Influence of DPH on the structure and dynamics of a DPPC bilayer

PHOSPHOLIPID ASSEMBLY ON SUPERPARAMAGNETIC NANOPARTICLES FOR THERMORESPONSIVE DRUG DELIVERY APPLICATIONS

Thermoresponsive nanocomposites were prepared by immobilizing a 2–3 nm thick phospholipid layer on the surface of superparamagnetic Fe3O4 nanoparticles via high-affinity avidin/biotin interactions. Morphological and physicochemical surface properties were assessed using transmission electron microscopy, confocal laser scanning microscopy, differential scanning calorimetry, and attenuated total reflectance Fourier transform infrared spectroscopy. The zeta potential of Fe3O4 colloids in phosphate buffered saline (PBS) decreased from -23.6 to -5.0 mV as a consequence of phospholipid immobilization. Nevertheless, heating properties of these superparamagnetic nanoparticles within an alternating magnetic field were not significantly affected. Hyperthermia-relevant temperatures > 40°C were achieved within 10–15 min using a 7-mT magnetic field alternating at a frequency of 1 MHz. Loading of the surface-associated phospholipid layer with the hydrophobic dye dansylcadaverine was accomplished at an efficiency of 479 ng/mg Fe3O4 . Release of this drug surrogate was temperature-dependent, resulting in a 2.5-fold greater release rate when nanoparticles were exposed to a temperature above the experimentally determined melting temperature of 39.7°C. These data underline the feasibility of preparing novel, stimulus-induced drug delivery systems where payload release from a colloid-immobilized phospholipid assembly is triggered by hyperthermia.

Interaction of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled fatty amines with 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine bilayers: a molecular dynamics study

A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled fatty amines of varying alkyl chain length, NBD-C(n), inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics (MD) simulations. For all amphiphiles, the NBD fluorophore locates near the glycerol backbone/carbonyl region of POPC and establishes stable hydrogen bonding with POPC ester oxygen atoms. Small differences observed in the transverse location of the fluorophore correlate with other calculated parameters and with small discrepancies recently measured in the photophysical properties of the molecules. The longer-chained NBD-C(n) amphiphiles show significant mass density near the bilayer midplane, and the chains of these derivatives interdigitate to some extent the opposite bilayer leaflet. This phenomenon leads to a slower lateral diffusion for the longer-chained derivatives (n > 12). Effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain-labeled NBD probes. The molecular details obtained by this work allow the rationalization of the nonmonotonic behavior, recently obtained experimentally, for the photophysical parameters of the amphiphiles and the kinetic and thermodynamic parameters for their interaction with the POPC membranes.

Recent developments in molecular dynamics simulations of fluorescent membrane probes

Due to their sensitivity and versatility, the use of fluorescence techniques in membrane biophysics is widespread. Because membrane lipids are non-fluorescent, extrinsic membrane probes are widely used. However, the behaviour of these probes when inserted in the bilayer is often poorly understood, and it can be hard to distinguish between legitimate membrane properties and perturbation resulting from probe incorporation. Atomistic molecular dynamics simulations present a convenient way to address these issues and have been increasingly used in recent years in this context. This article reviews the application of molecular dynamics to the study of fluorescent membrane probes, focusing on recent work with complex design fluorophores and ordered bilayer systems.

The impact of Texas red on lipid bilayer properties

We investigated the impact of fluorescent labeling on the properties of a lipid bilayer using atomistic molecular dynamics simulation. The system consisted of 24 Texas Red-1,2-dihexadecanoyl-sn-glycero-3-phophoethanolamine (TR-DHPE) in a bilayer of 488 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids. We found binding between TR-DHPE and DPPC caused by electrostatic interactions. On average, TR-DHPE is bound to 1.2 DPPC molecules. Binding reduced the diffusion coefficient of TR-DHPE by 34% relative to unlabeled DPPC molecules. We estimate that binding would lead to a ∼ 5 °C increase in the liquid to liquid-ordered transition temperature of a ternary lipid system. These results emphasize the importance of considering the impact of fluorescence labeling when interpreting experimental results.

Lipid simulations: a perspective on lipids in action

In this article, we provide an overview of lipid simulations, describing how a computer can be used as a laboratory for lipid research. We briefly discuss the methodology of lipid simulations followed by a number of topical applications that show the benefit of computer modeling for complementing experiments. In particular, we show examples of cases in which simulations have made predictions of novel phenomena that have later been confirmed by experimental studies. Overall, the applications discussed in this article focus on the most recent state of the art and aim to provide a perspective of where the field of lipid simulations stands at the moment.

The interactivities with lipid membranes differentially characterize selective and nonselective beta1-blockers

BACKGROUND AND OBJECTIVE

beta-Adrenoceptor-blocking agents have been used for perioperative management during anaesthesia, in which selective beta1-blockers are advantageous over nonselective beta-blockers. Apart from the different affinity for beta-adrenoceptors, beta1-blockers were differentially characterized in light of their different interaction with lipid membranes.

METHODS

Selective (atenolol, metoprolol and esmolol) and nonselective (alprenolol, oxprenolol and propranolol) beta1-blockers were reacted at 0.2-1 mmol l with 1,2-dipalmitoylphosphatidylcholine liposomes and biomimetic membranes consisting of phospholipids, sphingolipid and cholesterol. Their membrane interactivities were comparatively determined using the potency to modify membrane fluidity by measuring fluorescence polarization. Their relative hydrophobicities were evaluated by reversed-phase liquid chromatography.

RESULTS

The chromatographic evaluation divided the tested drugs into more hydrophobic ones containing nonselective beta-blockers and less hydrophobic ones containing selective beta1-blockers. Nonselective beta-blockers, but not selective beta1-blockers, fluidized liposomal membranes, with the potency being oxprenolol < alprenolol < propranolol. Membrane-active alprenolol preferentially acted on the hydrophobic deeper regions of phospholipid bilayers. The potency of nonselective beta-blockers to fluidize biomimetic membranes was greatest in propranolol, followed by alprenolol and oxprenolol, whereas all selective beta1-blockers were inactive.

CONCLUSION

The membrane-fluidizing effects of beta-blockers are correlated with their relative hydrophobicities and their respective conformations to perturb the alignment of phospholipid acyl chains. The membrane-interacting characteristics differentiate beta-blockers as nonselective propranolol, alprenolol and oxprenolol vs. beta1-selective atenolol, metoprolol and esmolol. Such differentiation reflects not only the structural difference but also the beta-adrenoceptor-blocking difference. The membrane fluidization may be partly responsible for the nonselective blockade of beta-adrenoceptors.

Material properties of lipid microdomains: force-volume imaging study of the effect of cholesterol on lipid microdomain rigidity

The effect of cholesterol (CHOL) on the material properties of supported lipid bilayers composed of lipid mixtures that mimic the composition of lipid microdomains was studied by force-volume (FV) imaging under near-physiological conditions. These studies were carried out with lipid mixtures of dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and sphingomyelin. FV imaging enabled simultaneous topology and force measurements of sphingomyelin-rich domains (higher domain (HD)) and phospholipid-rich domains (lower domain (LD)), which allowed quantitative measurement of the force needed to puncture the lipid bilayer with or without CHOL. The force required to penetrate the various domains of the bilayer was probed using high- and low-ionic-strength buffers as a function of increasing amounts of CHOL in the bilayer. The progressive addition of CHOL also led to a decreasing height difference between HD and LD. FV imaging further demonstrated a lack of adhesion between the atomic force microscope tip and the HD or LD at loads below the breakthrough force. These results can lead to a better understanding of the role that CHOL plays in the mechanical properties of cellular membranes in modulating membrane rigidity, which has important implications for cellular mechanotransduction.

Cardiolipin, a key component to mimic the E. coli bacterial membrane in model systems revealed by dynamic light scattering and steady-state fluorescence anisotropy

The phase transition temperatures of several lipidic systems were determined using two different techniques: dynamic light scattering (DLS) and steady-state fluorescence anisotropy, using two fluorescent probes that report different membrane regions (TMA-DPH and DPH). Atomic force microscopy (AFM) was used as a complementary technique to characterize different lipid model systems under study. The systems were chosen due to the increased interest in bacterial membrane studies due to the problem of antibiotic drug resistance. The simpler models studied comprised of mixtures of POPE and POPG lipids, which form a commonly used model system for Escherichia coli membranes. Given the important role of cardiolipin (CL) in natural membranes, a ternary model system, POPE/POPG/CL, was then considered. The results obtained in these mimetic systems were compared with those obtained for the natural systems E. coli polar and total lipid extract. DLS and fluorescence anisotropy are not commonly used to study lipid phase transitions, but it was shown that they can give useful information about the thermotropic behaviors of model systems for bacterial membranes. These two techniques provided very similar results, validating their use as methods to measure phase transitions in lipid model systems. The temperature transitions obtained from these two very different techniques and the AFM results clearly show that cardiolipin is a fundamental component to mimic bacteria membranes. The results suggest that the less commonly used ternary system is a considerably better mimic for natural E. coli membranes than binary lipid mixture.

Use of liposomes to evaluate the role of membrane interactions on antioxidant activity

Cellular membranes, which contain abundant phospholipids, such as phosphatidylcholine, are major targets subjected to the damage caused by free radicals. Cellular damage due to lipid oxidation is strongly associated with ageing, carcinogenesis and other diseases. In addition, lipid oxidation is an important deteriorative reaction in the processing and storage of lipid-containing foods. Liposomes have been used extensively as biological models for in vitro lipid oxidation studies. The resemblance between the liposomal and membrane bilayer core makes liposomes a very useful tool to investigate the significance of the antioxidant-membrane interactions for antioxidant activity. The antioxidant activity of a compound is strongly influenced by numerous factors including the nature of the lipid substrate, the hydrophilic-lipophilic balance of the antioxidant, the physical and chemical environments of the lipids, and various other interfacial interactions. Thus, compounds that are effective antioxidants in one model system or food matrix may be unsuitable in other systems.This chapter describes fluorescent probes-based methods commonly used for testing antioxidant activity in liposomes and stresses the need to combine antioxidant assays and drug-membrane interaction studies to get a better description of the antioxidants' profile considering their location in lipid bilayer and their effect on membrane fluidity and consequently provide additional information to that obtained currently from assays performed in aqueous buffer media.

Fluorescent membrane probes' behavior in lipid bilayers: insights from molecular dynamics simulations

Fluorescence spectroscopy and microscopy have been used as tools to study membrane biophysics for decades now. Because phospholipids are non-fluorescent, the use of extrinsic membrane probes in this context is commonplace. Two major points of concern arise regarding this matter, namely the incomplete understanding of the probe behavior inside the bilayer and the perturbation of the latter resulting from probe incorporation. To this effect, molecular dynamics (MD) simulations, by providing detailed atomic-scale information, represent a valuable way to characterize the location and dynamics of bilayer-inserted membrane probes, as well as the magnitude of perturbation they induce on the host lipid structure, and several important classes of reporter molecules have been studied in recent years. This article reviews the state of the art of MD simulations of bilayer-inserted fluorescent probes, focusing on the information that has been obtained from previous studies and hinting at future perspectives in this rapidly emerging field.

Ceramide-1-phosphate, in contrast to ceramide, is not segregated into lateral lipid domains in phosphatidylcholine bilayers

Sphingolipids are key lipid regulators of cell viability: ceramide is one of the key molecules in inducing programmed cell death (apoptosis), whereas other sphingolipids, such as ceramide 1-phosphate, are mitogenic. The thermotropic and structural behavior of binary systems of N-hexadecanoyl-D-erythro-ceramide (C(16)-ceramide) or N-hexadecanoyl-D-erythro-ceramide-1-phosphate (C(16)-ceramide-1-phosphate; C(16)-C1P) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with DSC and deuterium nuclear magnetic resonance ((2)H-NMR). Partial-phase diagrams (up to a mole fraction of sphingolipids X = 0.40) for both mixtures were constructed based on DSC and (2)H-NMR observations. For C(16)-ceramide-containing bilayers DSC heating scans showed already at X(cer) = 0.025 a complex structure of the main-phase transition peak suggestive of lateral-phase separation. The transition width increased significantly upon increasing X(cer), and the upper-phase boundary temperature of the mixture shifted to approximately 65 degrees C at X(cer) = 0.40. The temperature range over which (2)H-NMR spectra of C(16)-ceramide/DPPC-d(62) mixtures displayed coexistence of gel and liquid crystalline domains increased from approximately 10 degrees for X(cer) = 0.1 to approximately 21 degrees for X(cer) = 0.4. For C16-C1P/DPPC mixtures, DSC and (2)H-NMR observations indicated that two-phase coexistence was limited to significantly narrower temperature ranges for corresponding C1P concentrations. To complement these findings, C(16)-ceramide/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and C16-C1P/POPC mixtures were also studied by (2)H-NMR and fluorescence techniques. These observations indicate that DPPC and POPC bilayers are significantly less perturbed by C(16)-C1P than by C(16)-ceramide and that C(16)-C1P is miscible within DPPC bilayers at least up to X(C1P) = 0.30.

Nanoscopic description of biomembrane electrostatics: results of molecular dynamics simulations and fluorescence probing

Electrostatic fields generated on and inside biological membranes are recognized to play a fundamental role in key processes of cell functioning. Their understanding requires an adequate description on the level of elementary charges and the reconstruction of electrostatic potentials by integration over all elementary interactions. Out of all the available research tools, only molecular dynamics simulations are capable of this, extending from the atomic to the mesoscopic level of description on the required time and space scale. A complementary approach is that offered by molecular probe methods, with the application of electrochromic dyes. Highly sensitive to intermolecular interactions, they generate integrated signals arising from electric fields produced by elementary charges at the sites of their location. This review is an attempt to provide a critical analysis of these two approaches and their present and potential applications. The results obtained by both methods are consistent in that they both show an extremely complex profile of the electric field in the membrane. The nanoscopic view, with two-dimensional averaging over the bilayer plane and formal separation of the electrostatic potential into surface (Psi(s)), dipole (Psi(d)) and transmembrane (Psi(t)) potentials, is constructive in the analysis of different functional properties of membranes.

Probing membrane order and topography in supported lipid bilayers by combined polarized total internal reflection fluorescence-atomic force microscopy

Determining the local structure, dynamics, and conformational requirements for protein-protein and protein-lipid interactions in membranes is critical to understanding biological processes ranging from signaling to the translocating and membranolytic action of antimicrobial peptides. We report here the application of a combined polarized total internal reflection fluorescence microscopy-in situ atomic force microscopy platform. This platform's ability to image membrane orientational order was demonstrated on DOPC/DSPC/cholesterol model membranes containing the fluorescent membrane probe, DiI-C(20) or BODIPY-PC. Spatially resolved order parameters and fluorophore tilt angles extracted from the polarized total internal reflection fluorescence microscopy images were in good agreement with the topographical details resolved by in situ atomic force microscopy, portending use of this technique for high-resolution characterization of membrane domain structures and peptide-membrane interactions.

Effect of nonsteroidal anti-inflammatory drugs on the cellular membrane fluidity

In this work, fluorescence measurements were performed using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) to evaluate the effects of the interaction of nonsteroidal anti-inflammatory drugs--NSAIDs (meloxicam, lornoxicam, and nimesulide) with several membrane systems (liposomes with and without cholesterol, mouse splenocytes, mouse macrophages cell line--J774, human leukemia monocyte cell line--THP-1, and human granulocytes and mononuclear cells). DPH fluorescence quenching studies revealed that the NSAIDs studied were able to efficiently quench the probe located in membrane hydrocarbon region. Fluorescence anisotropy measurements were also made to investigate the effects on membrane fluidity resulting from the interaction between the drugs and membrane systems. All the anti-inflammatory drugs studied show an increase in the membrane fluidity in a concentration dependent manner. Results obtained provide an insight into NSAIDs capacity to be inserted in lipid bilayers and alter the lipid dynamics. The induced changes in lipid dynamics may modulate the activity of inflammatory enzymes or may be related with deleterious topical action of NSAIDs on gastric phospholipid fluidity.

Simulations of lipid adsorption on TiO2 surfaces in solution

Molecular dynamics simulations are carried out to study the adsorption of three lipids, namely, DOPC, DOPS, and DMTAP, on TiO2(110) rutile surfaces and the influence of the interface on their conformational properties. Three types of rutile (110) surfaces, characterized by a different degree of hydroxylation (the neutral nonhydroxylated and hydroxylated surfaces and a partially hydroxylated surface with charge density corresponding to physiological pH) are investigated using force fields derived from ab initio calculations and experimental data. It is found that the stability of the adsorbate and the strength of the attachment are strictly connected with the nature of both the lipid and the surface. Direct coordination of the phosphate or carbonyl oxygens of the lipids with available titanium sites, observed in the case of partially or nonhydroxylated layers, determines stronger adsorption and, as a consequence, reduced dynamics. For a given hydration state of the surface, the adsorption strengths are in the order DOPS > DOPC >> DMTAP, in agreement with experimental data according to which the presence of DOPS units inside lipid bilayers favors stronger adsorption and lower mobility. The adsorption geometry, the hydration state of the lipid headgroups, and the dynamical processes (detachment, diffusion, etc.) occurring at the lipid/oxide interface are analyzed in detail, putting on a roughly quantitative basis time scales and energy barriers of the latter processes.

BODIPY-cholesterol: a new tool to visualize sterol trafficking in living cells and organisms

Analysis of sterol distribution and transport in living cells has been hampered by the lack of bright, photostable fluorescent sterol derivatives that closely resemble cholesterol. In this study, we employed atomistic simulations and experiments to characterize a cholesterol compound with fluorescent boron dipyrromethene difluoride linked to sterol carbon-24 (BODIPY-cholesterol). This probe packed in the membrane and behaved similarly to cholesterol both in normal and in cholesterol-storage disease cells and with trace amounts allowed the visualization of sterol movement in living systems. Upon injection into the yolk sac, BODIPY-cholesterol did not disturb zebrafish development and was targeted to sterol-enriched brain regions in live fish. We conclude that this new probe closely mimics the membrane partitioning and trafficking of cholesterol and, because of its excellent fluorescent properties, enables the direct monitoring of sterol movement by time-lapse imaging using trace amounts of the probe. This is, to our knowledge, the first cholesterol probe that fulfills these prerequisites.

Molecular dynamics simulations of DiI-C18(3) in a DPPC lipid bilayer

We performed a 40 ns simulation of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI-C18(3)) in a 1,2-dipalmitoyl-sn-glycero-3-phosphatidyl choline (DPPC) bilayer in order to facilitate interpretation of lipid dynamics and membrane structure from fluorescence lifetime, anisotropy, and fluorescence correlations spectroscopy (FCS). Incorporation of DiI of 1.6 to 3.2 mol% induced negligible changes in area per lipid but detectable increases in bilayer thickness, each of which are indicators of membrane structural perturbation. The DiI chromophore angle was 77 +/- 17 degrees with respect to the bilayer normal, consistent with rotational diffusion inferred from polarization studies. The DiI headgroup was located 0.63 nm below the lipid head group-water interface, a novel result in contrast to some popular cartoon representations of DiI but consistent with DiI's increase in quantum yield when incorporated into lipid bilayers. Importantly, the fast component of rotational anisotropy matched published experimental results demonstrating that sufficient free volume exists at the sub-interfacial region to support fast rotations. Simulations with non-charged DiI head groups exhibited DiI flip-flop, demonstrating that the positively-charged chromophore stabilizes the orientation and location of DiI in a single monolayer. DiI induced detectable changes in interfacial properties of water ordering, electrostatic potential, and changes in P-N vector orientation of DPPC lipids. The diffusion coefficient of DiI (9.7 +/- 0.02 x 10(-8) cm2 s(-1)) was similar to the diffusion of DPPC molecules (10.7 +/- 0.04 x 10(-8) cm2 s(-1)), supporting the conclusion that DiI dynamics reflect lipid dynamics. These results provide the first atomistic level insight into DiI dynamics, results essential in elucidating lipid dynamics through single molecule fluorescence studies.

Effects of fluorescent probe NBD-PC on the structure, dynamics and phase transition of DPPC. A molecular dynamics and differential scanning calorimetry study

We present a combined theoretical (molecular dynamics, MD) and experimental (differential scanning calorimetry, DSC) study of the effect of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) acyl chain-labeled fluorescent phospholipid analogs (C6-NBD-PC and C12-NBD-PC) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. DSC measurements reveal that <1 mol% of NBD-PC causes elimination of the pre-transition and a large loss of cooperativity of the main transition of DPPC. Labeling with C6-NBD-PC or C12-NBD-PC shifts the main transition temperature to lower or higher values, respectively. Following our recent report on the location and dynamics of these probes (BBA 1768 (2007) 467-478) in fluid phase DPPC, we present a detailed analysis of 100-ns MD simulations of systems containing either C6-NBD-PC or C12-NBD-PC, focused on their influence on several properties of the host bilayer. Whereas most monitored parameters are not severely affected for 1.6 mol% of probe, for the higher concentration studied (6.2 mol%) important differences are evident. In agreement with published reports, we observed that the average area per phospholipid molecule increases, whereas DPPC acyl chain order parameters decrease. Moreover, we predict that incorporation of NBD-PC should increase the electrostatic potential across the bilayer and, especially for C12-NBD-PC, slow lateral diffusion of DPPC molecules and rotational mobility of DPPC acyl chains.

Influence of pyrene-labeling on fluid lipid membranes

We elucidate the influence of pyrene-labeled phospholipids on the structural properties of a fluid dipalmitoylphosphatidylcholine lipid membrane. To this end, we employ extensive atomic-scale molecular dynamics simulations with varying concentrations of pyrene-linked lipids. We find pyrene labeling to perturb the membrane structure significantly in the vicinity of the probe, the correlation length in the bilayer plane being about 1.0-1.5 nm. The local perturbations lead to enhanced ordering and packing of lipid acyl chains located in the vicinity of the probe. Surprisingly, this holds true not only for lipids that reside in the same leaflet as the pyrene-labeled probe but also for lipids in the opposite monolayer. The latter is due to substantial interdigitation of the pyrene moiety into the opposite leaflet, suggesting that occasional excimer formation may take place for probes in different leaflets. As a related issue, we also discuss the location and conformational orientation of the pyrene moieties. In particular, the orientational distribution of pyrene turns out to be more broad and diverse than the distribution of the corresponding acyl tails of nonlabeled lipids.

Characterizing assembly morphology changes during solubilization process of dimyristoyl phosphocholine vesicles by n-dodecyl triethylammonium bromide

In the present work, the assembly morphology changes during the solubilization process of the sonicated unilamellar vesicles from dimyristoyl phosphocholine (DMPC) by a cationic surfactant, n-dodecyl triethylammonium bromide (DTEAB) were well characterized with DSC, FF-TEM and DLS and fluorescence probes technique. Based on an analysis on the above results, a primary multi-stage model was brought forward to sketch the assembly morphology changes during the DMPC vesicle solubilization by DTEAB. In comparison with classical models, vesicles division, tubule-like structure formation and fission to vesicle were found in the middle stages of this model. Additionally, it is the first time that the transversally-cut profiles of tubule-like structures were observed during vesicle solubilization process.

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.

Polyunsaturation in lipid membranes: dynamic properties and lateral pressure profiles

We elucidate the influence of unsaturation on single-component membrane properties, focusing on their dynamical aspects and lateral pressure profiles across the membrane. To this end, we employ atomistic molecular dynamics simulations to study five different membrane systems with varying degrees of unsaturation, starting from saturated membranes and systematically increasing the level of unsaturation, ending up with a bilayer of phospholipids containing the docosahexaenoic acid. For an increasing level of unsaturation, we find considerable effects on dynamical properties, such as accelerated dynamics of the phosphocholine head groups and glycerol backbones and speeded up rotational dynamics of the lipid molecules. The lateral pressure profile is found to be altered by the degree of unsaturation. For an increasing number of double bonds, the peak in the middle of the bilayer decreases. This is compensated for by changes in the membrane-water interface region in terms of increasing peak heights of the lateral pressure profile. Implications of the findings are briefly discussed.

Location and dynamics of acyl chain NBD-labeled phosphatidylcholine (NBD-PC) in DPPC bilayers. A molecular dynamics and time-resolved fluorescence anisotropy study

100-ns molecular dynamics simulations of fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, both pure and containing 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) acyl-chain labeled fluorescent analogs (C6-NBD-PC and C12-NBD-PC), are described. These molecules are widely used as probes for lipid structure and dynamics. The results obtained here for pure DPPC agree with both experimental and theoretical published works. We verified that the NBD fluorophore of both derivatives loops to a transverse location closer to the interface than to the center of the bilayer. Whereas this was observed previously in experimental literature works, conflicting transverse locations were proposed for the NBD group. According to our results, the maximum of the transverse distribution of NBD is located around the glycerol backbone/carbonyl region, and the nitro group is the most external part of the fluorophore. Hydrogen bonds from the NH group of NBD (mostly to glycerol backbone lipid O atoms) and to the nitro O atoms of NBD (from water OH groups) are continuously observed. Rotation of NBD occurs with approximately 2.5-5 ns average correlation time for these probes, but very fast, unresolved reorientation motions occur in <20 ps, in agreement with time-resolved fluorescence anisotropy measurements. Finally, within the uncertainty of the analysis, both probes show lateral diffusion dynamics identical to DPPC.

Effect of Gd3+ on the colloidal stability of liposomes

Lanthanide ions such as La3+ and Gd3+ are well known to have large effects on the structure of phospholipid membranes. Unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) were prepared by sonication method and confirmed by transmission electron microscopy. The effects of concentration of gadolinium ions Gd3+ on DPPC unilamellar vesicles in aqueous media were studied by different techniques. As physical techniques, photon correlation spectroscopy, electrophoretic mobility, and differential scanning calorimetry were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersities values were observed as Gd3+ concentration increases, suggesting that this cation induces the aggregation of vesicles. Electrophoretic mobility measurements on unilamellar vesicles as a function of Gd3+ ion concentration show that the vesicles adsorb Gd3+ ions. Above Gd3+ concentrations of 0.1 mol dm-3, the zeta potential and light scattering measurements indicate the beginning of aggregation process. For comparison with similar phospholipids, the zeta potential of phosphatidylcholine interacting with Gd3+ was measured, showing an analogous behavior. Differential scanning calorimetry has been used to determine the effect of Gd3+ on the transition temperature (Tc) and on the enthalpy (DeltaHc) associated with the process.

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.

Endophilin BAR domain drives membrane curvature by two newly identified structure-based mechanisms

The crescent-shaped BAR (Bin/Amphiphysin/Rvs-homology) domain dimer is a versatile protein module that senses and generates positive membrane curvature. The BAR domain dimer of human endophilin-A1, solved at 3.1 A, has a unique structure consisting of a pair of helix-loop appendages sprouting out from the crescent. The appendage's short helices form a hydrophobic ridge, which runs across the concave surface at its center. Examining liposome binding and tubulation in vitro using purified BAR domain and its mutants indicated that the ridge penetrates into the membrane bilayer and enhances liposome tubulation. BAR domain-expressing cells exhibited marked plasma membrane tubulation in vivo. Furthermore, a swinging-arm mutant lost liposome tubulation activity yet retaining liposome binding. These data suggested that the rigid crescent dimer shape is crucial for the tubulation. We here propose that the BAR domain drives membrane curvature by coordinate action of the crescent's scaffold mechanism and the ridge's membrane insertion in addition to membrane binding via amino-terminal amphipathic helix.

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.

Membrane perturbation effects of peptides derived from the N-termini of unprocessed prion proteins

Peptides derived from the unprocessed N-termini of mouse and bovine prion proteins (mPrPp and bPrPp, respectively), comprising hydrophobic signal sequences followed by charged domains (KKRPKP), function as cell-penetrating peptides (CPPs) with live cells, concomitantly causing toxicity. Using steady-state fluorescence techniques, including calcein leakage and polarization of a membrane probe (diphenylhexatriene, DPH), as well as circular dichroism, we studied the membrane interactions of the peptides with large unilamellar phospholipid vesicles (LUVs), generally with a 30% negative surface charged density, comparing the effects with those of the CPP penetratin (pAntp) and the pore-forming peptide melittin. The prion peptides caused significant calcein leakage from LUVs concomitant with increased membrane ordering. Fluorescence correlation spectroscopy (FCS) studies of either rhodamine-entrapping (REVs) or rhodamine-labeled (RLVs) vesicles, showed that addition of the prion peptides resulted in significant release of rhodamine from the REVs without affecting the overall integrity of the RLVs. The membrane leakage effects due to the peptides had the following order of potency: melittin>mPrPp>bPrPp>pAntp. The membrane perturbation effects of the N-terminal prion peptides suggest that they form transient pores (similar to melittin) causing toxicity in parallel with their cellular trafficking.