Phosphatidylethanolamine Head Research Articles

Magnesium Ion Responses of Zwitterionic Phosphatidylethanolamine Head and Tail Groups Elucidated by Frequency-Resolved SFG-VS.

In the present study, the effects of magnesium ions on the conformational changes of the deuterated 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (D54-DMPE) monolayer were elucidated by frequency-resolved sum frequency generation vibrational spectroscopy (SFG-VS) and surface pressure-area isotherm measurements. It is found that the tilt angles of the methyl in tail groups decrease, while the tilt angles of the phosphate and methylene in head groups increase during the compression of the DMPE monolayers at both the air/water interface and the air/MgCl2 solution interfaces. It is also shown that the tilt angle of the methyl in the tail groups slightly decreases, while the tilt angles of the phosphate and methylene in the head groups significantly increase as the MgCl2 concentration increases from 0 to 1.0 M. These results indicate that both the tail groups and the head groups of the DMPE molecules become closer to the surface normal, as the MgCl2 concentration increases in the subphase.

Mass sensitivity of Langmuir-Blodgett monolayer film coated surface acoustic wave resonators to volatile organic solvents

Langmuir-Blodgett (LB) monolayer films deposited from a Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD), have been studied for their ability to be used as sensing layer for chemical gas sensors. DPPE-NBD molecules are known for their similarity to bio membrane molecules. Therefore, it is expected that they can serve as a matrix for immobilization of proteins, enzymes, aptamers, while preserving their function for selective reaction with organic analytes. In this study, LB monolayers of DPPE-NBD were deposited on Rayleigh Surface Acoustic Wave (RSAW) resonant devices working at 411 MHz and simultaneously on ultra-flat Si wafer substrates for Atomic Force Microscopy (AFM) inspection. Depositions were carried out both at a low surface pressure where the liquid-expanded phase dominates, and at a high surface pressure, where the liquid-condensed (solid) phase dominates. AFM topography reveals the liquid and solid phase coexistence as well as the formation of 3D pyramids of 3 to 30 nm height and 50 to 1000 nm in diameter. In a vapour sensing experiment, the RSAW devices were exposed subsequently to vapours of 6 volatile organic compounds (VOCs) and water. The most significant resonant frequency shift of 225 kHz which corresponds to 11.42 ng mass change was observed with chloroform vapours when the substrate with the solid phase LB monolayer was used as a sensing layer. Adsorption and desorption of the vapours was very fast (a few seconds) and completely reversible. The higher-pressure deposited LB film demonstrates higher sensitivity to all gases. Mechanisms of this behaviour are discussed.

Extending SDP Modeling to the Inverted Hexagonal Phase

The SDP (Scattering Density Profile) technique was developed to model the X-ray and neutron scattering profiles of lipid bilayers. The lipids were divided up into coarse grained components, with the probability of a component being found at a given point in the bilayer represented by an appropriate function. Most of the lipid component probability densities were modeled as Gaussians, with the hydrocarbon tail region represented by an error function. Building on this powerful method developed by others, we have extended it to the inverted hexagonal phase. As the technique was originally developed for use for lipids with phosphatidylcholine head groups, we have also modified it for use with phosphatidylethanolamine head groups. We have been able to generate models whose Fourier components agree well with the amplitudes acquired from X-ray diffraction. Additionally, the Luzzati interface from the models is close to the experimentally determined electron density peak, as one would expect for these systems.

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.

Substrate Selectivity of a P4‐ATPase Flippase

Phospholipids, the primary structural components of cell membranes, are distributed unequally across the membrane. The sphingolipids, including sphingomyelin (SM) and glycosphingolipids, are enriched on the exofacial surface of the plasma membrane or the luminal surfaces of internal organelles, while the cytofacial leaflet is enriched in the aminophospholipids, phophatidylserine (PS) and phosphatidylethanolamine (PE). Contributing to maintenance of this asymmetry is a combination of slow passive transbilayer phospholipid movement, trapping by cytofacial proteins and the activity of a number of energy‐dependent transporters. The substrate specificity and directionality of these pumps defines the transbilayer asymmetry of phospholipids. Two transporter superfamilies, ABC transporters and P‐type ATPases, account for these activities and move lipids in either the exofacial (floppase) or cytofacial (flippase) directions, respectively. In previous studies we have characterized the biochemical requirements and substrate specificity for a PS specific flippase activity in human red blood cells. Transport is ATP‐dependent and sensitive to vanadate and sulfhydryl‐modification. This flippase is selective for the sn‐1,2‐isomer of PS. Transport is diminished by modification of the PS headgroup amine (with the exception of mono‐methylation), carboxyl and phosphate groups, while a variety of fatty acyl groups and the D‐serine analog are accommodated. Red blood cells express two members of the P4‐type ATPase subfamily, ATP8A1 and ATP11C, that are likely responsible for this activity. Here, we report on the heterologous expression, purification, and functional assaying of hexahistidine‐tagged constructs of Atp8a1 with and without its cognate accessory subunit, Cdc50a. Purified Atp8a1 is vanadate‐sensitive and dependent upon Cdc50a co‐expression and PS for robust ATPase activity, although lipid specificity is not dependent on the presence of Cdc50a. In addition to reconstituting the flippase activity of Atp8a1 and ATP11c in vitro, our efforts are focused on rational mutagenesis of Atp8a1 to identify the lipid transport path and the determinants of substrate specificity, directed by homology modeling and the lipid‐binding characteristics of the closest members of the P‐type superfamily. X‐ray crystal structures of the SERCA pump in conformation E2 (PDB 2AGV) display a PE glycerol backbone and head group tightly pinched by helices M2 and M4 with PE's acyl chains extending between helices M2 and M6. This PE‐binding site implies a mechanism for how the P4lipid‐binding site and transport pathway evolved from ancestor P‐types designed to move simple cations. With these clues, we aim to identify the PS binding site and deduce the origin of the sn‐1,2‐stereospecificity of Atp8a1.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Langmuir-Blodgett Films from Fluorescently Labelled Phospholipids Deposited on Surface Acoustic Wave Devices

Over the years several new effects in fluorescently labelled Langmuir monolayers or Langmuir-Blodgett (LB) films, deposited on solid support plates, have been reported. Mostly layers from Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD) have been studied. This molecule behaves exactly as the DPPE molecule which is part of biological membranes. So it is expected that DPPE-NBD molecules can serve as an appropriate matrix for implementation of selectively reacting proteins or enzymes while preserving their function. This combination can act as an active layer in biosensor applications for operation in either gas or liquid environments and this molecule could further increase the system sensitivity in such applications. Addition of the chromophore NBD head group also stabilizes the film and allows for multilayer deposition which is not possible for phospholipids. In this study, LB monolayers were deposited on glass substrates at different surface pressures and with or without Cadmium ions in the water subphase. They were studied using fluorescence spectroscopy and Atomic Force Microscopy (AFM). In view of possible biosensor applications, LB layer by layer deposition was performed on Rayleigh Surface Acoustic Wave (SAW) resonant devices working at 440 MHz. These can effectively be used to transduce the biosensor signal from the active layer by providing a mass proportional frequency output. Layers from DPPE-NBD that might also be suitable for possible SAW based biosensor applications were deposited at a surface pressure of 10 mN/m. AFM spectroscopy reveals the phase coexistence of liquid and solid phases as well as higher bilayer nanosized cylinders. Fluorescence intensity was strongly self-quenched at this pressure and fluorescence lifetime spectroscopy shows a complex behaviour with a 3-rd order exponential decay. A frequency downshift of 10 KHz per monolayer was measured with the coated SAW device which results in a deposited mass of about 1 ng/monolayer.

Boundary Lipids of the Nicotinic Acetylcholine Receptor in Quasi-Native Membranes

The nicotinic acetylcholine receptor (nAChR) is a neurotransmitter receptor and pentameric ligand gated ion channel (pLGIC) critical for signaling across synapses, including the neuromuscular junction. In reconstituted membranes, the nAChR function depends heavily on cholesterol, leading to the hypothesis that nAChR will partition into cholesterol-enriched liquid-ordered domains ("rafts”). Native nAChR membranes are rich in lipids with saturated fatty acid (like palmitic acid) or polyunsaturated fatty acid (PUFA) acyl chains (like docosahexaenoic acid (DHA)). Using coarse-grained molecular dynamics simulations we characterized preferential lipid interactions and partitioning behavior of nAChR in binary membranes (cholesterol and lipids with two palmitic acid acyl chains) ternary membranes (cholesterol, a lipid with two palmitic acid acyl chains, and lipids with either two chains composed of the long-chain omega-3 PUFA DHA or of the omega-6 PUFA linoleic acid). We quantify occupation of non-annular and annular regions by cholesterol and saturated and polyunsaturated lipids. In the absence of PUFAs, cholesterol is enriched in the nAChR annulus in a concentration-dependent manner. Cholesterol is also distributed throughout the non-annular (embedded) sites, while palmatic acyl chains persistently occupy only one interface between the beta and alpha subunits. When lipids containing long-chain PUFA acyl chains are introduced, they displace cholesterol from two additional interfaces. Contrary to expectations, in domain-forming membranes containing PUFAs, the nAChR is observed to consistently partition into PUFA-rich, cholesterol-poor domains. Saturated lipids with either phosphatidylcholine and phosphatidylethanolamine head groups are significantly depleted in such systems, although the extent of depletion is reduced for phosphatidylethanolamine. While nAChR consistently partitions into the cholesterol poor domain, alpha-gamma and delta-beta faces interact more than other faces with the cholesterol-rich domains. We extend this approach to more complex membranes of interest, including more realistic synaptic membranes and the Xenopus oocyte membranes used for electrophysiology.

First observation of 3D aggregates in a single-component Langmuir film below the equilibrium spreading pressure

Single component monolayers from Dipalamitoyl Phosphatidyl Ethanolamine head labelled with the fluorescent chromophore NitroBenzoxaDiazole (DP-NBD-PE) were investigated at the air-water interface as Langmuir films and deposited on silicon wafers or glass plates as Langmuir-Blodgett (LB) films. A step compression and monitoring of the pressure relaxation together with domain formation as observed with Brewster angle microscopy suggests that main transition from liquid expanded to liquid compressed state start at around 6.9 mN/m though on the isotherm it starts around 9 mN/m at 25° C. Brewster angle microscopy also reveals a nonuniform structure in the monolayer. 3D aggregates - cylinders with 50 – 150 nm diameter and bilayer height were observed with Atomic Force Microscopy when deposition was carried at 7 mN/m, above the main phase transition but considerably lower than the equilibrium spreading pressure of 19.6 mN/m for this molecule. When LB film deposition is carried just below the main phase transition a uniform height layer of film in a liquid phase is observed with AFM with no structures. Both compression and deposition were carried at very low speeds with large time to relax in ordered to avoid kinetic effects. These 3D aggregates are not due to the transfer process or interaction with the substrate. These aggregates provide a highly developed area combined with monolayer thick structure which can produce very fast and highly sensitive biosensors.

Chemoenzymatic synthesis of fluorogenic phospholipids and evaluation in assays of phospholipases A, C and D

Phospholipases are ubiquitous in nature and the target of significant research aiming at both their physiological roles and technical applications in e.g. the food industry. In the search for sensitive and selective phospholipase assays, we have focused on synthetic FRET (Förster resonance energy transfer) substrates. This has led to the development of a facile, easily scalable and low cost synthesis of fluorogenic phospholipids featuring the dansyl/dabcyl fluorophore/quencher-pair on the fatty acid ω-position and on the phosphatidylethanolamine head group, respectively. Hence, the two substrates lyso-(dansyl-FA)-GPE-dabcyl (6) and (dansyl-FA)2-GPE-dabcyl (7) were synthesized by a chemoenzymatic strategy, in which preparation of (6) further included a novel selective enzymatic esterification step. As proof of concept, activity of a handful of phospholipases, one from each of the PLA1, PLA2, PLC and PLD classes, were assayed using substrates (6) and (7), and the kinetic parameter kcat/KM was determined. The PLA1 (Lecitase Ultra™) was found to be highly active on both substrates, whereas the PLD (from white cabbage) had no activity, presumably due to steric effects associated with the dabcyl-functionalization of the head group. It was further substantiated that the substrates are specific towards phospholipase activity as the tested lipase (Lipolase™) showed close to zero activity.

Lipid-Lipid Coupling to Membrane Curvature by Simulation and NMR

The material properties of a bilayer must be determined by the chemical structure of its lipid components and the interactions between them. The preferred (“spontaneous”) curvature of a lipid leaflet is one of the most interesting properties as it is the most direct connection between the stability of leaflet structures and lipid chemistry. For example, the curved lipid hexagonal phase is stabilized by phosphatidylethanolamine head groups.Nuclear magnetic resonance spectroscopy and molecular dynamics can probe structure on the molecular level, but relating these techniques to material properties requires new models and theoretical methods.In this work, we use computer simulation of lipid bilayers (in particular, sphingomyelin and DPPC) to predict the impact of chemical structure on spontaneous curvature. Then, NMR applied to small vesicles measures how lipid identity controls population of the inner and outer leaflet, reflecting the lipid's spontaneous curvature. Simulations predict that at low concentration of sphingomyelin, strong coupling between lipids determines this behavior in a non-linear way, in contrast to DPPC.

The application of pH-sensitive polymer-lipids to antigen delivery for cancer immunotherapy

For production of pH-sensitive liposomes, we developed pH-sensitive polymer-lipids that consists of pH-sensitive fusogenic polymer moieties such as 3-methyl glutarylated poly(glycidol) and 2-carboxycyclohexane-1-carboxylated poly(glycidol), connected to a phosphatidylethanolamine head group. Incorporation of these pH-sensitive polymer-lipids into egg yolk phosphatidylcholine liposomes produced highly pH-sensitive liposomes that were stable at neutral pH but which destabilized markedly in response to very small pH change in weakly acidic pH region. These liposomes delivered their contents (pyranine) into cytosol of dendritic cell-derived DC2.4 cells. When these polymer-lipid-incorporated liposomes loaded with antigenic protein ovalbumin (OVA) were administered subcutaneously to mice, the antigen-specific cellular immunity was induced efficiently in the mice. Furthermore, immunization of mice with these OVA-loaded pH-sensitive polymer-lipid-incorporated liposomes induced strong OVA-specific immunity, which achieved complete rejection of OVA-expressing E.G7-OVA cells and marked regression of E.G7-OVA tumors.

A new synthetic approach to phosphatidylethanolamine

Abstract A new synthetic method for phosphatidylethanolamine head group was developed via ring-opening of cyclic dioxaphospholane 2 with sodium azide and subsequent hydrogenation. The advantage of this strategy included short reaction steps, readily available materials and good yields.

Bioreducible Liposomes for Gene Delivery: From the Formulation to the Mechanism of Action

BackgroundA promising strategy to create stimuli-responsive gene delivery systems is to exploit the redox gradient between the oxidizing extracellular milieu and the reducing cytoplasm in order to disassemble DNA/cationic lipid complexes (lipoplexes). On these premises, we previously described the synthesis of SS14 redox-sensitive gemini surfactant for gene delivery. Although others have attributed the beneficial effects of intracellular reducing environment to reduced glutathione (GSH), these observations cannot rule out the possible implication of the redox milieu in its whole on transfection efficiency of bioreducible transfectants leaving the determinants of DNA release largely undefined.Methodology/Principal FindingsWith the aim of addressing this issue, SS14 was here formulated into binary and ternary 100 nm-extruded liposomes and the effects of the helper lipid composition and of the SS14/helper lipids molar ratio on chemical-physical and structural parameters defining transfection effectiveness were investigated. Among all formulations tested, DOPC/DOPE/SS14 at 25∶50∶25 molar ratio was the most effective in transfection studies owing to the presence of dioleoyl chains and phosphatidylethanolamine head groups in co-lipids. The increase in SS14 content up to 50% along DOPC/DOPE/SS14 liposome series yielded enhanced transfection, up to 2.7-fold higher than that of the benchmark Lipofectamine 2000, without altering cytotoxicity of the corresponding lipoplexes at charge ratio 5. Secondly, we specifically investigated the redox-dependent mechanisms of gene delivery into cells through tailored protocols of transfection in GSH-depleted and repleted vs. increased oxidative stress conditions. Importantly, GSH specifically induced DNA release in batch and in vitro.Conclusions/SignificanceThe presence of helper lipids carrying unsaturated dioleoyl chains and phosphatidylethanolamine head groups significantly improved transfection efficiencies of DOPC/DOPE/SS14 lipoplexes. Most importantly, this study shows that intracellular GSH levels linearly correlated with transfection efficiency while oxidative stress levels did not, highlighting for the first time the pivotal role of GSH rather than oxidative stress in its whole in transfection of bioreducible vectors.

Crystal Structure of Soluble Domain of Malaria Sporozoite Protein UIS3 in Complex with Lipid

Malaria parasite UIS3 (up-regulated in infective sporozoites gene 3) is essential for sporozoite development in infected hepatocytes. UIS3 encodes for a membrane protein that is localized to the parasite parasitophorous vacuolar membrane in infected hepatocytes. We describe here 2.5-Å resolution crystal structure of Plasmodium falciparum UIS3 soluble domain (PfUIS3130-229) in complex with the lipid phosphatidylethanolamine (PE). PfUIS3130-229 is a novel, compact, and all α-helical structure bound to one molecule of PE. The PfUIS3130-229-PE complex structure reveals a novel binding site with specific interactions between PfUIS3130-229 and the PE head group. One acyl chain of PE wraps around part of PfUIS3130-229 and docks onto a hydrophobic channel. We additionally provide new structural and biochemical evidence of PfUIS3130-229 interactions with lipids (phosphatidylethanolamine), with phospholipid liposomes, and with the human liver fatty acid-binding protein. The direct interaction of PfUIS3130-229 with liver fatty acid-binding protein most likely provides the parasite with a conduit for importing essential fatty acids/lipids. Therefore, our analyses have implications for lipid transport into the parasite during the rapid growth phases of sporozoites. Given that PfUIS3 is essential for establishment of liver stage infection by P. falciparum, our data provide a new target for abrogating parasite development within liver cells before typical symptoms of malaria can manifest.

Separation of fluorescent phosphatidyl inositol phosphates by CE

Phosphatidyl inositol 4,5-bisphosphate (PIP2) and phosphatidyl inositol 3,4,5-trisphosphate (PIP3) labeled with 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionic acid (BODIPY FL) on the acyl chain or a phosphatidyl ethanolamine head group were separated by CE with LIF detection. Several methods and capillary-coating procedures were tested for the separation of these phosphatidyl inositol phosphates (PIPs) at 20 degrees C. Separation of the PIPs in less than 20 min with excellent resolution was achieved using a buffer containing sodium deoxycholate (SDC), 1-propanol, MgCl2 and the polymer coating reagent, EOTrol LR. The efficiency of the optimized method was as high as 1.3x10(5) plates. The dependence of the separation on the concentration of 1-propanol, SDC, and MgCl2 was determined. The separation of PIP2 and PIP3 was primarily due to differential binding of the lipids to Mg2+ rather than to different solubilities in the micellar phase. The role of the SDC was to prevent adsorption of the hydrophobic lipids to the capillary wall and thus enhance the efficiency. The fluorescent PIPs are of value for both in vitro and in vivo assays of phospholipid metabolism. In particular, the use of these lipids with the optimized capillary-based separation will be of utility for drug screening as well as cell-based assays.

Pyrithione biocide interactions with bacterial phospholipid head groups.

Sodium pyrithione and zinc pyrithione (NaPT and ZnPT, respectively) are antimicrobial agents widely used in both the cosmetics and fuel industries. They are also utilized in the mining industry because of their metal chelating properties. They have been shown to depolarize membrane electropotential in fungi and are also known to inhibit fungal and bacterial substrate transport processes. Recent work has shown that both pyrithiones cause the leakage of intracellular material (potassium ions and O.D.260 nm absorbing material) from exposed bacterial cells. The work here reports studies on the interactions between the pyrithiones and the bacterial phospholipid head group structures, at both a practical and a theoretical level, utilizing tube dilution neutralizer studies, scanning spectrophotometry and molecular modelling. The tube dilution neutralizer studies exhibited a decrease in minimum inhibitory concentration (MIC) for both pyrithiones in the presence of extracellular phosphatidyl-ethanolamine and EDTA. Scanning spectrophotometry exhibited the chelation of the central zinc atom from the ZnPT chelate by the addition of EDTA. Molecular modelling studies exhibited the chelation of the phosphatidyl-ethanolamine head group by ZnPT. Zinc pyrithione also exhibited an interaction with the ammonium tail of the head group structures. Sodium pyrithione exhibited electrostatic interactions with the phospholipid head groups in the molecular modelling studies.

Specific interactions of mercury chloride with membranes and other ligands as revealed by mercury-NMR

High resolution mercury nuclear magnetic resonance ( 199Hg-NMR) experiments have been performed in order to monitor mercury chemical speciation when HgCl 2 is added to water solutions and follow mercury binding properties towards biomembranes or other ligands. Variations of 199Hg chemical shifts by several hundred ppm depending upon pH and/or pCl changes or upon ligand or membrane addition afforded to determine the thermodynamic parameters which describe the equilibria between the various species in solution. By comparison to an external reference, the decrease in concentration of mercury species in solution allowed to estimate the amount as well as the thermodynamic paramaters of unlabile mercury-ligand or mercury-membrane complexes. Hence, some buffer molecules can be classified in a scale of increasing complexing power towards Hg(II): EGTA > Tris > HEPES. In contrast, MOPS, Borax, phosphates and acetates show little complexation properties for mercury, in our experimental conditions. Evidence for complexation with phosphatidylethanolamine (PE), phosphatidylserine (PS) and human erythrocyte membranes has been found. Hg(II) does not form complexes with egg phosphatidylcholine membranes. Interaction with PE and PS model membranes can be described by the presence of two mercury sites, one labile, the other unlabile, in the NMR time scale. In the labile site Hg(PE) and Hg(PS) 2 would be formed whereas in the unlabile site Hg(II) would establish bridges between three PE or PS molecules. Calculated thermodynamic data clearly indicate that PE is a better complexing agent than PS. Evidence is also found that complexation with lipids uses at first the HgCl 2 species. Interestingly, mercury complexation with ligands or membranes can be completely reversed by addition of decimolar NaCl solutions. Minute mechanisms for mercury complexation with the primary amine of PE or PS membrane head groups are discussed.

1-Palmitoyl-2-[3-(diphenylhexatrienyl) propanoyl]- sn-glycero-3-phosphoethanolamine as a fluorescent membrane probe. Synthesis and partitioning properties

We have synthesized the fluorophore 1-palmitoyl-2-[3-(diphenylhexatrienyl) propanoyl]- sn-glycero-3-phosphoethanolamine, a fluorescent phospholipid which has a polar phosphatidylethanolamine head group, but one of the two chains is replaced by a diphenylhexatriene moiety. The partitioning of this probe, between gel-phase and liquid crystalline-phase of multilamellar vesicles of binary composition (either PC-PC or PC-PE mixtures) was measured and systematically compared to that of the choline analogue DPHpPC (R.A. Parente and B.R. Lentz (1985) Biochemistry 24, 6178–6185). We found in PC mixtures that DPHpPE partitions preferentially in the gel-phase of the vesicles ( K f/s = 0.8), contrary to the choline analogue which partitions preferentially in the fluid-phase. In heterogeneous PC-PE mixtures, DPHpPE never shows preferential partitioning for a PE-rich phase ( K f/s = 1.35 for DEPC/DPPE vesicles: K f/s = 0.5 for L oPE/DPPC vesicles). These partitioning data are compared to previous results in the literature and tentative explanations are given. In conclusion, this new molecule cannot be used as a specific probe for membrane domains rich in phosphatidylethanolamine.

Lipid-protein interactions mediate the photochemical function of rhodopsin.

We have investigated the molecular features of recombinant membranes that are necessary for the photochemical function of rhodopsin. The magnitude of the metarhodopsin I to metarhodopsin II phototransient following a 25% +/- 3% bleaching flash was used as a criterion of photochemical activity at 28 degrees C and pH 7.0. Nativelike activity of rhodopsin can be reconstituted with an extract of total lipids from rod outer segment membranes, demonstrating that the protein is minimally perturbed by the reconstitution protocol. Rhodopsin photochemical activity is enhanced by phosphatidylethanolamine head groups and docosahexaenoyl (22:6 omega 3) acyl chains. An equimolar mixture of phosphatidylethanolamine and phosphatidylcholine containing 50 mol% docosahexaenoyl chains results in optimal photochemical function. These results suggest the importance of both the head-group and acyl chain composition of the rod outer segment lipids in the visual process. The extracted rod lipids and those lipid mixtures favoring the conformational change from metarhodopsin I to II can undergo lamellar (L alpha) to inverted hexagonal (HII) phase transitions near physiological temperature. Interaction of rhodopsin with membrane lipids close to a L alpha to HII (or cubic) phase boundary may thus lead to properties which influence the energetics of conformational states of the protein linked to visual function.

Lateral interactions among phosphatidylcholine and phosphatidylethanolamine head groups in phospholipid monolayers and bilayers.

We develop theory for the lateral interactions among the zwitterionic head groups of phospholipids in monolayers and bilayers, particularly phosphatidylcholine (PC) and phosphatidylethanolamine (PE). With the P- end of the head group anchored at the water/hydrocarbon interface, a balance of two effects dictates the angle that the P--N+ dipole makes with respect to the plane of the bilayer: N+ is driven toward water due to the (Born) electrostatic free energy, but the hydrophobic effect drives the methyl and methylene groups around the N+ charge toward the hydrocarbon. The only adjustable parameter of the model is the average fluctuation of the oil/water interface or, alternatively, the dielectric constant of the hydrocarbon phase. The model predicts that at 5 degrees C the head group dipole should lie largely in the bilayer plane, in accord with X-ray, neutron diffraction, and NMR studies. The theory makes the novel prediction that the N+ end of the dipole becomes increasingly submerged in hydrocarbon with increasing temperature, leading to strongly enhanced lateral repulsion between PC head groups. This prediction is in good agreement with second and third viral coefficients of monolayer lateral pressures, and with the temperature dependence of the former. The theoretical model is consistent with head group fluctuations measured by neutron diffraction of PC and PE bilayers. Because PE has a smaller hydrophobic cluster near N+, its lateral repulsion should be much smaller and less temperature dependent than for PC, also in agreement with equation-of-state measurements. This suggests why at high density PE monolayers have higher melting temperatures than PC monolayers and more propensity for reversed curvature.