Lateral diffusion coefficients of phospholipids in spherical bilayers on a solid support measured by 2H-nuclear-magnetic-resonance relaxation

Fat SIRAH: Coarse-Grained Phospholipids To Explore Membrane-Protein Dynamics

The capability to handle highly heterogeneous molecular assemblies in a consistent manner is among the greatest challenges faced when deriving simulation parameters. This is particularly the case for coarse-grained (CG) simulations in which chemical functional groups are lumped into effective interaction centers for which transferability between different chemical environments is not guaranteed. Here, we introduce the parametrization of a set of CG phospholipids compatible with the latest version of the SIRAH force field for proteins. The newly introduced lipid species include different acylic chain lengths and partial unsaturation, as well as polar and acidic head groups that show a very good reproduction of structural membrane determinants, such as areas per lipid, thickness, order parameter, etc., and their dependence with temperature. Simulation of membrane proteins showed unprecedented accuracy in the unbiased description of the thickness-dependent membrane-protein orientation in systems where this information is experimentally available (namely, the SarcoEndoplasmic Reticulum Calcium-SERCA-pump and its regulator Phospholamban). The interactions that lead to this faithful reproduction can be traced down to the single amino acid-lipid interaction level and show full agreement with biochemical data present in the literature. Finally, the present parametrization is implemented in the GROMACS and AMBER simulation packages facilitating its use by a wide portion of the biocomputing community.

A fluid membrane enhances the velocity of cargo transport by small teams of kinesin-1

Kinesin-1 (hereafter referred to as kinesin) is a major microtubule-based motor protein for plus-end-directed intracellular transport in live cells. While the single-molecule functions of kinesin are well characterized, the physiologically relevant transport of membranous cargos by small teams of kinesins remains poorly understood. A key experimental challenge remains in the quantitative control of the number of motors driving transport. Here we utilized "motile fraction" to overcome this challenge and experimentally accessed transport by a single kinesin through the physiologically relevant transport by a small team of kinesins. We used a fluid lipid bilayer to model the cellular membrane in vitro and employed optical trapping to quantify the transport of membrane-enclosed cargos versus traditional membrane-free cargos under identical conditions. We found that coupling motors via a fluid membrane significantly enhances the velocity of cargo transport by small teams of kinesins. Importantly, enclosing a cargo in a fluid lipid membrane did not impact single-kinesin transport, indicating that membrane-dependent velocity enhancement for team-based transport arises from altered interactions between kinesins. Our study demonstrates that membrane-based coupling between motors is a key determinant of kinesin-based transport. Enhanced velocity may be critical for fast delivery of cargos in live cells.

Effect of ergosterol on the fungal membrane properties. All-atom and coarse-grained molecular dynamics study

Cell membranes are complex multicomponent systems consisting of thousands of different lipids with numerous embedded membrane proteins and many types of sterols. We used all-atom and coarse-grained molecular dynamics simulations to study the structural and dynamical properties of phospholipid bilayers containing four types of phospholipids and different amount of ergosterol, main sterol component in the fungal membranes. To characterize the influence of ergosterol on the membrane properties we analyzed the surface area per lipid, bilayer thickness, area compressibility modulus, mass density profiles, deuterium order parameters, and lateral diffusion coefficients. The presence of ergosterol induces the ordering of lipids leading to their denser packing, to reducing the lateral diffusion of lipids and lipid surface area, to increasing the thickness of bilayer and compressibility modulus. In addition, we evaluated each calculated property between the two simulation methods.

Probing Membrane Viscosity and Interleaflet Friction of Supported Lipid Bilayers by Tracking Electrostatically Adsorbed, Nano-Sized Vesicles

Particle tracking is used to measure the diffusional motion of nanosized (≈100 nm), lipid vesicles that are electrostatically adsorbed onto a solid supported lipid bilayer. It is found that the motion of membrane-adhering vesicles is Brownian and depends inversely on the vesicle size, but is insensitive to the vesicle surface charge. The measured diffusivity agrees well with the Evans-Sackmann model for the diffusion of inclusions in supported, fluidic membranes. The agreement implies that the vesicle motion is coupled to that of a nanoscopic lipid cluster in the upper leaflet, which slides over the lower leaflet. The diffusivity of membrane-adhering vesicles is therefore predominantly governed by the interleaflet friction coefficient, while the diffusivity of single lipids is mainly governed by the membrane viscosity. Combined with fluorescence recovery after photobleaching analysis, the interleaflet friction coefficient and the membrane viscosity are determined by applying the Evans-Sackmann model to the measured diffusivity of membrane adhering vesicles and that of supported membrane lipids. This approach provides an alternative to existing methods for measuring the interleaflet friction coefficient and the membrane viscosity.

A Photoswitchable Fluorophore for the Real-Time Monitoring of Dynamic Events in Living Organisms

This study reports the synthesis of a photoactivatable fluorophore with optimal photochemical and photophysical properties for the real-time tracking of motion in vivo. The photoactivation mechanism designed into this particular compound permits the conversion of an emissive reactant into an emissive product with resolved fluorescence, under mild illumination conditions that are impossible to replicate with conventional switching schemes based on bleaching. Indeed, the supramolecular delivery of these photoswitchable probes into the cellular blastoderm of Drosophila melanogaster embryos allows the real-time visualization of translocating molecules with no detrimental effects on the developing organisms. Thus, this innovative mechanism for fluorescence photoactivation can evolve into a general chemical tool to monitor dynamic processes in living biological specimens.

Self-assembly formation of lipid bilayer coatings on bare aluminum oxide: overcoming the force of interfacial water

Widely used in catalysis and biosensing applications, aluminum oxide has become popular for surface functionalization with biological macromolecules, including lipid bilayer coatings. However, it is difficult to form supported lipid bilayers on aluminum oxide, and current methods require covalent surface modification, which masks the interfacial properties of aluminum oxide, and/or complex fabrication techniques with specific conditions. Herein, we addressed this issue by identifying simple and robust strategies to form fluidic lipid bilayers on aluminum oxide. The fabrication of a single lipid bilayer coating was achieved by two methods, vesicle fusion under acidic conditions and solvent-assisted lipid bilayer (SALB) formation under near-physiological pH conditions. Importantly, quartz crystal microbalance with dissipation (QCM-D) monitoring measurements determined that the hydration layer of a supported lipid bilayer on aluminum oxide is appreciably thicker than that of a bilayer on silicon oxide. Fluorescence recovery after photobleaching (FRAP) analysis indicated that the diffusion coefficient of lateral lipid mobility was up to 3-fold greater on silicon oxide than on aluminum oxide. In spite of this hydrodynamic coupling, the diffusion coefficient on aluminum oxide, but not silicon oxide, was sensitive to the ionic strength condition. Extended-DLVO model calculations estimated the thermodynamics of lipid-substrate interactions on aluminum oxide and silicon oxide, and predict that the range of the repulsive hydration force is greater on aluminum oxide, which in turn leads to an increased equilibrium separation distance. Hence, while a strong hydration force likely contributes to the difficulty of bilayer fabrication on aluminum oxide, it also confers advantages by stabilizing lipid bilayers with thicker hydration layers due to confined interfacial water. Such knowledge provides the basis for improved surface functionalization strategies on aluminum oxide, underscoring the practical importance of surface hydration.

Behaviour of NBD-head group labelled phosphatidylethanolamines in POPC bilayers: a molecular dynamics study

An NBD-diC₁₆PE/POPC bilayer with typical fluorophore inverted-snorkelling configurations, and mass density profiles across the membrane. The wide distribution of the NBD fluorophore lies at the origin of its complex emission kinetics.

Solvent-assisted lipid bilayer formation on silicon dioxide and gold

Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

High-density arrays of submicron spherical supported lipid bilayers

Lipid bilayer membranes found in nature are heterogeneous mixtures of lipids and proteins. Model systems, such as supported lipid bilayers (SLBs), are often employed to simplify experimental systems while mimicking the properties of natural lipid bilayers. Here, we demonstrate a new method to form SLB arrays by first forming spherical supported lipid bilayers (SSLBs) on submicrometer-diameter SiO(2) beads. The SSLBs are then arrayed into microwells using a simple physical assembly method that requires no chemical modification of the substrate nor modification of the lipid membrane with recognition moieties. The resulting arrays have submicrometer SSLBs with 3 μm periodicity where >75% of the microwells are occupied by an individual SSLB. Because the arrays have high density, fluorescence from >1000 discrete SSLBs can be acquired with a single image capture. We show that 2-component random arrays can be formed, and we also use the arrays to determine the equilibrium dissociation constant for cholera toxin binding to ganglioside GM1. SSLB arrays are robust and are stable for at least one week in buffer.

Molecular dynamics simulation of HIV fusion inhibitor T-1249: insights on peptide-lipid interaction

T-1249 is a peptide that inhibits the fusion of HIV envelope with the target cell membrane. Recent results indicate that T-1249, as in the case of related inhibitor peptide T-20 (enfuvirtide), interacts with membranes, more extensively in the bilayer liquid disordered phase than in the liquid ordered state, which could be linked to its effectiveness. Extensive molecular dynamics simulations (100 ns) were carried out to investigate the interaction between T-1249 and bilayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and POPC/cholesterol (1 : 1). It was observed that T-1249 interacts to different extents with both membrane systems and that peptide interaction with the bilayer surface has a local effect on membrane structure. Formation of hydrogen bonding between certain peptide residues and several acceptor and donor groups in the bilayer molecules was observed. T-1249 showed higher extent of interaction with bilayers when compared to T-20. This is most notable in POPC/Chol membranes, owing to more peptide residues acting as H bond donors and acceptors between the peptide and the bilayer lipids, including H-bonds formed with cholesterol. This behavior is at variance with that of T-20, which forms no H bonds with cholesterol. This higher ability to interact with membranes is probably correlated with its higher inhibitory efficiency.

NMR determination of protein partitioning into membrane domains with different curvatures and application to the influenza M2 peptide

The M2 protein of the influenza A virus acts both as a drug-sensitive proton channel and mediates virus budding through membrane scission. The segment responsible for causing membrane curvature is an amphipathic helix in the cytoplasmic domain of the protein. Here, we use (31)P and (13)C solid-state NMR to examine M2-induced membrane curvature. M2(22-46), which includes only the transmembrane (TM) helix, and M2(21-61), which contains an additional amphipathic helix, are studied. (31)P chemical shift lineshapes indicate that M2(21-61) causes a high-curvature isotropic phase to both cholesterol-rich virus-mimetic membranes and 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayers, whereas M2(22-46) has minimal effect. The lamellar and isotropic domains have distinct (31)P isotropic chemical shifts, indicating perturbation of the lipid headgroup conformation by the amphipathic helix. (31)P- and (13)C-detected (1)H T(2) relaxation and two-dimensional peptide-lipid correlation spectra show that M2(21-61) preferentially binds to the high-curvature domain. (31)P linewidths indicate that the isotropic vesicles induced by M2(21-61) are 10-35 nm in diameter, and the virus-mimetic vesicles are smaller than the 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles. A strong correlation is found between high membrane curvature and weak drug-binding ability of the TM helix. Thus, the M2 amphipathic helix causes membrane curvature, which in turn perturbs the TM helix conformation, abolishing drug binding. These NMR experiments are applicable to other curvature-inducing membrane proteins such as fusion proteins and antimicrobial peptides.

Diffusion NMR and bicelle morphology

This minireview focuses on diffusion NMR studies in bicelles. Following a discourse on diffusion fundamentals, and a comparative overview of fluorescence and NMR-based techniques for measuring diffusion, the pulsed field gradient (PFG) NMR diffusion method is introduced, emphasizing its specific advantages and limitations when applied to diffusion measurements in macroscopically oriented lamellar systems such as magnetically aligned bicelles. The utility of PFG NMR diffusion measurements in bicellar model membrane systems for examining lateral diffusion of membrane-bound molecular species is demonstrated, along with certain features of lateral diffusion that such studies illuminate. Further, those aspects of bicelle morphology that have been resolved using PFG NMR diffusion studies of various molecular weight soluble polymeric species are reviewed. The discussion concludes with an outline of future prospects for diffusion NMR studies in bicelles.

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.

Phase-dependent lateral diffusion of α-tocopherol in DPPC liposomes monitored by fluorescence quenching

The temperature-dependent fluorescence quenching of an amphiphilic palmitoyl derivative of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-L) by α-tocopherol (α-Toc) has been determined in liposomes composed of a saturated lipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The mutual lateral diffusion coefficients (D(L)) were extracted according to a laterally diffusion-controlled dynamic quenching model. Three distinct temperature regimes were identified: one between 65 and 39 °C, where the lateral diffusion coefficients were in the range of 10(-7) cm(2) s(-1) and the lifetime of the probe was monoexponential in the absence of α-Toc, a second one between 39 and 30 °C, where the lateral diffusion coefficients were in the range of 10(-8) cm(2) s(-1) and the lifetime of the probe was biexponential in the absence of α-Toc, and a third one below 30 °C, in which no diffusion was detectable, suggesting D(L) < 10(-9) cm(2)s (-1). These temperature domains were assigned, supported by differential scanning calorimetry (DSC) measurements, to the liquid-crystalline, ripple, and solid-gel phases of DPPC liposomes in the presence of the two additives. The absolute values of the individual lateral diffusion coefficients (taken as (1)/(2) of the D(L) values) of the Fluorazophore-L/α-Toc (ca. 2.5 × 10(-7) cm(2) s(-1) at 52 °C) couple demonstrates that α-Toc does not diffuse at an unexpectedly high rate in comparison to the self-diffusion of DPPC (1.5 × 10(-7) cm(2) s(-1) at 52 °C). However, diffusion in DPPC liposomes is distinctly slower than that in POPC ones (e.g., D(L) = 4.9 × 10(-7) cm(2) s(-1) versus 6.4 × 10(-7) cm(2) s(-1) at 50 °C), with an activation energy of 49 ± 5 kJ mol(-1) (value for POPC: 47 ± 5 kJ mol(-1)), in the temperature range of the liquid-crystalline phase. Diffusion in the ripple phase, that is, below the main phase transition temperature, was found to be non-negligible, with an apparent activation energy of 175 ± 50 kJ mol(-1).

Formation, stability, and mobility of one-dimensional lipid bilayers on polysilicon nanowires

Curved lipid membranes are ubiquitous in living systems and play an important role in many biological processes. To understand how curvature and lipid composition affect membrane formation and fluidity, we have assembled and studied mixed 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) supported lipid bilayers on amorphous silicon nanowires grown around carbon nanotube cores with controlled wire diameters ranging from 20 to 200 nm. We found that lipid vesicles fused onto nanowire substrates and formed continuous bilayers for all DOPC-DOPE mixtures tested (with the DOPE content of up to 30%). Our measurements demonstrate that nanowire-supported bilayers are mobile, exhibit fast recovery after photobleaching, and have a low concentration of defects. Lipid diffusion coefficients in these high-curvature tubular membranes are comparable to the values reported for flat supported bilayers and increase slightly with decreasing nanowire diameter. A free space diffusion model adequately describes the effect of bilayer curvature on the lipid mobility for nanowire substrates with diameters greater than 50 nm, but shows significant deviations from the experimental values for smaller diameter nanowires.

Templated assembly of biomembranes on silica microspheres using bacteriorhodopsin conjugates as structural anchors

Membrane proteins are some of the most sophisticated molecules found in nature. These molecules have extraordinary recognition properties; hence, they represent a vast source of specialized materials with potential uses in sensing and screening applications. However, the strict requirement of the native lipid environment to preserve their structure and functionality presents an impediment in building biofunctional materials from these molecules. In general, the purification protocols remove the native lipid support structures found in the cellular environment that stabilize the membrane proteins. Furthermore, the membrane protein structure is often highly complex, typified by large, multisubunit complexes that not only span the lipid bilayer but also contain large (>2 nm) cytoplasmic and extracellular domains that protrude from the membrane. The present study is focused on using a biomimetic approach to build a stable, fluid microenvironment to be used to incorporate larger membrane proteins of interest into a tether-supported lipid bilayer membrane adequately spaced above a substrate passivated to liposome fusion and nonspecific adsorption. Our aim is to reintroduce the supporting structures of the native cell membrane using self-assembled supramolecular complexes constructed on microspheres in an artificial cytoskeleton motif. Central to our architecture is to utilize bacteriorhodopsin (bR), a transmembrane protein, as a biomembrane anchoring molecule to be tethered to surfaces of interest as a sparse structural element in the design. Compared to a typical lipid tether, which inserts into one leaflet of the lipid bilayer, bR anchoring provides an over 8-fold greater hydrophobic surface area in contact with the bilayer. In the work presented here, the silica microsphere surface was biofunctionalized with streptavidin to make it a suitable supporting interface. This was achieved by self-assembly of (p-aminophenyl)trimethoxysilane on the silica surface followed by subsequent conjugation of biotin-PEG3400 (PEG = poly(ethylene glycol) and PEG2000 for further passivation and the binding of streptavidin. We have conjugated bR with biotin-PEG3400 through amine-based coupling to use it as a tether. The biotin-PEG-bR conjugate was further labeled with Texas Red to facilitate localization via fluorescence imaging. Confocal microscopy was utilized to analyze the microsphere surface at different stages of surface modification by employing fluorescent staining techniques. Sparely tethered supported lipid bilayer membranes were constructed successfully on streptavidin-functionalized silica particles (5 mum) using a detergent-based method in which tethered bR nucleates self-assembly of the bilayer membrane. The fluidity of the supported membranes was analyzed using fluorescence recovery after photobleaching in confocal imaging detection mode. The phospholipid diffusion coefficients obtained from these studies indicated that nativelike fluidity was achieved in the tether-supported membranes, thus providing a prospective microenvironment for insertion of membrane proteins of interest.

An overview of lipid membrane supported by colloidal particles

In recent years, original hybrid assemblies composed of a particle core surrounded by a lipid shell emerged as promising entities for various biotechnological applications. Their broadened bio-potentialities, ranging from model membrane systems or biomolecule screening supports, to substance delivery reservoirs or therapeutic vectors, are furthered by their versatility of composition due to the possible wide variation in the particle nature and size, as well as in the lipid formulation. The synthesis, the characteristics, and the uses of these Lipid/Particle assemblies encountered in the literature so far are reviewed, and classified according to the spherical core size in order to highlight general trends. Moreover, several criteria are particularly discussed: i) the interactions involved between the particles and the lipids, and implicitly the assembly elaboration mechanism, ii) the most suited techniques for an accurate characterization of the entities from structural and physicochemical points of view, and iii) the remarkable properties of the solid-supported lipid membrane obtained.

Structure and mobility of lipid membranes on a thermoplastic substrate

Supported lipid membranes constitute one of the most important model systems for cell membranes. The properties of lipid membranes supported by the hydrophobic solid polymer cyclic olefin copolymer (COC) were investigated. Lipid layers consisting of varying amounts of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP, cationic) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, neutral) prepared by vesicle fusion and solvent exchange were compared. All lipid mixtures coated the COC surface homogeneously forming a fluid membrane as verified by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). The exact structure of the supported membranes was determined by synchrotron reflectivity experiments using a microfluidic chamber. The X-ray data are in agreement with a compressed (head-to-head distance = 29 angstroms) and less densely packed bilayer.

Membrane Curvature Change Induced by an Antimicrobial Peptide Detected by 31P Exchange NMR

The influence of an antimicrobial peptide, protegrin-1 (PG-1), on the curvature and lateral diffusion coefficient (D(L)) of phosphocholine bilayers is investigated using one- (1D) and two-dimensional (2D) (31)P exchange NMR. The experiments utilize the fact that lipid lateral diffusion over the curved surface of vesicles changes the molecular orientation and thus the (31)P chemical shift anisotropy. This reorientation is manifested in 2D spectra as off-diagonal intensities and in 1D stimulated-echo experiments as reduced echo heights. The 2D spectra give information on the reorientation-angle distribution while the decay of the stimulated-echo intensity, which closely tracks the second-order correlation function in our experiments, yields the correlation times of the reorientation. The relationships among the 2D exchange spectra, stimulated-echo intensities, the correlation function, and reorientation-angle distributions are analyzed in detail. In the absence of PG-1, both dilaurylphosphotidylcholine (DLPC) and palmitoyloleoylphosphatidylcholine (POPC) vesicles show biexponential decays of the stimulated-echo intensities to equilibrium values of 0.20-0.25, suggesting that the curvature of the lipid vesicles has a bimodal distribution. The addition of PG-1 to DLPC vesicles increased the decay time constants, indicating that D(L) decreases due to peptide binding. In contrast, the addition of PG-1 to POPC vesicles decreased the decay constants by three to fivefold, indicating that the POPC vesicles are fragmented into smaller vesicles. On the basis of the changes in D(L) and the decay constants, we estimate that the radius of the POPC vesicles decreases by threefold due to PG-1 binding. Simulations of the 2D exchange spectra yielded quantitative reorientation-angle distributions that are consistent with the bimodal distributions of the vesicle curvature and the effects of the peptide on the two types of lipid bilayers. Thus, (31)P exchange NMR provides useful insights into the membrane morphological changes induced by this antimicrobial peptide.

New insights into self-organization of a model lipid mixture and quantification of its adsorption on spherical polymer particles

The adsorption of lipids onto spherical polymer colloids led to original assemblies presenting structural characteristics adjustable with the lipid formulation. The model system selected for this work involved sulfate-charged poly(styrene) submicrometer particles and zwitterionic/cationic lipid mixtures composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP). According to the theoretical packing parameter calculations and whatever the DPPC/DPTAP ratio, the two lipids self-assembled in aqueous media to spontaneously form vesicles. A phase transition investigation of these DPPC/DPTAP vesicles using differential scanning calorimetry revealed particular thermotropic behaviors, especially for the equimolar formulation where very strong interactions occurred between DPPC and DPTAP. Furthermore, the coating of the lipids around particles was monitored versus DPPC/DPTAP ratio by means of numerous appropriate techniques. First, a thermogravimetric analysis, providing decomposition profiles of lipid/polymer particle assemblies with temperature, was atypically carried out for such nanostructures. Then, 1H NMR spectroscopy enabled the exact DPPC/DPTAP molar ratios adsorbed on particles to be determined by differentiating both lipids. Subsequently, it also pointed out the major role of electrostatic interactions as driving forces in the assembly elaboration process. In addition to these findings, quantitative information has been collected and correlated with chemical lipid assays and permitted the statement of a lipid bilayer coverage for the assemblies prepared in water, in agreement with quasi-elastic light scattering data.

Effect of Temperature, Cholesterol Content, and Antioxidant Structure on the Mobility of Vitamin E Constituents in Biomembrane Models Studied by Laterally Diffusion-Controlled Fluorescence Quenching

Kinetic parameters relevant for the antioxidant activity of the vitamin E constituents (alpha, beta, gamma, and delta homologues of tocopherols and tocotrienols) and of an amphiphilic vitamin C derivative, l-ascorbyl 6-palmitate, were determined. Fluorescence quenching experiments of 2,3-diazabicyclo[2.2.2]oct-2-ene in homogeneous acetonitrile-water mixtures afforded reactivity trends in terms of intermolecular quenching rate constants, while the quenching of Fluorazophore-L in liposomes provided the lateral diffusion coefficients relevant for understanding their biological activity in membranes. The reactivity in homogeneous solution was not influenced by the nature of the isoprenoid tail (tocopherol versus tocotrienol), but was dependent on the methylation pattern. The resulting order (alpha > beta = gamma > delta) was found to be in line with their reactivities toward peroxyl radicals as well as the phenolic O-H bond dissociation energies. The mutual lateral diffusion coefficient in POPC liposomes was the same, within error, for different tocopherols and tocotrienols (D(L) = (1.6 +/- 0.2) x 10(-7) cm(2) s(-1)). l-Ascorbyl 6-palmitate exhibited a reactivity similar to that of delta-tocopherol in homogeneous solution, but displayed a 1 order of magnitude lower fluorescence quenching efficiency in liposomes than the vitamin E constituents. Temperature effects on the laterally diffusion-controlled fluorescence quenching were large, with activation energies of 44 +/- 6 kJ mol(-1). The addition of cholesterol (0-30%) to POPC liposomes resulted only in slightly reduced diffusion coefficients. The combined results demonstrate that Fluorazophore-L can provide important physicochemical parameters for the understanding of antioxidant activity in biological environments.

Physicochemical and interfacial investigation of lipid/polymer particle assemblies

A model study was investigated to develop colloidal supramolecular assemblies consisting of particles coated with lipid layers. The interactions between monodisperse sulfate-charged poly(styrene) submicrometer particles and zwitterionic/cationic lipid vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-3-trimethylammonium propane were considered. The influence of relevant experimental parameters on the final associations was examined by quasi-elastic light scattering to point out some new phenomena occurring in these colloidal systems. The major role of electrostatic interactions as driving forces to control the organization between cationic lipids and oppositely charged poly(styrene) particles was clearly evident, whereas this influence was less pronounced when considering the zwitterionic lipids. The characterization of these original complex assemblies was completed by a thorough study of the surface modification. The combination of zeta potential measurements, X-ray photoelectron spectroscopy analyses, and microscopy observations proved that the envisioned model can really correspond to polymer particles surrounded by lipids.

Lipid bilayer tethered inside a nanoporous support: a solid-state nuclear magnetic resonance investigation

(31)P and (1)H solid-state nuclear magnetic resonance (NMR) experiments have been designed with the aim of studying directly the formation of supported bilayers tethered inside nanoporous aluminum oxide supports as a model of biomimetic membranes. The static and magic angle spinning (31)P NMR spectra of the supported bilayers have been compared with the experimental and simulated spectra of a simpler model with cylindrical geometry, namely a phospholipid bilayer adsorbed on an oriented polymer sheet. The broadening observed for the nanoporous model is most likely due to the presence of paramagnetic ions in the aluminum oxide. A phospholipid lateral diffusion coefficient of (2.8 +/- 0.4) x 10(-8) cm(2)/s has been measured for the tethered bilayer on a spherical support, indicating a good fluidity as compared with adsorbed membrane models.

Evidence of a tendency to self-association of the transmembrane domain of ErbB-2 in fluid phospholipid bilayers

The transmembrane domains of receptor tyrosine kinases are single-span helical structures suggested to participate directly in the formation of side-to-side receptor homodimers/homooligomers that modulate signal transduction. Transmembrane peptides from the class I receptor tyrosine kinase, ErbB-2, were examined directly by 2H NMR spectroscopy as a means of following such phenomena under the dynamic conditions that characterize fluid, fully hydrated bilayers of natural phospholipids. Appropriate peptides were expressed as 50-mers, containing the transmembrane domain of ErbB-2 plus contiguous stretches of amino acids from the cytoplasmic and extracellular domains. Deuterium probes were incorporated in place of 1H at a site within the helical intramembranous portion (the -CH3 side chain of Ala657), and the peptides were assembled into bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) for study. An analogous peptide corresponding to the oncogenic variant characterized by a Val659-->Glu point mutation was also examined. At high peptide concentration, prominent spectral features could be assigned to rapidly rotating transmembrane monomers and to large oligomers rotating very slowly relative to a time scale of 10(-5) s. As peptide concentration was lowered, the latter feature was greatly reduced, and an additional population of mobile species became identifiable, consistent with the presence of homodimers and/or small oligomers. The defined nature of these latter spectral features suggests that preferred interaction sites exist on the peptides. The appearance of similar phenomena in the case of transmembrane peptides from both wild-type ErbB-2 and the transforming mutant argues for the involvement of additional factors in signal modulation, such as limitations normally imposed by the missing extramembranous portions.

Polyelectrolyte molecular weight and electrostatically-induced domains in lipid bilayer membranes

Polyelectrolyte-induced domain formation in charged lipid bilayer membranes was investigated as a function of polyelectrolyte molecular weight using 2H nuclear magnetic resonance (NMR) spectroscopy. Lipid bilayers consisting of mixtures of alpha- or beta-choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-alpha-d2 or POPC-beta-d2) plus the cationic amphiphile 1,2-dioleoyl-3-(dimethylamino)propane (DODAP) were exposed to the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSSS) of various molecular weights. Regardless of molecular weight, PSSS produced dual component 2H NMR spectra, indicating two distinct POPC populations, corresponding to PSSS-bound and PSSS-free lipid, in slow exchange with one another. Analysis of the 2H NMR subspectra quadrupolar splittings and intensities showed the PSSS-bound domain to be enriched in DODAP, with the PSSS-free domain correspondingly depleted. At polyelectrolyte loadings below global charge equivalence, PSSS bound DODAP stoichiometrically for all PSSS molecular weights, indicating that the polyelectrolyte chain lies flat upon the membrane surface. At higher PSSS loadings the domains dissipated, leading to single component 2H NMR spectra. At high NaCl concentrations PSSS dissociated from the bilayer surface. Domain size on a per PSSS chain basis increased while the degree of enrichment with DODAP decreased progressively as the PSSS chain length decreased. Such molecular weight-dependent domain characteristics have not been predicted theoretically and need to be taken into account in future refinements of domain models.

Polylysine-induced 2H NMR-observable domains in phosphatidylserine/phosphatidylcholine lipid bilayers

The interaction of three polylysines, Lys(5) (N = 5), Lys(30) (N = 30), and Lys(100) (N = 100), where N is the number of lysine residues per chain, with phosphatidylserine-containing lipid bilayer membranes was investigated using 2H NMR spectroscopy. Lys(30) and Lys(100) added to multilamellar vesicles composed of (70:30) (mol:mol) mixtures of choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) + 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) produced two resolvable 2H NMR spectral components under conditions of low ionic strength and for cases where the global anionic lipid charge was in excess over the global cationic polypeptide charge. The intensities and quadrupolar splittings of the two spectral components were consistent with the existence of polylysine-bound domains enriched in POPS, in coexistence with polylysine-free domains depleted in POPS. Lys(5), however, yielded no 2H NMR resolvable domains. Increasing ionic strength caused domains to become diffuse and eventually dissipate entirely. At physiological salt concentrations, only Lys(100) yielded 2H NMR-resolvable domains. Therefore, under physiological conditions of ionic strength, pH, and anionic lipid bilayer content, and in the absence of other, e.g., hydrophobic, contributions to the binding free energy, the minimum number of lysine residues sufficient to produce spectroscopically resolvable POPS-enriched domains on the 2H NMR millisecond timescale may be fewer than 100, but is certainly greater than 30.

Microelectrophoresis of a bilayer-coated silica bead in an optical trap: application to enzymology

We describe an apparatus that combines microelectrophoresis and laser trap technologies to monitor the activity of phosphoinositide-specific phospholipase C-delta1 (PLC-delta) on a single bilayer-coated silica bead with a time resolution of approximately 1 s. A 1-microm-diameter bead was coated with a phospholipid bilayer composed of electrically neutral phosphatidylcholine (PC) and negatively charged phosphatidylinositol 4,5-bisphosphate (2% PIP2) and captured in a laser trap. When an AC field was applied (160 Hz, 20 V/cm), the electrophoretic force produced a displacement of the bead, Delta(x), from its equilibrium position in the trap; Delta(x), which was measured using a fast quadrant diode detector, is proportional to the zeta potential and thus to the number of PIP2 molecules on the outer leaflet (initially, approximately 10(5)). When a solution containing PLC-delta flows past the bead, the enzyme adsorbs to the surface and hydrolyzes PIP2 to form the neutral lipid diacylglycerol. We observed a nonexponential decay of PIP2 on the bead with time that is consistent with a model based on the known structural properties of PLC-delta.

Effect of cationic lipids in the formation of asymmetries in supported bilayers

We have studied the formation of a supported bilayer containing both cationic and zwitterionic lipids by fusion of small unilamellar vesicles (SUV) onto the solid surface at low salt conditions using a combination of attenuated total reflection infrared (ATR-IR) and deuterium NMR spectroscopy with microcalorimetry. The data suggest that a significant cationic lipid asymmetry between the outer (distal) and the inner (proximal) monolayer of a supported bilayer results under conditions of prolonged incubation times of the solid support with the SUV coating solution. For a SUV composition of DPPC/DHDAB (4:1) we observed an enrichment of the cationic component in the proximal monolayer of up to 200% compared to the distal monolayer after 12 h incubation. It is suggested that the electrostatic potential arising from the solid surface is the driving force for the creation of this asymmetry by means of directed flip-flop between the monolayers and/or by temporary fusion between SUV from the bulk with the supported bilayer.

Surface charge response of the phosphatidylcholine head group in bilayered micelles from phosphorus and deuterium nuclear magnetic resonance

Solid-state phosphorus (31P) and deuterium (2H) nuclear magnetic resonance (NMR) spectroscopy over the temperature range of 25-50 degreesC were used to investigate bilayered micelles (bicelles) composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1, 2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) in the presence of either the anionic lipid 1,2-dimyristoyl-sn-3-phosphoglycerol (DMPG) or the cationic lipid 1,2-dimyristoyl-3-trimethylammonium-propane (DMTAP). The 31P-NMR spectra demonstrate that bicellar structures form with DMPG/DMPC ratios ranging from 0 to 50/50 and with DMTAP/DMPC ratios from 0 to 40/60, while the overall concentration of DHPC remains constant. The formation of bicelles containing charged amphiphiles is contingent upon the presence of NaCl, with 50 mM NaCl being sufficient for bicelle formation at all concentrations of charged amphiphile investigated, while 150 mM NaCl affords better resolution of the various 31P-NMR resonance signals. The 2H-NMR spectra demonstrate that the quadrupolar splittings (Deltanu) of head group-deuterated DMPC change inversely as a function of the amount of negative versus positive charge present, and that the changes for deuterons on the alpha-carbon are opposite in sense to those for deuterons on the beta-carbon. This indicates that head group-deuterated phosphatidylcholine functions as a molecular voltmeter in bicelles in much the same fashion as it does in spherical vesicles.

Measurement of the lateral diffusion of dipalmitoylphosphatidylcholine adsorbed on silica beads in the absence and presence of melittin: a 31P two-dimensional exchange solid-state NMR study

31P two-dimensional exchange solid-state NMR spectroscopy was used to measure the lateral diffusion, D(L), in the fluid phase of dipalmitoylphosphatidylcholine (DPPC) in the presence and absence of melittin. The use of a spherical solid support with a radius of 320 +/- 20 nm, on which lipids and peptides are adsorbed together, and a novel way of analyzing the two-dimensional exchange patterns afforded a narrow distribution of D(L) centered at a value of (8.8 +/- 0.5) x 10(-8) cm2/s for the pure lipid system and a large distribution of D(L) spanning 1 x 10(-8) to 10 x 10(-8) cm2/s for the lipids in the presence of melittin. In addition, the determination of D(L) for nonsupported DPPC multilamellar vesicles (MLVs) suggests that the support does not slow down the lipid diffusion and that the radii of the bilayers vary from 300 to 800 nm. Finally, the DPPC-melittin complex is stabilized at the surface of the silica beads in the gel phase, opening the way to further study of the interaction between melittin and DPPC.

Globoside as a membrane receptor: a consideration of oligosaccharide communication with the hydrophobic domain

Recognition of macromolecules by glycosphingolipids is closely correlated with the nature of the glycolipid carbohydrate; however, it is also thought to be secondarily modulated by the structure of the single fatty acid. In the present work, we sought insight into what physical effect a change in this fatty acid has on the extramembranous portion of globosides at liposomal surfaces mimicking systems for which modulated receptor function has been recorded in the past. Protons of the exocyclic hydroxymethyl group on the terminal Gal residue of globotriaosylceramide (Gb3) were replaced with deuterium. In this location, the nonperturbing probe nuclei sampled cumulative conformational and orientational characteristics of the oligosaccharide chain at a sugar residue that is critical in specific binding of verotoxins. Deuterated Gb3 having 18:1 fatty acid was compared to the same species having 22:1 fatty acid, at 6.3 mol % in unsonicated bilayers of dipalmitoylphosphatidylcholine/cholesterol. Both produced narrow, apparently axially asymmetric 2H NMR spectra over a wide temperature range. Motional properties of the terminal sugar were measurably influenced by the fluidity of the host matrix; however, evidence was not found for conformational or orientational variation in this sugar brought about by the fatty acid alteration. In related experiments, acetate protons on the terminal N-acetyl galactosamine (GalNAc) residue of globotetraosylceramide (Gb4) were substituted with deuterium, and the natural fatty acid was replaced with 18:0 or 24:0 species deuterated at C2. Once again, species with short vs long fatty acid were examined for evidence of headgroup differences. Spectra of Gb4 were compared at 10 mol % in unsonicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine, and at 5 mol % in membranes containing 33 mol% cholesterol. Spectral splittings reflecting cumulative effects on conformation and order at the terminal deuterated sugar remained unchanged between species having 18:0 vs 24:0 fatty acid in POPC/cholesterol. In a pure POPC host matrix, there was clear evidence of a motional difference between the two--the longer chain Gb4 demonstrating spectral asymmetry--but the spectral width was unchanged. Transverse relaxation times, T2, were measured. Our findings appear to help correlate the conclusions of a number of workers dealing with the molecular basis of crypticity. We suggest that changes in glycolipid receptor function based on ceramide fatty acid variation have a major origin in the fatty acid's ability to determine the thermodynamics of interaction with the host matrix, as reflected in such parameters as glycolipid motional properties, local membrane curvature, and likely glycolipid time-dependent lateral associations. The result at low concentrations of glycolipid may often be only a subtly altered collective surface epitope, best detected by a specific recognition event.