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

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

Phospholipid-protein adducts of lipid peroxidation: synthesis and study of new biotinylated phosphatidylcholines.

Oxidative stress gives rise to a number of electrophilic aldehydes from membrane phospholipids, and these compounds have been linked to pathophysiologic events associated with the progression of cardiovascular disease. A headgroup biotinylated phosphatidylcholine (PC) has been prepared, and its oxidation chemistry has been studied. Biotin or biotin-sulfoxide groups were attached to PC at the ammonium headgroup via a di-ethylene glycol link. The modified phospholipids have calorimetric and colloidal properties similar to those of the parent. The oxidation of PLPBSO (the biotin-sulfoxide analogue of 1-palmitoyl-2-linoleoylglycerylphosphatidylcholine, PLPC) was studied under a variety of conditions. PLPBSO, like PLPC, undergoes oxidation to give electrophiles that adduct to small model peptides as well as to isolated proteins such as human serum albumin. PLPBSO incorporates into human blood plasma, and treatment of the plasma with water soluble free radical initiators gives rise to a number of biotinylated plasma proteins that can be isolated via (strept)avidin affinity. Isolated peptide or protein-lipid adducts can be identified by proteomics analyses, and studies on model peptides show that phospholipid-protein adduction sites can be identified by known algorithms. Biotinylated lipids such as PLPBSO and modern proteomics tools would appear to provide a new approach to exploring the chemistry and biology of membrane peroxidation associated with oxidative stress.

Structure, composition, and peptide binding properties of detergent soluble bilayers and detergent resistant rafts.

Lipid bilayers composed of unsaturated phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol are thought to contain microdomains that have similar detergent insolubility characteristics as rafts isolated from cell plasma membranes. We chemically characterized the fractions corresponding to detergent soluble membranes (DSMs) and detergent resistant membranes (DRMs) from 1:1:1 PC:SM:cholesterol, compared the binding properties of selected peptides to bilayers with the compositions of DSMs and DRMs, used differential scanning calorimetry to identify phase transitions, and determined the structure of DRMs with x-ray diffraction. Compared with the equimolar starting material, DRMs were enriched in both SM and cholesterol. Both transmembrane and interfacial peptides bound to a greater extent to DSM bilayers than to DRM bilayers, likely because of differences in the mechanical properties of the two bilayers. Thermograms from 1:1:1 PC:SM:cholesterol from 3 to 70 degrees C showed no evidence for a liquid-ordered to liquid-disordered phase transition. Over a wide range of osmotic stresses, each x-ray pattern from equimolar PC:SM:cholesterol or DRMs contained a broad wide-angle band at 4.5 A, indicating that the bilayers were in a liquid-crystalline phase, and several sharp low-angle reflections that indexed as orders of a single lamellar repeat period. Electron density profiles showed that the total bilayer thickness was 57 A for DRMs, which was approximately 5 A greater than that of 1:1:1 PC:SM:cholesterol and 10 A greater than the thickness of bilayers with the composition of DSMs. These x-ray data provide accurate values for the widths of raft and nonraft bilayers that should be important in understanding mechanisms of protein sorting by rafts.

The energetics of binding of a signal peptide to lipid bilayers: the role of bilayer properties.

The interactions between a signal peptide and a variety of lipid bilayers have been studied with direct binding assays, CD and isothermal titration calorimetry. We find that the binding of this peptide is influenced by charges and dipoles located in the bilayer interfacial region, as well as by the presence of cholesterol in the bilayer. These studies show that bilayer compositional and mechanical variations found in different biological membranes can affect the partitioning of peptides into the bilayer.

The effect of ethanol on the phase transition temperature and the phase structure of monounsaturated phosphatidylcholines.

Previous studies from our laboratories have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines, C(X):C(Y)PC, and the effect of ethanol concentration, [EtOH], on the main phase transition temperature, T(m), and the phase structure of the lipid bilayer composed of C(X):C(Y)PC using differential scanning calorimetry and X-ray diffraction techniques [Huang and McIntosh, Biophys. J. 72 (1997) 2702--2709]. In the present work, we have extended these studies to characterize the effect of [EtOH] on the T(m) and the phase structure of the lipid bilayer composed of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines with various positions of the cis double bond. Specifically, five positional isomers of 1-eicosanoyl-2-eicosenoyl-sn-glycero-3-phosphocholines, C(20):C(20:1 Delta(n))PC with n=5, 8, 11, 13 and 17, were synthesized and studied. For C(20):C(20:1 Delta(n))PC with n=5 and 8, results from the calorimetric experiments showed that in response to various concentrations of ethanol, the change in T(m) of the lipid bilayer composed of monounsaturated lipids was characterized by a sigmoidal or biphasic profile in the plot of T(m) versus [EtOH]. In contrast, a continuous depression of the T(m) by ethanol was observed calorimetrically for C(20):C(20:1 Delta(n))PC with n> or =11. The X-ray diffraction experiments further demonstrated that C(20):C(20:1 Delta(5))PC and C(20):C(20:1 Delta(8))PC can undergo the ethanol-induced gel-to-fully interdigitated phase transition at T

Influence of lipid composition on physical properties and peg-mediated fusion of curved and uncurved model membrane vesicles: "nature's own" fusogenic lipid bilayer.

Poly(ethylene glycol) (PEG)-mediated fusion of phosphatidylcholine model membranes has been shown to mimic the protein-mediated biomembrane process [Lee, J., and Lentz, B. R. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 9274-9279]. Unlike the simple model membranes used in this earlier study, the lipid composition of fusogenic biomembranes is quite complex. The purpose of this paper was to examine PEG-mediated fusion of highly curved (SUV) and largely uncurved (LUV) membrane vesicles composed of different lipids in order to identify lipid compositions that produce highly fusogenic membranes. Starting with liposomes composed of five lipids with different physical properties, dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylserine (DOPS), bovine brain sphingomyelin (SM), and cholesterol (CH), we systematically varied the composition and tested for the extent of PEG-mediated fusion after 5 min of treatment. We found that a vesicle system composed of four lipids, DOPC/DOPE/SM/CH, fused optimally at a 35/30/15/20 molar ratio. Each lipid seemed to play a part in optimizing the membrane for fusion. PE disrupted outer leaflet packing as demonstrated with TMA-DPH lifetime, C(6)-NBD-PC partitioning, and DPH anisotropy measurements, and thus significantly enhanced fusion and rupture, without significantly altering interbilayer approach (X-ray diffraction). An optimal ratio of PC/PE (35/30) produced a balance between fusion and rupture. CH and SM, when present at an optimal ratio of 3/4 in vesicles containing the optimal PC/PE ratio, reduced rupture without significantly reducing fusion. This optimal CH/SM ratio also enhanced outer leaflet packing, suggesting that fusion is dependent not only on outer leaflet packing but also on the properties of the inner leaflet. Addition of CH without SM enhanced rupture relative to fusion, while SM alone reduced both rupture and fusion. The optimal lipid composition is very close to the natural synaptic vesicle composition, suggesting that the synaptic vesicle composition is optimized with respect to fusogenicity.

Electrostatic control of phospholipid polymorphism.

A regular progression of polymorphic phase behavior was observed for mixtures of the anionic phospholipid, cardiolipin, and the cationic phospholipid derivative, 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine. As revealed by freeze-fracture electron microscopy and small-angle x-ray diffraction, whereas the two lipids separately assume only lamellar phases, their mixtures exhibit a symmetrical (depending on charge ratio and not polarity) sequence of nonlamellar phases. The inverted hexagonal phase, H(II,) formed from equimolar mixtures of the two lipids, i.e., at net charge neutrality (charge ratio (CR((+/-))) = 1:1). When one type of lipid was in significant excess (CR((+/-)) = 2:1 or CR((+/-)) = 1:2), a bicontinuous cubic structure was observed. These cubic phases were very similar to those sometimes present in cellular organelles that contain cardiolipin. Increasing the excess of cationic or anionic charge to CR((+/-)) = 4:1 or CR((+/-)) = 1:4 led to the appearance of membrane bilayers with numerous interlamellar contacts, i.e., sponge structures. It is evident that interactions between cationic and anionic moieties can influence the packing of polar heads and hence control polymorphic phase transitions. The facile isothermal, polymorphic interconversion of these lipids may have important biological and technical implications.

Preparation and preclinical evaluation of a novel liposomal complete-core lipopolysaccharide vaccine.

Our objective is to develop a prophylactic vaccine strategy that can be evaluated for surgical and other high-risk hospitalized patients. In this paper, we describe the preparation and preclinical evaluation of a liposomal complete-core lipopolysaccharide (LPS) vaccine that is nontoxic and broadly antigenic. Complete-core (Ra-chemotype) LPSs were isolated from four gram-negative bacterial strains (Escherichia coli K-12, E. coli R1, Pseudomonas aeruginosa PAC608, and Bacteroides fragilis), mixed together to form a cocktail of complete-core LPSs, and then incorporated into multilamellar liposomes consisting of dimyristoyl phosphatidyl choline, dimyristoyl phosphatidylglycerol, and cholesterol in a 4:1:4 molar ratio. The endotoxic activities of these LPS-containing liposomes were less than 0.1% of the endotoxicities of the original free LPSs as measured by the Limulus amoebocyte lysate assay. In vivo administration of liposomal complete-core LPS mixed with Al(OH)(3) to rabbits resulted in no pyrogenicity or overt toxicity over a 7-day period. In immunoblots, sera from rabbits following active immunization elicited cross-reactive antibodies to a large panel of rough and smooth LPSs from numerous clinically relevant gram-negative bacteria, including E. coli (serotypes O1, O4, O6, O8, O12, O15, O18, O75, O86, O157, and O111), P. aeruginosa (Fisher-Devlin serotypes 1, 2, and 3, which correspond to International Antigenic Typing Scheme types 6, 11, and 2, respectively), Klebsiella pneumoniae (serotypes O1, O2ab, and O3), B. fragilis, and Bacteroides vulgatus. Active immunization of mice with liposomal complete-core LPS provided protection against a lethal challenge with E. coli O18 LPS. The vaccine tested was nontoxic, nonpyrogenic, and immunogenic against a wide variety of pathogens found in clinical settings.

The lipopolysaccharide barrier: correlation of antibiotic susceptibility with antibiotic permeability and fluorescent probe binding kinetics.

Lipopolysaccharide (LPS), the primary lipid on the surface of Gram-negative bacteria, is thought to act as a permeability barrier, making the outer membrane relatively impermeable to hydrophobic antibiotics, detergents, and host proteins. Mutations in the LPS biosynthetic apparatus increase bacterial susceptibility to such agents. To determine how this increased susceptibility is mediated, we have correlated antibiotic susceptibilities of rough (antibiotic resistant) and deep rough (antibiotic susceptible) bacterial strains with antibiotic permeabilities and fluorescent probe binding kinetics for bilayers composed of LPS purified from the same strains. Bilayer permeabilities of two hydrophobic beta-lactam antibiotics were measured by encapsulating the appropriate beta-lactamases in large unilamellar vesicles. In the presence of MgCl(2), permeabilities of LPS bilayers from rough and deep rough bacteria were similar and significantly lower than those of bacterial phospholipids (BPL). Addition of BPL to the LPS bilayers increased their antibiotic permeability to approximately the level of the BPL bilayers. Binding rates of the fluorescent probe bis-aminonaphthylsulfonic acid (BANS) were 2 orders of magnitude slower for both rough and deep rough LPS bilayers compared to that of bilayers composed of BPL or mixtures of LPS and BPL. On the basis of these results and the observation that deep rough bacteria have higher levels of phospholipid on their surface than do rough bacteria (Kamio, Y., and Nikaido, H. (1976) Biochemistry 15, 2561-2569), we argue that the high susceptibility of deep rough bacteria is due to the presence of phospholipids on their surface. Experiments with phospholipid bilayers showed that the addition of PEG-lipids (containing covalently attached hydrophilic polymers) had little effect on permeability and binding rates, whereas the addition of cholesterol reduced permeability and slowed binding to levels approaching those of LPS. Therefore, we argue that the barrier provided by LPS is primarily due to its tight hydrocarbon chain packing (Snyder et al., (1999) Biochemistry 38, 10758-10767) rather than to its polysaccharide headgroup.

Short-range interactions between lipid bilayers measured by X-ray diffraction.

Interactions between lipid bilayers are critical in many biological processes in which membrane surfaces come close together. Recent X-ray diffraction analyses of bilayers subjected to known osmotic pressures have provided critical information on the magnitude of both the repulsive and the attractive forces that exist between phospholipid and glycolipid membranes.

O-Alkyl dioleoylphosphatidylcholinium compounds: the effect of varying alkyl chain length on their physical properties and in vitro DNA transfection activity.

1,2-Dioleoyl-sn-3-ethylphosphocholine (EDOPC) has been previously shown be a highly effective DNA transfection reagent in vitro. To assess the effect of alkyl chain length on transfection efficiency, the O-methyl, O-propyl, O-hexyl, O-decyl, and O-octadecyl derivatives have been prepared from dioleoylphosphatidylcholine using the corresponding alkyl trifluoromethylsulfonate. The methyl, ethyl, and propyl derivatives formed liposomes which were very large and unilamellar. The ethyl and propyl derivatives were equally efficient at mediating transfection (even in the presence of serum) of BHK cells, but the chemically labile methyl derivative was a much weaker transfection agent. The O-decyl and O-octadecyl compounds, which assume the inverted hexagonal phase in excess water (as determined by X-ray diffraction), were almost inactive after manual agitation in both water and in saline; however, after sonication, these compounds exhibited good transfection activity. The O-hexyl derivative displayed novel behavior, assuming the lamellar phase at low and a cubic phase at high ionic strength. All compounds, whether lamellar or not, formed lamellar structures when complexed with DNA. In water, where the hexyl compound dispersed well, sonication diminished transfection activity, whereas at physiological ionic strength, which led to poor manual dispersion, sonication was essential for good transfection. These results emphasize the importance of optimal dispersion of a cationic lipid: too little, and interaction with DNA is handicapped, too much, and the resultant particle transfects poorly. Lipid dispersibility is thus an important variable in assessing lipid transfection agents, and caution is advised in attributing too much significance to chemical structure until interaction with DNA has been optimized.

Synthesis, structure, and thermal properties of 1,2-dipalmitoylgalloylglycerol (DPGG), a novel self-adhering lipid.

A novel diacyl glycerol-based lipid with a polyphenolic head group has been synthesized and characterized. X-ray diffraction experiments show that this lipid, 1,2-dipalmitoylgalloylglycerol (DPGG), hydrates to form gel phase bilayers at 20 degrees C with extremely narrow interbilayer fluid separations, indicating that apposing DPGG bilayers strongly adhere to each other. Differential scanning calorimetry shows that fully hydrated DPGG exhibits a pretransition exotherm (3.7 kcal/mol) at 52 degrees C and a high enthalpy (11.3 kcal/mol) main endothermic transition at 69 degrees C. These thermal properties are similar to those of galactosylceramides with similar hydrocarbon chain compositions. The adhesive and thermal properties of DPGG are likely due to both intermolecular hydrogen-bonding and hydrophobic interactions between the aromatic rings on the gallic acids.

Adhesion between cerebroside bilayers.

The structure, hydration properties, and adhesion energy of the membrane glycolipid galactosylceramide (GalCer) were studied by osmotic stress/X-ray diffraction analysis.(1) Fully hydrated GalCer gave a repeat period of 67 A, which decreased less than 2 A with application of applied osmotic pressures as large as 1.6 x 10(9) dyn/cm(2). These results, along with the invariance of GalCer structure obtained by a Fourier analysis of the X-ray data, indicated that there was an extremely narrow fluid space (less than the diameter of a single water molecule) between fully hydrated cerebroside bilayers. Electron density profiles showed that the hydrocarbon chains from apposing GalCer monolayers partially interdigitated in the center of the bilayer. To obtain information on the adhesive properties of GalCer bilayers, we incorporated into the bilayer various mole ratios of the negatively charged lipid dipalmitoylphosphatidylglycerol (DPPG) to provide known electrostatic repulsion between the bilayers. Although 17 and 20 mol % DPPG swelled (disjoined) the GalCer bilayers by an amount predictable from electrostatic double-layer theory, 5, 10, 13, and 15 mol % DPPG did not disjoin the bilayers. By calculating the magnitude of the electrostatic pressure necessary to disjoin the bilayers, we estimated the adhesion energy for GalCer bilayers to be about -1.5 erg/cm(2), a much larger value than that previously measured for phosphatidylcholine bilayers. The observed discontinuous disjoining with increased electrostatic pressure and this relatively large value for adhesion energy indicated the presence of an attractive interaction, in addition to van der Waals attraction, between cerebroside bilayers. Possible attractive interactions are hydrogen bond formation and hydrophobic interactions between the galactose headgroups of apposing GalCer bilayers.

Physical and biological properties of cationic triesters of phosphatidylcholine.

The properties of a new class of phospholipids, alkyl phosphocholine triesters, are described. These compounds were prepared from phosphatidylcholines through substitution of the phosphate oxygen by reaction with alkyl trifluoromethylsulfonates. Their unusual behavior is ascribed to their net positive charge and absence of intermolecular hydrogen bonding. The O-ethyl, unsaturated derivatives hydrated to generate large, unilamellar liposomes. The phase transition temperature of the saturated derivatives is very similar to that of the precursor phosphatidylcholine and quite insensitive to ionic strength. The dissociation of single molecules from bilayers is unusually facile, as revealed by the surface activity of aqueous liposome dispersions. Vesicles of cationic phospholipids fused with vesicles of anionic lipids. Liquid crystalline cationic phospholipids such as 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine triflate formed normal lipid bilayers in aqueous phases that interacted with short, linear DNA and supercoiled plasmid DNA to form a sandwich-structured complex in which bilayers were separated by strands of DNA. DNA in a 1:1 (mol) complex with cationic lipid was shielded from the aqueous phase, but was released by neutralizing the cationic charge with anionic lipid. DNA-lipid complexes transfected DNA into cells very effectively. Transfection efficiency depended upon the form of the lipid dispersion used to generate DNA-lipid complexes; in the case of the O-ethyl derivative described here, large vesicle preparations in the liquid crystalline phase were most effective.

Lipopolysaccharide bilayer structure: effect of chemotype, core mutations, divalent cations, and temperature.

Lipopolysaccharide (LPS), the primary lipid on the surface of Gram-negative bacteria, is thought to act as a protective and permeability barrier. X-ray diffraction analysis of osmotically stressed LPS multilayers was used to determine the structure and interactive properties of LPSs from strains containing the minimum number of sugars necessary for bacterial survival (Re chemotype) to the maximum number of sugars found in rough bacteria (Ra chemotype). At 20 degrees C in the absence of divalent cations, LPS suspensions gave a sharp wide-angle reflection at 4.23 A and a broad low-angle band centered at 50-68 A depending on the chemotype, indicating the presence of gel phase bilayers separated by large fluid spaces. As osmotic pressure was applied, the apposing bilayers were squeezed together and lamellar diffraction at 6 A resolution was obtained. At low applied pressures (<10(6) dyn/cm2), the total repulsive pressure between bilayers could be explained by electrostatic double layer theory. At higher applied pressures, there was a sharp upward break in each pressure-distance relation, indicating the presence of a hydrophilic steric barrier whose range depended strongly on the LPS chemotype. The positions of these upward breaks, along with electron density profiles, showed that the sugar core width systematically increased from 10 A for the Re chemotype to 27 A for the Ra chemotype. In excess buffer, the addition of divalent cations brought the bilayers into steric contact. Electron density profiles were used to determine the locations of cation binding sites and polar substituents on the LPS oligosaccharide core. The area per hydrocarbon chain was approximately 26 A2 in liquid-crystalline LPS bilayers, an indication of an acyl chain packing that is much tighter than that found in bilayers composed of typical membrane lipids. This unusually tight packing could be a critical factor in the permeability barrier provided by LPS.

Orientation of LamB signal peptides in bilayers: influence of lipid probes on peptide binding and interpretation of fluorescence quenching data.

The orientation in lipid bilayers of the signal sequence of the bacterial protein LamB was studied using binding, circular dichroism, and fluorescence quenching experiments. Measurements were made of binding modifications caused by the incorporation of lipid probes (brominated or nitroxide-labeled phospholipids) used in the parallax fluorescence quenching method of determining peptide penetration depth [Abrams, F. S., and London, E. (1992) Biochemistry 31, 5312-5322]. The signal peptide bound to a similar extent to neutral bilayers composed of either egg phosphatidylcholine (EPC) or phosphatidylcholines brominated at various positions on their acyl chains. The fluorescence of a tryptophan in either the 18 or 24 position of the peptide was quenched more by bromines in the 6 and 7 than in the 9 and 10 positions on the lipid hydrocarbon chain. Parallax calculations showed that tryptophan-18 was located only 4 A from the hydrocarbon-water interface, consistent with the peptide adopting a "hammock" configuration in the bilayer, with both termini exposed to the aqueous phase and the central alpha-helix located near the hydrocarbon-water interface. In contrast, the incorporation of 10% nitroxide-labeled lipids into EPC bilayers modified peptide binding in a manner dependent on the position of the nitroxide on the hydrocarbon chain; 7-Doxyl PC reduced the percent peptide bound by about one-half, whereas 12-Doxyl PC had little effect on binding. These binding differences modified tryptophan quenching by these probes, making parallax analysis problematical. In the presence of the positively charged LamB peptide, the incorporation of negatively charged phospholipids into EPC bilayers increased the level of peptide binding and modified tryptophan quenching by nitroxide probes. These results suggest that the nitroxide probe could be partially excluded from negatively charged lipid domains where the peptide preferentially bound. Quite different binding and quenching results were obtained with a negatively charged peptide analogue, showing that the charge on both the peptide and bilayer affects peptide-nitroxide probe interactions.

Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.

It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.

A new monofluorinated phosphatidylcholine forms interdigitated bilayers.

16-Fluoropalmitic acid was synthesized from 16-hydroxypalmitic acid using diethylaminosulfur trifluoride. This monofluorinated fatty acid then was used to make 1-palmitoyl-2-[16-fluoropalmitoyl]-phosphatidylcholine (F-DPPC) as a fluorinated analog of dipalmitoylphosphatidylcholine (DPPC). Surprisingly, we found that the phase transition temperature (Tm) of F-DPPC occurs near 50 degrees C, approximately 10 degrees C higher than its nonfluorinated counterpart, DPPC, as judged by both differential scanning calorimetry and infrared spectroscopy. The pretransition observed for DPPC is absent in F-DPPC. A combination of REDOR, rotational-echo double-resonance, and conventional solid-state NMR experiments demonstrates that F-DPPC forms a fully interdigitated bilayer in the gel phase. Electron paramagnetic resonance experiments show that below Tm, the hydrocarbon chains of F-DPPC are more motionally restricted than those of DPPC. X-ray scattering experiments confirm that the thickness and packing of gel phase F-DPPC is similar to that of heptanetriol-induced interdigitated DPPC. F-DPPC is the first phosphoglyceride containing sn-1 and sn-2 ester-linked fatty acyl chains of equal length that spontaneously forms interdigitated bilayers in the gel state in the absence of inducing agents such as alcohols.

Modulation of the binding of signal peptides to lipid bilayers by dipoles near the hydrocarbon-water interface.

Interactions between signal (leader) sequences and membranes are critical to protein insertion and translocation across membranes. In this paper, circular dichroism, tryptophan fluorescence, electrophoretic mobility, dipole potential, and binding measurements were used to study the interaction of the signal sequence of the Escherichia coli LamB protein with various lipid bilayers. By modifying specific chemicophysical properties of both the signal sequence and bilayer, we analyzed some of the key factors underlying peptide-lipid interactions. We synthesized three analogues of the LamB signal peptide differing in their net charge (-2 to +4) and studied their binding to bilayers containing combinations of neutral lipids [egg phosphatidylcholine (EPC), sphingomyelin, cholesterol, ketocholesterol, and nitroxide-containing phospholipid] and a charged lipid (phosphatidylserine). All three peptides bound to EPC bilayers and underwent a random coil to alpha-helix transition upon binding. Microelectrophoresis experiments revealed that both the N and C termini were near the outer surface of the bilayer, suggesting that the peptides adopted a "hammock" configuration with both termini exposed to the aqueous phase and the core of the alpha-helix located near the hydrocarbon-water interface. The binding of these LamB peptides was not markedly dependent on the bilayer area per molecule, compressibility modulus, or dipole potential, but did depend on the charge of the peptide and bilayer interfacial region. Moreover, the binding of LamB peptides was essentially eliminated in bilayers composed of phospholipids with a nitroxide moiety at the 7 position in one of their acyl chains or in EPC bilayers containing equimolar ketocholestanol. We propose that the incorporation of nitroxide or ketone groups into the hydrocarbon region near the lipid headgroup increases the effective width of the hydrophilic interfacial region and prevents some of the hydrophobic amino acids in the alpha-helix from reaching the nonpolar hydrocarbon core, thereby diminishing the free energy of partitioning and inhibiting peptide binding. These results point to an important role for interfacial dipoles in peptide-lipid interactions.