Phases and phase transitions of the phosphatidylcholines

Membrane properties of sphingomyelins

Sphingomyelin and phosphatidylcholine are important components in the external leaflet of cellular plasma membranes. In this review we compare the structure of these lipid molecules, with emphasis on the differences in hydrogen bonding capacity and membrane properties that arise from the small but significant differences in molecular structure. The membrane properties of sphingomyelins and the implications that these have, or might have, in biological membranes and for raft function are further discussed.

Identification of a novel family of oxidized phospholipids that serve as ligands for the macrophage scavenger receptor CD36

The macrophage scavenger receptor CD36 plays an important role in the uptake of oxidized forms of low density lipoprotein (LDL) and contributes to lesion development in murine models of atherosclerosis. However, the structural basis of CD36 lipoprotein ligand recognition is unknown. We now identify a novel class of oxidized phospholipids that serve as high affinity ligands for CD36 and mediate recognition of oxidized forms of LDL by CD36 on macrophages. Small unilamellar vesicles of homogeneous phosphatidylcholine (PC) molecular species were oxidized by the myeloperoxidase (MPO)-H(2)O(2)-NO(2)(-) system, and products were separated by sequential LC/ESI/MS/MS. In parallel, fractions were tested for their ability to bind to CD36. Four major structurally related phospholipids with CD36 binding activity were identified from oxidized 1-palmitoyl-2-arachidonyl-PC, and four corresponding structural analogs with CD36 binding activity were identified from oxidized 1-palmitoyl-2-linoleoyl-PC. Each was then synthetically prepared, its structure confirmed by multinuclear NMR and high resolution mass spectrometry, and shown to possess identical CD36 binding activity and LC/ESI/MS/MS characteristics in both native and derivatized forms. Based upon the structures of the active compounds identified, and structure-function studies with a variety of synthetic analogs, we conclude that the structural characteristics required for high affinity binding of oxidized PC species to CD36 are a phospholipid with an sn-2 acyl group that incorporates a terminal gamma-hydroxy(or oxo)-alpha,beta-unsaturated carbonyl (oxPC(CD36)). LC/ESI/MS/MS studies demonstrate that oxPC(CD36) are formed during LDL oxidation by multiple distinct pathways. Formation of this novel class of oxidized PC species contributes to CD36-mediated recognition of LDL oxidized by MPO and other biologically relevant mechanisms. The present results offer structural insights into the molecular patterns recognized by the scavenger receptor CD36 and provide a platform for the development of potential therapeutic inhibitory agents.

Component and state separation in DMPC/DSPC lipid bilayers: a Monte Carlo simulation study

In this paper a two-state, two-component, Ising-type model is used to simulate the lateral distribution of the components and gel/fluid state acyl chains in dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) lipid bilayers. The same model has been successful in calculating the excess heat capacity curves, the fluorescence recovery after photobleaching (FRAP) threshold temperatures, the most frequent center-to-center distances between DSPC clusters, and the fractal dimensions of gel clusters (Sugar, I. P., T. E. Thompson, and R. L. Biltonen, 1999. Biophys. J. 76:2099-2110). Depending on the temperature and mole fraction the population of the cluster size is either homogeneous or inhomogeneous. In the inhomogeneous population the size of the largest cluster scales with the size of the system, while the rest of the clusters remain small with increasing system size. In a homogeneous population, however, every cluster remains small with increasing system size. For both compositional and fluid/gel state clusters, threshold temperatures-the so-called percolation threshold temperatures-are determined where change in the type of the population takes place. At a given mole fraction, the number of percolation threshold temperatures can be 0, 1, 2, or 3. By plotting these percolation threshold temperatures on the temperature/mole fraction plane, the diagrams of component and state separation of DMPC/DSPC bilayers are constructed. In agreement with the small-angle neutron scattering measurements, the component separation diagram shows nonrandom lateral distribution of the components not only in the gel-fluid mixed phase region, but also in the pure gel and pure fluid regions. A combined diagram of component and state separation is constructed to characterize the lateral distribution of lipid components and gel/fluid state acyl chains in DMPC/DSPC mixtures. While theoretical phase diagrams of two component mixtures can be constructed only in the case of first-order transitions, state and component separation diagrams can be constructed whether or not the system is involved in first-order transition. The effects of interchain interactions on the component and state separation diagrams are demonstrated on three different models. The influences of state and component separation on the in-plane and off-plane membrane reactions are discussed.

Specific mode of interaction between components of model pulmonary surfactants using computer simulations

Atomistic molecular dynamics simulations and structural bioinformatics tools enable the identification of the exact mode of interaction between model pulmonary surfactant components. Two nanosecond long simulations of the N-terminal region of human surfactant protein-B (SP-B(1-25)) in dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) monolayers of different lipid surface densities reveal the preferential affinity of SP-B(1-25) for anionic phospholipids. In particular, arginine 12 and lysine 24 interact strongly and with high specificity with the phosphate group of the DPPG lipids, stabilizing the position, the orientation, and the secondary structure of the peptide in the monolayer. The peptide lies at an oblique angle to the interfacial plane, ranging between 47 degrees and 62 degrees, increasing with decreasing lipid surface density. In DPPC monolayers the interaction is largely determined by hydrophobic interactions. The non-specific nature of DPPC-SP-B(1-25) interactions allows for significant flexibility in the topology of the peptide in the lipid matrix. Bioinformatics tools are employed to generalize the simulation results to the sequences of SP-B(1-25) in other organisms. The importance of specific residues, and the role of the largely helical and amphiphilic nature of the peptide in the functionality of SP-B(1-25) are established. The synergy of classical mechanics tools with bioinformatics methods greatly enhances the molecular-level interpretation of pulmonary surfactant action and facilitates the development of design rules for synthetic surfactant analogues.

Micromanipulation of phospholipid bilayers by atomic force microscopy

The molecular details of adhesion mechanics in phospholipid bilayers have been studied using atomic force microscopy (AFM). Under tension fused bilayers of dipalmitoylphosphatidylcholine (DPPC) yield to give non-distance dependent and discrete force plateaux of 45.4, 81.6 and 113+/-3.5 pN. This behaviour may persist over distances as great as 400 nm and suggests the stable formation of a cylindrical tube which bridges the bilayers on the two surfaces. The stability of this connective structure may have implications for the formation of pili and hence for the initial stage of bacterial conjugation. Dimyristoylphosphatidylcholine (DMPC) bilayers also exhibit force plateaux but with a much less pronounced quantization. Bilayers composed of egg PC, sterylamine and cholesterol stressed in a similar way show complex behaviour which can in part be explained using the models demonstrated in the pure lipids.

Different interactions of egg-yolk phosphatidylcholine and sphingomyelin with detergent bile salts

To examine physical-chemical aspects of bile salt-phospholipid interactions that could contribute to preferential phosphatidylcholine (PC) secretion into bile, we have compared transitions between vesicles and micelles in model systems containing taurocholate (TC) and either egg-yolk PC (EYPC), egg-yolk sphingomyelin (EYSM), buttermilk SM (BMSM) or dipalmitoyl PC (DPPC). Phase transitions from micelles to vesicles were observed at 4-fold dilution of serially diluted EYPC/TC systems, but not earlier than at 16-fold dilution of SM/TC or DPPC/TC systems, indicating lower concentrations of the detergent required for micellization in the case of SM or DPPC. Cryo-transmission electron microscopy of phase transitions initiated by addition of TC to phospholipid vesicles revealed extremely long SM-containing intermediate structures, but shorter EYPC-containing intermediate structures. Again, larger amounts of bile salt were required to induce phase transitions in the case of EYPC compared to SM. Sizes of TC-phospholipid micelles increased progressively upon increasing phospholipid contents in the rank order: DPPC-TC<EYSM-TC<BMSM-TC<EYPC-TC, consistent with higher micellization concentrations in the case of EYPC. Micelles were also separated from vesicular phases in two-phase model systems composed with TC, both EYPC and EYSM and 0, 10, 20 or 30 mol% cholesterol, by ultracentrifugation and ultrafiltration of the supernatant. At increasing cholesterol contents, EYPC preferentially distributed into the micellar phase. In contrast, no preferential micellar EYPC distribution occurred in the absence of the sterol. These results indicate different structural arrangements of EYPC-TC micelles compared to SM-TC micelles and lower detergent concentrations required for micellization in the case of SM-containing vesicles.

An index of lipid phase diagrams

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

Proton diffusion at phospholipid assemblies

A new scanning electrochemical microscopy proton feedback method has been developed for investigating lateral proton diffusion at phospholipid assemblies: specifically Langmuir monolayers at the water/air interface. In this approach, a base is electrogenerated by the reduction of a weak acid (producing hydrogen) at a "submarine" ultramicroelectrode (UME) placed in the aqueous subphase of a Langmuir trough close to a monolayer. The electrogenerated base diffuses to and titrates monolayer-bound protons and is converted back to its initial form, so enhancing the current response at the UME. Local deprotonation of the monolayer creates a concentration gradient for lateral proton diffusion. A numerical model has been developed, taking into account the potential-dependent association/dissociation constant of the interfacial acid groups. A comparison is made of monolayers comprising either acidic DL-alpha-phosphatidyl-L-serine, dipalmitoyl (DPPS) or zwitterionic L-alpha-phosphatidylcholine, dipalmitoyl (DPPC) monolayers at a range of surface pressures. It is demonstrated that lateral proton fluxes at DPPS are significant, but the lateral proton diffusion coefficient is lower than in bulk solution. In contrast, lateral proton diffusion cannot be detected at DPPC, suggesting that the acid/base character of the phospholipid is important in determining the magnitude of interfacial proton fluxes.

Application of pressure perturbation calorimetry to lipid bilayers

Pressure perturbation calorimetry (PPC) is a new method that measures the heat consumed or released by a sample after a sudden pressure jump. The heat change can be used to derive the thermal volume expansion coefficient, alpha(V), as a function of temperature and, in the case of phase transitions, the volume change, DeltaV, occurring at the phase transition. Here we present the first report on the application of PPC to determine these quantities for lipid bilayers. We measure the volume changes of the pretransition and main transition of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and the thermal expansivity of the fluid phase of DMPC and of two unsaturated lipids, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine. The high sensitivity of PPC instrumentation gives accurate data for alpha(V) and DeltaV even upon the application of relatively low pressures of approximately 5 bar.

Cyclodextrin-induced lipid lateral separation in DMPC membranes: (2)H nuclear magnetic resonance study

Cholesteryl cyclodextrins, obtained by grafting a cholesterol moiety on the oligosaccharide core, combine the size selectivity of the cyclodextrin cavity with the carrier properties of model membrane systems such as micelles or liposomes. The cholesteryl cyclodextrins were incorporated as guests in chain perdeuterated dimyristoyl phosphatidylcholine (DMPC-d54) membranes. The deuterium nuclear magnetic resonance (NMR) spectra obtained with the A form of cholesteryl-beta-cyclodextrin (beta CC(A)), with a succinyl spacer inserted between the cholesterol moiety and the cyclodextrin headgroup, indicated that this compound induces a lateral phase separation of DMPC-d54, into a pure lipid phase and a cholesteryl cyclodextrin-rich phase. The lipid exchange rate between the two phases was slow on the NMR timescale (>10(-5) s), and two well-resolved spectral components could be detected. The laterally segregated mixed phase was observed at various membrane concentrations of cholesteryl cyclodextrin, even with dispersions containing only 5% of the derivative. The dePaked spectra allowed the determination of the relative amount of DMPC-d54 molecules contained in each phase, giving approximately 1 to 1.5 DMPC molecules per unit of beta CC(A). This ratio was found to be independent of the total membrane concentration of beta CC(A). The cholesteryl cylodextrin-rich phase was detected on a large range of temperature from -12 degrees C to 25 degrees C and exhibits a smooth transition from a fluid environment to a more ordered state, occurring approximately 0 degrees C. A boundary phase between the pure lipid and cyclodextrin-rich phase was detected at 19 degrees C just below the fluid-to-gel transition. The average orientational order was reduced in the cholesteryl cyclodextrin-rich phase, and quasi-independent of temperature, as opposed to the order parameters measured for the NMR signals of the pure lipid phase. However, the NMR data obtained with beta CC(A) deuterated on the cyclodextrin headgroup indicated that the latter was quasistatic, with very large order parameters (approximately 120 kHz) at all temperatures, suggesting strong interactions between neighboring cyclodextrin headgroups. The interactions of DMPC-d54 membranes with the B form of cholesteryl-beta-cyclodextrin, lacking the succinyl spacer, was also investigated in a parallel study. No lateral phase separation was found with this compound, indicating that the spatial location and a precise positioning (allowed by the spacer) of the cyclodextrin headgroup at the membrane interface was crucial for the stability of the cholesteryl cyclodextrin lamellar phase.

Cholesterol is not crucial for the existence of microdomains in kidney brush-border membrane models

The external membrane leaflet plays a key role in the organization of the cell plasma membrane as a mosaic of ordered microdomains enriched in sphingolipids and cholesterol and of fluid domains. In this study, the thermotropic behavior and the topology of bilayers made of a phosphatidylcholine/sphingomyelin mixture, which mimicks the lipid composition of the external leaflet of renal brush-border membranes, were examined by differential scanning calorimetry and atomic force microscopy. In the absence of cholesterol, a broad phase separation process occurred where ordered gel phase domains of size varying from the mesoscopic to the microscopic scale, enriched in sphingomyelin, occupied half of the bilayer surface at room temperature. Increasing amounts of cholesterol progressively decreased the enthalpy of the transition and modified the topology of membranes domains up to a concentration of 33 mol % for which no membrane domains were detected. These results strongly suggest that, in membranes highly enriched in sphingolipids like renal and intestinal brush borders, there is a threshold close to the physiological concentration above which cholesterol acts as a suppressor rather than as a promoter of membrane domains. They also suggest that cholesterol depletion does not abolish the lateral heterogenity in brush-border membranes.

Enzymes inside lipid vesicles: preparation, reactivity and applications

There are a number of methods that can be used for the preparation of enzyme-containing lipid vesicles (liposomes) which are lipid dispersions that contain water-soluble enzymes in the trapped aqueous space. This has been shown by many investigations carried out with a variety of enzymes. A review of these studies is given and some of the main results are summarized. With respect to the vesicle-forming amphiphiles used, most preparations are based on phosphatidylcholine, either the natural mixtures obtained from soybean or egg yolk, or chemically defined compounds, such as DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) or POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). Charged enzyme-containing lipid vesicles are often prepared by adding a certain amount of a negatively charged amphiphile (typically dicetylphosphate) or a positively charged lipid (usually stearylamine). The presence of charges in the vesicle membrane may lead to an adsorption of the enzyme onto the interior or exterior site of the vesicle bilayers. If (i) the high enzyme encapsulation efficiencies; (ii) avoidance of the use of organic solvents during the entrapment procedure; (iii) relatively monodisperse spherical vesicles of about 100 nm diameter; and (iv) a high degree of unilamellarity are required, then the use of the so-called 'dehydration-rehydration method', followed by the 'extrusion technique' has shown to be superior over other procedures. In addition to many investigations in the field of cheese production--there are several studies on the (potential) medical and biomedical applications of enzyme-containing lipid vesicles (e.g. in the enzyme-replacement therapy or for immunoassays)--including a few in vivo studies. In many cases, the enzyme molecules are expected to be released from the vesicles at the target site, and the vesicles in these cases serve as the carrier system. For (potential) medical applications as enzyme carriers in the blood circulation, the preparation of sterically stabilized lipid vesicles has proven to be advantageous. Regarding the use of enzyme-containing vesicles as submicrometer-sized nanoreactors, substrates are added to the bulk phase. Upon permeation across the vesicle bilayer(s), the trapped enzymes inside the vesicles catalyze the conversion of the substrate molecules into products. Using physical (e.g. microwave irradiation) or chemical methods (e.g. addition of micelle-forming amphiphiles at sublytic concentration), the bilayer permeability can be controlled to a certain extent. A detailed molecular understanding of these (usually) submicrometer-sized bioreactor systems is still not there. There are only a few approaches towards a deeper understanding and modeling of the catalytic activity of the entrapped enzyme molecules upon externally added substrates. Using micrometer-sized vesicles (so-called 'giant vesicles') as simple models for the lipidic matrix of biological cells, enzyme molecules can be microinjected inside individual target vesicles, and the corresponding enzymatic reaction can be monitored by fluorescence microscopy using appropriate fluorogenic substrate molecules.

Thermodynamic stability of the bacteriorhodopsin lattice as measured by lipid dilution

To determine the strength of noncovalent interactions that stabilize a membrane protein complex, we have developed an in vitro method for quantifying the dissociation of the bacteriorhodopsin (BR) lattice, a naturally occurring two-dimensional crystal. A lattice suspension was titrated with a short- and long-chain phosphatidylcholine mixture to dilute BR within the lipid bilayer. The fraction of BR in the lattice form as a function of added lipid was determined by visible circular dichroism spectroscopy and fit with a cooperative self-assembly model to obtain a critical concentration for lattice assembly. Critical concentration values of wild-type and mutant proteins were used to calculate the change in lattice stability upon mutation (DeltaDeltaG). By using this method, a series of mutant proteins was examined in which residues at the BR-BR interface were replaced with smaller amino acids, either Ala or Gly. Most of the mutant lattices were destabilized, with DeltaDeltaG values of 0.2-1.1 kcal/mol at 30 degrees C, consistent with favorable packing of apolar residues in the membrane. One mutant, I45A, was stabilized by approximately 1.0 kcal/mol, possibly due to increased lipid entropy. The DeltaDeltaG values agreed well with previous in vivo measurements, except in the case of I45A. The ability to measure the change in stability of mutant protein complexes in a lipid bilayer may provide a means of determining the contributions of specific protein-protein and protein-lipid interactions to membrane protein structure.

Cholesterol organization in membranes at low concentrations: effects of curvature stress and membrane thickness

Cholesterol is often found distributed nonrandomly in domains in biological and model membranes and has been reported to be distributed heterogeneously among various intracellular membranes. Although a large body of literature exists on the organization of cholesterol in plasma membranes or membranes with high cholesterol content, very little is known about organization of cholesterol in membranes containing low amounts of cholesterol. Using a fluorescent cholesterol analog (25-[N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol, or NBD-cholesterol), we have previously shown that cholesterol may exhibit local organization even at very low concentrations in membranes, which could possibly be attributable to transbilayer tail-to-tail dimers. This is supported by similar observations reported by other groups using cholesterol or dehydroergosterol, a naturally occurring fluorescent cholesterol analog which closely mimics cholesterol. In this paper, we have tested the basic features of cholesterol organization in membranes at low concentrations using spectral features of dehydroergosterol. More importantly, we have investigated the role of membrane surface curvature and thickness on transbilayer dimer arrangement of cholesterol using NBD-cholesterol. We find that dimerization is not favored in membranes with high curvature. However, cholesterol dimers are observed again if the curvature stress is relieved. Further, we have monitored the effect of membrane thickness on the dimerization process. Our results show that the dimerization process is stringently controlled by a narrow window of membrane thickness. Interestingly, this type of local organization of NBD-cholesterol at low concentrations is also observed in sphingomyelin-containing membranes. These results could be significant in membranes that have very low cholesterol content, such as the endoplasmic reticulum and the inner mitochondrial membrane, and in trafficking and sorting of cellular cholesterol.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Dehydration induces lateral expansion of polyunsaturated 18:0-22:6 phosphatidylcholine in a new lamellar phase

To gain a better understanding of the biological role of polyunsaturated phospholipids, infrared (IR) linear dichroism, NMR, and x-ray diffraction studies have been conducted on the lyotropic phase behavior and bilayer dimensions of sn-1 chain perdeuterated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (SDPC-d35), a mixed-chain saturated (18:0)-polyunsaturated (22:6 omega 3) lipid. SDPC films were hydrated at definite values of temperature (T) and relative humidity (RH). In excess water, the lipid forms exclusively lamellar phases in the temperature range 0--50 degrees C. Upon dehydration the lipid undergoes the main phase transition between the liquid-crystalline (L(alpha)) and gel (L(beta)) phase at T < 15 degrees C. Both the saturated and polyunsaturated chains adopt a stretched conformation in the L(beta) phase, presumably the all-trans (stearoyl) and angle iron or helical (docosahexaenoyl) one. A new fluid lamellar phase (L(alpha)') was found in partially hydrated samples at T > 15 degrees C. SDPC membranes expand laterally and contract vertically in the L(alpha)' phase when water was removed. This tendency is in sharp contrast to typical dehydration-induced changes of membrane dimensions. The slope of the phase transition lines in the RH-T phase diagram reveal that the lyotropic L(alpha)'-L(alpha) and L(beta)-L(alpha) transitions are driven by enthalpy and entropy, respectively The possible molecular origin of the phase transitions is discussed. The properties of SDPC are compared with that of membranes of monounsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31).

Effects of bilayer thickness on the activity of diacylglycerol kinase of Escherichia coli

We have developed a procedure for the reconstitution of Escherichia coli diacylglycerol kinase (DGK) into phospholipid bilayers containing diacylglycerol substrate. When DGK is reconstituted into a series of phosphatidylcholines containing monounsaturated fatty acyl chains, activity against dihexanoylglycerol (DHG) as a substrate was found to be markedly dependent on the fatty acyl chain length with the highest activity in dioleoylphosphatidylcholine [di(C18:1)PC] and a lower activity in bilayers with shorter or longer fatty acyl chains. Low activities in the short chain phospholipid dimyristoleoylphosphatidylcholine [di(C14:1)PC] followed from an increase in the K(m) value for DHG and ATP, with no effect on v(max). In contrast, in the long chain lipid dierucoylphosphatidylcholine [di(C24:1)PC], the low activity followed from a decrease in v(max) with no effect on K(m). In mixtures of two phosphatidylcholines with different chain lengths, the activity corresponded to that expected for the average chain length of the mixture. Cholesterol increased the activity in di(C14:1)PC but slightly decreased it in di(C18:1)PC or di(C24:1)PC, effects that could follow from changes in bilayer thickness caused by cholesterol.

The physico-chemical characteristics of the phosphocholine-containing glycoglycerolipid MfGL-II govern the permeability properties of Mycoplasma fermentans

Mycoplasma fermentans seems to be involved in several pathogenic conditions in humans, and is among other things capable of fusing with T-cells and lymphocytes. The choline-containing phosphoglycolipid 6'-O-(3"-phosphocholine-2"-amino-1"-phospho-1",3"-propanediol)-alpha-D-glucopyranosyl-(1'-->3)-1,2-diacylglycerol (MfGL-II) in the membrane of M. fermentans has been suggested to enhance the fusion process, and the characteristics of MfGL-II were therefore investigated. When a cell culture ages the fraction of MfGL-II increases, and the fraction of the other major membrane lipid, phosphatidylglycerol (PtdGro), decreases concomitantly. Swelling experiments showed that the permeability and osmotic fragility are markedly reduced in aged cells. MfGL-II is selectively released into the surrounding medium when aged M. fermentans cells are incubated in buffer containing EDTA. The physico-chemical properties of MfGL-II were studied by NMR spectroscopy and differential scanning calorimetry, and they can explain the biochemical results. The temperature for the transition between gel and lamellar liquid crystalline (Lalpha) phases is 35-45 degrees C higher for MfGL-II than for PtdGro, which most probably gives rise to the reduced permeability in aged cells. At high water contents MfGL-II forms an Lalpha phase and isotropic aggregates which were interpreted to be vesicles with a radius of approximately 450 A. It is proposed that MfGL-II forms vesicles in the surrounding medium when it is released from the cell membrane. Neither EDTA nor Ca2+ ions have a significant influence on the aggregate structures formed by MfGL-II. Our results indicate that MfGL-II has no fusogenic properties. It is more probable that a recently identified lysolipid in the M. fermentans membrane acts as a fusogen.

New ordered metastable phases between the gel and subgel phases in hydrated phospholipids

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 microl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC(15)PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L(beta) phase transforms directly into subgel L(c) phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L(R1) and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L(R1) phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L(R1) and SGII phases are metastable phases, which may only be formed by cooling the respective gel L(beta) and L(beta') phases, and not by heating the subgel L(c) phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase.

Effect of cholesterol on interaction of dibucaine with phospholipid vesicles: a fluorescence study

Interaction of the local anesthetic dibucaine with small unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and dioleoyl phosphatidylcholine (DOPC) containing different mol percents of cholesterol has been studied by fluorescence spectroscopy. Fluorescence measurements on dibucaine in presence of phospholipid vesicles containing various amounts of cholesterol yielded a pattern of variation of wavelength at emission maximum and steady-state anisotropy which indicated that the microenvironment of dibucaine is more polar and flexible in membranes that contain cholesterol than in membranes without cholesterol. Experiments on quenching of fluorescence from membrane-associated dibucaine by potassium iodide showed a marked increase in quenching efficiency as the cholesterol content of the vesicles was increased, demonstrating increased accessibility of the iodide quenchers to dibucaine in the presence of cholesterol, when compared to that in its absence. Total emission intensity decay profiles of dibucaine yielded two lifetime components of approximately 1 ns and approximately 2.8--3.1 ns with mean relative contributions of approximately 25 and approximately 75%, respectively. The mean lifetime in vesicles was 20--30% smaller than in the aqueous medium and showed a moderate variation with cholesterol content. Fluorescence measurements at two different temperatures in DMPC SUVs, one at 33 degrees C, above the phase transition temperature and another at 25 degrees C, around the main phase transition, indicated two different mode of dibucaine localization. At 25 degrees C dibucaine partitioned differentially in presence and absence of cholesterol. However, at 33 degrees C the apparent partition coefficients remained unaltered indicating differences in the microenvironment of dibucaine in presence and absence of cholesterol in the phospholipid membranes.

Physical and chemical stability of marine lipid-based liposomes under acid conditions

Liposomes made from a marine lipid extract containing a high polyunsaturated fatty lipid ratio were submitted to large pH variations, ranging from 1 to 8. Shape transformations were followed by video microscopy using giant liposomes and micromanipulation experiments. Acidification induced a decrease of the vesicle size simultaneous to the appearance of invaginations. These pH-dependent structural rearrangements were interpreted in terms of osmotic shocks and chemical modifications of the membranes. Liposomes produced by direct filtration were studied using turbidity measurements and optical microscopy observations. A low pH led to an instantaneous vesicle aggregation and to complex supramolecular and/or morphological changes as a function of time. The subsequent buffer neutralization of the liposome suspensions induced a partial reversion of the aggregation phenomenon while the structural membrane rearrangements were persisting. Furthermore, weak chemical degradations (oxidation and hydrolysis) were evidenced when the vesicles were incubated at low pH up to a 24-h incubation time. Thus, although acidification revealed liposome size and shape changes, the bilayer structure was maintained indicating that marine lipid-based liposomes could be used as oral administration vectors.

Capabilities of liposomes for topological transformation

Dynamic behaviors of liposomes caused by interactions between liposomal membranes and surfactant were studied by direct real-time observation by using high-intensity dark-field microscopy. Solubilization of liposomes by surfactants is thought to be a catastrophic event akin to the explosion of soap bubbles in the air; however, the actual process has not been clarified. We studied this process experimentally and found that liposomes exposed to various surfactants exhibited unusual behavior, namely continuous shrinkage accompanied by intermittent quakes, release of encapsulated liposomes, opening up, and inside-out topological inversion.

Chromatographic retention of drug molecules on immobilised liposomes prepared from egg phospholipids and from chemically pure phospholipids

The partitioning of a chemically diverse set of drugs into liposomes was studied by immobilised liposome chromatography (ILC). For this purpose liposomes composed of (i) purified egg phospholipids (EPL), (ii) synthetic phosphatidylcholine (PC), (iii) PC--synthetic phosphatidylethanolamine (PE) 80:20 (mol/mol) and (iv) PC--synthetic phosphatidylserine (PS) 80:20 (mol/mol) were immobilised in gel beads by freeze-thawing. The drug partitioning was assessed from the retention volume, which was expressed as a capacity factor, K(s), normalised with respect to the amount of immobilised phospholipid. The drug retention on EPL, PC and PC--PE liposomes was very similar, whereas the negatively charged PC--PS liposomes increased the retention of positively charged and decreased retention of negatively charged drugs. The partitioning of drugs on liposome columns (log K(s)) versus their octanol--water partitioning (log P(oct)) showed three separate rectilinear relationships, depending on the charge of the compound (neutral, positive, or negative). Statistical analysis (ANCOVA) proved that the lines had similar slopes. Repeated analysis of four reference compounds showed a low variation (<0.12 log units) over time (about 250 days). A close relationship was observed between the drug retention in short EPL columns with a low content of phospholipids and the retention in longer standard EPL columns. The short 'quick screen bilayer columns' permit analysis of highly lipophilic compounds within 30 min and are thus applicable for medium-throughput screening in drug discovery settings. A very strong rectilinear relationship (r(2)=0.95, n=13) between log K(s) (EPL) and published liposome partitioning data (log D(mem)) confirmed that the ILC drug retention reflects the drug partitioning into the lipid bilayers. A moderate to fair rectilinear relationship was observed between the normalised retention on PC, PC-PE and EPL liposomes (r(2)=0.79, 0.86 and 0.85, respectively, n=24) and corresponding published log k'(IAM) data obtained on immobilised artificial membrane (IAM) columns. Transport across Caco-2 cell monolayers (log P(c)) showed curvilinear relationships with log K(s), log k'(IAM), log P(oct) and log D(oct). The drug fraction absorbed in humans showed a similar relationship to log K(s) values as to surface plasmon resonance signals representing drug-liposome interaction (Danelian et al., 2000 J Med Chem, 43, 2083--2086).

A macroscopic description of lipid bilayer phase transitions of mixed-chain phosphatidylcholines: chain-length and chain-asymmetry dependence

A macroscopic model is presented to quantitatively describe lipid bilayer gel to fluid phase transitions. In this model, the Gibbs potential of the lipid bilayer is expressed in terms of a single order parameter q, the average chain orientational order parameter. The Gibbs potential is based on molecular mean-field and statistical mechanical calculations of inter and intrachain interactions. Chain-length and chain-asymmetry are incorporated into the Gibbs potential so that one equation provides an accurate description of mixed-chain phosphatidylcholines of a single class. Two general classes of lipids are studied in this work: lipid bilayers of partially or noninterdigitated gel phases, and bilayers of mixed interdigitated gel phases. The model parameters are obtained by fitting the transition temperature and enthalpy data of phosphatidylcholines to the model. The proposed model provides estimates for the transition temperature and enthalpy, van der Waals energy, number of gauche bonds, chain orientational order parameter, and bond rotational and excluded volume entropies, achieving excellent agreement with existing data obtained with various techniques.

Insolubility of lipids in triton X-100: physical origin and relationship to sphingolipid/cholesterol membrane domains (rafts)

The insolubility of lipids in detergents is a useful method for probing the structure of biological membranes. Insolubility in detergents like Triton X-100 is observed in lipid bilayers that exist in physical states in which lipid packing is tight. The Triton X-100-insoluble lipid fraction obtained after detergent extraction of eukaryotic cells is composed of detergent-insoluble membranes rich in sphingolipids and cholesterol. These insoluble membranes appear to arise from sphingolipid- and cholesterol-rich membrane domains (rafts) in the tightly packed liquid ordered state. Because the degree of lipid insolubility depends on the stability of lipid-lipid interactions relative to lipid-detergent interactions, the quantitative relationship between rafts and detergent-insoluble membranes is complex, and can depend on lipid composition, detergent and temperature. Nevertheless, when used conservatively detergent insolubility is an invaluable tool for studying cellular rafts and characterizing their composition.

Structure of lipid bilayers

The quantitative experimental uncertainty in the structure of fully hydrated, biologically relevant, fluid (L(alpha)) phase lipid bilayers has been too large to provide a firm base for applications or for comparison with simulations. Many structural methods are reviewed including modern liquid crystallography of lipid bilayers that deals with the fully developed undulation fluctuations that occur in the L(alpha) phase. These fluctuations degrade the higher order diffraction data in a way that, if unrecognized, leads to erroneous conclusions regarding bilayer structure. Diffraction measurements at high instrumental resolution provide a measure of these fluctuations. In addition to providing better structural determination, this opens a new window on interactions between bilayers, so the experimental determination of interbilayer interaction parameters is reviewed briefly. We introduce a new structural correction based on fluctuations that has not been included in any previous studies. Updated measurements, such as for the area compressibility modulus, are used to provide adjustments to many of the literature values of structural quantities. Since the gel (L(beta)') phase is valuable as a stepping stone for obtaining fluid phase results, a brief review is given of the lower temperature phases. The uncertainty in structural results for lipid bilayers is being reduced and best current values are provided for bilayers of five lipids.

Estimations of lipid bilayer geometry in fluid lamellar phases

The excess water bilayer thickness, d(l,0), and molecular area, A(0), of lipid amphiphiles in the fluid lamellar phases of dioleoylphosphatidylcholine (DOPC) and dipalmitoleoylphosphatidylcholine (DPolPC) have been estimated between 15 and 50 degrees C and for dimyristoylphosphatidylcholine (DMPC) between 25 and 50 degrees C. These determinations have been made from X-ray measurements on samples of known water composition. With respect to temperature, T, d(l,0) and A(0) are well fitted to a linear equation. We find d(l,0) (A)=(35.68+/-0.02)-(0.0333+/-0.0006)T (degrees C) and A(0) (A(2))=(70.97+/-0.05)+(0.136+/-0.001)T (degrees C) for DOPC, d(l,0) (A)=(35.2+/-0.1)-(0.068+/-0.003)T (degrees C) and A(0) (A(2))=(59.7+/-0.2)+(0.210+/-0.006)T (degrees C) for DMPC, and d(l,0) (A)=(34.54+/-0.03)-(0.0531+/-0.0009)T (degrees C) and A(0) (A(2))=(67.12+/-0.09)+(0.173+/-0.003)T (degrees C) for DPolPC. The accuracy of these estimates depends largely on how accurately the excess water point is determined. Ideally, reliable X-ray and compositional data will be available around the excess water and it may be found by simple inspection, but this is the exception rather than the rule, since samples close to water excess normally sequester sizeable amounts of water in defects, which lead to an underestimate of d(l,0). and overestimate of A(0). In this paper, we report a methodology for identifying and removing such data points and fitting the remaining data in order to determine the excess water point.

Fourier transform infrared spectroscopy and differential scanning calorimetry studies of fatty acid homogeneous ceramide 2

Ceramides provide a major component of the barrier function of skin. An understanding of barrier organization requires a detailed characterization of ceramide phase behavior and molecular interactions. Toward this end, Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) studies of ceramide 2 analogues (non-hydroxylated fatty acid N-acyl sphingosines) of specific chain lengths (C(14), C(16), C(18), C(20)) are presented. In addition, the molecular interactions of the individual chains in each molecule are elucidated through thermotropic FTIR studies of derivatives possessing perdeuterated fatty acid chains. DSC data showed a much smaller chain length variation (for the C(16), C(18), C(20) derivatives) in the main order-disorder transition temperature (approx. 93+/-1 degrees C) than is observed in the corresponding series of phosphatidylcholines, consistent with minimal ceramide hydration. The temperature dependence of the methylene stretching and scissoring modes revealed a solid-solid phase transition at 20-25 degrees C below the main order-disorder transition accompanied by chain packing alterations from orthorhombic-->hexagonal subcells. The chain packing transition was accompanied by enhanced penetration of water into the polar region. This was deduced from the temperature dependence of the amide I and II modes, which provide direct evidence for H-->D exchange. The CD(2) scissoring mode splitting of the deuterated fatty acid constituent of the C(16), C(18), C(20) chains revealed preferential segregation of microdomains (3-5 chains) of this species within the orthorhombic phase. In contrast, the sphingosine base chains appeared to be sufficiently separated so as to inhibit interchain vibrational coupling between them. FTIR spectroscopy provides a convenient means for characterizing domain formation, chain packing, and hydration sites of these phases, which are highly ordered under physiological conditions.

Oxidation of unsaturated phospholipids in membrane bilayer mixtures is accompanied by membrane fluidity changes

Steady-state and time-resolved fluorescence spectroscopy has been used to obtain information on oxidation processes and associated dynamical and structural changes in model membrane bilayers made from single unilamellar vesicles (SUV's) of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) mixed with increasing amounts of 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (SAPC). The highly unsaturated arachidonoyl chain containing four double bonds is prone to oxidation. Lipid oxidation was initiated chemically by a proper oxidant and could be followed on line via the fluorescence changes of an incorporated fluorescent lipophilic fatty acid: 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid (BP-C11). The oxidation rate increases with an increasing amount of SAPC. Size measurements of different SUV's incorporated with a trace amount of a phosphatidylcholine analogue of BP-C11 using fluorescence correlation spectroscopy have demonstrated that an increase of lipid unsaturation results in smaller sized SUV's and therefore to a larger curvature of the outer bilayer leaflet. This suggests that the lipid-lipid spacing has increased and that the unsaturated fatty acyl chains are better accessible for the oxidant. Oxidation results in some characteristic physical changes in membrane dynamics and structure, as indicated by the use of specific fluorescence probes. Fluorescence measurements of both dipyrenyl- and diphenylhexatriene-labelled PC introduced in non-oxidised and oxidised DOPC-SAPC membranes clearly show that the microfluidity (local fluidity at the very site of the probes) significantly decreases when the oxidised SAPC content increases in the lipid mixture. A similar effect is observed from the lateral diffusion experiments using monopyrenyl PC in the same membrane systems: the lateral diffusion is distinctly slower in oxidised membranes.

Pretransitional effects in dimyristoylphosphatidylcholine vesicle membranes: optical dynamometry study

We used micron-sized latex spheres to probe the phase state and the viscoelastic properties of dimyristoylphosphatidylcholine (DMPC) bilayers as a function of temperature. One or two particles were manipulated and stuck to a DMPC giant vesicle by means of an optical trap. Above the fluid-gel main transition temperature, T(m) congruent with 23.4 degrees C, the particles could move on the surface of the vesicle, spontaneously (Brownian motion) or driven by an external force, either gravity or the laser beam's radiation pressure. From the analysis of the particle motions, we deduced the values of the membrane hydrodynamic shear viscosity, eta(s), and found that it would increase considerably near T(m). Below T(m), the long-distance motion of the particles was blocked. We performed experiments with two particles stuck on the membrane. By optical dynamometry, we measured the elastic resistance of the membrane to a variation in the interparticle distance and found that it would decrease considerably (down to zero) when the temperature was increased to T(m). We propose an interpretation relating the elastic response to the membrane curvature modulus, k(C). In this scheme, the two-bead dynamometry experiments provide a direct measurement of k(C) in the P'(beta) phase of lipid bilayers.

Detection of the metastable rippled gel phase in hydrated phosphatidylcholine by fluorescence spectroscopy

Steady-state and time-resolved emission spectroscopy of 1-anilinonaphthalene-8-sulfonic acid (ANS) have been used for characterization of the metastable rippled gel phase, Pbeta'(mst), formed in fully-hydrated dipalmitoylphosphatidylcholine (DPPC) upon cooling from the liquid crystalline phase Lalpha [Tenchov et al., Biophys. J. 56 (1989) 757]. The Pbeta'(mst) phase of DPPC clearly differs from the stable Pbeta' phase by increased (approximately 27%) ANS emission intensity, by enhanced (approximately 23%) average radiative rate constant, and by reduced (approximately 18%) non-radiative quenching rate constant. The fluorescence intensity peak at the Pbeta'-->Lalpha transition temperature is replaced by a large, reversible stepwise intensity drop at the Pbeta'(mst)-->Lalpha transition. No such effects have been found for dimiristoylphosphatidylcholine (DMPC) dispersions confirming previous results that DMPC does not form a Pbeta'(mst) phase. Since ANS is known to predominantly reside in the interfacial region, the observed effects indicate differences between the stable and metastable rippled phases in the organization and dynamics of their lipid/water interfaces. The data demonstrate that the metastable rippled phase manifests its appearance also through interactions with small molecules (ANS size approximately 8 A).

Hydration of DMPC and DPPC at 4 degrees C produces a novel subgel phase with convex-concave bilayer curvatures

Hydration of dimyristoyl- and dipalmitoylphosphatidylcholines at 4 degrees C results in the formation of a characteristic subgel phase designated Pcc. Examination of the phase by freeze-fracture electron microscopy shows convex-concave deformations of the planar bilayer which are of two types. A smaller type with a radius of curvature of about 20 nm predominates in DMPC, and a larger type with about 70 nm radii of curvatures dominates in DPPC. The Pcc phase can also be formed in samples hydrated at temperatures above the main phase transition if the dispersion is frozen slowly and subsequently incubated at 4 degrees C for several days. The subgel Pcc phase was distinguished from the subgel Lc phase by the temperature of transition, packing of the acyl chains on the basis of wide-angle X-ray diffraction, and 2H-NMR spectra characteristic of a 'solid-ordered' phase. Vibrational spectra of the carbonyl and phosphate regions are consistent with a partially reduced hydration state. The origin of the convex-concave bilayer deformation is believed to result from constraints imposed by limiting hydration of the headgroup and a frustration arising from the spontaneous curvature of both monolayers.

Protein-induced membrane disorder: a molecular dynamics study of melittin in a dipalmitoylphosphatidylcholine bilayer

A molecular dynamics simulation of melittin in a hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer was performed. The 19, 000-atom system included a 72-DPPC phospholipid bilayer, a 26-amino acid peptide, and more than 3000 water molecules. The N-terminus of the peptide was protonated and embedded in the membrane in a transbilayer orientation perpendicular to the surface. The simulation results show that the peptide affects the lower (intracellular) layer of the bilayer more strongly than the upper (extracellular) layer. The simulation results can be interpreted as indicating an increased level of disorder and structural deformation for lower-layer phospholipids in the immediate vicinity of the peptide. This conclusion is supported by the calculated deuterium order parameters, the observed deformation at the intracellular interface, and an increase in fractional free volume. The upper layer was less affected by the embedded peptide, except for an acquired tilt relative to the bilayer normal. The effect of melittin on the surrounding membrane is localized to its immediate vicinity, and its asymmetry with respect to the two layers may result from the fact that it is not fully transmembranal. Melittin's hydrophilic C-terminus anchors it at the extracellular interface, leaving the N-terminus "loose" in the lower layer of the membrane. In general, the simulation supports a role for local deformation and water penetration in melittin-induced lysis. As for the peptide, like other membrane-embedded polypeptides, melittin adopts a significant 25 degree tilt relative to the membrane normal. This tilt is correlated with a comparable tilt of the lipids in the upper membrane layer. The peptide itself retains an overall helical structure throughout the simulation (with the exception of the three N-terminal residues), adopting a 30 degree intrahelical bend angle.

Calorimetric and molecular mechanics studies of the thermotropic phase behavior of membrane phospholipids

In this review, we summarize the results of recent studies on the main phase transition behavior of phospholipid bilayers using the combined approaches of molecular mechanics simulations and high-resolution differential scanning calorimetry. Following a brief overview of the phase transition phenomenon exhibited by the lipid bilayer, we begin with the review by showing how several structural parameters underlying various phospholipids including phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol are defined and determined. Specifically, these structural parameters are obtained with saturated lipids packed in the gel-state bilayer using computer-based molecular mechanics calculations. Then we proceed to present the calorimetric data obtained with the lipid bilayer composed of saturated phospholipids as it undergoes the gel-to-liquid-crystalline phase transition in excess water. The general equations that can correlate the gel-to-liquid-crystalline phase transition temperature (T(m)) of the lipid bilayer with the structural parameters of the lipid molecule constituting the lipid bilayer are subsequently presented. From these equations, two tables of predicated T(m) values for well over 400 molecular species of saturated phosphatidylcholine and saturated phosphatidylethanolamine are generated. We further review the structure and chain-melting behavior of a large number of sn-1 saturated/sn-2 unsaturated phospholipids. Two T(m)-diagrams are shown, from which the effects of the number and the position of one to five cis carbon-carbon double bonds on T(m) can be viewed simultaneously. Finally, in the last part of this review, simple molecular models that have been invoked to interpret the characteristic T(m) trends exhibited by lipid bilayers composed of unsaturated lipids with different numbers and positions of cis carbon-carbon double bonds as seen in the T(m)-diagram are presented.

Interactions of angiotensin II non-peptide AT(1) antagonist losartan with phospholipid membranes studied by combined use of differential scanning calorimetry and electron spin resonance spectroscopy

We used differential scanning calorimetry (DSC) and electron spin resonance (ESR) spectroscopy to investigate the interactions of Losartan, a potent, orally active Angiotensin II AT(1) receptor antagonist with phospholipid membranes. DSC results showed that Losartan sensitively affected the chain-melting behavior of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes. ESR spectroscopy showed that phosphatidylcholines spin-labeled at the 5-position of the sn-2 acyl chain (n-PCSL with n=5), incorporated either in DMPC or DPPC bilayers containing Losartan, were restricted in motion both in the gel and in the liquid-crystalline membrane phases, indicating a location of the antagonist close to the interfacial region of the phosphatidylcholine bilayer. At high drug concentrations (mole fraction >/= x=0.60), the decrease in chain mobility registered by 5-PCSL in fluid-phase membranes is smaller than that found at lower concentrations, whereas that registered by 14-PCSL is further increased. This indicates a different mode of interaction with Losartan at high concentrations, possibly arising from a location deeper within the bilayer. Additionally, Losartan reduced the spin-spin broadening of 12-PCSL spin labels in the gel-phase of DMPC and DPPC bilayers. As a conclusion, our study has shown that Losartan interacts with phospholipid membranes by affecting both their thermotropic behavior and molecular mobility.

Two-photon fluorescence microscopy observation of shape changes at the phase transition in phospholipid giant unilamellar vesicles

Using the sectioning effect of the two-photon fluorescence microscope, we studied the behavior of phospholipid giant unilamellar vesicles (GUVs) composed of pure diacylphosphatidylcholine phospholipids during the gel-to-liquid crystalline phase transition. We used the well-characterized excitation generalized polarization function (GP(ex)) of 6-dodecanoyl-2-dimethylamine-naphthalene (LAURDAN), which is sensitive to the changes in water content in the lipid vesicles, to monitor the phase transition in the GUVs. Even though the vesicles do not show temperature hysteresis at the main phase transition, we observed different behaviors of the vesicle shape, depending on how the GUV sample reaches the main phase transition. During the cooling cycles, we observed an increase in the vesicle diameter at the phase transition ( approximately 0.5-1%), followed by a decrease in the diameter when the vesicle reached the gel phase. During the heating cycles and close to the phase transition temperature, a surprising behavior is observed, showing a sequence of different vesicle shapes as follows: spherical-polygonal-ellipsoidal. We attribute these changes to the effect of lipid domain coexistence on the macroscopic structure of the GUVs. The "shape hysteresis" in the GUVs is reversible and largely independent of the temperature scan rate. In the presence of 30 mol% of cholesterol the events observed at the phase transition in the GUVs formed by pure phospholipids were absent.