acid in the sn-1 position had the lower temperature. The phase transition temperatures of pairs of isomers containing palmitate and oleate at the sn-1 and -2 positions were different by at least 6.7 degrees C, while those containing stearate and oleate were different by at least 2.3 degrees C. Differences in the chain lengths of the fatty acids at the two positions of the glycerol appear to predominate over differences in the depths of the double bonds in the bilayer in determining the transition temperatures.

The effect of 5-(n-alk(en)yl) resorcinols from rye on membrane structure

The increased membrane permeability for K+, glycerol and erythritol, and membrane lysis induced by alkyl and alkenyl resorcinols, respectively, might be due to the interaction with membrane proteins and the formation of reversed micelles. The 5-(n-alk(en)yl) resorcinols show a very high stability at the air/water interface. The molecular area is 0.28 and 0.37 nm2 (at 30 mN/m) for alkyl and alkenyl resorcinols from rye, respectively. Differential scanning calorimetry experiments show a miscibility of alk(en)yl resorcinols with phosphatidylcholines. Only for alkenyl resorcinols is a small reduction found in the free energy of dipalmitoyl phosphatidylcholine. Electron microscopy studies show protein patching in erythrocyte membranes after the addition of resorcinols. The resorcinol-induced K+ release is not influenced by the presence of proteolytic enzymes, but strongly reduced by bovine serum albumin and glycophorin. 31P-NMR measurements show the occurrence of an isotropic and hexagonal signal in egg phosphatidylcholine in the presence of about 30 mol% alk(en)yl resorcinol.

Structure of 1-acyl lysophosphatidylcholine and fatty acid complex in bilayers

The phase transition characteristics of bilayers formed in a codispersion of 1-acyl lysophosphatidylcholine and a fatty acid depend on the chain length of both the components and on the pH of the aqueous medium. Incorporation of cholesterol as a third component abolishes the transition. It is suggested that acyl chain interactions between fatty acid and 1-acyl lysophosphatidylcholine molecules in their aqueous codispersions are maximized by close-packing such that the acyl chains of both molecules are aligned parallel to each other and the carboxyl group is located in the vicinity of the 2-hydroxyl group of lysophosphatidylcholine. The shape and size of a functional dimer thus formed are similar but not identical to those of 1,2-diacyl phosphatidylcholine. Several predictions arising from this suggestion, including phase separation in codispersions of fatty acid + 1-acyl lysophosphatidylcholine + diacyl phosphatidylcholine, are experimentally confirmed.

Association of lysophosphatidylcholine with fatty acids in aqueous phase to form bilayers

Lysophospholipids have been implicated in a variety of physiological process. In vivo lysophospholipids are invariably produced with fatty acids as the product of hydrolysis by phospholipase A2 (ref. 2). Therefore, it is of considerable interest to examine the properties of the mixture of these lipids in aqueous suspensions. Results presented here demonstrate that a mixture of fatty acid and lysophosphatidylcholine forms a bilayer type of organisation even though the individual components form micelles when dispersed in an aqueous phase.