Thermotropic and barotropic phase transitions of N-methylated dipalmitoylphosphatidylethanolamine bilayers

Insights into Molecular Interactions between a GAPDH-Related Fish Antimicrobial Peptide, Analogs Thereof, and Bacterial Membranes

Interactions between SJGAP (skipjack tuna GAPDH-related antimicrobial peptide) and four analogs thereof with model bacterial membranes were studied using Fourier-transform infrared spectroscopy (FTIR) and molecular dynamics (MD) simulations. MD trajectory analyses showed that the N-terminal segment of the peptide analogs has many contacts with the polar heads of membrane phospholipids, while the central α helix interacts strongly with the hydrophobic core of the membranes. The peptides also had a marked influence on the wave numbers associated with the phase transition of phospholipids organized as liposomes in both the interface and aliphatic chain regions of the infrared spectra, supporting the interactions observed in the MD trajectories. In addition, interesting links were found between peptide interactions with the aliphatic chains of membrane phospholipids, as determined by FTIR and from the MD trajectories, and the membrane permeabilization capacity of these peptide analogs, as previously demonstrated. To summarize, the combined experimental and computational efforts have provided insights into crucial aspects of the interactions between the investigated peptides and bacterial membranes. This work thus makes an original contribution to our understanding of the molecular interactions underlying the antimicrobial activity of these GAPDH-related antimicrobial peptides from Scombridae.

The effect of methylated phosphatidylethanolamine derivatives on the ionization properties of signaling phosphatidic acid

Phosphatidylethanolamine (PE) and Phosphatidylcholine (PC) are the most abundant glycerophospholipids in eukaryotic membranes. The differences in the physicochemical properties of their headgroups have contrasting modulatory effects on their interaction with intracellular macromolecules. As such, their overall impact on membrane structure and function differs significantly. Enzymatic methylation of PE's amine headgroup produces two methylated derivatives namely monomethyl PE (MMPE) and dimethyl PE (DMPE) which have physicochemical properties that generally range between that of PE and PC. Additionally, their influence on membrane properties differs from both PE and PC. Although variations in headgroup methylation have been reported to affect signaling pathways, the direct influence that these differences exert on the ionization properties of signaling phospholipids have not been investigated. Here, we briefly review membrane function and structure that are mediated by the differences in headgroup methylation between PE, MMPE, DMPE and PC. In addition, using ³¹P MAS NMR, we investigate the effect of these four phospholipids on the ionization properties of the ubiquitous signaling anionic lipid phosphatidic acid (PA). Our results show that PA's ionization properties are differentially affected by changes in phospholipid headgroup methylation. This could have important implications for PA-protein binding and hence physiological functions in cells where signaling events lead to changes in abundance of methylated PE derivatives in the membrane.

Location of the TEMPO moiety of TEMPO-PC in phosphatidylcholine bilayers is membrane phase-dependent

The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) moiety tethered to the headgroup of phosphatidylcholine (PC) lipid is employed in spin labeling electron paramagnetic resonance (EPR) spectroscopy to probe the water dynamics near lipid bilayer interfaces. Due to its amphiphilic character, however, TEMPO spin label could partition between aqueous and lipid phases, and may even be stabilized in the lipid phase. Accurate assessment of the TEMPO-PC configuration in bilayer membranes is essential for correctly interpreting the data from measurements. Here, we carry out all-atom molecular dynamics (MD) simulations of TEMPO-PC probe in single-component lipid bilayers at varying temperatures, using two standard MD force fields. We find that for DPPC membrane whose gel-to-fluid lipid phase transition occurs at 314 K, while the TEMPO spin label is stabilized above the bilayer interface in the gel phase, there is a preferential location of TEMPO below the membrane interface in the fluid phase. For bilayers made of unsaturated lipids, DOPC and POPC, which adopt the fluid phase at ambient temperature, TEMPO is unequivocally stabilized inside the bilayers. Our finding of membrane phase-dependent positioning of TEMPO moiety highlights the importance of assessing the packing order and fluidity of lipids under a given measurement condition.

Polymer-Lipid Hybrid Materials

Hierarchic self-assembly underpins much of the form and function seen in synthetic or biological soft materials. Lipids are paramount examples, building themselves in nature or synthetically in a variety of meso/nanostructures. Synthetic block copolymers capture many of lipid's structural and functional properties. Lipids are typically biocompatible and high molecular weight polymers are mechanically robust and chemically versatile. The development of new materials for applications like controlled drug/gene/protein delivery, biosensors, and artificial cells often requires the combination of lipids and polymers. The emergent composite material, a "polymer-lipid hybrid membrane", displays synergistic properties not seen in pure components. Specific examples include the observation that hybrid membranes undergo lateral phase separation that can correlate in registry across multiple layers into a three-dimensional phase-separated system with enhanced permeability of encapsulated drugs. It is timely to underpin these emergent properties in several categories of hybrid systems ranging from colloidal suspensions to supported hybrid films. In this review, we discuss the form and function of a vast number of polymer-lipid hybrid systems published to date. We rationalize the results to raise new fundamental understanding of hybrid self-assembling soft materials as well as to enable the design of new supramolecular systems and applications.

Externally Triggered Novel Rapid-Release Sonosensitive Folate-Modified Liposomes for Gemcitabine: Development and Characteristics

PURPOSE

To develop an externally triggered rapid-release targeted system for treating ovarian cancer, gemcitabine (GMC) was entrapped into sonosensitive (SoS) folate (Fo)-modified liposomes (LPs).

METHODS

GMC-loaded LPs (GMC LPs), GMC-loaded Fo-targeted LPs (GMC-Fo LPs), and GMC-loaded Fo-targeted SoS LPs (GMC-SoS Fo LPs) were prepared utilizing a film-hydration technique and evaluated based on particle size, ζ-potential, and percentage entrapped drug. Cellular uptake of the fluorescent delivery systems in Fo-expressing ovarian cancer cells was quantified using flow cytometry. Finally, tumor-targeting ability, in vivo evaluation, and pharmacokinetic studies were performed.

RESULTS

GMC LPs, GMC-Fo LPs, and GMC-SoS Fo LPs were successfully prepared, with sizes of <120.3±2.4 nm, 39.7 mV ζ-potential, and 86.3%±1.84% entrapped drug. Cellular uptake of GMC-SoS Fo LPs improved 6.51-fold over GMC LPs (under ultrasonic irradiation - p<0.05). However, cellular uptake of GMC-Fo LPs improved just 1.24-fold over GMC LPs (p>0.05). Biodistribution study showed that of GMC concentration in tumors treated with GMC-SoS-Fo LPs (with ultrasound) improved 2.89-fold that of free GMC (p<0.05). In vivo, GMC-SoS Fo LPs showed the highest antiproliferative and antitumor action on ovarian cancer.

CONCLUSION

These findings showed that externally triggered rapid-release SoS Fo-modified LPs are a promising system for delivering rapid-release drugs into tumors.

Membrane States of Saturated Glycerophospholipids: A Thermodynamic Study of Bilayer Phase Transitions

Bilayer membranes formed by phospholipids vary in their membrane states by undergoing phase transitions in response to various external environmental factors. Pressure is one of these important environmental factors, but there are very few studies on the effects of pressure on phospholipid bilayer membranes. It is possible to deepen our understanding of the membrane states of phospholipid bilayer membranes by combining information regarding temperature- and/or ligand-responsivity with that regarding pressure-responsivity. In this review, we thermodynamically characterize the bilayer phase transitions of three kinds of saturated glycerophospholipids, each with a different polar head group (phosphatidyl-ethanolamine (PE), -choline (PC) or -glycerol (PG)), and explain their various membrane states depending on temperature and pressure. Both temperature- and pressure-responsivity reveal inherent features of these bilayer membranes: the metastability of the gel phase for PE bilayer membranes, the polymorphism of the gel phases for PC bilayer membranes and morphological changes in bilayer aggregates for PG bilayer membranes.

The acetylcholinesterase (AChE) inhibitor and anti-Alzheimer drug donepezil interacts with human erythrocytes

Donepezil is used to treat symptomatically the Alzheimer's disease (AD). This drug is a specific inhibitor of the enzyme acetylcholinesterase (AChE), whose main physiological function is to hydrolyze the neurotransmitter acetylcholine. The main objective of this work was to study the effect of donepezil on human erythrocytes as AChE is present in its membrane. For this purpose, human erythrocytes and molecular model of its membrane built-up of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) were used. The latter correspond to classes of phospholipids present in the outer and inner monolayers of the human erythrocyte membrane, respectively. Our experimental evidences obtained from X-ray diffraction and differential scanning calorimetry (DSC) analysis indicated that donepezil was capable of interacting with both phospholipids. Fluorescence spectroscopy results showed a moderate increase in the fluidity of the hydrophobic tails of DMPC and isolated unsealed human erythrocyte membranes (IUM). On the other hand, results by scanning electron microscopy (SEM) and optical defocusing microscopy (DM) showed that the drug changed the normal biconcave shape of the erythrocytes inducing the formation of stomatocytes (cup-shaped cells). This effect was explained by the incorporation of donepezil molecules into the erythrocyte membrane and interactions with AChE.

Dependence on acyl chain length of energy and volume parameters of the gel to liquid-crystalline transition of 1,2-diacylphosphatidylcholines. Theoretical consideration

The effect of acyl chain length on energy and volume parameters of gel to liquid-crystal transitions in phospholipids is analyzed. It is demonstrated that simple structural and thermodynamic considerations allow predicting some thermodynamic and volume characteristics of transitions and their dependencies on the acyl chains length.

Effect of pressure on bilayer phase behavior of N-methylated di-O-hexadecylphosphatidylethanolamines: relevance of head-group modification on the bilayer interdigitation

The phase transitions of N-methylated di-O-hexadecylphosphatidylethanolamines (DHPE, DH-N-methyl-PE (DHMePE) and DH-N,N-dimethyl-PE (DHMe₂PE)) were observed by differential scanning calorimetry (DSC) and fluorometry under atmospheric pressure and by light-transmittance measurements under high pressure. The DSC thermograms showed that the N-methylated DHPE bilayers underwent the phase transition from the gel phase to the liquid crystalline (L_α) phase under atmospheric pressure. The gel phase was identified by fluorometry as the lamellar gel (L_β) phase, and not interdigitated gel (L_βI) phase. The gel/L_α transition temperature increased with pressure while decreased stepwise with increasing polar head-group size. This stepwise depression of the transition temperature may be caused by the inverse-proportional hydrogen-bonding capabilities of the head-group to the head-group size. The thermodynamic quantities of the gel/L_α transition were comparable for the N-methylated DHPE bilayers. The pressure-induced L_βI phase was not found in these bilayers although the bilayer of di-O-hexadecylphosphatidylcholine (DHPC), which is a kind of N-methylated DHPEs, forms the L_βI phase only by hydration under atmospheric pressure. Taking into account that the bilayers of diacyl-homologs of N-methylated DHPEs, N-methylated dipalmitoyl-PEs except for dipalmitoylphosphatidylcholine (DPPC), do not form the L_βI phase in the whole pressure range investigated but the DPPC bilayer forms the L_βI phase under high pressure, we can say that the interdigitation requires weaker interaction between large-sized head groups like the bulky choline group.

Location and Effects of an Antitumoral Catechin on the Structural Properties of Phosphatidylethanolamine Membranes

Green tea catechins exhibit high diversity of biological effects including antioncogenic properties, and there is enormous interest in their potential use in the treatment of a number of pathologies. It is recognized that the mechanism underlying the activity of catechins relay in part in processes related to the membrane, and many studies revealed that the ability of catechins to interact with lipids plays a probably necessary role in their mechanism of action. We present in this work the characterization of the interaction between an antitumoral synthetically modified catechin (3-O-(3,4,5-trimethoxybenzoyl)-(-)-catechin, TMCG) and dimiristoylphosphatidyl-ethanolamine (DMPE) membranes using an array of biophysical techniques which include differential scanning calorimetry, X-ray diffraction, infrared spectroscopy, atomic force microscopy, and molecular dynamics simulations. We found that TMCG incorporate into DMPE bilayers perturbing the thermotropic transition from the gel to the fluid state forming enriched domains which separated into different gel phases. TMCG does not influence the overall bilayer assembly of phosphatidylethanolamine systems but it manages to influence the interfacial region of the membrane and slightly decrease the interlamellar repeat distance of the bilayer. TMCG seems to be located in the interior of the phosphatidylethanolamine bilayer with the methoxy groups being in the deepest position and some portion of the molecule interacting with the water interface. We believe that the reported interactions are significant not only from the point of view of the known antitumoral effect of TMCG, but also might contribute to understanding the basic molecular mechanism of the biological effects of the catechins found at the membrane level.

How Do Membranes Respond to Pressure?

Bilayers formed by phospholipids are fundamental structures of biological membranes. The mechanical perturbation brought about by pressure significantly affects the membrane states of phospholipid bilayers. In this chapter, we focus our attention on the pressure responsivity for bilayers of some major phospholipids contained in biological membranes. At first, the membrane states and phase transitions of phospholipid bilayers depending on water content, temperature and pressure are explained by using the bilayer phase diagrams of dipalmitoylphosphatidylcholine (DPPC), which is the most familiar phospholipid in model membrane studies. Subsequently, the thermotropic and barotropic bilayer phase behavior of various kinds of phospholipids with different molecular structures is discussed from the comparison of their temperature--pressure phase diagrams to that of the DPPC bilayer. It turns out that a slight change in the molecular structure of the phospholipids produces a significant difference in the bilayer phase behavior. The systematic pressure studies on the phase behavior of the phospholipid bilayers reveal not only the pressure responsivity for the bilayers but also the role and meaning of several important phospholipids existing in real biological membranes.

Structure and phase equilibria of the soybean lecithin/PEG 40 monostearate/water system

PEG stearates are extensively used as emulsifiers in many lipid-based formulations. However, the scheme of the principles of the lipid-surfactant polymer interactions are still poorly understood and need more studies. A new phase diagram of a lecithin/PEG 40 monostearate/water system at 30 °C is reported. First, we have characterized the binary PEG 40 monostearate/water system by the determination of the critical micelle concentration value and the viscous properties. Then, the ternary phase behavior and the influence of phase structure on their macroscopic properties are studied by a combination of different techniques, namely, optical microscopy, small-angle X-ray scattering, differential scanning calorimetry, and rheology. The phase behavior is complex, and some samples evolve even at long times. The single monophasic regions correspond to micellar, swollen lamellar, and lamellar gel phases. The existence of extended areas of phase coexistence (hexagonal, cubic, and lamellar liquid crystalline phases) may be a consequence of the low miscibility of S40P in the lecithin bilayer as well as of the segregation of the phospholipid polydisperse hydrophobic chains. The presence of the PEG 40 monostearate has less effect in the transformation to the cubic phase for lecithin than that found in other systems with simple glycerol-based lipids.

Physicochemical characterization of liposomes after ultrasound exposure - mechanisms of drug release

Ultrasound is investigated as a novel drug delivery tool within cancer therapy. Non-thermal ultrasound treatment of solid tumours post i.v.-injection of drug-carrying liposomes may induce local drug release from the carrier followed by enhanced intracellular drug uptake. Recently, ultrasound-mediated drug release of liposomes (sonosensitivity) was shown to strongly depend on liposome membrane composition. In the current study the ultrasound-mediated drug release mechanism of liposomes was investigated. The results showed that differences in ultrasound drug release kinetics obtained for different liposomal compositions were caused by distinctive release mechanisms of the carriers. Two types of liposomes composed of 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) and hydrogenated soy L-α-phosphatidylcholine (HSPC) as main lipids, respectively, were recently shown to vary in sonosensitivity. Here, these liposomes were analyzed prior to and after a given ultrasound-exposure for their mean size, size distribution and morphology. Cryo-transmission electron microscopy, dynamic light scattering and asymmetric flow field-flow fractionation in combination with multi-angle light scattering revealed a significant change in mean size, size distribution and morphology of DOPE-based liposomes after ultrasound, pointing to an irreversible disruption of the vesicles and concomitant drug release. In contrast, the HSPC-based liposomes remained unchanged in size and structure after ultrasound application, indicating pore-mediated release mechanisms. The results show that the release mechanisms and interactions between ultrasound and liposomes depend on the liposome membrane-composition, explaining their sonosensitive properties.

Thermotropic and barotropic phase behavior of phosphatidylcholine bilayers

Bilayers formed by phospholipids are frequently used as model biological membranes in various life science studies. A characteristic feature of phospholipid bilayers is to undergo a structural change called a phase transition in response to environmental changes of their surroundings. In this review, we focus our attention on phase transitions of some major phospholipids contained in biological membranes, phosphatidylcholines (PCs), depending on temperature and pressure. Bilayers of dipalmitoylphosphatidylcholine (DPPC), which is the most representative lipid in model membrane studies, will first be explained. Then, the bilayer phase behavior of various kinds of PCs with different molecular structures is revealed from the temperature-pressure phase diagrams, and the difference in phase stability among these PC bilayers is discussed in connection with the molecular structure of the PC molecules. Furthermore, the solvent effect on the phase behavior is also described briefly.

Study on the subgel-phase formation using an asymmetric phospholipid bilayer membrane by high-pressure fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.

Gd(III)-DOTA-modified sonosensitive liposomes for ultrasound-triggered release and MR imaging

Ultrasound-sensitive (sonosensitive) liposomes for tumor targeting have been studied in order to increase the antitumor efficacy of drugs and decrease the associated severe side effects. Liposomal contrast agents having Gd(III) are known as a nano-contrast agent system for the efficient and selective delivery of contrast agents into pathological sites. The objective of this study was to prepare Gd(III)-DOTA-modified sonosensitive liposomes (GdSL), which could deliver a model drug, doxorubicin (DOX), to a specific site and, at the same time, be capable of magnetic resonance (MR) imaging. The GdSL was prepared using synthesized Gd(III)-DOTA-1,2-distearoyl-sn-glycero-3-phosphoethanolamine lipid. Sonosensitivity of GdSL to 20-kHz ultrasound induced 33% to 40% of DOX release. The relaxivities (r1) of GdSL were 6.6 to 7.8 mM-1 s-1, which were higher than that of MR-bester®. Intracellular uptake properties of GdSL were evaluated according to the intensity of ultrasound. Intracellular uptake of DOX for ultrasound-triggered GdSL was higher than that for non-ultrasound-triggered GdSL. The results of our study suggest that the paramagnetic and sonosensitive liposomes, GdSL, may provide a versatile platform for molecular imaging and targeted drug delivery.

Thermotropic and barotropic phase transitions of dialkyldimethylammonium bromide bilayer membranes: effect of chain length

The bilayer phase transitions of dialkyldimethylammonium bromides (2C(n)Br; n = 12, 14, 16) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. Under atmospheric pressure, the 2C(12)Br bilayer membrane underwent the stable transition from the lamellar crystal (L(c)) phase to the liquid crystalline (L(α)) phase. The 2C(14)Br bilayer underwent the main transition from the metastable lamellar gel (L(β)) phase to the metastable L(α) phase in addition to the stable L(c)/L(α) transition. For the 2C(16)Br bilayer, moreover, three kinds of phase transitions were observed: the metastable main transition, the metastable transition from the metastable lamellar crystal (L(c(2))) phase to the metastable L(α) phase, and the stable lamellar crystal (L(c(1)))/L(α) transition. The temperatures of all the phase transitions elevated almost linearly with increasing pressure. The temperature (T)-pressure (p) phase diagrams of the 2C(12)Br and 2C(14)Br bilayers were simple, but that of the 2C(16)Br bilayer was complex; that is, the T-p curves for the metastable main transition and the L(c(2))/L(α) transition intersect at ca. 25 MPa, which means the inversion of the relative phase stability between the metastable phases of L(β) and L(c(2)) above and below the pressure. Moreover, the T-p curve of the L(c(2))/L(α) transition was separated into two curves under high pressure, and as a result, the pressure-induced L(c(2P)) phase appeared in between. Thermodynamic quantities for phase transitions of the 2C(n)Br bilayers increased with an increase in alkyl-chain length. The chain-length dependence of the phase-transition temperature for all kinds of transitions observed suggests that the stable L(c(1))/L(α) transition incorporates the metastable L(c(2))/L(α) transition in the bilayers of 2C(n)Br with shorter alkyl chains, and the main-transition of the 2C(12)Br bilayer would occur at a temperature below 0 °C.

Effect of sphingomyelin headgroup size on molecular properties and interactions with cholesterol

Sphingomyelins (SMs) and sterols are important constituents of the plasma membrane and have also been identified as major lipid components in membrane rafts. Using SM analogs with decreasing headgroup methylation, we systemically analyzed the effect of headgroup size on membrane properties and interactions with cholesterol. An increase in headgroup size resulted in a decrease in the main phase transition. Atom-scale molecular-dynamics simulations were in agreement with the fluorescence anisotropy experiments, showing that molecular areas increased and acyl chain order decreased with increasing headgroup size. Furthermore, the transition temperatures were constantly higher for SM headgroup analogs compared to corresponding phosphatidylcholine headgroup analogs. The sterol affinity for phospholipid bilayers was assessed using a sterol-partitioning assay and an increased headgroup size increased sterol affinity for the bilayer, with a higher sterol affinity for SM analogs as compared to phosphatidylcholine analogs. Moreover, the size of the headgroup affected the formation and composition of cholesterol-containing ordered domains. Palmitoyl-SM (the largest headgroup) seemed to attract more cholesterol into ordered domains than the other SM analogs with smaller headgroups. The ordering and condensing effect of cholesterol on membrane lipids was also largest for palmitoyl-SM as compared to the smaller SM analogs. The results show that the size of the SM headgroup is crucially important for SM-SM and SM-sterol interactions. Our results further emphasize that interfacial electrostatic interactions are important for stabilizing cholesterol interactions with SMs.

Hydrostatic pressure reveals bilayer phase behavior of dioctadecyldimethylammonium bromide and chloride

Bilayer phase transitions of dioctadecyldimethylammonium bromide (2C(18)Br) and chloride (2C(18)Cl) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. The 2C(18)Br bilayer membrane showed different kinds of transitions depending on preparation methods of samples under atmospheric pressure. Under certain conditions, the 2C(18)Br bilayer underwent three kinds of transitions, the metastable transition from the metastable lamellar crystal (L(c(2))) phase to the metastable lamellar gel (L(β)) phase at 35.4 °C, the metastable main transition from the metastable L(β) phase to the metastable liquid crystalline (L(α)) phase at 44.5 °C, and the stable transition from the stable lamellar crystal (L(c(1))) phase to the stable L(α) phase at 52.8 °C. On the contrary, the 2C(18)Cl bilayer underwent two kinds of transitions, the stable transition from the stable L(c) phase to the stable L(β) phase at 19.7 °C and the stable main transition from the stable L(β) phase to the stable L(α) phase at 39.9 °C. The temperatures of the phase transitions of the 2C(18)Br and 2C(18)Cl bilayers were almost linearly elevated by applying pressure. It was found from the temperature (T)-pressure (p) phase diagram of the 2C(18)Br bilayer that the T-p curves for the main transition and the L(c(1))/L(α) transition intersect at ca. 130 MPa because of the larger slope of the former transition curve. On the other hand, the T-p phase diagram of the 2C(18)Cl bilayer took a simple shape. The thermodynamic properties for the main transition of the 2C(18)Br and 2C(18)Cl bilayers were comparable to each other, whereas those for the L(c(1))/L(α) transition of the 2C(18)Br bilayer showed considerably high values, signifying that the L(c(1)) phase of the 2C(18)Br bilayer is extremely stable. These differences observed in both bilayers are attributable to the difference in interaction between a surfactant and its counterion.

A peculiar phase transition of plasmalogen bilayer membrane under high pressure

The bilayer phase transition of plasmalogen, monounsaturated plasmenylcholine 1-O-1'-(Z)-octadecenyl-2-oleoyl-sn-glycero-3-phosphocholine (Plg-SOPC), was examined by differential scanning calorimetry, high-pressure transmittance, and fluorescence techniques. The bilayer properties of Plg-SOPC such as the temperature-pressure phase diagram, the thermodynamic quantities of the transition, and the location of a fluorescent membrane probe in the bilayer, were compared with those of a similar phospholipid 1-stearoyl-2-oleoyl-phosphatidylcholine (SOPC). It turned out that a vinyl-ether bond in the sn-1 position of the glycerol backbone in the Plg-SOPC molecule produces a peculiar phase transition under high pressure and significantly affects the membrane properties.

High-pressure study on bilayer phase behavior of oleoylmyristoyl- and myristoyloleoyl-phosphatidylcholines

We investigated the thermotropic and barotropic bilayer phase behavior of 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC) and 1-oleoyl-2-myristoyl-sn-glycero-3-phosphocholine (OMPC) by means of the differential scanning calorimetry (DSC) and high-pressure light-transmittance technique. Water could be used as a solvent for measurements at high pressures because of the elevation of the transition temperatures above 0 degrees C by pressurization, whereas aqueous 50 wt.% ethylene glycol solution was used mainly for those at low pressures. Only one phase transition was observed in the DSC thermogram of the MOPC bilayer membrane as an endothermic peak, and also observed at high pressures as an abrupt change of the light-transmittance. The transition was assigned as a main transition between the lamellar gel (L(beta)) and liquid-crystalline (L(alpha)) phases on the basis of the values of enthalpy change (DeltaH) and slope of the transition temperature with respect to pressure (dT/dP). The DSC thermogram of the OMPC bilayer membrane similarly showed a single endothermic peak but two kinds of phase transitions were observed at different temperatures in the light-transmittance profile at high pressures. The extrapolation of the lower-temperature transition in the high-pressure range to an ambient pressure coincided with the transition observed in the DSC thermogram. This transition was identified as a transition between the lamellar crystal (L(c)) and L(alpha) (or L(beta)) phases from the DeltaH and dT/dP values. The higher-temperature transition, appearing only at high pressures, was identified as the L(beta)/L(alpha) transition considering the topological resemblance of its temperature-pressure phase diagram as that of the dioleoylphosphatidylcholine bilayer membrane. The phase diagram of the OMPC bilayer membrane demonstrated that the L(beta) phase cannot exist at pressures below ca. 190 MPa while it can exist stably in a finite temperature range at pressures above the pressure.

Pressure-induced phase transitions of lipid bilayers observed by fluorescent probes Prodan and Laurdan

The fluorescence spectra of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and 6-dodecanoyl-2-(dimethylamino)naphthalene (Laurdan) in bilayer membranes of 1,2-distearoylphosphatidylcholine (DSPC) were observed as a function of pressure at constant temperature. The emission spectra of Prodan and Laurdan varied with the pressure-induced states of bilayer membranes. The maximum emission wavelength (lambda(max)) of Prodan characteristic of the liquid crystalline (L(alpha)), lamellar gel (L(beta)') and pressure-induced interdigitated gel (L(beta)I) phases of the DSPC bilayer was 480, 440 and 500 nm, respectively. On the other hand, the lambda(max) of Laurdan characteristic of the L(alpha) and L(beta)' phases was 480 and 440 nm in a similar manner as Prodan probe. However, no change in the lambda(max) was observed in spite of the occurrence of the interdigitation of bilayer. Since the lambda(max) reflects the solvent property around the probe molecules, we could speculate about the location of fluorescent probe in the bilayer membranes. In the L(alpha) phase the same chromophore group of Prodan and Laurdan probes distributes around phosphate group of lipid (i.e., polar region). The transformation of bilayer into the L(beta)' phase causes the Prodan and Laurdan molecules to move into the glycerol backbone (i.e., less polar) region. In the ripple gel (P(beta)') phase, the emission spectrum of Prodan shows a broad peak at about 480 nm and a shoulder around 440 nm, which means that the Prodan molecules are widespread over the wide range from the glycerol backbone to the hydrophilic part of bilayer. The P(beta)'/L(beta)I phase transition causes the Prodan molecule to squeeze out from the glycerol backbone region and to move the hydrophilic region near the bilayer surface. Contrarily, the Laurdan molecule was not squeezed out from the glycerol backbone region because the long acyl chain of Laurdan serves as an anchor in the hydrophobic core of bilayer. The ratio of fluorescence intensity of Prodan at 480 nm to that at 440 nm, F(480)/F(440), is available to observation of bilayer phase transitions. The plot of F(480)/F(440) versus pressure seems to be useful for the recognition of bilayer phase transition, especially the bilayer interdigitation.

Thermotropic and barotropic phase transitions in bilayer membranes of ether-linked phospholipids with varying alkyl chain lengths

The bilayer phase transitions of a series of ether-linked phospholipids, 1,2-dialkylphosphatidylcholines containing linear saturated alkyl chain (C(n)=12, 14, 16 and 18), were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the pre- and main-transitions for the ether-linked PC bilayer membranes were calculated and compared with those of a series of ester-linked PCs, 1,2-diacylphosphatidylcholines. The thermodynamic quantities of the main transition for the ether-linked PC bilayers showed distinct dependence on alkyl-chain length and were slightly different from those of the ester-linked PC bilayers. From the comparison of thermodynamic quantities for the main transition between both PC bilayers, we revealed that the attractive interaction in the gel phase for the ether-linked PC bilayers is weaker than that for the ester-linked PC bilayers. Regarding the pretransition, although changes in enthalpy and entropy for both PC bilayers were comparable to each other, the volume changes of the ether-linked PC bilayers roughly doubled those of the ester-linked PC bilayers. The larger volume change results from the smallest partial molar volume of the ether-linked PC molecule in the interdigitated gel phase. Further, we constructed the temperature-pressure phase diagrams for the ether-linked PC bilayers by using the phase-transition data. The region of the interdigitated gel phase in the phase diagrams was extended by applying pressure and by increasing the alkyl-chain length of the molecule. Comparing the phase diagrams with those for the ester-linked PC bilayers, it was proved that the phase behavior of the ester-linked PC bilayers under high temperature and pressure is almost equivalent to that of the ether-linked PC bilayers in the vicinity of ambient pressure.

Pressure-jump X-ray studies of liquid crystal transitions in lipids

In this paper, we give an overview of our studies by static and time-resolved X-ray diffraction of inverse cubic phases and phase transitions in lipids. In [section sign] 1, we briefly discuss the lyotropic phase behaviour of lipids, focusing attention on non-lamellar structures, and their geometric/topological relationship to fusion processes in lipid membranes. Possible pathways for transitions between different cubic phases are also outlined. In [section sign] 2, we discuss the effects of hydrostatic pressure on lipid membranes and lipid phase transitions, and describe how the parameters required to predict the pressure dependence of lipid phase transition temperatures can be conveniently measured. We review some earlier results of inverse bicontinuous cubic phases from our laboratory, showing effects such as pressure-induced formation and swelling. In [section sign] 3, we describe the technique of pressure-jump synchrotron X-ray diffraction. We present results that have been obtained from the lipid system 1:2 dilauroylphosphatidylcholine/lauric acid for cubic-inverse hexagonal, cubic-cubic and lamellar-cubic transitions. The rate of transition was found to increase with the amplitude of the pressure-jump and with increasing temperature. Evidence for intermediate structures occurring transiently during the transitions was also obtained. In [section sign] 4, we describe an IDL-based 'AXcess' software package being developed in our laboratory to permit batch processing and analysis of the large X-ray datasets produced by pressure-jump synchrotron experiments. In [section sign] 5, we present some recent results on the fluid lamellar-Pn3m cubic phase transition of the single-chain lipid 1-monoelaidin, which we have studied both by pressure-jump and temperature-jump X-ray diffraction. Finally, in [section sign] 6, we give a few indicators of future directions of this research. We anticipate that the most useful technical advance will be the development of pressure-jump apparatus on the microsecond time-scale, which will involve the use of a stack of piezoelectric pressure actuators. The pressure-jump technique is not restricted to lipid phase transitions, but can be used to study a wide range of soft matter transitions, ranging from protein unfolding and DNA unwinding and transitions, to phase transitions in thermotropic liquid crystals, surfactants and block copolymers.

Bilayer phase transitions of N-methylated dioleoylphosphatidylethanolamines under high pressure

The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)-pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T-p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127-0.145 K MPa-1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lalpha) phase for DOPC bilayer (0.131 K MPa-1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lbeta) phase to the Lalpha phase for DOPC bilayer (0.233 K MPa-1) was significantly different from that of the Lc/Lalpha transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lalpha transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lalpha phase although they have been regarded as the main transition in the previous studies. The Lbeta phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.