Structure and fluctuations of phosphatidylcholines in the vicinity of the main phase transition

Polymorphism of a Highly Asymmetrical Triacylglycerol in Milk Fat: 1-Butyryl 2-Stearoyl 3-Palmitoyl-glycerol

Milk fat has more than 200 triacylglycerols (TAGs), which play a pivotal role in its crystallization behavior. Asymmetrical TAGs containing short butyryl chains contribute to a significant portion of milk fat TAGs. This work aims to elucidate the crystallization behavior of asymmetrical milk fat TAGs by employing the pure compound of 1-butyryl 2-stearoyl 3-palmitoyl-glycerol (BuSP). The structural evolution of BuSP after being cooled down to 20 °C from the melt is evaluated by small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry (DSC). The temporal structural observation shows that BuSP crystallizes into the α-form with short and long spacings of 4.10 and 56.9 Å, respectively, during the first hour of isothermal hold at 20 °C. The polymorphic transformation of the α to β' phase occurred after 4 h of isothermal hold, and the β'- to α-form fraction ratio was about 70:30 at the end of the isothermal experiment (18 h). Pure β'-form X-ray patterns are obtained from the BuSP powder with short spacings of 4.33, 4.14, and 3.80 Å, while the long spacing of 51.2 Å depicts a three-chain-length lamellar structure with a tilt angle of 32°. Corresponding DSC measurements display that BuSP crystallizes from the melt at 29.1 °C, whereas the melting of α- and β'-forms was recorded at 30.3 and 47.8 °C, respectively. In the absence of the β-form, the β'-polymorph is the most stable observed form in BuSP. This work exemplarily explains the crystallization behavior of asymmetrical milk fat TAGs and thus provides new insights into their role in overall milk fat crystallization.

Deciphering the origin of the melting profile of unilamellar phosphatidylcholine liposomes by measuring the turbidity of its suspensions

The elucidation of the thermal properties of phosphatidylcholine liposomes is often based on the analysis of the thermal capacity profiles of multilamellar liposomes (MLV), which may qualitatively disagree with those of unilamellar liposomes (LUV). Experiments and interpretation of LUV liposomes is further complicated by aggregation and lamellarization of lipid bilayers in a short time period, which makes it almost impossible to distinguish the signatures of the two types of bilayers. To characterize independently MLV and LUV of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the latter were prepared with the addition of small amounts of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) which, due to the sterical hindrance and negative charge at a given pH value, cause LUV repellence and contribute to their stability. Differential scanning calorimetry curves and temperature-dependent UV/Vis spectra of the prepared MLV and LUV were measured. Multivariate analysis of spectrophotometric data determined the phase transition temperatures (pretransition at T_p and the main phase transition at T_m), and based on the changes in turbidities, the thickness of the lipid bilayer in LUV was determined. The obtained data suggested that the curvature change is a key distinguishing factor in MLV and LUV heat capacity profiles. By combining the experimental results and those obtained by MD simulations, the interfacial water layer was characterized and its contribution to the thermal properties of LUV was discussed.

Restoring structural parameters of lipid mixtures from small-angle X-ray scattering data

Small-angle X-ray scattering (SAXS) is widely utilized to study soluble macromolecules, including those embedded into lipid carriers and delivery systems such as surfactant micelles, phospho-lipid vesicles and bilayered nanodiscs. To adequately describe the scattering from such systems, one needs to account for both the form factor (overall structure) and long-range-order Bragg reflections emerging from the organization of bilayers, which is a non-trivial task. Presently existing methods separate the analysis of lipid mixtures into distinct procedures using form-factor fitting and the fitting of the Bragg peak regions. This article describes a general approach for the computation and analysis of SAXS data from lipid mixtures over the entire angular range of an experiment. The approach allows one to restore the electron density of a lipid bilayer and simultaneously recover the corresponding size distribution and multilamellar organization of the vesicles. The method is implemented in a computer program, LIPMIX, and its performance is demonstrated on an aqueous solution of layered lipid vesicles undergoing an extrusion process. The approach is expected to be useful for the analysis of various types of lipid-based systems, e.g. for the characterization of interactions between target drug molecules and potential carrier/delivery systems.

µDrop: a system for high-throughput small-angle X-ray scattering measurements of microlitre samples

An automatic sample changer system for measurements of large numbers of liquid samples - the µDrop Sample Changer - is presented. It is based on a robotic arm equipped with a pipetting mechanism, which is combined with a novel drop-based sample holder. In this holder a drop of liquid is suspended between two parallel plates by surface tension. The absence of a transfer line benefits the cleaning, improving the background as well as making it faster and more efficient than most comparable capillary-based systems. The µDrop Sample Changer reaches cycle times below 35 s and can process up to 480 samples in a single run. Sample handling is very reliable, with a drop misplacement chance of about 0.2%. Very low sample volumes (<20 µl) are needed and repeatable measurements were performed down to 6 µl. Using measurements of bovine serum albumin and lysozyme, the performance of the instrument and quality of the gathered data of low and high concentrations of proteins are presented. The temperature of samples can also be controlled during storage and during measurement, which is demonstrated by observing a phase transition of a mesophase-forming lipid solution. The instrument has been developed for use in small-angle X-ray scattering experiments, which is a well established technique for measuring (macro-)molecules. It is commonly used in biological studies, where often large sets of rare samples have to be measured.

Structure and Dynamics of Dioleoyl-Phosphatidylcholine Bilayers under the Influence of Quercetin and Rutin

Quercetin and rutin, two widely studied flavonoids with applications foreseen in the sectors of pharmaceutical and cosmetic industries, have been chosen as model compounds for a detailed structural and dynamical investigation onto their influence on fluid lipid bilayers. Combining global small angle X-ray scattering analysis with molecular dynamics, various changes in the properties of dioleoyl-phosphatidylcholine (DOPC) bilayers have been determined. The solubility of quercetin in DOPC membranes is assured up to 12 mol %, whereas rutin, with additional glucose and rhamnose groups, are fully soluble only up to 6 mol %. Both flavonoids induce an increase in membrane undulations and thin the bilayers slightly (<1 Å) in a concentration dependent manner, wherein quercetin shows a stronger effect. Concomitantly, in the order of 2-4%, the adjacent bilayer distance increases with the flavonoid's concentration. Partial molecular areas of quercetin and rutin are determined to be 26 and 51 Ų, respectively. Simulated averaged areas per molecule confirm these estimates. A 60° tilted orientation of quercetin is observed with respect to the bilayer normal, whereas the flavonoid moiety of rutin is oriented more perpendicular (α-angle 30°) to the membrane surface. Both flavonoid moieties are located at a depth of 12 and 16 Å for quercetin and rutin, respectively, while their anionic forms display a location closer to the polar interface. Finally, at both simulated concentrations (1.5 and 12 mol %), DOPC-rutin systems induce a stronger packing of the pure DOPC lipid bilayer, mainly due to stronger attractive electrostatic interactions in the polar lipid head region.

Ordering of Saturated and Unsaturated Lipid Membranes near Their Phase Transitions Induced by an Amphiphilic Cyclodextrin and Cholesterol

When inserted in membranes of dimyristoyl phosphatidylcholine (DMPC), methylated β-cyclodextrins with one (TrimβMLC) or two (TrimβDLC) lauryl acyl chains grafted onto the hydrophilic cavity exert a "cholesterol-like ordering effect", by straightening the acyl chains in the fluid phase at temperatures near the chain melting transition. This effect may be related to pretransitional events such as the "anomalous swelling" known to occur with saturated phosphatidylcholine membranes. To investigate this model, order profiles and bilayer thicknesses of DMPC and unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) membranes containing amphiphilic cyclodextrins or cholesterol were determined by deuterium NMR. The pure lipid membranes display both a qualitatively similar chain ordering upon cooling in the fluid phase, more important at the chain extremity, which gets more pronounced near their fluid-to-gel transitions. Both membranes show a bilayer thickness increase by ∼0.5 Å just above their transition, as observed previously with saturated phosphatidylcholines of various chain lengths. Membrane-insertion of 5% TrimβMLC or cholesterol induces an important ordering of the DMPC acyl chains just above the transition, which is also more pronounced at the chain extremity. There is an additional increase of the bilayer thickness, most probably due to a deep insertion of these amphiphilic molecules, facilitated by increased bilayer softness in the anomalous swelling regime. These effects are more important with TrimβMLC than with cholesterol. By contrast, no enhanced acyl chain ordering was observed when approaching the transition of TrimβMLC-containing POPC membranes, as a possible consequence of an eventual lack of anomalous swelling in unsaturated lipid membranes. Insertion of higher concentrations of TrimβMLC was found to induce a magnetic orientation of the DMPC membranes in the fluid phase with 10% of this derivative, coupled with the appearance of a broad isotropic component when the concentration is raised to 20%. No membrane orientation or isotropic component was detected with TrimβMLC-containing POPC membranes.

Molecular Dynamics of POPC Phospholipid Bilayers through the Gel to Fluid Phase Transition: An Incoherent Quasi-Elastic Neutron Scattering Study

The microscopic dynamics for the gel and liquid-crystalline phase of highly aligned D2O-hydrated bilayers of 1-palmitoyl-oleoyl-sn-glycero-phosphocholine (POPC) were investigated in the temperature range from 248 to 273 K by using incoherent quasi-elastic neutrons scattering (QENS). We develop a model for describing the molecular motions of the liquid phase occurring in the 0.3 to 350 ps time range. Accordingly, the complex dynamics of hydrogen are described in terms of simple dynamical processes involving different parts of the phospholipid chain. The analysis of the data evidences the existence of three different motions: the fast motion of hydrogen vibrating around the carbon atoms, the intermediate motion of carbon atoms in the acyl chains, and the slower translational motion of the entire phospholipid molecule. The influence of the temperature on these dynamical processes is investigated. In particular, by going from gel to liquid-crystalline phase, we reveal an increase of the segmental motion mainly affecting the terminal part of the acyl chains and a change of the diffusional dynamics from a localized rattling-like motion to a confined diffusion.

On scattered waves and lipid domains: detecting membrane rafts with X-rays and neutrons

In order to understand the biological role of lipids in cell membranes, it is necessary to determine the mesoscopic structure of well-defined model membrane systems. Neutron and X-ray scattering are non-invasive, probe-free techniques that have been used extensively in such systems to probe length scales ranging from angstroms to microns, and dynamics occurring over picosecond to millisecond time scales. Recent developments in the area of phase separated lipid systems mimicking membrane rafts will be presented, and the underlying concepts of the different scattering techniques used to study them will be discussed in detail.

In situ determination of structure and fluctuations of coexisting fluid membrane domains

Biophysical understanding of membrane domains requires accurate knowledge of their structural details and elasticity. We report on a global small angle x-ray scattering data analysis technique for coexisting liquid-ordered (L_o) and liquid-disordered (L_d) domains in fully hydrated multilamellar vesicles. This enabled their detailed analysis for differences in membrane thickness, area per lipid, hydrocarbon chain length, and bending fluctuation as demonstrated for two ternary mixtures (DOPC/DSPC/CHOL and DOPC/DPPC/CHOL) at different cholesterol concentrations. L_o domains were found to be ∼10 Å thicker, and laterally up to 20 Å²/lipid more condensed than L_d domains. Their bending fluctuations were also reduced by ∼65%. Increase of cholesterol concentration caused significant changes in structural properties of L_d, while its influence on L_o properties was marginal. We further observed that temperature-induced melting of L_o domains is associated with a diffusion of cholesterol to L_d domains and controlled by L_o/L_d thickness differences.

Bilayer undulation dynamics in unilamellar phospholipid vesicles: effect of temperature, cholesterol and trehalose

We report a combined dynamic light scattering (DLS) and neutron spin-echo (NSE) study on the local bilayer undulation dynamics of phospholipid vesicles composed of 1,2-dimyristoyl-glycero-3-phosphatidylcholine (DMPC) under the influence of temperature and the additives cholesterol and trehalose. The additives affect vesicle size and self-diffusion. Mechanical properties of the membrane and corresponding bilayer undulations are tuned by changing lipid headgroup or acyl chain properties through temperature or composition. On the local length scale, changes at the lipid headgroup influence the bilayer bending rigidity κ less than changes at the lipid acyl chain: We observe a bilayer softening around the main phase transition temperature Tm of the single lipid system, and stiffening when more cholesterol is added, in concordance with literature. Surprisingly, no effect on the mechanical properties of the vesicles is observed upon the addition of trehalose.

Structure and Volta potential of lipid multilayers: effect of X-ray irradiation

The effect of hard X-ray radiation on the structure and electrostatics of solid-supported lipid multilayer membranes is investigated using a scanning Kelvin probe (SKP) integrated with a high-energy synchrotron beamline to enable in situ measurements of the membranes' local Volta potential (V(p)) during X-ray structural characterization. The undulator radiation employed does not induce any detectable structural damage, but the V(p) of both bare and lipid-modified substrates is found to undergo strong radiation-induced shifts, almost immediately after X-ray exposure. Sample regions that are macroscopically distant (~cm) from the irradiated region experience an exponential V(p) growth with a characteristic time constant of several minutes. The V(p) variations occurring upon periodic on/off X-ray beam switching are fully or partially reversible depending on the location and time-scale of the SKP measurement. The general relevance of these findings for synchrotron-based characterization of biomolecular thin films is critically reviewed.

Losartan's affinity to fluid bilayers modulates lipid-cholesterol interactions

Losartan is an angiotensin II receptor antagonist mainly used for the regulation of high blood pressure. Since it was anticipated that losartan reaches the receptor site via membrane diffusion, the impact of losartan on model membranes has been investigated by small angle X-ray scattering. For this purpose 2-20 mol% losartan was incorporated into dimyristoyl-phosphatidylcholine (DMPC) and palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers and into their binary mixtures with cholesterol in the concentration range of 0 to 40 mol%. Effects of losartan on single component bilayers are alike. Partitioning of losartan into the membranes confers a negative charge to the lipid bilayers that causes the formation of unilamellar vesicles and a reduction of the bilayer thickness by 3-4%. Analysis of the structural data resulted in an estimate for the partial area of losartan, A(Los) ≈ 40 Å(2). In the presence of cholesterol, differences between the effects of losartan on POPC and DMPC are striking. Membrane condensation by cholesterol is retarded by losartan in POPC. This contrasts with DMPC, where an increase of the cholesterol content shifts the partitioning equilibrium of losartan towards the aqueous phase, such that losartan gets depleted from the bilayers from 20 mol% cholesterol onwards. This indicates (i) a chain-saturation dependent competition of losartan with lipid-cholesterol interactions, and (ii) the insolubility of losartan in the liquid ordered phase of PCs. Consequently, losartan's action is more likely to take place in fluid plasma membrane patches rather than in domains rich in cholesterol and saturated lipid species such as in membrane rafts.

GLOBAL PROPERTIES OF BIOMIMETIC MEMBRANES: PERSPECTIVES ON MOLECULAR FEATURES

Global properties of biological model membranes such as, e.g., structure or elasticity, are known to be closely related to their local features. If a membrane active compound interacts with the membrane assembly, the membrane will primarily be affected on the local, molecular level. The local perturbation may than, through some coupling, translate into a global adjustment of the membrane. In order to address this coupling x-ray and neutron diffraction data analysis techniques have been developed that allow accurate monitoring of changes in global properties. This offers new perspectives on molecular membrane features that in combination with complementary techniques, such as differential scanning calorimetry, spectroscopy or dynamic scattering lead to a better understanding of biomimetic membranes. The present article reviews these aspects giving application examples for single- and multicomponent membranes, respectively.

Thermal, dynamic and structural properties of drug AT1 antagonist olmesartan in lipid bilayers

It is proposed that AT1 antagonists (ARBs) exert their biological action by inserting into the lipid membrane and then diffuse to the active site of AT1 receptor. Thus, lipid bilayers are expected to be actively involved and play a critical role in drug action. For this reason, the thermal, dynamic and structural effects of olmesartan alone and together with cholesterol were studied using differential scanning calorimetry (DSC), 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR), cross-polarization (CP) MAS NMR, and Raman spectroscopy as well as small- and wide angle X-ray scattering (SAXS and WAXS) on dipalmitoyl-phosphatidylcholine (DPPC) multilamellar vesicles. 13C CP/MAS spectra provided direct evidence for the incorporation of olmesartan and cholesterol in lipid bilayers. Raman and X-ray data revealed how both molecules modify the bilayer's properties. Olmesartan locates itself at the head-group region and upper segment of the lipid bilayers as 13C CP/MAS spectra show that its presence causes significant chemical shift changes mainly in the A ring of the steroidal part of cholesterol. The influence of olmesartan on DPPC/cholesterol bilayers is less pronounced. Although, olmesartan and cholesterol are residing at the same region of the lipid bilayers, due to their different sizes, display distinct impacts on the bilayer's properties. Cholesterol broadens significantly the main transition, abolishes the pre-transition, and decreases the membrane fluidity above the main transition. Olmesartan is the only so far studied ARB that increases the gauche:trans ratio in the liquid crystalline phase. These significant differences of olmesartan may in part explain its distinct pharmacological profile.

Fluid phase lipid areas and bilayer thicknesses of commonly used phosphatidylcholines as a function of temperature

The structural parameters of fluid phase bilayers composed of phosphatidylcholines with fully saturated, mixed, and branched fatty acid chains, at several temperatures, have been determined by simultaneously analyzing small-angle neutron and X-ray scattering data. Bilayer parameters, such as area per lipid and overall bilayer thickness have been obtained in conjunction with intrabilayer structural parameters (e.g. hydrocarbon region thickness). The results have allowed us to assess the effect of temperature and hydrocarbon chain composition on bilayer structure. For example, we found that for all lipids there is, not surprisingly, an increase in fatty acid chain trans-gauche isomerization with increasing temperature. Moreover, this increase in trans-gauche isomerization scales with fatty acid chain length in mixed chain lipids. However, in the case of lipids with saturated fatty acid chains, trans-gauche isomerization is increasingly tempered by attractive chain-chain van der Waals interactions with increasing chain length. Finally, our results confirm a strong dependence of lipid chain dynamics as a function of double bond position along fatty acid chains.

Effects of ethanol on the organization of phosphocholine lipid bilayers

We have investigated the consequences of the addition of ethanol to aqueous solutions containing 100 nm diameter phosphocholine unilamellar vesicles. We have studied the effect of ethanol addition on both gel phase and fluid phase phospholipid bilayers of 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC), using time-resolved fluorescence measurements of perylene incorporated into the vesicles. We observe an increase in the perylene rotational diffusion time constants for ethanol concentrations of ca. 0.6 M in both the gel phase (289 K) and the fluid phase (303 K), indicating a change in the bilayer interacyl chain spacing and/or organization. While the change in rotational diffusion behavior of perylene is seen for both phospholipid phases, the details of the change in chromophore dynamics are not the same for the two phases, likely due to the differing extents of disorder in the phospholipid acyl chain region at the two temperatures. These data provide insight into the effects of ethanol on the local environment of the probe in both gel phase and fluid phase lipid bilayers.

Influence of cholesterol on the collective dynamics of the phospholipid acyl chains in model membranes

We have studied the packing and collective dynamics of the phospholipid acyl chains in a model membrane composed of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and cholesterol in varied phase state. After a structural characterization of this two-component model bilayer using X-ray reflectivity, we have carried out coherent inelastic neutron scattering to investigate the chain dynamics. Both DMPC/cholesterol membranes exhibited much sharper and more pronounced low-energy inelastic excitations than a pure DMPC membrane. In the high-energy regime above 10 meV, the insertion of cholesterol into the membrane was found to shift the position of the inelastic excitation towards values otherwise found in the pure lipids gel phase. Thus, the dissipative collective short-range dynamics of the acyl chains is strongly influenced by the presence of cholesterol.

Thermotropic phase behavior of multilamellar membranes of dioleoylphosphatidylcholine

We use the X-ray diffraction method to examine the thermotropic phase behavior of multilamellar membranes of dioleoylphosphatidylcholine. We find that when the temperature is reduced from room temperature to below 0 degrees C, both the lipid bilayers and the amount of water in the bilayers increase. But the interbilayer distance descends abruptly at a certain temperature between -6 and -15 degrees C, the actual value depending on the relative humidity of the atmosphere, solely due to the thinning of the water layer, d(w). There are several L(alpha) and L(c) phase coexistence states both in the cooling process and in the heating process. In the cooling process, only a part of the lipid molecules accomplish the L(alpha)-to-L(c) main phase transition at -16 degrees C, with the rest of the lipids being frozen down to a very low temperature. In the heating process, however, these frozen lipid molecules are able to move to complete the L(alpha)-to-L(c) main phase transition at -12 degrees C. The reverse of the main phase transition begins at -9 degrees C and is completed at -5 degrees C, after which the water is absorbed into the lipid bilayer to increase the thickness of the water layer, while the thickness of the lipid membranes remain unchanged. This process continues until all the ice on top of the samples melts.

Mechanical properties of interacting lipopolysaccharide membranes from bacteria mutants studied by specular and off-specular neutron scattering

Specular and off-specular neutron scattering are used to study the influence of molecular chemistry (mutation) on the intermembrane interactions and mechanical properties of the outer membrane of Gram-negative bacteria consisting of lipopolysaccharides (LPSs). For this purpose, solid-supported multilayers of mutant LPS membranes are deposited on silicon wafers and hydrated either at defined humidity or in bulk buffers. The planar sample geometry allows to identify out-of-plane and in-plane scattering vector components. The measured two-dimensional reciprocal space maps are simulated with membrane displacement correlation functions determined by two mechanical parameters (vertical compression modulus and bending rigidity) and an effective cutoff radius for the membrane fluctuation wavelength. Experiments at controlled humidity enable one to examine the influence of the disjoining pressure on the saccharide-mediated intermembrane interactions, while experiments in bulk buffers (i.e., in the absence of an external osmotic stress) reveal the effect of divalent cations on LPS membranes, highlighting the role of divalent cations in the survival mechanism of bacteria in the presence of antimicrobial molecules.

Membrane thickening by the antimicrobial peptide PGLa

Using x-ray diffraction, solid-state 2H-NMR, differential scanning calorimetry, and dilatometry, we have observed a perturbation of saturated acyl chain phosphatidylglycerol bilayers by the antimicrobial peptide peptidyl-glycylleucine-carboxyamide (PGLa) that is dependent on the length of the hydrocarbon chain. In the gel phase, PGLa induces a quasi-interdigitated phase, previously reported also for other peptides, which is most pronounced for C18 phosphatidylglycerol. In the fluid phase, we found an increase of the membrane thickness and NMR order parameter for C14 and C16 phosphatidylglycerol bilayers, though not for C18. The data is best understood in terms of a close hydrophobic match between the C18 bilayer core and the peptide length when PGLa is inserted with its helical axis normal to the bilayer surface. The C16 acyl chains appear to stretch to accommodate PGLa, whereas tilting within the bilayer seems to be energetically favorable for the peptide when inserted into bilayers of C14 phosphatidylglycerol. In contrast to the commonly accepted membrane thinning effect of antimicrobial peptides, the data demonstrate that pore formation does not necessarily relate to changes in the overall bilayer structure.

Bending modulus of lipid bilayers in a liquid-crystalline phase including an anomalous swelling regime estimated by neutron spin echo experiments

Membrane fluctuations of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were investigated by neutron spin echo spectroscopy. The intermediate structure factor was analyzed in terms of the model proposed by Zilman and Granek (Phys. Rev. Lett. 77, 4788 (1996)), and the bending modulus of lipid bilayers was derived. The hardening of a lipid bilayer upon approaching the main transition point in the anomalous swelling regime was observed, which naturally connects the bending modulus in the gel phase below the main transition temperature.

Differential modulation of membrane structure and fluctuations by plant sterols and cholesterol

We have studied the concentration and temperature dependent influence of cholesterol, stigmasterol, and sitosterol on the global structure and the bending fluctuations of fluid dimyristoyl phosphatidylcholine and palmitoyl oleoyl phosphatidylcholine bilayers applying small-angle x-ray scattering, as well as dilatometry and ultrasound velocimetry. Independent of the lipid matrix, cholesterol was found to be most efficient in modulating bilayer thickness and elasticity, followed by sitosterol and stigmasterol. This can be attributed to the additional ethyl groups and double bond at the C(17) alkyl side-chain of the two plant sterols. Hence, it seems that some flexibility of the sterol hydrocarbon chain is needed to accommodate within the lipid bilayer. In addition, we did not observe two populations of membranes within the putative liquid-ordered/liquid-disordered phase coexistence regime of binary sterol/lipid mixtures. Instead, the diffraction patterns could be interpreted in terms of a uniform phase. This lends further support to the idea of compositional fluctuations of unstable sterol rich domains recently brought up by fluorescence microscopy experiments, which contrasts the formation of stable domains within the miscibility gap of binary lipid/sterol mixtures.

Critical temperatures and a critical chain length in saturated diacylphosphatidylcholines: calorimetric, ultrasonic and Monte Carlo simulation study of chain-melting/ordering in aqueous lipid dispersions

Chain-ordering/melting transition in a series of saturated diacylphosphatidylcholines (PCs) in aqueous dispersions have been studied experimentally (calorimetric and ultrasonic techniques) and theoretically (an Ising-like lattice model). The shape of the calorimetric curves was compared with the theoretical data and interpreted in terms of the lateral interactions and critical temperatures determined for each lipid studied. A critical chain length has been found (between 16 and 17 C-atoms per chain) which subdivides PCs into two classes with different phase behavior. In shorter lipids, the transition takes place above their critical temperatures meaning that this is an intrinsically continuous transition. In longer lipids, the transition occurs below the critical temperatures of the lipids, meaning that the transition is intrinsically discontinuous (first-order). This conclusion was supported independently by the ultrasonic relaxation sensitive to density fluctuations. Interestingly, it is this length that is the most abundant among the saturated chains in biological membranes.

Lipid lateral segregation driven by diacyl cyclodextrin interactions at the membrane surface

Cyclodextrins are hydrophilic molecular cages with a hydrophobic interior allowing the inclusion of water-insoluble drugs. Amphiphilic cyclodextrins obtained by appending a hydrophobic anchor were designed to improve the cell targeting of the drug-containing cavities through their liposome transportation in the organism. After insertion in model membranes, they were found to induce a lateral phase separation into a pure lipid phase and a fluid cyclodextrin-rich phase (L(CD)) with reduced acyl chain order parameters, as observed with a derivative containing a cholesterol anchor (M. Roux, R. Auzely-Velty, F. Djedaïni-Pilard, and B. Perly. 2002. Biophysical Journal, 8:813-822). We present another class of amphiphilic cyclodextrins obtained by grafting aspartic acid esterified by two lauryl chains on the oligosaccharide core via a succinyl spacer. The obtained dilauryl-beta-cyclodextrin (betaDLC) was inserted in chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) membranes and studied by deuterium NMR ((2)H-NMR). A laterally segregated mixed phase was found to sequester three times more lipids than the cholesteryl derivative (approximately 4-5 lipids per monomer of betaDLC), and a quasipure L(CD) phase could be obtained with a 20% molar concentration of betaDLC. When cooled below the main fluid-to-gel transition of DMPC-d54 the betaDLC-rich phase stays fluid, coexisting with pure lipid in the gel state, and exhibits a sharp transition to a gel phase with frozen DMPC acyl chains at 12.5 degrees C. No lateral phase separation was observed with partially or fully methylated betaDLC, confirming that the stability of the segregated L(CD) phase was governed through hydrogen-bond-mediated intermolecular interactions between cyclodextrin headgroups at the membrane surface. As opposed to native betaDLC, the methylated derivatives were found to strongly increase the orientational order of DMPC acyl chains as the temperature reaches the membrane fluid-to-gel transition. The results are discussed in relation to the "anomalous swelling" of saturated phosphatidylcholine multilamellar membranes known to occur in the vicinity of the main fluid-to-gel transition.

Gauging the effect of impurities on lipid bilayer phase transition temperature

We report on the gel-to-fluid phase transition behavior of unilamellar vesicles formed with 1,2-dimyristoyl-sn-phosphatidylcholine (14:0 DMPC). We have interrogated the gel-to-fluid transition temperature of these bilayer structures using the chromophore perylene incorporated in their nonpolar region. We observe a discontinuous change in the reorientation time of perylene sequestered within the bilayer at the known melting transition temperature of 14:0 DMPC, 24 degrees C. The perylene reorientation data reveal a local viscosity of 14.5 +/- 2.5 cP in the gel phase, and 8.5 +/- 1.5 cP in the fluid phase. We have also incorporated small amounts of 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (14:1 DMPC) into these unilamellar vesicles and find that the melting transition temperature for these bilayers varies in a regular manner with the amount of 14:1 DMPC present. These data demonstrate that very little "contaminant" is required to cause a substantial change in the gel-to-fluid transition temperature, even though these contaminants do not alter the viscosity of the bilayer sensed by perylene, either above or below the melting transition.

Influence of Perfluorinated Compounds on the Properties of Model Lipid Membranes

The influence of selected perfluorinated compounds (PFCs), perfluorooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS), on the structure and organization of lipid membranes was investigated using model membranes-lipid monolayers and bilayers. The simplest model--a lipid monolayer--was studied at the air-water interface using the Langmuir-Blodgett technique with surface pressure and surface potential measurements. Lipid bilayers were characterized by NMR techniques and molecular dynamics simulations. Two phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), characterized by different surface properties have been chosen as components of the model membranes. For a DPPC monolayer, a phase transition from the liquid-expanded state to the liquid-condensed state can be observed upon compression at room temperature, while a DMPC monolayer under the same conditions remains in the liquid-expanded state. For each of the two lipids, the presence of both PFOA and PFOS leads to the formation of a more fluidic layer at the air-water interface. Pulsed field gradient NMR measurements of the lateral diffusion coefficient (DL) of DMPC and PFOA in oriented bilayers reveal that, upon addition of PFOA to DMPC bilayers, DL of DMPC decreases for small amounts of PFOA, while larger additions produce an increased DL. The DL values of PFOA were found to be slightly larger than those for DMPC, probably as a consequence of the water solubility of PFOA. Furthermore, 31P and 2H NMR showed that the gel-liquid crystalline phase transition temperature decreased by the addition of PFOA for concentrations of 5 mol % and above, indicating a destabilizing effect of PFOA on the membranes. Deuterium order parameters of deuterated DMPC were found to increase slightly upon increasing the PFOA concentration. The monolayer experiments reveal that PFOS also penetrates slowly into already preformed lipid layers, leading to a change of their properties with time. These experimental observations are in qualitative agreement with the computational results obtained from the molecular dynamics simulations showing a slow migration of PFCs from the surrounding water phase into DPPC and DMPC bilayers.

On possible microscopic origins of the swelling of neutral lipid bilayers induced by simple salts

It was recently suggested that the swelling of neutral multilipid bilayers upon addition of a salt can be simply explained only by the electrolyte screening of the van der Waals attractions, while assuming that the hydration force and the repulsion due to thermal undulations of membranes are unaffected by the salt. While we agree that the screening of the van der Waals interactions plays a role, we suggest that the increase in the hydration force upon addition of a salt has also to be taken into account. In a statistical model, which accounts for the membrane undulations, parameters could be found to explain the multibilayer swelling even when the van der Waals attraction is considered unaffected by the electrolyte screening. These results point out that the decrease by a factor of three of the Hamaker constant upon addition of a salt, suggested recently to be responsible for the swelling of neutral multilipid bilayers, is perhaps too large, and a smaller decrease in Hamaker constant, coupled with the above mentioned effects might explain the swelling.

Control of the structure of self-spreading lipid membrane by changing electrolyte concentration

We have controlled the structure of self-spreading lipid bilayer membranes prepared on surface-oxidized silicon substrates by changing electrolyte concentration. Analysis of the fluorescence intensity, considering the optical interference effect, clarified the stacking structure of the lipid membrane. By varying the electrolyte concentration, we can vary the number of single multilamellar lobes adsorbed on the underlying self-spreading bilayer. This dependence of the stacking ability on the electrolyte concentration was investigated on the basis of changes in the bilayer-lobe interaction energies, including van der Waals, electrostatic double layer, and hydration interaction energies. Theoretical estimation suggests that the observed electrolyte concentration dependence can be explained by the combination of the van der Waals attractive interaction energy and the repulsive double-layer interaction energy.

Transient ordered domains in single-component phospholipid bilayers

We report evidence of dense, ordered nanodomains in single-component fluid lipid bilayers. Our atomic-scale molecular dynamics simulations suggest that the area available to a lipid acyl chain exhibits large fluctuations, resulting in denser and sparser domains. The sizes of the dense domains can be up to approximately 10 nm, and their lifetimes are of the order of approximately 10 ns. In addition, our simulations suggest that domains of lipids with highly ordered acyl chains form predominantly within the dense regions, their sizes ranging from a few chains up to a few nanometers, and with lifetimes between approximately 10 ps-10 ns. These observations shed light on the origin of experimentally observed fluctuations, as well as on the mechanisms of phase transitions in lipid membranes.

Structure and interaction potentials in solid-supported lipid membranes studied by X-ray reflectivity at varied osmotic pressure

Highly oriented solid-supported lipid membranes in stacks of controlled number N approximately 16 (oligo-membranes) have been prepared by spin-coating using the uncharged lipid model system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The samples have been immersed in aqueous polymer solutions for control of osmotic pressure and have been studied by X-ray reflectivity. The bilayer structure and fluctuations have been determined by modelling the data over the full q-range. Thermal fluctuations are described using the continuous smectic Hamiltonian with the appropriate boundary conditions at the substrate and at the free surface of the stack. The resulting fluctuation amplitudes and the pressure-distance relation are discussed in view of the inter-bilayer potential.

Anomalous swelling of lipid bilayer stacks is caused by softening of the bending modulus

Arrays of bilayers of the lipid dimyristoylphosphatidylcholine (DMPC) exhibit anomalous swelling as the temperature decreases from T=27 degrees C toward the main phase transition at T(M) =24 degrees C, within the fluid L(alpha) thermodynamic phase. Analysis of diffuse x-ray scattering data from oriented stacks of biological lipid bilayers now makes it possible to obtain the bending modulus K(C) and the bulk compressibility modulus B separately. We report results that show that the measured bending modulus K(C) for DMPC decreases by almost a factor of 2 between T=27 degrees C and the transition temperature at T(M) =24 degrees C, which is the same temperature range where the anomalous swelling occurs. We also report Monte Carlo simulations that show that the anomalous swelling can be fully accounted for by the measured decrease in K(C) with no changes in the van der Waals or hydration forces.