Impact of cholesterol on voids in phospholipid membranes

Compressibilities and volume fluctuations of archaeal tetraether liposomes

Bipolar tetraether lipids (BTLs) are abundant in crenarchaeota, which thrive in both thermophilic and nonthermophilic environments, with wide-ranging growth temperatures (4-108°C). BTL liposomes can serve as membrane models to explore the role of BTLs in the thermal stability of the plasma membrane of crenarchaeota. In this study, we focus on the liposomes made of the polar lipid fraction E (PLFE). PLFE is one of the main BTLs isolated from the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. Using molecular acoustics (ultrasound velocimetry and densimetry), pressure perturbation calorimetry, and differential scanning calorimetry, we have determined partial specific adiabatic and isothermal compressibility, their respective compressibility coefficients, partial specific volume, and relative volume fluctuations of PLFE large unilamellar vesicles (LUVs) over a wide range of temperatures (20-85°C). The results are compared with those obtained from liposomes made of dipalmitoyl-L-α-phosphatidylcholine (DPPC), a conventional monopolar diester lipid. We found that, in the entire temperature range examined, compressibilities of PLFE LUVs are low, comparable to those found in gel state of DPPC. Relative volume fluctuations of PLFE LUVs at any given temperature examined are 1.6-2.2 times more damped than those found in DPPC LUVs. Both compressibilities and relative volume fluctuations in PLFE LUVs are much less temperature-sensitive than those in DPPC liposomes. The isothermal compressibility coefficient (β(T)(lipid)) of PLFE LUVs changes from 3.59 × 10(-10) Pa(-1) at 25°C to 4.08 × 10(-10) Pa(-1) at 78°C. Volume fluctuations of PLFE LUVs change only 0.25% from 30°C to 80°C. The highly damped volume fluctuations and their low temperature sensitivity, echo that PLFE liposomes are rigid and tightly packed. To our knowledge, the data provide a deeper understanding of lipid packing in PLFE liposomes than has been previously reported, as well as a molecular explanation for the low solute permeation and limited membrane lateral motion. The obtained results may help to establish new strategies for rational design of stable BTL-based liposomes for drug/vaccine delivery.

The influence of additives on the nanoscopic dynamics of the phospholipid dimyristoylphosphatidylcholine

The influence of additives on the molecular dynamics of the phospholipid dimyristoylphosphatidylcholine (DMPC) in its fully hydrated liquid crystalline phase was studied. Quasielastic neutron scattering (QENS) was used to detect motions with dimensions of some Ångstroms on two different time scales, namely 60ps and 900ps. The effects of myristic acid, farnesol, cholesterol, and sodium glycocholate could consistently be explained on the basis of collective, flow-like motions of the phospholipid molecules. The influence of the additives on these motions was explained by packing effects, corresponding to the reduction of free volume. Cholesterol was found to decrease the mobility of DMPC seen on the 900ps time scale with increasing cholesterol content. In contrast, all other studied additives have no significant effect on the mobility.

Free volume theory applied to lateral diffusion in Langmuir monolayers: atomistic simulations for a protein-free model of lung surfactant

We hereby present a study on lateral diffusion of lipids in Langmuir monolayers. We apply atomistic molecular dynamics simulations to a model system whose composition is consistent with protein-free lung surfactant. Our main focus is on the assessment of the validity of the free volume theory for lateral diffusion and on the interpretation of the cross-sectional area and activation energy parameters appearing in the theory. We find that the diffusion results can be fitted to the description given by the free volume theory, but the interpretation of its parameters is not straightforward. While the cross-sectional area appears to be related to the hard-core cross-sectional area of a lipid, its role in the lateral diffusion process is unclear. Also, the activation energy derived using the free volume theory is different from the activation energy found through Arrhenius analysis, and its physical interpretation remains elusive. Finally, we find that lipid diffusion does not occur via rapid single-particle "jumps". Instead, lipids move in a concerted manner as loosely defined transient clusters, as observed earlier for lipid bilayers.

Temperature-induced structural transition in-situ in porcine lens--changes observed in void size distribution

The function of mammalian ocular lens is to provide a sharp image to the retina. Accordingly, the lens needs to be transparent and minimize light scattering. To do so the lens fiber cells first loose intracellular organelles, organize the cytoplasm and arrange the fiber cell membranes. Because the fiber cells are metabolically inactive, the plasma membrane becomes the only cellular organelle and consequently, the phase behavior of these membranes determines the physiological state of the lens. Previous studies have shown that lipids extracted from the nuclear and cortical region of human lens show a temperature-induced phase transition close to the body temperature. Yet, the physiological function of this phase transition is not known, and even the presence of the phase transition in intact lenses is unknown. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the sub-nanometer-sized local structure of intact porcine lens and these studies were complemented with differential scanning calorimeter and mass spectrometric analysis in extracted porcine lens lipids. Using PALS, we present evidence for the presence of a temperature-dependent structural transition centered at 35.5 degrees C in-situ in clear extracted porcine lenses. Further studies employing extracted lens lipids and purified egg-yolk sphingomyelin and cholesterol mixtures suggest that the nano-scale transition emerges from the phase behavior of lens lipids. Based on our results, PALS seems to be a viable method for gaining additional information on biological tissues, especially since it enables non-destructive studies on intact tissues.

Comparison of cholesterol and its direct precursors along the biosynthetic pathway: effects of cholesterol, desmosterol and 7-dehydrocholesterol on saturated and unsaturated lipid bilayers

Despite extensive studies, the remarkable structure-function relationship of cholesterol in cellular membranes has remained rather elusive. This is exemplified by the fact that the membrane properties of cholesterol are distinctly different from those of many other sterols. Here we elucidate this issue through atomic-scale simulations of desmosterol and 7-dehydrocholesterol (7DHC), which are immediate precursors of cholesterol in its two distinct biosynthetic pathways. While desmosterol and 7DHC differ from cholesterol only by one additional double bond, we find that their influence on saturated lipid bilayers is substantially different from cholesterol. The capability to form ordered regions in a saturated (dipalmitoyl-phosphatidylcholine) membrane is given by cholesterol > 7DHC > desmosterol, indicating the important role of cholesterol in saturated lipid environments. For comparison, in an unsaturated (dioleoyl-phosphatidylcholine) bilayer, the membrane properties of all sterols were found to be essentially identical. Our studies indicate that the different membrane ordering properties of sterols can be characterized by a single experimentally accessible parameter, the sterol tilt. The smaller the tilt, the more ordered are the lipids around a given sterol. The molecular level mechanisms responsible for tilt modulation are found to be related to changes in local packing around the additional double bonds.

Liquid ordered and gel phases of lipid bilayers: fluorescent probes reveal close fluidity but different hydration

Hydration and fluidity of lipid bilayers in different phase states were studied using fluorescent probes selectively located at the interface. The probe of hydration was a recently developed 3-hydroxyflavone derivative, which is highly sensitive to the environment, whereas the probe of fluidity was the diphenylhexatriene derivative, 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene. By variation of the cholesterol content and temperature in large unilamellar vesicles composed of sphingomyelin or dipalmitoylphosphatidlycholine, we generated different phases: gel, liquid ordered (raft), liquid crystalline, and liquid disordered (considered as liquid crystalline phase with cholesterol). For these four phases, the hydration increases in the following order: liquid ordered << gel approximately liquid disordered < liquid crystalline. The membrane fluidity shows a somewhat different trend, namely liquid ordered approximately gel < liquid disordered < liquid crystalline. Thus, gel and liquid ordered phases exhibit similar fluidity, whereas the last phase is significantly less hydrated. We expect that cholesterol due to its specific H-bonding interactions with lipids and its ability to fill the voids in lipid bilayers expels efficiently water molecules from the highly ordered gel phase to form the liquid ordered phase. In this study, the liquid ordered (raft) and gel phases are for the first time clearly distinguished by their strong difference in hydration.

Statistical thermodynamics through computer simulation to characterize phospholipid interactions in membranes

This chapter describes the major issues that are involved in the statistical thermodynamics of phospholipid membranes at the atomic level. The ingredients going into models of lipid bilayers are summarized: force fields, representation of long-range interactions, and boundary conditions. Next, the choice of thermodynamic ensembles, and the two main options for the generation of a representative sample of configurations: molecular dynamics and Monte Carlo are discussed. The final issue that is dealt with describes the various ways the generated ensembles can be analyzed.

Molecular dynamics studies of the molecular structure and interactions of cholesterol superlattices and random domains in an unsaturated phosphatidylcholine bilayer membrane

The effect of the molecular organization of lipid components on the properties of the bilayer membrane has been a topic of increasing interest. Several experimental and theoretical studies have suggested that cholesterol is not randomly distributed in the fluid-state lipid bilayer but forms nanoscale domains. Several cholesterol-enriched nanodomain structures have been proposed, including rafts, regular or maze arrays, complexes, and superlattices. At present, the molecular mechanisms by which lipid composition influences the formation and stability of lipid nanodomains remain unclear. In this study, we have used molecular dynamics (MD) simulations to investigate the effects of the molecular organization of cholesterol--superlattice versus random--on the structure of and interactions between lipids and water in lipid bilayers of cholesterol and 1-palmitoyl-2-oleoylphosphatidylcholine (cholesterol/POPC) at a fixed cholesterol mole fraction of 0.40. On the basis of four independent replicates of 200-ns MD simulations for a superlattice or random bilayer, statistically significant differences were observed in the lipid structural parameters, area per lipid, density profile, and acyl chain order profile, as well as the hydrogen bonding between various pairs (POPC and water, cholesterol and water, and POPC and cholesterol). The time evolution of the radial distribution of the cholesterol hydroxy oxygen suggests that the lateral distribution of cholesterol in the superlattice bilayer is more stable than that in the random bilayer. Furthermore, the results indicate that a relatively long simulation time, more than 100 ns, is required for these two-component bilayers to reach equilibrium and that this time is influenced by the initial lateral distribution of lipid components.

Glycolipid Membranes through Atomistic Simulations: Effect of Glucose and Galactose Head Groups on Lipid Bilayer Properties

Though glycolipids are involved in a multitude of cellular functions, the understanding of their atom-scale properties in lipid membranes has remained very limited due to the lack of atomistic simulations. In this work, we employ extensive simulations to characterize one-component membranes comprised of glycoglycerolipids, focusing on two common glyco head groups, namely glucose and galactose. The properties of these two glycoglycerolipid bilayers are compared in a systematic manner with membranes consisting of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) lipids, whose structures aside from the head group are identical with those of the two glycolipids. We find that the glycolipid systems are characterized by a substantial number of hydrogen bonds in the head group region, leading to membrane packing that is stronger than in a PC but less significant than that in a PE bilayer. The role played by the glyco head group is especially evident in the electrostatic membrane potential, which is particularly large in the glycolipid membranes. For the same reason, the interfacial forces near glycolipid bilayers are significantly different from those found in PC and PE bilayers, affecting, e.g., the ordering of water close to the membrane. These effects are particularly important for the case of galactose, an important component in thylacoids.

Free Pyrene Probes in Gel and Fluid Membranes: Perspective through Atomistic Simulations

We consider the properties of free pyrene probes inside gel- and fluidlike phospholipid membranes and unravel their influence on membrane properties. For this purpose, we employ atomic-scale molecular dynamics simulations at several temperatures for varying pyrene concentrations. Molecular dynamics simulations show that free pyrene molecules prefer to be located in the hydrophobic acyl chain region close to the glycerol group of lipid molecules. Their orientation is shown to depend on the phase of the membrane. In the fluid phase, pyrenes favor orientations where they are standing upright in parallel to the membrane normal, while, in the gel phase, the orientation is affected by the tilt of lipid acyl chains. Pyrenes are found to locally perturb membrane structure, while the nature of perturbations in the gel and fluid phases is completely different. In the gel phase, pyrenes break the local packing of lipids and decrease the ordering of lipid acyl chains around them, while, in the fluid phase, pyrenes increase the ordering of nearby acyl chains, thus having an opposite effect. Interestingly, this proposes a similarity to effects induced by cholesterol on structural membrane properties above and below the gel-fluid transition temperature. Further studies express a view that the orientational ordering of pyrene is not a particularly good measure of the acyl chain ordering of lipids. While pyrene ordering provides the correct qualitative behavior of acyl chain ordering in the fluid phase, its capability to predict the correct temperature dependence is limited.

What happens if cholesterol is made smoother: importance of methyl substituents in cholesterol ring structure on phosphatidylcholine-sterol interaction

Although sterols constitute one of the most important molecular species in cells, the reasons for their structure-function relationships in lipid membranes are not well understood. The main objective of this work is to elucidate the recently suggested possibility that the ordering and condensing effects of sterols on phospholipid membranes are related to the smoothness of a sterol. We focus on cholesterol, which has two methyl groups attached to its beta-face, and compare its properties to those of demethylated cholesterol (Dchol), from which the two methyl groups have been removed. Atomic-scale molecular dynamics simulations of lipid membranes comprised of saturated lipids and sterols, either cholesterol or Dchol, provide compelling evidence that despite its smoother structure, the ordering and condensing effects of Dchol are less effective than those of cholesterol. The ordering capability of both cholesterol and Dchol is highly asymmetric with respect to their ring structure, but whereas cholesterol favors the alpha-face, Dchol favors the beta-face. The origin and implications of this difference are analyzed in detail. The picture that emerges from this study supports a view that the two methyl groups at the steroid ring system of cholesterol play an important role in cholesterol-lipid interactions by reducing sterol tilt in the bilayer and hence allowing for an optimal orientation for cholesterol.

Insight into the putative specific interactions between cholesterol, sphingomyelin, and palmitoyl-oleoyl phosphatidylcholine

The effects of cholesterol (Chol) on phospholipid bilayers include ordering of the fatty acyl chains, condensing of the lipids in the bilayer plane, and promotion of the liquid-ordered phase. These effects depend on the type of phospholipids in the bilayer and are determined by the nature of the underlying molecular interactions. As for Chol, it has been shown to interact more favorably with sphingomyelin than with most phosphatidylcholines, which in given circumstances leads to formation of lateral domains. However, the exact origin and nature of Chol-phospholipid interactions have recently been subjects of speculation. We examine interactions between Chol, palmitoylsphingomyelin (PSM) and palmitoyl-oleoyl-phosphatidylcholine (POPC) in hydrated lipid bilayers by extensive atom-scale molecular dynamics simulations. We employ a tailored lipid configuration: Individual PSM and Chol monomers, as well as PSM-Chol dimers, are embedded in a POPC lipid bilayer in the liquid crystalline phase. Such a setup allows direct comparison of dimeric and monomeric PSMs and Chol, which ultimately shows how the small differences in PSM and POPC structure can lead to profoundly different interactions with Chol. Our analysis shows that direct hydrogen bonding between PSM and Chol does not provide an adequate explanation for their putative specific interaction. Rather, a combination of charge-pairing, hydrophobic, and van der Waals interactions leads to a lower tilt in PSM neighboring Chol than in Chol with only POPC neighbors. This implies improved Chol-induced ordering of PSM's chains over POPC's chains. These findings are discussed in the context of the hydrophobic mismatch concept suggested recently.

Cholesterol-sphingomyelin interactions: a molecular dynamics simulation study

Stearoylsphingomyelin (SSM) bilayers containing 0, 22, and 50 mol % cholesterol (Chol) and a pentadecanoyl-stearoylphosphatidylcholine (15SPC) bilayer containing 22 mol % Chol were molecular dynamics simulated at two temperatures (37 degrees C and 60 degrees C). 15SPC is the best PC equivalent of SSM. The Chol effect on the SSM bilayer differs significantly from that on the 15SPC bilayer. At the same temperature and Chol content, H-bonding of Chol with SSM is more extensive than with 15SPC. SSM-Chol H-bonding anchors the OH group of Chol in the lower regions of the SSM-Chol bilayer interface. Such a location strengthens the influence of Chol on the SSM chains. In effect, the phase of the SSM-Chol bilayer containing 22 mol % Chol at 37 degrees C is shifted from the gel to the liquid-ordered phase, and the bilayer displays similar properties below and above the main phase-transition temperature for a pure SSM bilayer of approximately 45 degrees C. In contrast, due to a higher location, Chol is not able to change the phase of the 15SPC-Chol bilayer, which at 37 degrees C remains in the gel phase. Chol affects both the core and interface of the SSM bilayer. With increasing Chol content, the order of SSM chains and hydration of SSM headgroups increase, whereas polar interactions between lipids decrease.

Phospholipid Bilayer Free Volume Analysis Employing the Thermal Ring-Closing Reaction of Merocyanine Molecular Switches

The free volume properties of phospholipid bilayers have been determined using a new assay that applies the photochromic and solvatochromic properties of merocyanines. The orientation and embedding depth of the merocyanines in the bilayer are controlled using substitution on the merocyanine indole moiety. The free volume changes at the aqueous interface (region 1), the phospholipid headgroup (region 2), and the aliphatic interior (region 3) of the bilayer are compared by analyzing the rate constants for the merocyanine ring-closing reaction. Free volume variations during the P(beta)(')(gel) <--> L(alpha)(liquid) phase transition are observed in region 1, in accordance with large structural rearrangements between the gel and the liquid phases in this region. The largest free volume is found in region 3, and the smallest is found in region 2. This distribution of free volume in the bilayer agrees with computational studies of these systems. Comparison of the free volume in region 2 of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipids shows that this method is sensitive to small structural differences between lipids. In region 2, the free volume is found to be approximately 2 times larger in DPPC bilayers, which could be related to different merocyanine interactions with the two phosphatidylcholines. Free volume properties determined on picosecond and second time scales are compared based on an analysis of merocyanine formation and decoloration reactions.

Interaction of fusidic acid with lipid membranes: Implications to the mechanism of antibiotic activity

We have studied the effects of cholesterol and steroid-based antibiotic fusidic acid (FA) on the behavior of lipid bilayers using a variety of experimental techniques together with atomic-scale molecular dynamics simulations. Capillary electrophoretic measurements showed that FA was incorporated into fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes. Differential scanning calorimetry in turn showed that FA only slightly altered the thermodynamic properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, whereas cholesterol abolished all endotherms when the mole fraction of cholesterol (X(chol)) was >0.20. Fluorescence spectroscopy was then used to further characterize the influence of these two steroids on DPPC large unilamellar vesicles. In the case of FA, our result strongly suggested that FA was organized into lateral microdomains with increased water penetration into the membrane. For cholesterol/DPPC mixtures, fluorescence spectroscopy results were compatible with the formation of the liquid-ordered phase. A comparison of FA and cholesterol-induced effects on DPPC bilayers through atomistic molecular dynamics simulations showed that both FA and cholesterol tend to order neighboring lipid chains. However, the ordering effect of FA was slightly weaker than that of cholesterol, and especially for deprotonated FA the difference was significant. Summarizing, our results show that FA is readily incorporated into the lipid bilayer where it is likely to be enriched into lateral microdomains. These domains could facilitate the association of elongation factor-G into lipid rafts in living bacteria, enhancing markedly the antibiotic efficacy of FA.

Cholesterol effects on a mixed-chain phosphatidylcholine bilayer: a molecular dynamics simulation study

A molecular dynamics simulation of a mono-cis-unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer containing approximately 22 mol% of cholesterol (POPC-Chol) was carried out for 15 ns. An 8-ns trajectory was analysed to determine the effects of Chol on the membrane properties and compare it with that on the fully saturated 1,2-dimyristoyl-phosphatidylcholine bilayer containing approximately 22 mol% of Chol (DMPC-Chol). The study suggests that the experimentally observed weaker effect of Chol on the POPC than DMPC bilayer might result from a different vertical localisation of the Chol hydroxyl group (OH-Chol) in both bilayers: in the POPC-Chol bilayer, OH-Chol is placed approximately 3 A higher in the bilayer interface than in the DMPC-Chol bilayer. Because of the rigid cis double bond in the beta-chain of POPC, Chol fits worse to the POPC-Chol membrane environment and is pushed up, in effect all Chol ring atoms are, on average, located above the double bond. Both in mono-cis-unsaturated and fully saturated PC bilayers, Chol induces stronger van der Waals interactions among the chains, whereas its interactions with the chains are weak. In contrast to DMPC, the smooth alpha-face of the Chol ring lowers the order of POPC chains, whereas the rough beta-face increases the order.

A New Fluorescent Squaraine Probe for the Measurement of Membrane Polarity

The present study was undertaken to evaluate the sensitivity of newly synthesized squaraine dye 1 to the changes in lipid bilayer physical properties and compared it with the well-known dye 2. Partitioning of the dye 1 into lipid bilayer was found to be followed by significant increase of its fluorescence intensity and red-shift of emission maximum, while intensity of the dye 2 fluorescence increased only slightly on going from aqueous to lipidic environment. This suggests that dye 1 is more sensitive to the changes in membrane properties as compared to dye 2. Partition coefficients of the dye 1 have been determined for the model membranes composed of zwitterionic phospholipid phosphatidylcholine (PC) and its mixtures with positively charged detergent cetyltrimethylammonium bromide (CTAB), anionic phospholipid cardiolipin (CL), and sterol (Chol). The spectral responses of the dye 1 in different liposome media proved to correlate with the increase of bilayer polarity induced by Chol and CL or its decrease caused by CTAB. It was concluded that dye 1 can be used as fluorescent probe for examining membrane-related processes.

Conformation study of the membrane models by the Maxwell displacement current technique and oxidative stress

The role of biological membranes as a target in biological radiation damage is still unclear. Recently much attention has been paid to the dynamic behaviour of the cell membrane. Maxwell displacement current technique (MDC) provides new possibility of conformation study of the membrane models. Oxidative stress can impair macromolecules in the cell on a molecular level. MDC technique enables to study the changes in molecular orientations and/or conformations of cell membranes. The combination of different methods in structural biology can clarify membrane chemical and physical properties.

Modulating Membrane Properties: The Effect of Trehalose and Cholesterol on a Phospholipid Bilayer

The protective properties of trehalose on cholesterol-containing lipid dipalmitoylphosphatidylcholine (DPPC) bilayers are studied through molecular simulations. The ability of the disaccharide to interact with the phospholipid headgroups and stabilize the membrane persists even at high cholesterol concentrations and restricts some of the changes to the structure that would otherwise be imposed by cholesterol molecules. Predictions of bilayer properties such as area per lipid, tail ordering, and chain conformation support the notion that the disaccharide decreases the main melting transition in these multicomponent model membranes, which correspond more closely to common biological systems than pure bilayers. Molecular simulations indicate that the membrane dynamics are slowed considerably by the presence of trehalose, indicating that high sugar concentrations would serve to avert possible phase separations that could arise in mixed phospholipid systems. Various time correlation functions suggest that the character of the modifications in lipid dynamics induced by trehalose and cholesterol is different in the hydrophilic and hydrophobic regions of the membrane.

Predicting permeability coefficient in ADMET evaluation by using different membranes-interaction QSAR

Membrane-interaction quantitative structure activity relationship (MI-QSAR) analysis was applied to a data set with 18 compounds in 18 different membranes. MI-QSAR was used to estimate the ADMET properties including the transport of organic solutes through biological membranes. The most important descriptors are the aqueous solvation free energy, FH2O, and diffusion coefficient for all membranes. The correlation coefficient, r2, and cross-validation correlation coefficient, q2, for DMPG membrane is 0.850 and 0.770, respectively. The relationship between FH2O and permeability is nonlinear. But the detail effect of aqueous solvation free energy and diffusion coefficient to the permeability depends on the type of membrane. The final models also support the solution-diffusion mechanism of transport is important in membrane.

Isomerization Dynamics of Photochromic Spiropyran Molecular Switches in Phospholipid Bilayers

Transient absorption spectroscopy was used to investigate the dynamics of the photochromic indolinobenzospiropyran reaction in toluene solution and in phosphatidylcholine bilayers (1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)). After excitation with UV light, colorless (R/S)-2-(3',3'-dimethyl-6-nitro-3'H-spiro[chromene-2,2'-indol]-1'-yl)ethanol derivatives are converted to colored merocyanine products in high yield; Phi = 0.45 in DMPC liposomes. We find that the reaction occurs in the bilayer aliphatic region in the gel (P(beta)(')) and liquid (L(alpha)) phases. The Arrhenius activation energy for the isomerization in DMPC bilayers was approximately 3.5 times larger in the liquid phase (L(alpha), E(a) = 26.0 +/- 1.0 kJ mol(-1)) than that in the gel phase (P(beta)('), E(a) = 7.3 +/- 1.6 kJ mol(-1)). Analysis of the isomerization rate constant temperature dependence allows an estimation of the bilayer viscosity and free volume properties in the L(alpha) phase.

Significance of sterol structural specificity. Desmosterol cannot replace cholesterol in lipid rafts

Desmosterol is an immediate precursor of cholesterol in the Bloch pathway of sterol synthesis and an abundant membrane lipid in specific cell types. The significance of the difference between the two sterols, an additional double bond at position C24 in the tail of desmosterol, is not known. Here, we provide evidence that the biophysical and functional characteristics of the two sterols differ and that this is because the double bond at C24 significantly weakens the sterol ordering potential. In model membranes, desmosterol was significantly weaker than cholesterol in promoting the formation or stability of ordered domains, and in mammalian cell membranes, desmosterol associated less avidly than cholesterol with detergent-resistant membranes. Atomic scale molecular dynamics simulations showed that the double bond gives rise to additional stress in the tail, creating a rigid structure between C24 and C27 and favoring tilting of desmosterol distinct from cholesterol. Functional effects of desmosterol in cell membranes were assessed upon acutely exchanging approximately 70% of cholesterol to desmosterol. This led to impaired raft-dependent signaling via the insulin receptor, whereas non-raft-dependent protein secretion was not affected. We suggest that the choice of cholesterol synthesis route may provide a physiological mechanism to modulate raft-dependent functions in cells.

Lateral pressure profiles in cholesterol-DPPC bilayers

By means of atomistic molecular dynamics simulations, we study cholesterol-DPPC (dipalmitoyl phosphatidylcholine) bilayers of different composition, from pure DPPC bilayers to a 1:1 mixture of DPPC and cholesterol. The lateral pressure profiles through the bilayers are computed and separated into contributions from the different components. We find that the pressure inside the bilayer changes qualitatively for cholesterol concentrations of about 20% or higher. The pressure profile in the inside of the bilayer then turns from a rather flat shape into an alternating sequence of regions with large positive and negative lateral pressure. The changes in the lateral pressure profile are so characteristic that specific interaction between cholesterol and molecules such as membrane proteins mediated solely via the lateral pressure profile might become possible.

Effect of Cholesterol on the Properties of Phospholipid Membranes. 4. Interatomic Voids

The properties of the interatomic voids present in fully hydrated dimyristoylphosphatidylcholine (DMPC)-cholesterol mixed membranes of different compositions are analyzed in detail using a generalized variant of the Voronoi-Delaunay method on the basis of computer simulation results. The systems investigated are chosen from both sides of the DMPC-cholesterol miscibility gap; the pure DMPC bilayer has also been included in the analysis as a reference system. The results obtained show that the empty space is organized in a more compact way, forming larger voids in the presence than in the absence of cholesterol. The voids located in the region of the rigid cholesterol rings become, on average, less spherical, oriented more parallel with the membrane normal axis with increasing cholesterol concentration, whereas an opposite effect of cholesterol is observed in the middle of the membrane among the chain terminal methyl groups. In general, the preferential orientation of the voids is found to strongly correlate with that of the molecules in the hydrocarbon phase of the membranes. The membranes are found to contain rather large voids, the volume of which can be an order of magnitude larger than the largest spherical cavities present in the systems. These voids are elongated or branching channels rather than big empty holes. The voids located among the DMPC and cholesterol molecules are lying preferably parallel with the membrane normal axis. The existence of such empty channels can be of great importance in the cross-membrane permeation of small, uncharged penetrants, in particular, of polar molecules.

Response to comment by Almeida et al.: free area theories for lipid bilayers--predictive or not?

Free area theories for lateral diffusion in lipid bilayers are reviewed and discussed. It has been suggested by Almeida et al. that free area theories yield quantitative predictions for lateral diffusion coefficients of lipids. We investigate the plausibility of this suggestion by first sketching what is to be expected of a quantitative theory with predictive power, and subsequently examining whether existing free area theories comply with these expectations. Our conclusion is that current free area theories for lipid bilayers are not quantitative theories with predictive power. They involve a number of adjustable parameters, all of which are not estimated independently, but derived from fitting the theory to the very data whose behavior the theory is supposed to predict. Further, the interpretation and behavior of some of the parameters are ambiguous. The best example is the so-called activation barrier, whose qualitative behavior with the cholesterol concentration in a DMPC bilayer varies depending on the experimental method used to generate the input data and the exact assumptions made to formulate the theory. Independent determination of the activation barrier from numerical simulations or experiments appears to be very difficult.