Modulation of the orientational order profile of the lipid acyl chain in the L alpha phase

Anticancer drugs tamoxifen and 4hydroxytamoxifen as effectors of phosphatidylethanolamine lipid polymorphism

The interaction of tamoxifen (TMX) and its metabolite 4-hydroxytamoxifen (HTMX) with a biomimetic membrane model system composed of 1,2-dielaidoylphosphatidylethanolamine (DEPE) has been studied using a biophysical approach. Incorporation of TMX into DEPE bilayers gives rise to a progressive broadening of the L_β/L_α phase transition and a downward temperature shift. The L_β/L_α phase transition presents multiple endotherms, indicating a lateral segregation of TMX/DEPE domains within the plane of the bilayer. TMX and HTMX also widen and shift the L_α to hexagonal-H_II transition toward lower values, the phase diagrams showing that both compounds facilitate formation of the H_II phase. TMX increases motional disorder of DEPE acyl chains in the L_β, L_α and H_II phases, whereas the effect of HTMX is clearly different. In addition, neither TMX nor HTMX significantly perturb the hydration state of the polar headgroup region of DEPE. Molecular dynamics (MD) simulations indicate that these drugs do not affect membrane thickness, area per lipid, or the conformation of DEPE molecules. As a general rule, the interaction of HTMX with DEPE is qualitatively similar to TMX but less intense. However, a significant difference shown by MD is that HTMX is mainly placed around the center of each monolayer while TMX is located mainly at the center of the membrane, also having a greater tendency to cluster formation. These results are discussed to understand the modulation of phosphatidylethanolamine lipid polymorphism carried out by these drugs, which could be of relevance to explain their effects on enzyme activity or membrane permeabilization.

Capturing Membrane Phase Separation by Dual Resolution Molecular Dynamics Simulations

Understanding the lateral organization in plasma membranes remains an open problem and is of great interest to many researchers. Model membranes consisting of coexisting domains are commonly used as simplified models of plasma membranes. The coarse-grained (CG) Martini force field has successfully captured spontaneous separation of ternary membranes into a liquid-disordered and a liquid-ordered domain. With all-atom (AA) models, however, phase separation is much harder to achieve due to the slow underlying dynamics. To remedy this problem, here, we apply the virtual site (VS) hybrid method on a ternary membrane composed of saturated lipids, unsaturated lipids, and cholesterol to investigate the phase separation. The VS scheme couples the two membrane leaflets at CG and AA resolution. We found that the rapid phase separation reached by the CG leaflet can accelerate and guide this process in the AA leaflet.

Lipid selectivity in detergent extraction from bilayers

Despite numerous studies on detergent-induced solubilization of membranes and on the underlying mechanisms associated with this process, very little is known regarding the selectivity of detergents for lipids during their extraction from membranes. To get insights about this phenomenon, solubilization of model bilayers prepared from binary lipid mixtures by different detergents was examined. Three commonly used detergents were used: the non-ionic Triton X-100 (TX), the negatively-charged sodium dodecylsulfate (SDS), and the positively-charged n-dodecyltrimethylammonium chloride (DTAC). Two model membranes were used in order to identify if specific intermolecular interactions can lead to lipid selectivity: bilayers made of a binary mixture of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and of a binary mixture of POPC and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG). Therefore, it was possible to describe systems presenting a combination of detergents bearing different charges with bilayers with different polymorphic propensities and charge. In conditions for which partial solubilization was observed, the composition of the extracted lipid phase was quantified with Liquid Chromatography coupled to Mass Spectrometry to elucidate whether a lipid selectivity occurred in the solubilization process. On one hand, it is found that repulsive or attractive electrostatic interactions did not lead to any lipid selectivity. On the other hand, POPE was systematically less extracted than POPC, regardless of the detergent nature. We propose that this lipid selectivity is inherent to the molecular shape of POPE unsuited for micelles curvature properties.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Structure and Dynamics of Cinnamycin-Lipid Complexes: Mechanisms of Selectivity for Phosphatidylethanolamine Lipids

Cinnamycin is a lantibiotic peptide, which selectively binds to and permeabilizes membranes containing phosphatidylethanolamine (PE) lipids. As PE is a major component of many bacterial cell membranes, cinnamycin has considerable potential for destroying these. In this study, molecular dynamics simulations are used to elucidate the structure of a lipid-cinnamycin complex and the origin of selective lipid binding. The simulations reveal that cinnamycin selectively binds to PE by forming an extensive hydrogen-bonding network involving all three hydrogen atoms of the primary ammonium group of the PE head group. The substitution of a single hydrogen atom with a methyl group on the ammonium nitrogen destabilizes this hydrogen-bonding network. In addition to binding the primary ammonium group, cinnamycin also interacts with the phosphate group of the lipid through a previously uncharacterized phosphate-binding site formed by the backbone Phe10-Abu11-Phe12-Val13 moieties (Abu = 1-α-aminobutyric acid). In addition, hydroxylation of Asp15 at Cβ plays a role in selective binding of PE due to its tight interaction with the charged amine of the lipid head group. The simulations reveal that the position and orientation of the peptide in the membrane depend on the type of lipid to which it binds, suggesting a reason for why cinnamycin selectively permeabilizes PE-containing membranes.

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.

DHA Modifies the Size and Composition of Raftlike Domains: A Solid-State 2H NMR Study

Docosahexaenoic acid is an omega-3 polyunsaturated fatty acid that relieves the symptoms of a wide variety of chronic inflammatory disorders. The structural mechanism is not yet completely understood. Our focus here is on the plasma membrane as a site of action. We examined the molecular organization of [²H₃₁]-N-palmitoylsphingomyelin (PSM-d₃₁) mixed with 1-palmitoyl-2-docosahexaenoylphosphatylcholine (PDPC) or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), as a monounsaturated control, and cholesterol (chol) (1:1:1 mol) in a model membrane by solid-state ²H NMR. The spectra were analyzed in terms of segregation into ordered SM-rich/chol-rich (raftlike) and disordered PC-rich/chol-poor (nonraft) domains that are nanoscale in size. An increase in the size of domains is revealed when POPC was replaced by PDPC. Spectra that are single-component, attributed to fast exchange between domains (<45 nm), for PSM-d₃₁ mixed with POPC and chol become two-component, attributed to slow exchange between domains (r > 30 nm), for PSM-d₃₁ mixed with PDPC and chol. The resolution of separate signals from PSM-d₃₁, and correspondingly from [3α-²H₁]cholesterol (chol-d₁) and 1-[²H₃₁]palmitoyl-2-docosahexaenoylphosphatidylcholine (PDPC-d₃₁), in raftlike and nonraft domains enabled us to determine the composition of the domains in the PDPC-containing membrane. Most of the lipid (28% SM, 29% chol, and 23% PDPC with respect to total lipid at 30°C) was found in the raftlike domain. Despite substantial infiltration of PDPC into raftlike domains, there appears to be minimal effect on the order of SM, implying the existence of internal structure that limits contact between SM and PDPC. Our results suggest a significant refinement to the model by which DHA regulates the architecture of ordered, sphingolipid-chol-enriched domains (rafts) in membranes.

Constrained Versus Free Cholesterol in DPPC Membranes: A Comparison of Chain Ordering Ability Using Deuterium NMR

We report here the first exploration of the nature of the hydrophobic region of bilayer membranes formed from sterol-modified phospholipids [Huang, Z.; Szoka, F. C., Sterol-Modified Phospholipids: Cholesterol and Phospholipid Chimeras with Improved Biomembrane Properties. J. Am. Chem. Soc. 2008, 130 (46), 15702-15712] & [Ding, J.; Starling, A. P.; East, J. M.; Lee, A. G., Binding Sites for Cholesterol on Ca(2+)-ATPase Studied by Using a Cholesterol-Containing Phospholipid. Biochemistry 1994, 33 (16), 4974-4979]. Using ²H NMR spectroscopy, we present our results for the phase behavior and acyl chain ordering of multilamellar vesicles (MLVs) of a sterol-modified phospholipid, 1-cholesterylhemisuccinoyl-2-palmitoyl(d₃₁)-sn-glycero-3-phosphocholine (hereafter referred to as CholPPC-d₃₁). We compared our results with the conformational order induced by cholesterol at various concentrations in 1-palmitoyl,2-palmitoyl(d₃₁)-sn-glycero-3-phosphocholine (DPPC-d₃₁)/cholesterol membranes. On the basis of the existing literature [Foglia, F.; Barlow, D. J.; Szoka, F. C.; Huang, Z.; Rogers, S. E.; Lawrence, M. J., Structural Studies of the Monolayers and Bilayers Formed by a Novel Cholesterol-Phospholipid Chimera. Langmuir 2011, 27 (13), 8275-8281], we expected to find that the deuterated palmitoyl chain in CholPPC-d₃₁ membranes had an order parameter profile similar to the deuterated palmitoyl chain of sn-2 labeled DPPC-d₃₁ in MLVs of a mixture of DPPC-d₃₁ with 40 mol % unconstrained cholesterol. Our data indicate that the ordering ability of cholesterol in CholPPC is significantly reduced compared to free cholesterol in DPPC. This result emphasizes that cholesterol molecules must be free to move in the bilayers to reach their maximum ordering ability. In other words, when compared to unconstrained cholesterol, the constrained cholesterol moiety in CholPPC causes nonoptimal chain packing.

Concepts and Methods of Solid-State NMR Spectroscopy Applied to Biomembranes

Concepts of solid-state NMR spectroscopy and applications to fluid membranes are reviewed in this paper. Membrane lipids with ²H-labeled acyl chains or polar head groups are studied using ²H NMR to yield knowledge of their atomistic structures in relation to equilibrium properties. This review demonstrates the principles and applications of solid-state NMR by unifying dipolar and quadrupolar interactions and highlights the unique features offered by solid-state ²H NMR with experimental illustrations. For randomly oriented multilamellar lipids or aligned membranes, solid-state ²H NMR enables direct measurement of residual quadrupolar couplings (RQCs) due to individual C-²H-labeled segments. The distribution of RQC values gives nearly complete profiles of the segmental order parameters S_CD⁽ⁱ⁾ as a function of acyl segment position (i). Alternatively, one can measure residual dipolar couplings (RDCs) for natural abundance lipid samples to obtain segmental S_CH order parameters. A theoretical mean-torque model provides acyl-packing profiles representing the cumulative chain extension along the normal to the aqueous interface. Equilibrium structural properties of fluid bilayers and various thermodynamic quantities can then be calculated, which describe the interactions with cholesterol, detergents, peptides, and integral membrane proteins and formation of lipid rafts. One can also obtain direct information for membrane-bound peptides or proteins by measuring RDCs using magic-angle spinning (MAS) in combination with dipolar recoupling methods. Solid-state NMR methods have been extensively applied to characterize model membranes and membrane-bound peptides and proteins, giving unique information on their conformations, orientations, and interactions in the natural liquid-crystalline state.

Ceramide-C16 Is a Versatile Modulator of Phosphatidylethanolamine Polymorphism

Ceramide-C16 (CerC16) is a sphingolipid associated with several diseases like diabetes, obesity, Parkinson disease, and certain types of cancers. As a consequence, research efforts are devoted to identify the impact of CerC16 on the behavior of membranes, and to understand how it is involved in these diseases. In this work, we investigated the impacts of CerC16 (up to 20 mol %) on the lipid polymorphism of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), using differential scanning calorimetry, and sequential ²H and ³¹P solid-state nuclear magnetic resonance spectroscopy. A partial phase diagram is proposed. The results indicate that the presence of CerC16 leads to an upshift of the temperature of the gel-to-liquid crystalline (L_β - L_α) phase transition, leading to a large L_β/L_α phase coexistence region where gel-phase domains contain ∼35 mol % CerC16. It also leads to a downshift of the temperature of the lamellar-to-inverted hexagonal (L - H_II) phase transition of POPE. The opposite influence on the two-phase transitions of POPE brings a three-phase coexistence line when the two transitions overlap. The resulting H_II phase can be ceramide enriched, coexisting with a L_α phase, or ceramide depleted, coexisting with a L_β phase, depending on the CerC16 proportions. The uncommon capability of CerC16 to modulate the membrane fluidity, its curvature propensity, and the membrane interface properties highlights its potential as a versatile messenger in cell membrane events.

Melittin-Induced Lipid Extraction Modulated by the Methylation Level of Phosphatidylcholine Headgroups

Protein- and peptide-induced lipid extraction from membranes is a critical process for many biological events, including reverse cholesterol transport and sperm capacitation. In this work, we examine whether such processes could display specificity for some lipid species. Melittin, the main component of dry bee venom, was used as a model amphipathic α-helical peptide. We specifically determined the modulation of melittin-induced lipid extraction from membranes by the change of the methylation level of phospholipid headgroups. Phosphatidylcholine (PC) bilayers were demethylated either by substitution with phosphatidylethanolamine (PE) or chemically by using mono- and dimethylated PE. It is shown that demethylation reduces the association of melittin with membranes, likely because of the resulting tighter chain packing of the phospholipids, which reduces the capacity of the membranes to accommodate inserted melittin. This reduced binding of the peptide is accompanied by an inhibition of the lipid extraction caused by melittin. We demonstrate that melittin selectively extracts PC from PC/PE membranes. This selectivity is proposed to be a consequence of a PE depletion in the surroundings of bound melittin to minimize disruption of the interphospholipid interactions. The resulting PC-enriched vicinity of melittin would be responsible for the observed formation of PC-enriched lipid/peptide particles resulting from the lipid efflux. These findings reveal that modulating the methylation level of phospholipid headgroups is a simple way to control the specificity of lipid extraction from membranes by peptides/proteins and thereby modulate the lipid composition of the membranes.

Effect of Sterol Structure on the Physical Properties of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine Membranes Determined Using (2)H Nuclear Magnetic Resonance

The effect of a series of phytosterols on lipid chain ordering in 1-palmitoyl((2)H31)-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31) multibilayer vesicles was examined by (2)H NMR spectroscopy at 25 °C. These results, along with existing data for other sterols, indicate that the ordering power of sterols in POPC-d31 depends on subtle aspects of sterol structure. Cholesterol, 7-dehydrocholesterol (7-DHC), campesterol, β-sitosterol, ergosterol, brassicasterol, and stigmasterol all increase the lipid chain order as sterol concentration is increased. However, saturation of the ordering occurs at different sterol concentrations for ergosterol (as previously reported), brassicasterol, β-sitosterol, and stigmasterol. Here our interest lies in finding which part of the sterol structure is responsible for the observed saturation of the palmitoyl chain order as a function of sterol concentration. In particular, we propose that the saturation of the ordering of POPC-d31/brassicasterol and POPC-d31/stigmasterol membranes at quite low sterol concentrations is due to the presence of a double bond at C22. We also discuss how the structural differences between the sterols affect their ability to intercalate between the POPC acyl chains. Furthermore, the effective solubility of sterols in POPC is discussed in relation to the dependence of maximum POPC-d31 chain order vs sterol concentration.

Atomistic resolution structure and dynamics of lipid bilayers in simulations and experiments

Accurate details on the sampled atomistic resolution structures of lipid bilayers can be experimentally obtained by measuring C-H bond order parameters, spin relaxation rates and scattering form factors. These parameters can be also directly calculated from the classical atomistic resolution molecular dynamics simulations (MD) and compared to the experimentally achieved results. This comparison measures the simulation model quality with respect to 'reality'. If agreement is sufficient, the simulation model gives an atomistic structural interpretation of the acquired experimental data. Significant advance of MD models is made by jointly interpreting different experiments using the same structural model. Here we focus on phosphatidylcholine lipid bilayers, which out of all model membranes have been studied mostly by experiments and simulations, leading to the largest available dataset. From the applied comparisons we conclude that the acyl chain region structure and rotational dynamics are generally well described in simulation models. Also changes with temperature, dehydration and cholesterol concentration are qualitatively correctly reproduced. However, the quality of the underlying atomistic resolution structural changes is uncertain. Even worse, when focusing on the lipid bilayer properties at the interfacial region, e.g. glycerol backbone and choline structures, and cation binding, many simulation models produce an inaccurate description of experimental data. Thus extreme care must be applied when simulations are applied to understand phenomena where the interfacial region plays a significant role. This work is done by the NMRlipids Open Collaboration project running at https://nmrlipids.blogspot.fi and https://github.com/NMRLipids. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Depolarization Laplace transform analysis of exchangeable hyperpolarized ¹²⁹Xe for detecting ordering phases and cholesterol content of biomembrane models

We present a highly sensitive nuclear-magnetic resonance technique to study membrane dynamics that combines the temporary encapsulation of spin-hyperpolarized xenon ((129)Xe) atoms in cryptophane-A-monoacid (CrAma) and their indirect detection through chemical exchange saturation transfer. Radiofrequency-labeled Xe@CrAma complexes exhibit characteristic differences in chemical exchange saturation transfer-driven depolarization when interacting with binary membrane models composed of different molecular ratios of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). The method is also applied to mixtures of cholesterol and POPC. The existence of domains that fluctuate in cluster size in DPPC/POPC models at a high (75-98%) DPPC content induces up to a fivefold increase in spin depolarization time τ at 297 K. In POPC/cholesterol model membranes, the parameter τ depends linearly on the cholesterol content at 310 K and allows us to determine the cholesterol content with an accuracy of at least 5%.

Cholesterol translocation in a phospholipid membrane

Cholesterol (CHOL) molecules play a key role in modulating the rigidity of cell membranes and controlling intracellular transport and signal transduction. Using an all-atom molecular dynamics approach, we study the process of CHOL interleaflet transport (flip-flop) in a dipalmitoylphosphatidycholine (DPPC)-CHOL bilayer over a time period of 15 μs. We investigate the effect of the flip-flop process on mechanical stress across the bilayer and the role of CHOL in inducing molecular order in bilayer leaflets. The simulations are carried out at physiologically relevant CHOL concentration (30%), temperature (323 K), and pressure (1 bar). CHOL flip-flop events are observed with a rate constant of 3 × 10⁴s⁻¹. Once a flip-flop event is triggered, a CHOL molecule takes an average of 73 nanoseconds to migrate from one bilayer leaflet to the other.

The molecular structure of the liquid-ordered phase of lipid bilayers

Molecular dynamics simulations reveal substructures within the liquid-ordered phase of lipid bilayers. These substructures, identified in a 10 μs all-atom trajectory of liquid-ordered/liquid-disordered coexistence (L(o)/L(d)) are composed of saturated hydrocarbon chains packed with local hexagonal order and separated by interstitial regions enriched in cholesterol and unsaturated chains. Lipid hydrocarbon chain order parameters calculated from the L(o) phase are in excellent agreement with (2)H NMR measurements; the local hexagonal packing is also consistent with (1)H-MAS NMR spectra of the L(o) phase, NMR diffusion experiments, and small-angle X-ray and neutron scattering. The balance of cholesterol-rich to local hexagonal order is proposed to control the partitioning of membrane components into the L(o) regions. The latter have been frequently associated with formation of so-called rafts, platforms in the plasma membranes of cells that facilitate interaction between components of signaling pathways.

Lamellar self-assemblies of single-chain amphiphiles and sterols and their derived liposomes: distinct compositions and distinct properties

Typically, single-chain amphiphiles and sterols do not form fluid lamellar phases once hydrated individually. Most of the single-chain amphiphiles form actually micelles in aqueous environments, while sterols display a very limited solubility in water. However, under certain conditions, mixtures of single-chain amphiphiles and sterols lead to the formation of stable fluid bilayers. Over the past decade, several of these systems leading to fluid lamellar self-assemblies have been identified and this article reviews the current knowledge relative to these non-phospholipid bilayers made of single-chain amphiphiles and sterols. It presents an integrated view about the molecular features that are required for their stability, the properties they share, and the origin of these characteristics. It was also shown that these lamellar systems could lead to the formation of unilamellar vesicles, similar to phospholipid based liposomes. These vesicles display distinct properties that make them potentially appealing for technological applications; they display a limited permeability, they are stable, they are formed with molecules that are relatively chemically inert (and relatively cheap), and they can be readily functionalized. The features of these distinct liposomes and their technological applications are reviewed. Finally, the putative biological implications of these non-phospholipid fluid bilayers are also discussed.

Using spin-label W-band EPR to study membrane fluidity profiles in samples of small volume

Conventional and saturation-recovery (SR) EPR at W-band (94GHz) using phosphatidylcholine spin labels (labeled at the alkyl chain [n-PC] and headgroup [T-PC]) to obtain profiles of membrane fluidity has been demonstrated. Dimyristoylphosphatidylcholine (DMPC) membranes with and without 50 mol% cholesterol have been studied, and the results have been compared with similar studies at X-band (9.4 GHz) (L. Mainali, J.B. Feix, J.S. Hyde, W.K. Subczynski, J. Magn. Reson. 212 (2011) 418-425). Profiles of the spin-lattice relaxation rate (T(1)(-1)) obtained from SR EPR measurements for n-PCs and T-PC were used as a convenient quantitative measure of membrane fluidity. Additionally, spectral analysis using Freed's MOMD (microscopic-order macroscopic-disorder) model (E. Meirovitch, J.H. Freed J. Phys. Chem. 88 (1984) 4995-5004) provided rotational diffusion coefficients (R(perpendicular) and R(||)) and order parameters (S(0)). Spectral analysis at X-band provided one rotational diffusion coefficient, R(perpendicular). T(1)(-1), R(perpendicular), and R(||) profiles reflect local membrane dynamics of the lipid alkyl chain, while the order parameter shows only the amplitude of the wobbling motion of the lipid alkyl chain. Using these dynamic parameters, namely T(1)(-1), R(perpendicular), and R(||), one can discriminate the different effects of cholesterol at different depths, showing that cholesterol has a rigidifying effect on alkyl chains to the depth occupied by the rigid steroid ring structure and a fluidizing effect at deeper locations. The nondynamic parameter, S(0), shows that cholesterol has an ordering effect on alkyl chains at all depths. Conventional and SR EPR measurements with T-PC indicate that cholesterol has a fluidizing effect on phospholipid headgroups. EPR at W-band provides more detailed information about the depth-dependent dynamic organization of the membrane compared with information obtained at X-band. EPR at W-band has the potential to be a powerful tool for studying membrane fluidity in samples of small volume, ~30 nL, compared with a representative sample volume of ~3 μL at X-band.

Peptide model helices in lipid membranes: insertion, positioning, and lipid response on aggregation studied by X-ray scattering

Studying membrane active peptides or protein fragments within the lipid bilayer environment is particularly challenging in the case of synthetically modified, labeled, artificial, or recently discovered native structures. For such samples the localization and orientation of the molecular species or probe within the lipid bilayer environment is the focus of research prior to an evaluation of their dynamic or mechanistic behavior. X-ray scattering is a powerful method to study peptide/lipid interactions in the fluid, fully hydrated state of a lipid bilayer. For one, the lipid response can be revealed by observing membrane thickening and thinning as well as packing in the membrane plane; at the same time, the distinct positions of peptide moieties within lipid membranes can be elucidated at resolutions of up to several angstroms by applying heavy-atom labeling techniques. In this study, we describe a generally applicable X-ray scattering approach that provides robust and quantitative information about peptide insertion and localization as well as peptide/lipid interaction within highly oriented, hydrated multilamellar membrane stacks. To this end, we have studied an artificial, designed β-helical peptide motif in its homodimeric and hairpin variants adopting different states of oligomerization. These peptide lipid complexes were analyzed by grazing incidence diffraction (GID) to monitor changes in the lateral lipid packing and ordering. In addition, we have applied anomalous reflectivity using synchrotron radiation as well as in-house X-ray reflectivity in combination with iodine-labeling in order to determine the electron density distribution ρ(z) along the membrane normal (z axis), and thereby reveal the hydrophobic mismatch situation as well as the position of certain amino acid side chains within the lipid bilayer. In the case of multiple labeling, the latter technique is not only applicable to demonstrate the peptide's reconstitution but also to generate evidence about the relative peptide orientation with respect to the lipid bilayer.

Deuterium NMR study of the effect of ergosterol on POPE membranes

We study the effect of ergosterol on the physical properties of 1-[(2)H(31)]palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) multibilayers using deuterium nuclear magnetic resonance. NMR spectra were taken as a function of temperature and ergosterol concentration up to 70 mol %. The spectral first moments show that there is a dramatic difference in the ability of ergosterol to disorder the gel phase and to order the liquid-crystalline phase of POPE membranes, an unusual behavior among lipid/sterol systems studied up to now. Further investigation of the liquid-crystalline phase shows that ergosterol (erg) increases the chain order of POPE-d31, but that this effect saturates at 10 mol % ergosterol. This is in marked contrast to the effect of cholesterol (chol) on POPE membranes: the chain order of POPE increases with cholesterol to at least 45 mol %. Moreover, we found that at higher ergosterol concentrations (>40 mol %) ergosterol decreases the POPE-d31 chain order, which, to our knowledge, has not been directly observed in other lipid/sterol systems. The temperature-composition phase diagram is presented. Finally, at all ergosterol concentrations, the chain order of liquid-crystalline-phase POPE is much smaller than that of comparable POPE/chol membranes. This implies that there is no liquid-ordered phase behavior for POPE/erg membranes.

Influence of the nature of the sterol on the behavior of palmitic acid/sterol mixtures and their derived liposomes

The phase behavior of mixtures formed with palmitic acid (PA) and one of the following sterols (dihydrocholesterol, ergosterol, 7-dehydrocholesterol, stigmasterol and stigmastanol), in a PA/sterol molar ratio of 3/7, has been characterized by IR and (2)H NMR spectroscopy at different pH. Our study shows that it is possible to form liquid-ordered (lo) lamellar phases with these binary non-phospholipid mixtures. The characterization of alkyl chain dynamics of PA in these systems revealed the large ordering effect of the sterols. It was possible to extrude these systems, using standard extrusion techniques, to form large unilamellar vesicles (LUVs), except in the case of ergosterol-containing mixture. The resulting LUVs displayed a very limited passive permeability consistent with the high sterol concentration. In addition, the stability of these PA/sterol self-assembled bilayers was also found to be pH-sensitive, therefore, potentially useful as nanovectors. By examining different sterols, we could establish some correlations between the structure of these bilayers and their permeability properties. The structure of the side chain at C17 of the sterol appears to play a prime role in the mixing properties with fatty acid.

Phase behavior of an equimolar mixture of N-palmitoyl-D-erythro-sphingosine, cholesterol, and palmitic acid, a mixture with optimized hydrophobic matching

The phase behavior and lipid mixing properties of an equimolar mixture of nonhydroxylated palmitoyl ceramide (Cer16), palmitic acid (PA), and cholesterol have been investigated using 2H NMR and vibrational spectroscopy. This mixture is formed by the three main classes of lipids found in the stratum corneum (SC), the top layer of the epidermis, and provides an optimized hydrophobic matching. Therefore, its behavior highlights the role played by hydrophobic matching on the phase behavior of SC lipids. We found that, below 45 degrees C, the mixture is essentially formed of coexisting crystalline domains with a small fraction of lipids (less than 20%) that forms a gel or fluid phase, likely ensuring cohesion between the solid domains. Upon heating, there is the formation of a liquid ordered phase mainly composed of PA and cholesterol, including a small fraction of Cer16. This finding is particularly highlighted by correlation vibrational microspectroscopy that indicates that domains enriched in cholesterol and PA include more disordered Cer16 than those found in the Cer16-rich domains. Solubilization of Cer16 in the fluid phase occurs progressively upon further heating, and this leads to the formation of a nonlamellar self-assembly where the motions are isotropic on the NMR time scale. It is found that the miscibility of Cer16 with cholesterol and PA is more limited than the one previously observed for ceramide III extracted from bovine brain, which is heterogeneous in chain composition and includes, in addition to Cer16, analogous ceramide with longer alkyl chains that are not hydrophobically matched with cholesterol and PA. Therefore, it is inferred that, in SC, the chain heterogeneity is a stronger criteria for lipid miscibility than chain hydrophobic matching.

Cationic detergent/sterol mixtures can form fluid lamellar phases and stable unilamellar vesicles

In recent studies, it has been shown that mixtures of palmitic acid (PA), and cholesterol or cholesterol sulfate (Schol), in a PA/sterol molar ratio of 30/70 lead to the formation of liquid-ordered (lo) lamellar phases. The extrusion of these systems gave large unilamellar vesicles (LUVs) that displayed a very limited passive permeability, a property associated with their high sterol content. In this study, we showed that the formation of lo-phase bilayers was also possible when mixing a cationic detergent (cetylpyridinium chloride, CPC) and sterol in a 30/70 molar ratio. The existence of this phase was established using IR and 2H NMR spectroscopy. Moreover, 2H NMR allowed us to study the orientation and dynamics of CPC and cholesterol in these self-assemblies. The extrusion of the CPC/Schol bilayers leads to the formation of LUVs, and their passive permeability was found to be very limited, making them interesting candidates as nanovectors.

Phase diagrams of lipid mixtures relevant to the study of membrane rafts

The present paper reviews the phase properties of phosphatidylcholine-sphingomyelin-cholesterol mixtures, that are often used as models for membrane "raft" microdomains. The available data based on X-ray, microscopic and spectroscopic observations, surface pressure and calorimetric measurements, and detergent solubilization assays, are critically evaluated and rationalized in terms of triangular phase diagrams. The remaining uncertainties are discussed specifically and separately from the data on which a consensus appears to exist.

NMR investigations of interactions between anesthetics and lipid bilayers

Interactions between anesthetics (lidocaine and short chain alcohols) and lipid membranes formed by dimyristoylphosphatidylcholine (DMPC) were studied using NMR spectroscopy. The orientational order of lidocaine was investigated using deuterium NMR on a selectively labelled compound whereas segmental ordering in the lipids was probed by two-dimensional 1H-13C separated local field experiments under magic-angle spinning conditions. In addition, trajectories generated in molecular dynamics (MD) computer simulations were used for interpretation of the experimental results. Separate simulations were carried out with charged and uncharged lidocaine molecules. Reasonable agreement between experimental dipolar interactions and the calculated counterparts was observed. Our results clearly show that charged lidocaine affects significantly the lipid headgroup. In particular the ordering of the lipids is increased accompanied by drastic changes in the orientation of the P-N vector in the choline group.

Elasticity, strength, and water permeability of bilayers that contain raft microdomain-forming lipids

Bilayers composed of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (CHOL) are commonly used as systems to model the raft-lipid domain structure believed to compartmentalize particular cell membrane proteins. In this work, micropipette aspiration of giant unilamellar vesicles was used to test the elasticities, water permeabilities, and rupture tensions of single-component PC, binary 1:1 PC/CHOL, and 1:1 SM/CHOL, and ternary 1:1:1 PC/SM/CHOL bilayers, one set of measurements with dioleoyl PC (DOPC; C18:1/C18:1 PC) and the other with stearoyloleoyl PC (SOPC; C18:0/C18:1 PC). Defining the elastic moduli (K(A)), the initial slopes of the increase in tension (sigma) versus stretch in lipid surface area (alpha(e)) were determined for all systems at low (15 degrees C) and high (32-33 degrees C) temperatures. The moduli for the single-component PC and binary phospholipid/CHOL bilayers followed a descending hierarchy of stretch resistance with SM/CHOL > SOPC/CHOL > DOPC/CHOL > PC. Although much more resistant to stretch than the single-component PC bilayers, the elastic response of vesicle bilayers made from the ternary phospholipid/CHOL mixtures showed an abrupt softening (discontinuity in slope), when immediately subjected to a steady ramp of tension at the low temperature (15 degrees C). However, the discontinuities in elastic stretch resistance at low temperature vanished when the bilayers were held at approximately 1 mN/m prestress for long times before a tension ramp and when tested at the higher temperature 32-33 degrees C. The elastic moduli of single-component PC and DOPC/CHOL bilayers changed very little with temperature, whereas the moduli of the binary SOPC/CHOL and SM/CHOL bilayers diminished markedly with increase in temperature, as did the ternary SOPC/SM/CHOL system. For all systems, increasing temperature increased the water permeability but decreased rupture tension. Concomitantly, the measurements of permeability exhibited a prominent correlation with the rupture tension across all the systems. Together, these micromechanical tests of binary and ternary phospholipid/CHOL bilayers demonstrate that PC hydrocarbon chain unsaturation and temperature are major determinants of the mechanical and permeation properties of membranes composed of raft microdomain-forming lipids.

Membrane interactions of ternary phospholipid/cholesterol bilayers and encapsulation efficiencies of a RIP II protein

Membrane interactions of liposomes of ternary phospholipid/cholesterol bilayers are investigated. These interactions lead to discoidal deformations and regular aggregations and are strongly enhanced by the presence of mistletoe lectin (ML), a RIP II type protein. The encapsulation of ML into liposomal nanocapsules is studied with a systematic variation of the lipid composition to monitor its effect on the physical properties: entrapment, mean size, morphology, and stability. Extrusion of multilamellar vesicles through filters 80 nm pore size was used for the generation of liposomes. The mean sizes of liposomes ranged between 120 and 200 nm in diameter with narrow size distributions. The increase in flow rate with pressure for three dioleoylphosphatidylcholine (DOPC)/cholesterol (Chol)/dipalmitoylphosphatidylcholine (DPPC) lipid mixtures was linear and allowed to extrapolate to the minimum burst pressure of the liposomal bilayers. From the minimum pressures P(min), the bilayer lysis tensions gamma(l) were determined. The increase in P(min) and gamma(l) with an increasing content of a saturated phosopholipid (DPPC) indicates that DPPC increases the mechanical strength of lipid bilayers. Apparently, DPPC, like cholesterol, leads to a less compressible surface and a more cohesive membrane. After preparation, vesicle solutions were purified by gel permeation chromatography to separate encapsulated ML from free ML in the extravesicular solution. Purified liposomes were then characterized. The content of entrapped and adsorbed ML was measured using ELISA. Repetitive freezing/thawing cycles prior to extrusion significantly increased ML uptake. On the contrary, adsorption was not affected neither by lipid composition, nor concentration and preparation. Differences in experimental encapsulation efficiency only reflect the differences in the mean vesicle sizes of the different samples as is revealed by a comparison to a theoretical estimate. Cryo-transmission electron microscopy (Cryo-TEM) images show that beside spherical, single-walled liposomes, there is a considerable fraction of discoidally deformed vesicles. Based on our results and those found in the literature, we speculate that the flattening of the vesicles is a consequence of lipid phase separation and the formation of condensed complexes and areas of different bending elasticities. This phenomenon eventually leads to agglomeration of deformed liposomal structures, becoming more pronounced with the increase in the relative amount of saturated fatty acids, presumably caused by hydrophobic interaction. For the same lipid mixture aggregation correlated linearly with the ML content. Finally, tested liposomal samples were kept at 4 degrees C to examine their stability. Only slight fluctuations in diameter and the increase in polydispersity after 3 weeks of storage occurred, with no statistically significant evidence of drug leakage during a time period of 12 days, illustrating physical stability of liposomes.

Phase Behavior of Palmitic Acid/Cholesterol/Cholesterol Sulfate Mixtures and Properties of the Derived Liposomes

The phase behavior of mixtures formed by palmitic acid (PA), cholesterol (Chol), and sodium cholesteryl sulfate (Schol) has been characterized by differential scanning calorimetry and infrared and 2H NMR spectroscopy. It is reported that it is possible to form, with PA/sterol mixtures, fluid lamellar phases where the sterol content is very high (a sterol mole fraction of 0.7). As a consequence of the rigidifying ability of the sterols, the PA acyl chains are very ordered. The stability of these self-assembled bilayers is found to be pH-dependent. This property can be controlled by the Chol/Schol molar ratio, and it is proposed that this parameter modulates the balance between the intermolecular interactions between the constituting species. A phase-composition diagram summarizing the behavior of these mixtures as a function of pH, at room temperature, is presented. It is also shown that it is possible to produce large unilamellar vesicles (LUVs) from these mixtures, using standard extrusion techniques. The resulting LUVs display a very limited passive release of the entrapped material. In addition, these LUVs constitute a versatile vector for pH-triggered release.

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.

Cholesterol supports headgroup superlattice domain formation in fluid phospholipid/cholesterol bilayers

Fluorescence and Fourier transform infrared (FTIR) spectroscopic techniques were used to explore the effect of added cholesterol on the composition-dependent formation of putative phospholipid headgroup superlattices in fluid 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers. Steady-state fluorescence anisotropy measurements of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) revealed significant dips at several POPE-to-phospholipid mole fractions (X(PE)'s) when the cholesterol-to-lipid mole fraction (X(CHOL)) was fixed at 0.00, 0.35, 0.40, and 0.50. Most of the observed dips occur at or close to critical X(PE)'s predicted by the Headgroup Superlattice (SL) model, suggesting that phospholipid headgroups of different structures tend to adopt regular distributions even in the presence of cholesterol. Time-resolved fluorescence anisotropy measurements revealed that DPH-PC senses a disordered and highly mobile microenvironment in the POPE/POPC/CHOL bilayers at those critical X(PE)'s, indicating that this probe may partition to defect regions in the bilayers. The presence of coexisting packing defect regions and regularly distributed SL domains is a key feature predicted by the Headgroup SL model. Importantly, probe-free FTIR measurements of acyl chain C-H, interfacial carbonyl, and headgroup phosphate stretching peak frequencies revealed the presence of abrupt changes at X(PE)'s close to those observed in the fluorescence data. When X(PE) was varied from 0.60 to 0.72 and X(CHOL) from 0.34 to 0.46, a clear dip at the lipid composition coordinates (X(PE), X(CHOL)) approximately (0.68, 0.40) was observed in the three-dimensional surface plots of DPH-PC anisotropy as well as the carbonyl and phosphate stretching frequencies. The critical X(CHOL) at 0.40 agrees with the Cholesterol SL model, which assumes that cholesterol and phospholipid form SL domains at the lipid acyl chain level. In conclusion, this study provides evidence that cholesterol supports formation of phospholipid headgroup SLs in fluid state ternary lipid bilayers. The feasibility of the parallel existence of SLs at the lipid headgroup and acyl chain levels supports the relevance of the lipid SL model for the membranes of eukaryotic cells that typically contain significant amounts of cholesterol. We speculate that lipid SL formation may play a central role in the regulation of membrane lipid compositions, maintenance of organelle boundaries, and other crucial phenomena in those cells.

Acyl chain order parameter profiles in phospholipid bilayers: computation from molecular dynamics simulations and comparison with 2H NMR experiments

Order parameters from deuterium NMR are often used to validate or calibrate molecular dynamics simulations. This paper gives a short overview of the literature in which experimental order parameters from (2)H NMR are compared to those calculated from MD simulations. The different ways in which order parameters from experiment are used to calibrate and validate simulations are reviewed. In the second part of this review, a case study of cholesterol in a DMPC bilayer is presented. It is concluded that the agreement between experimental data and simulation is favorable in the hydrophobic region of the membrane, for both the phospholipids and cholesterol. In the interfacial region the agreement is less satisfactory, probably because of the high polarity of this region which makes the correct computation of the electrostatics more complex.

Fatty acids influence "solid" phase formation in models of stratum corneum intercellular membranes

Stacked intercellular lipid membranes in the uppermost epidermal layer, the stratum corneum (SC), are responsible for skin's barrier function. These membranes are unique in composition, the major lipids being ceramides (Cer), cholesterol, and free fatty acids (FFA) in approximately equimolar proportions. Notably, SC lipids include chains much longer than those of most biological membranes. Previously we showed that Cer's small hydrophilic headgroup enabled SC model membranes composed of bovine brain ceramide (BBCer), cholesterol, and palmitic acid in equimolar proportion to solidify at pH 5.2. In order to determine the influence of FFA chain length on the phase behavior of such membranes, we used 2H NMR and FT-IR to study BBCer/cholesterol/FFA dispersions containing linear saturated FFA 14-22 carbons long. Independent of chain length, the solid phase dominated the FFA spectrum at physiological temperature. Upon heating, each dispersion underwent phase transitions to a liquid crystalline phase (only weakly evident for the membrane containing FFA-C22) and then to an isotropic phase. The phase behavior, the lipid mixing properties, and the transition temperatures are shown to depend strongly on FFA chain length. A distribution of FFA chain lengths is found in the SC and could be required for the coexistence of a proportion of solid lipids with some more fluid domains, which is known to be necessary for normal skin barrier function.

Fluorescence resonance energy transfer to characterize cholesterol-induced domains

Cholesterol is a major component of mammalian cell membranes. It has remarkable effects on the properties of phospholipids bilayers, and is implicated in the lipid raft model. Depending on the membrane composition, cholesterol-containing bilayers can exist either as single phase or as mixture of coexisting phases. These are organized in domains of variant size determined by composition, but can be smaller than the optical microscopy resolution limit. This chapter describes a methodology based on fluorescence resonance energy transfer, which is sensitive to phase separation and can provide estimates of domain size in these usually hard to characterize systems.

Effect of chain unsaturation on bilayer response to pressure

Using wide-line deuterium NMR, the effects of pressure on saturated-chain orientational order and gel-to-liquid-crystal phase transition temperature were observed in bilayers of 16:0-18:1 PC-d31 (POPC-d31) and 16:0-18:2 PC-d31 (PLPC-d31). Spectra were recorded for a range of pressures at selected temperatures and for a range of temperatures at selected pressures up to 193 MPa. For 16:0-18:1 PC-d31, the main transition temperature increased by approximately 0.18 K/MPa, a rate that is similar to what is found for bilayers of disaturated PC's. For 16:0-18:2 PC-d31, the increase in transition temperature with pressure was slightly smaller at approximately 0.13 K/MPa. To investigate the isothermal response of chain orientational order to pressure, spectra for each lipid were obtained for three pressures (ambient, 55 MPa, and 110 MPa) near room temperature (approximately 25 degrees C) and for three pressures (ambient, 110 MPa, and 193 MPa) at higher temperature (approximately 40 degrees C). These temperatures were chosen such that the difference between the higher observation temperature and the main transition of 16:0-18:1 PC-d31 would be similar to the difference between the lower observation temperature and the main transition of 16:0-18:2 PC-d31. Application of a given pressure was found to raise the orientational order for all methylene groups on the saturated chain of a particular lipid by roughly similar amounts. For comparable pressure differences, the pressure-induced ordering of the 16:0-18:1 PC-d31 saturated chain at approximately 40 degrees C was greater than that of the corresponding chain in 16:0-18:2 PC- d31 at approximately 25 degrees C. These observations suggest that increasing levels of chain unsaturation may reduce the sensitivity of bilayer order to variations in pressure at corresponding temperatures relative to their ambient pressure transitions.

Structure and lipid interaction of N-palmitoylsphingomyelin in bilayer membranes as revealed by 2H-NMR spectroscopy

Selectively deuterated N-palmitoyl sphingomyelins were studied by deuterium nuclear magnetic resonance spectroscopy ((2)H-NMR) to elucidate the backbone conformation as well as the interaction of the sphingolipids with glycerophospholipids. Macroscopic alignment of the lipid bilayers provided good spectral resolution and permitted the convenient control of bilayer hydration. Selective deuteration at the acyl chain carbons C(2) and C(3) revealed that the N-acyl chain performs a bend, similar to the sn-2 chain of the phosphatidylcholines. Profiles of C-D bond order parameters were derived from the segmental quadrupolar splittings for sphingomyelin alone and for sphingomyelin-phosphatidycholine mixtures. In the liquid-crystalline state, the N-acyl chain of sphingomyelin alone revealed significantly more configurational order than the chains of homologous disaturated or monounsaturated phosphatidylcholines. The average chain order parameters and the relative width of the order parameter distribution were correlated over a range of bilayer compositions. The temperature dependence of the (2)H-NMR spectra revealed phase separation in bilayers composed of sphingomyelin and monounsaturated phosphatidylcholine, in broad agreement with existing phase diagrams.

Transfer of lipophilic drugs between liposomal membranes and biological interfaces: Consequences for drug delivery

This review paper describes the present knowledge on the interaction of lipophilic, poorly water soluble, drugs with liposomal membranes and the reversibility of this interaction. This interaction is discussed in the context of equilibrium and spontaneous transfer kinetics of the drug, when the liposomes are brought in co-dispersion with other artificial or natural phospholipid membranes in an aqueous medium. The focus is on drugs, which have the potential to partition (dissolve) in a lipid membrane but do not perturb membranes. The degree of interaction is described as solubility of a drug in phospholipid membranes and the kinetics of transfer of a lipophilic drug between membranes. Finally, the consequences of these two factors on the design of lipid based carriers for oral, as well as parenteral use, for lipophilic drugs and lead selection of oral lipophilic drugs is described. Since liposomes serve as model-membranes for natural membranes, the assessment of lipid solubility and transfer kinetics of lipophilic drug using liposome formulations may additionally have predictive value for bioavailability and biodistribution and the pharmacokinetics of lipophilic drugs after parenteral as well as oral administration.