Gel-to-inverted hexagonal (L beta-HII) phase transitions in phosphatidylethanolamines and fatty acid-phosphatidylcholine mixtures, demonstrated by 31P-NMR spectroscopy and x-ray diffraction

The effect of headgroup methylation on polymorphic phase behaviour in hydrated N-methylated phosphoethanolamine:palmitic acid membranes

Mixtures of fatty acids and phospholipids can form hexagonal (HII) and inverse bicontinuous cubic phases, the latter of which are implicated in various cellular processes and have wide-ranging biotechnological applications in protein crystallisation and drug delivery systems. Therefore, it is vitally important to understand the formation conditions of inverse bicontinuous cubic phases and how their properties can be tuned. We have used differential scanning calorimetry and synchrotron-based small angle and wide angle X-ray scattering (SAXS/WAXS) to investigate the polymorphic phase behaviour of palmitic acid/partially-methylated phospholipid mixtures, and how headgroup methylation impacts on inverse bicontinuous cubic phase formation. We find that upon partial methylation of the phospholipid headgroup (1 or 2 methyl substituents) inverse bicontinuous cubic phases are formed (of the Im3m spacegroup), which is not the case with 0 or 3 methyl substituents. This shows how important headgroup methylation is for controlling phase behaviour and how a change in headgroup methylation can be used to controllably tune various inverse bicontinuous phase features such as their lattice parameter and the temperature range of their stability.

Assessing the Interactions of Auristatin Derivatives with Mixed Phospholipid-Sodium Dodecyl Sulfate Aggregate Dispersions

The aim of this study was to assess what properties of the pseudostationary phases in electrokinetic capillary chromatography affect the interactions between monomethyl auristatin E (MMAE) and hydrophilically modified structural analogues thereof with various lipophilic phases. MMAE is a widely used cytotoxic agent in antibody-drug conjugates (ADC), which are used as selective biopharmaceutical drugs in the treatment of cancers. MMAE and its derivatives are highly lipophilic, yet they fail to interact with biomimicking phosphatidylcholine-phosphatidylserine liposomes. To reveal what properties affect the interaction of the auristatin derivatives with cell plasma membrane-mimicking vesicles, capillary electrokinetic chromatography was used with four different types of micellar and vesicular pseudostationary phases: pure vesicles, mixed vesicles, mixed micelles, and pure micelles. Vesicular phases were composed of pure phospholipids [dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC)] and phospholipid-surfactant mixtures [sodium dodecyl sulfate, (SDS) with DMPC and DLPC] while the micellar phases comprised pure surfactant (SDS) and surfactant-phospholipid mixtures (SDS-DMPC and SDS-DLPC). In addition, differential scanning calorimetry and dynamic light scattering were used to monitor the aggregate composition. Our data shows that the interaction between hydrophobic auristatin derivatives and hydrophobic pseudostationary phases critically depends on the type, size, and hydrogen bonding capability of the pseudostationary phases.

Evidence that Listeria innocua modulates its membrane's stored curvature elastic stress, but not fluidity, through the cell cycle

This paper reports that the abundances of endogenous cardiolipin and phosphatidylethanolamine halve during elongation of the Gram-positive bacterium Listeria innocua. The lyotropic phase behaviour of model lipid systems that describe these modulations in lipid composition indicate that the average stored curvature elastic stress of the membrane is reduced on elongation of the cell, while the fluidity appears to be maintained. These findings suggest that phospholipid metabolism is linked to the cell cycle and that changes in membrane composition can facilitate passage to the succeding stage of the cell cycle. This therefore suggests a means by which bacteria can manage the physical properties of their membranes through the cell cycle.

Pressure-temperature phase behavior of mixtures of natural sphingomyelin and ceramide extracts

Ceramides are a group of sphingolipids that act as highly important signaling molecules in a variety of cellular processes including differentiation and apoptosis. The predominant in vivo synthetic pathway for ceramide formation is via sphingomyelinase catalyzed hydrolysis of sphingomyelin. The biochemistry of this essential pathway has been studied in detail; however, there is currently a lack of information on the structural behavior of sphingomyelin- and ceramide-rich model membrane systems, which is essential for developing a bottom-up understanding of ceramide signaling and platform formation. We have studied the lyotropic phase behavior of sphingomyelin-ceramide mixtures in excess water as a function of temperature (30-70 °C) and pressure (1-200 MPa) by small- and wide-angle X-ray scattering. At low ceramide concentrations the mixtures form the ripple gel phase (P(β)') below the gel transition temperature for sphingomyelin, and this observation has been confirmed by atomic force microscopy. Formation of the ripple gel phase can also be induced at higher temperatures via the application of hydrostatic pressure. At high ceramide concentration an inverse hexagonal phase (HII) is formed coexisting with a cubic phase.

Inertial cavitation to non-invasively trigger and monitor intratumoral release of drug from intravenously delivered liposomes

The encapsulation of cytotoxic drugs within liposomes enhances pharmacokinetics and allows passive accumulation within tumors. However, liposomes designed to achieve good stability during the delivery phase often have compromised activity at the target site. This problem of inefficient and unpredictable drug release is compounded by the present lack of low-cost, non-invasive methods to measure such release. Here we show that focused ultrasound, used at pressures similar to those applied during diagnostic ultrasound scanning, can be utilised to both trigger and monitor release of payload from liposomes. Notably, drug release was influenced by liposome composition and the presence of SonoVue® microbubbles, which provided the nuclei for the initiation of an event known as inertial cavitation. In vitro studies demonstrated that liposomes formulated with a high proportion of 1,2 distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) released up to 30% of payload following ultrasound exposure in the presence of SonoVue®, provided that the exposure created sufficient inertial cavitation events, as characterised by violent bubble collapse and the generation of broadband acoustic emissions. In contrast a 'Doxil'-like liposome formulation gave no such triggered release. In pre-clinical studies, ultrasound was used as a non-invasive, targeted stimulus to trigger a 16-fold increase in the level of payload release within tumors following intravenous delivery. The inertial cavitation events driving this release could be measured remotely in real-time and were a reliable predictor of drug release.

Nanostructured self assembled lipid materials for drug delivery and tissue engineering

Every living organism comprises of lipids as basic building blocks in addition to other components. Utilizing these lipids for pharmaceutical and biomedical applications can overcome biocompatibility and biodegradability issues. A well known example is liposomes (lipids arranged in lamellar structures), but other than that there are additional unique mesophasic structures of lipids formed as a result of lipid polymorphisms, which include cubic-, hexagonal- or sponge-phase structures. These structures provide the advantages of stability and production feasibility compared with liposomes. Cubosomes, which exist in a cubic structure, have improved stability, bioadhesivity and biocompatibility. Hexagonal phases or hexosomes exhibit hexagonal arrangements and can encapsulate different drugs with high stability. Lipids also forms tube-like structures known as tubules and ribbons that are also utilized in different biomedical applications, especially in tissue engineering. Immune stimulating complexes are nanocage-like structures formed as a result of interactions of lipid, antigen and Quillaja saponin. These lipidic mesophasic structures have been utilized for gene, vaccine and drug delivery. This article addresses lipid self-assembled supramolecular nanostructures, including cubosomes, hexosomes, tubules, ribbons, cochleates, lipoplexes and immune stimulating complexes and their biomedical applications.

Effect of mycolic acid on surface activity of binary surfactant lipid monolayers

In pulmonary tuberculosis, Mycobacterium tuberculosis lies in close physical proximity to alveolar surfactant. Cell walls of the mycobacteria contain loosely bound, detachable surface-active lipids. In this study, the effect of mycolic acid (MA), the most abundant mycobacterial cell wall lipid, on the surface activity of phospholipid mixtures from lung surfactant was investigated using Langmuir monolayers and atomic force microscopy (AFM). In the presence of mycolic acid, all the surfactant lipid mixtures attained high minimum surface tensions (between 20 and 40 mN/m) and decreased surface compressibility moduli <50 mN/m. AFM images showed that the smooth surface topography of surfactant lipid monolayers was altered with addition of MA. Aggregates with diverse heights of at least two layer thicknesses were found in the presence of mycolic acid. Mycolic acids could aggregate within surfactant lipid monolayers and result in disturbed monolayer surface activity. The extent of the effect of mycolic acid depended on the initial state of the monolayer, with fluid films of DPPC-POPC and DPPC-CHOL being least affected. The results imply inhibitory effects of mycolic acid toward lung surfactant lipids and could be a mechanism of lung surfactant dysfunction in pulmonary tuberculosis.

Lateral pressure profile, spontaneous curvature frustration, and the incorporation and conformation of proteins in membranes

Lipid-protein interactions are an important determinant of the stability and function of integral and transmembrane proteins. In addition to local interactions at the lipid-protein interface, global interactions such as the distribution of internal lateral pressure may also influence protein conformation. It is shown here that the effects of the membrane lateral pressure profile on the conformation or insertion of proteins in membranes are equivalent to the elastic response to the frustrated spontaneous curvature, c(o), of the component lipid monolayer leaflets. The chemical potential of the protein in the membrane is predicted to depend linearly on the spontaneous curvature of the lipid leaflets, just as does the contribution of the protein to the elastic bending energy of the lipid, and to be independent of the hydrophobic tension, gamma(phob), at the lipid-water interface. Analysis of the dependence of protein partitioning or conformational transitions on spontaneous curvature of the constituent lipids gives an experimental estimate for the cross-sectional intramembrane shape of the protein or its difference between conformations. Values in the region of 50-110 A(2) are estimated for the effective cross-sectional shape changes on the insertion and conductance transitions of alamethicin, and on the activation of CTP:phosphocholine cytidylyltransferase or rhodopsin in lipid membranes. Much larger values are estimated for the mechanosensitive channel, MscL. Values for the change in intramembrane shape may also be used, together with determinations of lipid relative association constants, to estimate contributions of direct lipid-protein interactions to the lateral pressure experienced by the protein. Changes in chemical potential approximately 12 kJ mol(-1) can be estimated for radial changes of 1 A in a protein of diameter 40 A.

Effect of fatty acids on phase behavior of hydrated dipalmitoylphosphatidylcholine bilayer: saturated versus unsaturated fatty acids

The effect of some fatty acids on the phase behavior of hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer was investigated with special interest in possible difference between saturated and unsaturated fatty acids. The phase behavior of hydrated DPPC bilayer was followed by a differential scanning calorimetry and a Fourier transform infrared spectroscopy. The addition of palmitic acid (PA) increased the bilayer phase transition temperature with the increase of the PA content in the mixture. In addition, DPPC molecules in gel phase bilayer became more rigid in the presence of PA compared with those in the absence of PA. This effect of PA on the phase behavior of hydrated DPPC bilayer is common to other saturated fatty acids, stearic acid, myristic acid, and also to unsaturated fatty acid with trans double bond, elaidic acid. Contrary to these fatty acids, oleic acid (OA), the unsaturated fatty acid with cis double bond in the acyl chain, exhibited quite different behavior. The effect of OA on the bilayer phase transition temperature was rather small, although a slight decrease in the temperature was appreciable. Furthermore, the IR spectral results demonstrated that the perturbing effect of OA on the gel phase bilayer of DPPC was quite small. These results mean that OA does not disturb the hydrated DPPC bilayer significantly.

Synthesis and thermotropic characterization of a homologous series of racemic beta-D-glucosyl dialkylglycerols

The phase behaviour of aqueous dispersions of a series of synthetic 1,2-di-O-alkyl-3-O-(beta-D-glucosyl)-rac-glycerols with both odd and even hydrocarbon chain lengths was studied by differential scanning calorimetry and low angle X-ray diffraction (XRD). Thermograms of these lipids show a single, strongly energetic phase transition, which was shown to correspond to either a lamellar gel/liquid crystalline (L(beta)/L(alpha)) phase transition (short chain compounds, n < or =14 carbon atoms) or a lamellar gel/inverted hexagonal (L(beta)/H(II)) phase transition (longer chain compounds, n > or =15 carbon atoms) by XRD. The shorter chain compounds may exhibit additional transitions at higher temperatures, which have been identified as lamellar/nonlamellar phase transitions by XRD. The nature of these nonlamellar phases and the number of associated intermediate transitions can be seen to vary with chain length. The thermotropic phase properties of these lipids are generally similar to those reported for the corresponding 1,2-sn-diacyl alpha- and beta-D-glucosyl counterparts, as well as the recently published 1, 2-dialkyl-3-O-(beta-D-glycosyl)-sn-glycerols. However, the racemic lipids studied here show no evidence of the complex patterns of gel phase polymorphism exhibited by the above mentioned compounds. This suggests that the chirality of the glycerol molecule, by virtue of its position in the interfacial region, may significantly alter the phase properties of a lipid, perhaps by controlling the relative positions of hydrogen bond donors and acceptors in the polar region of the membrane.

Lipid vesicles and membrane fusion

Membrane fusion is essential for cell survival and has attracted a great deal of both theoretical and experimental interest. Fluorescence (de)quenching measurements were designed to distinguish between bilayermerging and vesicle-mixing. Theoretical studies and various microscopic and diffraction methods have elucidated the mechanism of membrane fusion. These have revealed that membrane proximity and high defect density in the adjacent bilayers are the only prerequisites for fusion. Intermediates, such as stalk or inverse micellar structures can, but need not, be involved in vesicle fusion. Nonlamellar phase creation is accompanied by massive membrane fusion although it is not a requirement for bilayer merging. Propensity for membrane fusion is increased by increasing the local membrane disorder as well by performing manipulations that bring bilayers closer together. Membrane rigidification and enlarged bilayer separation opposes this trend. Membrane fusion is promoted by defects created in the bilayer due to the vicinity of lipid phase transition, lateral phase separation or domain generation, high local membrane curvature, osmotic or electric stress in or on the membrane; the addition of amphiphats or macromolecules which insert themselves into the membrane, freezing or other mechanical membrane perturbation have similar effects. Lowering the water activity by the addition of water soluble polymers or by partial system dehydration invokes membrane aggregation and hence facilitates fusion; as does the membrane charge neutralization after proton or other ion binding to the lipids and intermembrane scaffolding by proteins or other macromolecules. The alignment of defect rich domains and polypeptides or protein binding is pluripotent: not only does it increase the number of proximal defects in the bilayers, it triggers the vesicle aggregation and is fusogenic. Exceptions are the bound molecules that create steric or electrical barriers between the membranes which prevent fusion. Membrane fusion can be non-leaky but it is very common to lose material from the vesicle interior during the later stages of membrane unification, that is, after a few hundred microseconds following the induction of fusion.

X-Ray Diffraction Study of the Lamellar-Hexagonal Phase Transition in Phospholipid/Surfactant Mixtures

The lamellar-to-hexagonal phase transition of a phospholipid/ surfactant mixed system of 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC) and oligo(ethylene oxide)dodecyl ethers of type C12H25O(CH2CH2O)2H(C12E2) in molar surfactant/phospholipid ratio (RS/L) of 2 at low hydration driven by temperature has been studied by X-ray diffraction. The Lbeta-HII phase transition is a reversible two-state process showing hysteresis at fast temperature scan rates. The obtained hexagonal phase exhibits a temperature dependent structural change. The numbers of bound water molecules per composite particle (WS+L) absorbed in the lamellar and hexagonal phases are nearly the same, changing from WS+L = 5.0 to 4.7 during the phase transition. The fluidity of the alkyl chains on increasing the temperature and the close packing of the hydrophilic molecular parts are the driving parameters of the lamellar-to-hexagonal transformation. Copyright 1998 Academic Press.

The role of the cytoplasmic tail region of influenza virus hemagglutinin in formation and growth of fusion pores

The effect of the cytoplasmic tail of influenza hemagglutinin (HA) (H3 subtype) on fusion kinetics and pore growth was examined An SV40 recombinant virus was used to express wild-type (WT) HA and HA mutants containing changes in the HA cytoplasmic tail. HA and its mutants were expressed in CV-1 cells and the ability of these cells to fuse to either red blood cells (RBCs) or planar bilayer membranes was determined quantitatively. The percentage of cells expressing HA and the levels of expression were the same for WT HA or HA lacking its cytoplasmic tail (CT-), and for a mutant, MAY, in which the three HA C-terminal cysteine residues were replaced to block the addition of palmitate. When RBCs were colabeled with large and small aqueous dyes and fused to CV-1 cells expressing WT HA, transfer of the large dye was significantly slower and extent of transfer was lower than that of the small dye, indicating that pores did not expand quickly to large diameters. An absence of the HA cytoplasmic tail did not alter the time course of spread for either dye. When CV-1 cells expressing WT HA were fused to planar membranes, small pores tended to open and close repetitively ("flicker") before a pore would continue to either grow irreversibly to large conductances or grow to intermediate sizes and then contract. For HA mutants CT- and MAY, flickering was less likely to occur, but these pores did evolve in a manner identical to WT HA postflicker pores. We conclude that palmitate covalently linked to cysteine residues of the HA cytoplasmic tail is required for pore flickering, but that the tail does not play an important role in subsequent pore enlargement.

Phosphatidylcholine-fatty acid membranes: effects of headgroup hydration on the phase behaviour and structural parameters of the gel and inverse hexagonal (H(II)) phases

The phase behaviour and structural parameters of a homologous series of saturated diacyl phosphatidylcholine/fatty acid 1:2 (mol/mol) mixtures having chain lengths from C12 to C20 were studied by X-ray diffraction and calorimetry, as a function of water content. The chain-melting transition temperatures of the 1:2 PC/FA mixtures are found to be largely independent of the degree of hydration. For all chain lengths, the tilted L(beta') and rippled P(beta') gel phases of the pure PC component are replaced by an untilted L(beta) gel phase in the 1:2 PC/FA mixtures. This gel phase swells considerably upon hydration, with a limiting water layer thickness in the range 18-24 A, depending on the chain length. However, unlike pure phospholipid systems, the lateral chain packing within the gel phase bilayers is essentially identical in both the dry and the fully hydrated states. The fluid bilayer L(alpha) phase is suppressed in the 1:2 mixtures, being replaced by inverse non-lamellar phases for all chain lengths greater than C12, and at all levels of hydration. For chain lengths of C16 and greater, the inverse hexagonal H(II) phase is formed directly upon chain melting, at all water contents. For the shorter chain length mixtures, the behaviour is more complex, with the H(II) phase forming at low hydration, but with bicontinuous cubic phases appearing at higher levels of hydration. The implications of these surprising results are explored, in terms of the effective hydrophilicity of the associated PC and FA headgroups and the packing within the interfacial region. We suggest that the presence of the fatty acids significantly alters the lateral stress profile across the lipid monolayer in the fluid state, compared to that of the corresponding pure PC system, such that inverse phases, where the interface bends towards the water, become strongly favoured. Furthermore, for short chain lengths, packing constraints favour the formation of phases with negative interfacial Gaussian curvature, such as the bicontinuous cubic phases, rather than the H(II) phase, which has more severe chain packing frustration.

Stages of the bilayer-micelle transition in the system phosphatidylcholine-C12E8 as studied by deuterium- and phosphorous-NMR, light scattering, and calorimetry

The perturbation of phospholipid bilayer membranes by a nonionic detergent, octaethyleneglycol mono-n-dodecylether (C12E8), was investigated by 2H- and 31P-NMR, static and dynamic light scattering, and differential scanning calorimetry. Preequilibrated mixtures of the saturated phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC), and 1,2-dilauroyl-sn-glycero-3-phosphorylcholine (DLPC) with the detergent were studied over a broad temperature range including the temperature of the main thermotropic phase transition of the pure phospholipids. Above this temperature, at a phospholipid/detergent molar ratio 2:1, the membranes were oriented in the magnetic field. Cooling of the mixtures below the thermotropic phase transition temperatures of the pure phospholipids led to micelle formation. In mixtures of DPPC and DMPC with C12E8, a narrow calorimetric signal at the onset temperature of the solubilization suggested that micelle formation was related to the disorder-order transition in the phospholipid acyl chains. The particle size changed from 150 nm to approximately 7 nm over the temperature range of the bilayer-micelle transition. The spontaneous orientation of the membranes at high temperatures enabled the direct determination of segmental order parameters from the deuterium spectra. The order parameter profiles of the phospholipid acyl chains could be attributed to slow fluctuations of the whole membrane and to detergent-induced local perturbations of the bilayer order. The packing constraints in the mixed bilayers that eventually lead to bilayer solubilization were reflected by the order parameters of the interfacial phospholipid acyl chain segments and of the phospholipid headgroup. These results are interpreted in terms of the changing average shape of the component molecules. Considering the decreasing cross sectional areas in the acyl chain region and the increasing hydration of the detergent headgroups, the bilayer-micelle transition is the result of an imbalance in the chain and headgroup repulsion. A neutral or pivotal plane can be defined on the basis of the temperature dependence of the interfacial quadrupolar splittings.

Temperature- and pH-controlled fusion between complex lipid membranes. Examples with the diacylphosphatidylcholine/fatty acid mixed liposomes

The fusion capability of complex lipid bilayers and its pH as well as temperature sensitivity have been studied by optical and spectroscopic means. The aggregation and fusion efficiency of such lipid membranes can be optimized by controlling the phase characteristics of the individual membrane components. For a practically relevant illustration, the stoichiometric 1:2 (mol/mol) mixtures of phosphatidylcholines and fatty acids are used. Perhaps the most interesting liposomes of this kind, which are made of dipalmitoylphosphatidylcholine/elaidic acid (DPPC/ELA-COOH (1:2)), undergo a chain-melting phase transition between 42 degrees C and 48 degrees C, depending on the bulk pH value. The highest chain-melting phase transition temperatures are measured with the fully protonated fatty acids at pH < or = 5.5 and involve a change into the non-bilayer high-temperature state. Upon increasing pH, this transition reverts into an ordinary gel-to-fluid lamellar phase change and occurs at 42 degrees C, by and large. Simultaneously, the rate and the efficacy of fusion between the PC/FA and PC/FA- mixed vesicles decreases. The fusion efficacy of the PC/FA(-) mixed liposomes at pH > or = pK(FA) approximately 7.5 is practically negligible. This is largely due to the increased interbilayer repulsion and to the relatively high water-solubility of the deprotonated fatty acid molecules at high pH. While the pH-variability chiefly affects the efficacy of the intermembrane aggregation, the vesicle fusion itself is more sensitive to temperature variations. It is more likely that the temperature dependence of the intramembrane defect density is chiefly responsible for this. Optimal conditions for the fusion between DPPC/ELA-COOH (1:2) mixed vesicles are thus 3.5 < or = pH < or = 5.5 (6.3) (aggregation maximum) and T > or = 41.5 degrees C = Tm(DPPC) (defect density and fusion maximum). Under such conditions the average size of PC/FA (1:2) mixed vesicles in a 1 mM suspension increases by a factor of 10 over a period of 10 min. Interbilayer fusion can also be catalyzed by the mechanically induced local membrane defects. Freshly made liposomes thus always fuse more avidly than aged vesicles. This permits estimates of the kinetics of membrane defects annihilation based on the measured temporal dependence of the maximum fusion-rate. From such studies, a quasi-exponential decay on the time scale of 1.2 h is found for the thermolabile fusogenic DPPC/ELA-COOH liposomes.

The phase behavior of mixed aqueous dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol

The phases and transition sequences for aqueous dispersions of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycerol (1,2-DPG) have been studied by differential scanning calorimetry, dynamic x-ray diffraction, freeze-fracture electron microscopy, 31P-nuclear magnetic resonance spectroscopy, and Fourier-transform infrared spectroscopy. The results have been used to construct a dynamic phase diagram of the binary mixture as a function of temperature over the range 20 degrees-90 degrees C. It is concluded that DPPC and 1,2-DPG form two complexes in the gel phase, the first one with a DPPC/1,2-DPG molar ratio of 55:45 and the second one at a molar ratio of approximately 1:2, defining three different regions in the phase diagram. Two eutectic points are postulated to occur: one at a very low 1,2-DPG concentration and the other at a 1,2-DPG concentration slightly higher than 66 mol%. At temperatures higher than the transition temperature, lamellar phases were predominant at low 1,2-DPG concentrations, but nonlamellar phases were found to be predominant at high proportions of 1,2-DPG. A very important aspect of these DPPC/1,2-DPG mixtures was that, in the gel phase, they showed a ripple structure, as seen by freeze-fracture electron microscopy and consistent with the high lamellar repeat spacings seen by x-ray diffraction. Ripple phase characteristics were also found in the fluid lamellar phases occurring at concentrations up to 35.6 mol% of 1,2-DPG. Evidence was obtained by Fourier transform infrared spectroscopy of the dehydration of the lipid-water interface induced by the presence of 1,2-DPG. The biological significance of the presence of diacylglycerol in membrane lipid domains is discussed.

Phases and phase transitions of the hydrated phosphatidylethanolamines

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Here, a review of the LIPIDAT data subset referring to hydrated phosphatidylethanolamines (PE) is presented together with an analysis of these data. The PE subset represents 14% of all LIPIDAT records. It includes data collected over a 38-year period and consists of 1511 records obtained from 203 articles in 35 different journals. An analysis of the data in the subset has allowed us to identify trends in synthetic PE phase behavior reflecting changes in lipid chain length, chain unsaturation (number, isomeric type and position of double bonds), chain asymmetry and branching, type of chain-glycerol linkage (ether vs. ester) and headgroup modification. Also included is a summary of the data concerning the effect of pH, stereochemical purity, and different additives such as salts, saccharides, alcohols, amino adds and alkanes on PE phase behavior. Information on the phase behavior of biologically derived PE is also presented. This review includes 236 references.

Interaction of sphingosine and stearylamine with phosphatidylserine as studied by DSC and NMR

The interaction of sphingosine (SP) and stearylamine (SA) with dipalmitoylphosphatidylserine (DPPS) has been studied by using differential scanning calorimetry (DSC) and phosphorus nuclear magnetic resonance (31P-NMR). DSC showed that SP and SA rigidified the membranes, forming an azeotropic mixture with DPPS. The azeotropic mixture which was formed between DPPS and SP was found at a DPPS/SP molar ratio of 2:1 whereas SA and DPPS formed an azeotropic mixture at a DPPS/SA molar ratio of 1:1. An eutectic point was observed at 85 mol% of SP and 90 mol% of SA in DPPS. 31P-NMR showed the presence of a lamellar phase at DPPS/SP and DPPS/SA molar ratios lower than 1:1, whereas at higher molar ratios and at high temperatures, besides the lamellar phase, an isotropic component was detected. It was found that, at physiological pH, both SP and SA were protonated in a large extent, i.e., positively charged, since their apparent pK in the membrane were 9.1 and 8.9, respectively. The results reported in this work may be relevant to understand a number of biological effects produced by these positively charged molecules, due to their electrostatic interaction with negatively charged phospholipids.

Calorimetric and spectroscopic studies of the polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines

The polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines was investigated by differential scanning calorimetry, 31P-nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Upon heating, aqueous dispersions of dried samples of the short- and medium-chain homologues (n < or = 17) exhibit single, highly energetic transitions from a dry, crystalline form to the fully hydrated, liquid-crystalline bilayer at temperatures higher than the lamellar gel-liquid-crystalline phase transition exhibited by fully hydrated samples. In contrast, the longer chain homologues (n > or = 18) first exhibit a transition from a dehydrated solid form to the hydrated L beta gel phase followed by the gel-liquid-crystalline phase transition normally observed with fully hydrated samples. The fully hydrated, aqueous dispersions of these lipids all exhibit reversible, fairly energetic gel-liquid-crystalline transitions at temperatures that are significantly higher than those of the corresponding phosphatidylcholines. In addition, at still higher temperatures, the longer chain members of this series (n > or = 16) exhibit weakly energetic transitions from the lamellar phase to an inverted nonlamellar phase. Upon appropriate incubation at low temperatures, aqueous dispersions of the shorter chain members of this homologous series (n < or = 16) form a highly ordered crystal-like phase that, upon heating, converts directly to the liquid-crystalline phase at the same temperature as do the aqueous dispersions of the dried lipid. The spectroscopic data indicate that unlike the n-saturated diacyl phosphatidylcholines, the stable crystal-like phases of this series of phosphatidylethanolamines describe an isostructural series in which the hydrocarbon chains are packed in an orthorhombic subcell and the headgroup and polar/apolar interfacial regions of the bilayer are effectively immobilized and substantially dehydrated. Our results suggest that many of the differences between the properties of these phosphatidylethanolamine bilayers and their phosphatidylcholine counterparts can be rationalized on the basis of stronger intermolecular interactions in the headgroup and interfacial regions of the phosphatidylethanolamine bilayers. These are probably the result of differences in the hydration and hydrogen bonding interactions involving the phosphorylethanolamine headgroup and moieties in the polar/apolar interfacial regions of phosphatidylethanolamine bilayers.

Influence of retinoids on phosphatidylethanolamine lipid polymorphism

The interaction of all-trans-retinoic acid and all-trans-retinol with dielaidoylphosphatidylethanolamine has been studied by differential scanning calorimetry and 31P-NMR spectroscopy. Increasing concentrations of all-trans-retinoic acid up to a mol fraction of 0.09 were found to induce shifts to lower temperatures of both the L beta to L alpha and L alpha to hexagonal-HII phase transitions, with a slight decrease in the enthalpy change of the transitions. At higher concentrations no further effects on the transitions were observed, and this is interpreted as indicative of a limited miscibility of retinoic acid with the phospholipid. 31P-NMR spectroscopy confirmed that the L alpha to hexagonal-HII phase transition was shifted to lower temperatures in the presence of retinoic acid. On the other hand increasing concentrations of all-trans-retinol up to a mol fraction of 0.166, induced a progressive shift of the L beta to L alpha and the L alpha to hexagonal-HII phase transitions to lower temperatures. At higher concentrations the main gel to liquid-crystalline phase transition was further displaced to lower temperatures and the lamellar to hexagonal-HII phase transition was not observed in the thermograms. 31P-NMR spectroscopy indicated that retinol was able of inducing the phospholipid to adopt the hexagonal-HII phase at temperatures even below the main gel to liquid-crystalline phase transition temperature of the pure phospholipid.

Fusion of lipid bilayers: a model involving mechanistic connection to HII phase forming lipids

A model for the molecular mechanism of the fusion of lipid bilayers is described. A crucial feature of this model and related to the lamellar-->hexagonal phase HII transition is a novel, hypothetical lipid conformation, tentatively referred to here as extended. During fusion this conformation could manifest itself in the contact site between two vesicles in close proximity and involves the extension of the acyl chains of a phospholipid molecule in opposite directions, i.e. embedded into the two opposing bilayers while maintaining the headgroup in the interface. Although evidence for the occurrence of the extended conformation for phospholipids is sparse this conformation appears to be compatible with currently available experimental data. Of importance also is that the extended conformation allows for the fusion of two bilayer membranes to proceed with minimal exposure of the lipid hydrocarbon chains to water. It can also account for other features of membrane fusion such as lipid mixing in the intermediate state without mixing of the vesicle contents as well as for the molecular basis of the action of fusogenic lipids.

Ca(2+)-induced fusion of phospholipid vesicles containing free fatty acids: modulation by transmembrane pH gradients

The influence of a transmembrane pH gradient on the Ca(2+)-induced fusion of phospholipid vesicles, containing free fatty acids, has been investigated. Large unilamellar vesicles composed of an equimolar mixture of cardiolipin, dioleoylphosphatidylcholine, and cholesterol, containing 20 mol % oleic acid, were employed. Fusion was measured using a kinetic assay for lipid mixing, based on fluorescence resonance energy transfer. At pH 7.5, but not at pH 6.0, in the absence of a pH gradient, oleic acid stimulates the fusion of the vesicles by shifting the Ca2+ threshold concentration required for aggregation and fusion of the vesicles from about 13 mM to 10 mM. In the presence of a pH gradient (at an external pH of 7.5 and a vesicle interior pH of 10.5), the vesicles exhibit fusion characteristics similar to vesicles that do not contain oleic acid at all, consistent with an effective sequestration of the fatty acid to the inner monolayer of the vesicle bilayer induced by the imposed pH gradient. The kinetics of the fusion process upon simultaneous generation of the pH gradient across the vesicle bilayer and initiation of the fusion reaction show that the inward movement of oleic acid in response to the pH gradient is extremely fast, occurring well within 1 s. Conversely, dissipation of an imposed pH gradient, by addition of a proton ionophore during the course of the fusion process, results in a rapid enhancement of the rate of fusion due to reequilibration of the oleic acid between the two bilayers leaflets.

Partial dehydration of phosphatidylethanolamine phosphate groups during hexagonal phase formation, as seen by i.r. spectroscopy

The gel-to-fluid and lamellar-to-HII-hexagonal thermotropic phase transitions of egg-yolk phosphatidylethanolamine have been examined by Fourier-transform infrared spectroscopy under a variety of conditions, namely excess water at pH 5.0, excess water at pH 9.5 and low hydration. The various lamellar and hexagonal phases have been characterized by X-ray diffraction. At pH 5.0, gel-fluid and lamellar-hexagonal transitions were detected at 10 and 32 degrees C respectively, in accordance with previous data. At pH 9.5, only the first of these two transitions was detected. In the partially hydrated sample a single phenomenon was observed, probably encompassing both transitions, so that, in practice, a gel-HII-hexagonal transition appears to occur. The region of the i.r. spectrum corresponding to the phospholipid phosphate group reveals that the lamellar-hexagonal, but not the gel-fluid, transition is accompanied by a weakening in the shell of hydrogen-bonded water, thus providing direct evidence that, in a pure lipid/water system, hexagonal phase formation requires partial dehydration of the phospholipid phosphate group. X-ray diffraction data support this conclusion, since, at least in the low-hydration system, the average surface area per lipid polar group decreases with the thermotropic lamellar-hexagonal transition.

Anomalous mixing of zwitterionic and anionic phospholipids with double-chain cationic amphiphiles in lipid bilayers

High-sensitivity scanning calorimetry has been used to examine the thermotropic behavior of mixtures combining dipalmitoylphosphatidylcholine (DPPC), phosphatidylethanolamine (DPPE) and O-methylphosphatidic acid (DPPA-OMe) with the double-chain cationic amphiphiles N,N-dihexadecyl-N,N- dimethylammonium chloride (DHDAC), 1,2-dipalmitoyloxy-3-(trimethylammonio)propane (DPTAP) and the corresponding monomethylated tertiary amino compounds (DHMMA-H+ and DPDAP-H+). At physiological ionic strength, mixtures of these cationic amphiphiles with the anionic phospholipid DPPA-OMe can show gel-to-liquid-crystalline phase transitions at considerably higher temperatures than do either of the pure components. Surprisingly, binary mixtures of DPPC and these cationic amphiphiles also show strongly nonideal mixing, with phase diagrams exhibiting pronounced maxima in their solidus and liquidus curves. Similar behavior is not observed for mixtures of DPPC with DPPA-OMe, which closely resembles DPTAP and DPDAP-H+ in backbone configuration and polar headgroup size. The present results suggest that perturbation of the orientation of the phosphatidylcholine headgroup by cationic amphiphiles, as demonstrated previously by Seelig and co-workers (Biochemistry 28 [1989], 7720-7728), can significantly affect the thermotropic behavior of phospholipids such as DPPC. Such effects may exert a generally important (though not always easily recognizable) influence on the organization and thermotropic behavior of systems where zwitterionic phospholipids are combined with charged bilayer-associated molecules.

Cytochrome c-lipid interactions studied by resonance Raman and 31P NMR spectroscopy. Correlation between the conformational changes of the protein and the lipid bilayer

The interaction of cytochrome c with negatively charged lipids has been studied by resonance Raman spectroscopy of the protein heme group and 31P NMR of the phospholipid headgroups. The gel-to-fluid-phase transition of dimyristoylphosphatidylglycerol induces shifts in the conformational and coordination equilibria of the bound cytochrome c, as recorded by the resonance Raman spectra in the fingerprint and marker band regions. Conformational and coordination shifts of the bound cytochrome are also induced on admixture of dioleoylglycerol or dioleoylphosphatidylcholine with dioleoylphosphatidylglycerol. In the case of dioleoylglycerol, significant changes take place even at levels as low as 5 mol %. Binding of cytochrome c induces or increases the content of near isotropically diffusing lipid registered by the 31P NMR spectra of the different lipids studied. Admixture of dioleoylglycerol also increases the bilayer curvature of dioleoylphosphatidylglycerol, inducing an inverted hexagonal phase at 50 mol % concentration; the tendency to spontaneous curvature in the lipid appears to relax the conformational change detected in the protein.

Structure and interactions of ether- and ester-linked phosphatidylethanolamines

The ether-linked phospholipid 1,2-dihexadecylphosphatidylethanolamine (DHPE) was studied as a function of hydration and in fully hydrated mixed phospholipid systems with its ester-linked analogue 1,2-dipalmitoylphosphatidylethanolamine (DPPE). A combination of differential scanning calorimetry (DSC) and X-ray diffraction was used to examine the phase behavior of these lipids. By DSC, from 0 to 10 wt % H2O, DHPE displayed a single reversible transition that decreased from 95.2 to 78.8 degrees C and which was shown by X-ray diffraction data to be a direct bilayer gel to inverted hexagonal conversion, L beta----HII. Above 15% H2O, two reversible transitions were observed which stabilized at 67.1 and 92.3 degrees C above 19% H2O. X-ray diffraction data of fully hydrated DHPE confirmed the lower temperature transition to be a bilayer gel to bilayer liquid-crystalline (L beta----L alpha) phase transition and the higher temperature transition to be a bilayer liquid-crystalline to inverted hexagonal (L alpha----HII) phase transition. The lamellar repeat distance of gel-state DHPE increased as a function of hydration to a limiting value of 62.5 A at 19% H2O (8.6 mol of water/mol of DHPE), which corresponds to the hydration at which the transition temperatures are seen to stabilize by DSC. Electron density profiles of DHPE, in addition to calculations of the lipid layer thickness, confirmed that DHPE in the gel state forms a noninterdigitated bilayer at all hydrations. Fully hydrated mixed phospholipid systems of DHPE and DPPE exhibited two reversible transitions by DSC.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of small organic molecules on phospholipid phase transitions

Small organic molecules are known to exhibit a wide spectrum of physiological or pharmacological effects and many of them are thought to be membrane associated. Therefore a great number of studies is devoted to the interaction between these molecules and phospholipid model membranes. Results obtained for molecular species of varying hydrophobic/hydrophilic balances will be described. It will be shown that, in general, these different molecules induce similar effects on phospholipid phase transitions, although they are located differently in the membrane. Detailed studies of these interactions will help to understand these processes on a molecular level.

On the reversibility of the phase transitions in lipid-water systems

Heating and cooling phase sequences observed in phospholipid and glycolipid dispersions in excess water have been summarized. Differences between heating and cooling sequences and also with respect to a reference phase sequence "subgel-gel-lamellar liquid crystalline-cubic-inverted hexagonal" have been pointed out. Together with kinetic data obtained by alternating current (AC) calorimetry, these data have been used for a discussion on the reversibility of the lipid phase transitions. Several typical symptoms of irreversible behavior are (i) undercooling of stable phases; (ii) formation of phases which are metastable over the whole range of their existence; (iii) slow formation of the nascent phase requiring isothermal annealing out of the transition region; (iv) different nonconvergent transition pathways in heating and cooling. These phenomena are related to the appearance of slow rearrangement modes during the phase transitions with characteristic times longer than experimental time scales. Similarly slow relaxations supporting the existence of long-lived non-equilibrium lipid states in the biomembranes may have also certain physiological significance.