Configurations of fatty acyl chains in egg phosphatidylcholine-cholesterol mixed bilayers

Solubility Limits of Cholesterol, Lanosterol, Ergosterol, Stigmasterol, and β-Sitosterol in Electroformed Lipid Vesicles

Here we use nuclear magnetic resonance to measure the solubility limit of several biologically relevant sterols in electroformed giant unilamellar vesicle membranes containing phosphatidylcholine (PC) lipids in ratios of 1:1:X DOPC:DPPC:sterol. We find solubility limits of cholesterol, lanosterol, ergosterol, stigmasterol, and β-sitosterol to be 65-70%, ~35%, 30-35%, 20-25%, and ~40%, respectively. The low solubilities of stigmasterol and β-sitosterol, which differ from cholesterol only in their alkyl tails, show that subtle differences in tail structure can strongly affect sterol solubility. Below the solubility limits, the fraction of sterol to PC-lipid in electroformed vesicles linearly reflects the fraction in the original stock solutions used in the electroformation process.

Sterol structure determines miscibility versus melting transitions in lipid vesicles

Lipid bilayer membranes composed of DOPC, DPPC, and a series of sterols demix into coexisting liquid phases below a miscibility transition temperature. We use fluorescence microscopy to directly observe phase transitions in vesicles of 1:1:1 DOPC/DPPC/sterol within giant unilamellar vesicles. We show that vesicles containing the "promoter" sterols cholesterol, ergosterol, 25-hydroxycholesterol, epicholesterol, or dihydrocholesterol demix into coexisting liquid phases as temperature is lowered through the miscibility transition. In contrast, vesicles containing the "inhibitor" sterols androstenolone, coprostanol, cholestenone, or cholestane form coexisting gel (solid) and liquid phases. Vesicles containing lanosterol, a sterol found in the cholesterol and ergosterol synthesis pathways, do not exhibit coexisting phases over a wide range of temperatures and compositions. Although more detailed phase diagrams and precise distinctions between gel and liquid phases are required to fully define the phase behavior of these sterols in vesicles, we find that our classifications of promoter and inhibitor sterols are consistent with previous designations based on fluorescence quenching and detergent resistance. We find no trend in the liquid-liquid or gel-liquid transition temperatures of membranes with promoter or inhibitor sterols and measure the surface fraction of coexisting phases. We find that the vesicle phase behavior is related to the structure of the sterols. Promoter sterols have flat, fused rings, a hydroxyl headgroup, an alkyl tail, and a small molecular area, which are all attributes of "membrane active" sterols.

Dynamics of raft molecules in the cell and artificial membranes: approaches by pulse EPR spin labeling and single molecule optical microscopy

Lipid rafts in the plasma membrane, domains rich in cholesterol and sphingolipids, have been implicated in a number of important membrane functions. Detergent insolubility has been used to define membrane "rafts" biochemically. However, such an approach does not directly contribute to the understanding of the size and the lifetime of rafts, dynamics of the raft-constituent molecules, and the function of rafts in the membrane in situ. To address these issues, we have developed pulse EPR spin labeling and single molecule tracking optical techniques for studies of rafts in both artificial and cell membranes. In this review, we summarize our results and perspectives obtained by using these methods. We emphasize the importance of clearly distinguishing small/unstable rafts (lifetime shorter than a millisecond) in unstimulated cells and stabilized rafts induced by liganded and oligomerized (GPI-anchored) receptor molecules (core receptor rafts, lifetime over a few minutes). We propose that these stabilized rafts further induce temporal, greater rafts (signaling rafts, lifetime on the order of a second) for signaling by coalescing other small/unstable rafts, including those in the inner leaflet of the membrane, each containing perhaps one molecule of the downstream effector molecules. At variance with the general view, we emphasize the importance of cholesterol segregation from the liquid-crystalline unsaturated bulk-phase membrane for formation of the rafts, rather than the affinity of cholesterol and saturated alkyl chains. In the binary mixture of cholesterol and an unsaturated phospholipid, cholesterol is segregated out from the bulk unsaturated liquid-crystalline phase, forming cholesterol-enriched domains or clustered cholesterol domains, probably due to the lateral nonconformability between the rigid planar transfused ring structure of cholesterol and the rigid bend of the unsaturated alkyl chain at C9-C10. However, such cholesterol-rich domains are small, perhaps consisting of only several cholesterol molecules, and are short-lived, on the order of 1-100 ns. We speculate that these cholesterol-enriched domains may be stabilized by the presence of saturated alkyl chains of sphingomyelin or glycosphingolipids, and also by clustered raft proteins. In the influenza viral membrane, one of the simplest forms of a biological membrane, the lifetime of a protein and cholesterol-rich domain was evaluated to be on the order of 100 micro, again showing the short lifetime of rafts in an unstimulated state. Finally, we propose a thermal Lego model for rafts as the basic building blocks for signaling pathways in the plasma membrane.

1-[2-Hydroxy-3-octadecan-1'-oate]propyl-2'',2'',5'',5''-tetramethyl pyrolidine-N-oxyl-3''-carboxylate as a potential spin probe for membrane structure studies

The synthesis of a new minimum steric perturbing proxyl nitroxide, which is a derivative of glycerol and contains a stearic acid moiety, has been carried out. Its localization in model membrane L-alpha-dipalmitoyl phosphatidyl choline (DPPC) was ascertained with the help of ESR, DSC, 1H and 31P NMR techniques. The nitroxide was used for detecting the changes in the phase transition temperature of the model membranes in the presence and absence of drugs. The permeation of the vasodilating drug epinephrine has also been studied using this spin label. The results prove the potential applicability of the new spin probe in the spin labeling of biomembranes.

New fluorescent cholesterol analogs as membrane probes

New fluorescent cholesterol analogs, (22E, 20R)-3beta-hydroxy-23-(9-anthryl)-24-norchola-5,22-die ne (R-AV-Ch), and the 20S-isomer (S-AV-Ch) were synthesized, their spectral and membrane properties were characterized. The probes bear a 9-anthrylvinyl (AV) group instead of C22-C27 segment of the cholesterol alkyl chain. Computer simulations show that both of the probes have bulkier tail regions than cholesterol and predict some perturbation in the packing of membranes, particularly for R-AV-Ch. In monolayer experiments, the force-area behavior of the probes was compared with that of cholesterol, pure and in mixtures with palmitoyloleoyl phosphatidylcholine (POPC) and N-stearoyl sphingomyelin (SSM). The results show that pure R-AV-Ch occupies 35-40% more cross-sectional area than cholesterol at surface pressures below film collapse (0-22 mN/m); whereas S-AV-Ch occupies nearly the same molecular area as cholesterol. Isotherms of POPC or SSM mixed with 0.1 mol fraction of either probe are similar to isotherms of the corresponding mixtures of POPC or SSM with cholesterol. The probes show typical AV absorption (lambda 386, 368, 350 and 256 nm) and fluorescence (lambda 412-435 nm) spectra. Steady-state anisotropies of R-AV-Ch and S-AV-Ch in isotropic medium or liquid-crystalline bilayers are higher than the values obtained for other AV probes reflecting hindered intramolecular mobility of the fluorophore and decreased overall rotational rate of the rigid cholesterol derivatives. This suggestion is confirmed by time-resolved fluorescence experiments which show also, in accordance with monolayer data, that S-AV-Ch is better accommodated in POPC-cholesterol bilayers than R-AV-Ch. Model and natural membranes can be labeled by either injecting the probes via a water-soluble organic solvent or by co-lyophilizing probe and phospholipid prior to vesicle production. Detergent-solubilization studies involving 'raft' lipids showed that S-AV-Ch almost identically mimicked the behavior of cholesterol and that of R-AV-Ch was only slightly inferior. Overall, the data suggest that the AV-labeled cholesterol analogs mimic cholesterol behavior in membrane systems and will be useful in related studies.

Effect of cholesterol on molecular order and dynamics in highly polyunsaturated phospholipid bilayers

The effect of cholesterol on phospholipid acyl chain packing in bilayers consisting of highly unsaturated acyl chains in the liquid crystalline phase was examined for a series of symmetrically and asymmetrically substituted phosphatidylcholines (PCs). The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic structural properties of bilayers containing 30 mol % cholesterol. The bilayers were composed of symmetrically substituted PCs with acyl chains of 14:0, 18:1n9, 20:4n6, or 22:6n3, containing 0, 1, 4, or 6 double bonds, respectively, and mixed-chain PCs with a saturated 16:0 sn-1 chain and 1, 4, or 6 double bonds in the sn-2 chain. DPH excited-state lifetime was fit to a Lorentzian lifetime distribution, the center of which was increased 1-2 ns by 30 mol % cholesterol relative to the cholesterol-free bilayers. Lifetime distributions were dramatically narrowed by the addition of cholesterol in all bilayers except the two consisting of dipolyunsaturated PCs. DPH anisotropy decay was interpreted in terms of the Brownian rotational diffusion model. The effect of cholesterol on both the perpendicular diffusion coefficient D perpendicular and the orientational distribution function f(theta) varied with acyl chain unsaturation. In all bilayers, except the two dipolyunsaturated PCs, 30 mol % cholesterol dramatically slowed DPH rotational motion and restricted DPH orientational freedom. The effect of cholesterol was especially diminished in di-22:6n3 PC, suggesting that this phospholipid may be particularly effective at promoting lateral domains, which are cholesterol-rich and unsaturation-rich, respectively. The results are discussed in terms of a model for lipid packing in membranes containing cholesterol and PCs with highly unsaturated acyl chains.

The design, synthesis and transmembrane transport studies of a biomimetic sterol-based ion channel

A model sterol-based ion channel was rationally designed and synthesized. The potential ion channel is comprised of a tartrate-derived crown ether to which six steroids are appended. Macromolecule 1a was incorporated into phospholipid vesicles and shown to facilitate the transmembrane transport of sodium and lithium ions using alkali metal NMR spectroscopy.

Aggregates of saturated phospholipids at the air-water interface

Surface viscosities of phospholipid/steroid mixtures at the air-water interface were measured by means of an oscillating pendulum. Phospholipids studied included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-dipalmitoyl-sn-glycero-3-phosphodimethylethanolamine (DPPDME); 1,2-dipalmitoyl-sn-glycero-3-phosphomonomethylethanolamine (DPPMME); 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE); 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DPPG); 1-palmitoyl-2-elaidyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-linelaidyl-sn-glycero-3-phosphocholine. Each saturated phospholipid was studied in the presence of cholesterol, DPPC was also investigated in the presence of 5-androsten-3beta-ol; cholestanol; 5-cholestene;5alpha-cholesten-3beta-ol methyl ether; coprostanol; 7,(5alpha)-cholesten-3beta-ol; desmosterol; epicholestanol; lanosterol and lophenol. The surface viscosities of the trans-unsaturated phosphatidylcholines (PC) were undetectable by this method and in this respect they resembled cis- unsaturated PC. The surface viscosities of saturated phospholipids were very high but were reduced by low concentrations of steroids. Interpretation of the results for DPPC/cholesterol mixtures indicates that DPPC functions at the air-water interface as a one-dimensional linear aggregate. At 50 mN/m and 22 degrees C the average structure contains approximately 300 DPPC molecules. DPPDME acts similarly but DPPMME, DPPE and DPPG differ from DPPC in their response to cholesterol. All of the steroids examined except 5-androsten-3beta-ol and the lanosterol mixture paralleled cholesterol in their interaction with DPPC an indication that phospholipid/steroid interactions modifying surface viscosity have less stringent requirements for the steroid structure than interactions measured as condensation in surface area.

Computation of mixed phosphatidylcholine-cholesterol bilayer structures by energy minimization

The energetically preferred structures of dimyristoylphosphatidylcholine (DMPC)-cholesterol bilayers were determined at a 1:1 mole ratio. Crystallographic symmetry operations were used to generate planar bilayers of cholesterol and DMPC. Energy minimization was carried out with respect to bond rotations, rigid body motions, and the two-dimensional lattice constants. The lowest energy structures had a hydrogen bond between the cholesterol hydroxyl and the carbonyl oxygen of the sn-2 acyl chain, but the largest contribution to the intermolecular energy was from the nonbonded interactions between the flat alpha surface of cholesterol and the acyl chains of DMPC. Two modes of packing in the bilayer were found; in structure A (the global minimum), unlike molecules are nearest neighbors, whereas in structure B (second lowest energy) like-like intermolecular interactions predominate. Crystallographic close packing of the molecules in the bilayer was achieved, as judged from the molecular areas and the bilayer thickness. These energy-minimized structures are consistent with the available experimental data on mixed bilayers of lecithin and cholesterol, and may be used as starting points for molecular dynamics or other calculations on bilayers.

Proxyl nitroxide of lithocholic acid: a potential spin probe for model membranes

A new steroidal proxyl (2,2,5,5-tetramethylpyrrolidine-N-oxyl) nitroxide (SPN), with the proxyl nitroxide moiety in the pendant side chain of the steroid, has been synthesized. Its localization in lipid bilayers was ascertained with the help of 1H NMR and 31P NMR experiments. The effects of the nitroxide group in SPN incorporated into the bilayer on 13C relaxation times are interpreted qualitatively in terms of localization of the nitroxide group within the bilayer structure. The nitroxide SPN was used to monitor changes in membrane fluidity and permeability induced by local anaesthetics, mepivacaine and xylocaine and the antikeratinizing agent, azelaic acid. The results conclusively proved the applicability of the new steroidal proxyl nitroxide (SPN) as a potential spin probe for spin labeling studies.

Vitamin E and cancer prevention: recent advances and future potentials

Many animal and in vitro experiments have shown that the supplementation of diet with vitamin E within a certain dose range reduced the risk of chemical- and radiation-induced cancers. In vitro studies revealed that alpha-tocopheryl succinate (TS) induced differentiation and growth-inhibition in certain animal and human tumor cells in culture, whereas alpha-tocopherol (alpha-T), alpha-tocopheryl acetate (alpha-TA) and alpha-tocopheryl nicotinate (alpha-TN) were ineffective, alpha-TS also reduced basal and ligand-stimulated adenylate cyclase activity, and expression of c-myc and H-ras oncogenes in certain tumor cells in culture. The relative efficacy of various forms of vitamin E in cancer prevention in animal or human models has not been evaluated. Human epidemiologic studies utilizing retrospective and prospective case-control experimental designs are not suitable for evaluating the role of vitamin E in cancer prevention due to several inherent problems associated with these methodologies. Intervention trials utilizing vitamin E with appropriate biological and statistical rationales are most suitable for testing the role of vitamin E in cancer prevention in humans. Some human trials utilizing vitamin E alone or in combination with other nutrients are in progress.

Stereochemical and positional specificity of the lipase/acyltransferase produced by Aeromonas hydrophila

Aeromonas species secrete a glycerophospholipid-cholesterol acyltransferase (GCAT) which shares many properties with mammalian plasma lecithin-cholesterol acetyltransferase (LCAT). We have studied the stereochemical and positional specificity of GCAT against a variety of lipid substrates using NMR spectroscopy as well as other assay methods. The results show that both the primary and secondary acyl ester bonds of L-phosphatidylcholine can be hydrolyzed but only the sn-2 fatty acid can be transferred to cholesterol. The enzyme has an absolute requirement for the L configuration at the sn-2 position of phosphatidylcholine. The secondary ester bond of D-phosphatidylcholine cannot be hydrolyzed, and this lipid is not a substrate for acyl transfer. In contrast to the phospholipases, but similar to LCAT, the enzyme does not interact stereochemically with the phosphorus of phosphatidylcholine. In fact, the phosphorus is not required for enzyme activity, as GCAT will also hydrolyze monolayers of diglyceride, although at much lower rates.

Influence of phospholipid unsaturation on the cholesterol distribution in membranes

Over the last half decade, we have studied saturated and unsaturated phosphatidylcholine (PC)-cholesterol membranes, with special attention paid to fluid-phase immiscibility in cis-unsaturated PC-cholesterol membranes. The investigations were carried out with fatty acid and sterol analogue spin labels for which reorientational diffusion of the nitroxide was measured using conventional ESR technique. We also used saturation recovery ESR technique where dual probes were utilized. Bimolecular collision rates between a membrane-soluble square-planar copper complex,3-ethoxy-2-oxobutyraldehyde bis(N4,N4-dimethylthiosemicarbazonato)copper(II) (CuKTMS2) and one of several nitroxide radical lipid-type spin labels were determined by measuring the nitroxide spin-lattice relaxation time (T1). The results obtained in all these studies can be explained if the following model is assumed: 1) at physiological temperatures, fluid-phase micro-immiscibility takes place in cis-unsaturated PC-cholesterol membranes, which induces cholesterol-rich domains in the membrane due to the steric nonconformability between the rigid fused-ring structure of cholesterol and the 30 degrees bend at the cis double bond of the alkyl chains of unsaturated PC. 2) The cholesterol-rich domains are small and/or of short lifetime (10(-9) s to less than 10(-7) s). Our results also suggest that the extra space that is available for conformational disorder and accommodation of small molecules is created in the central part of the bilayer by intercalation of cholesterol in cis-unsaturated PC membrane due to the mismatch in the hydrophobic length and nonconformability between cis-unsaturated PC alkyl chains and the bulky tetracyclic ring of cholesterol.

Cholesterol and the cell membrane

Recent studies concerning cholesterol, its behavior and its roles in cell growth provide important new clues to the role of this fascinating molecule in normal and pathological states.

Lanosterol and cholesterol have different effects on phospholipid acyl chain ordering

2H nuclear magnetic resonance (2H-NMR) spectra of dioleoylphosphatidylcholine labelled at positions 9 and 10 in the acyl chains of the phospholipid were obtained in the presence of cholesterol and lanosterol. The spectra show in all cases three quadrupole splittings. One is due to the deuterium on position 10 of the sn-1 chain and another to the deuterium on position 10 of the sn-2 chain. The third deuterium quadrupole splitting arises from the deuterium at position 9 of both chains. Cholesterol, at increasing concentration, produces an increase in the quadrupole splitting from position 9, corresponding to an increase in order of that C-D bond segment arising from the inclusion of cholesterol in the membrane. Little effect is noted on the quadrupole splittings arising from position 10 of either chain. Lanosterol appears to have no effect on the quadrupole splittings from position 9. Lanosterol, likewise, has no effects on the quadrupole splittings from position 10 of both chains. These data therefore suggest little disorganization of the membrane structure due to the 14-methyl group. However, the 14-methyl group prevents lanosterol from causing the increase in motional order of the phospholipid hydrocarbon chains characteristic of cholesterol.

Steroid-lipid interactions in sonicated dipalmitoyl phosphatidyl choline vesicles: a steady-state and time-resolved fluorescence anisotropy study with all trans-1,6-diphenyl-1,3,5-hexatriene as probe

The effect of steroid chemical structure on steroid-lipid interaction is studied in sonicated dipalmitoyl phosphatidylcholine vesicles by steady-state and time-resolved fluorescence anisotropy techniques using diphenylhexatriene as probe. A large number of cholesterol derivatives and steroids, some of them being precursors involved in corticosteroids biosynthesis, has been studied. Addition of a polar group on the side chain in carbon-20 or 22 position evokes considerable difference for the interaction with the bilayer. The carbon-22 position appears to be of critical importance. This observation may be relevant for the functioning of mitochondria from steroid producing tissues, where a stereospecific hydroxylation of cholesterol at the carbon-22R position is required for pregnenolone synthesis.

Relationship between antifungal activity and inhibition of sterol biosynthesis in miconazole, clotrimazole, and 15-azasterol

The availability of Saccharomyces cerevisiae mutants which are defective in sterol biosynthesis makes it possible to determine whether the ability of several antifungal agents to inhibit cell growth is due to their effect on sterol production. 15-Aza-24-methylene-8,14-cholestadien-3 beta-ol (15-azasterol) is known to block the reduction of the sterol delta 14 bond following C-14 demethylation. This agent inhibits the growth of wild-type S. cerevisiae but does not inhibit the growth of a strain that is defective in the removal of the C-14 methyl group of lanosterol and in the introduction of the 5,6 double bond. 15-Azasterol does not inhibit the growth of a sterol auxotrophic strain growing on an exogenous supply of sterol. Therefore, the effect of 15-azasterol on sterol biosynthesis is clearly the cause of its ability to inhibit growth. On the other hand, growth inhibition by two imidazole antifungal agents, clotrimazole and miconazole, cannot be ascribed to their ability to prevent the removal of the C-14 methyl group of lanosterol, because they inhibit the growth of the sterol auxotrophic strain as well as that of the demethylase mutant.

Gel to liquid-crystalline transition temperatures of water dispersions of two pairs of positional isomers of unsaturated mixed-acid phosphatidylcholines

The gel to liquid-crystalline phase transition temperatures of dispersions of mixed-acid sn-1,2-lecithins which contain one unsaturated and one saturated fatty acid have been studied by differential scanning calorimetry. The temperature for 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (containing no reversed isomer) was -9.3 degrees C while that for 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (containing 8% of the reversed isomer) was -2.6 degrees C. The temperature for 2-oleoyl-1-stearoyl-sn-glycero-3-phosphocholine (containing 6% of the reversed isomer) was 6.3 degrees C while that for 1-oleoyl-2-stearoyl-sn-glycero-3-phosphocholine (containing 18% of the reversed isomer) was 8.6 degrees C. The differences in transition temperatures for the isomers of a pair containing the same two acids were consistent with those observed for positional isomers of saturated mixed-acid lecithins in that the isomer of the pair which had the longer fatty acid in the sn-1 position had the lower temperature. The phase transition temperatures of pairs of isomers containing palmitate and oleate at the sn-1 and -2 positions were different by at least 6.7 degrees C, while those containing stearate and oleate were different by at least 2.3 degrees C. Differences in the chain lengths of the fatty acids at the two positions of the glycerol appear to predominate over differences in the depths of the double bonds in the bilayer in determining the transition temperatures.

Erythrocyte water permeability. The effects of anesthetic alcohols and alterations in the level of membrane cholesterol

1. Treatment of human erythrocytes with anesthetic n-alkanols (pentanol, hexanol and hepatanol) results in a decrease in the osmotic water permeability of the red cell membrane. 2. The alcohol-induced changes in osmotic water permeability are proportional to the alcohol concentration and roughly parallel diphenylhexatriene that are induced by the alcohols. 3. Enrichment of the red cell membrane in cholesterol also results in a decrease in the osmotic water permeability. 4. Moderate depletion (9% or 40%) of membrane cholesterol is without effect on the osmotic water permeability, even though this treatment results in a significant increase in the rotational mobility of diphenylhexatriene in the membrane lipids.

Cholestryl-phosphoryl-choline in lipid bilayers

Cholesteryl-phosphoryl-choline (CPC), a hybrid between cholesterol and lecithin, is incorporated into sonicated liposomes and erythrocyte membranes similarly to cholesterol. The effect of CPC on lipid microviscosity and degree of order is smaller, but not significantly than that of cholesterol. It is proposed that CPC may be employed as an efficient modulator of lipid dynamics.

Nuclear magnetic resonance studies in liposomes: effects of steroids on lecithin fatty acyl chain mobility

The effects of fourteen sterols on the NMR spectra of liposomes derived from egg yolk phosphatidylcholines were studied by continuous-wave and Fourier-transform measurements at 60 MHz. Sterols were compared for their ability to broaden the acyl methylene resonances of phosphatidylcholine, when incorporated into liposomes at 25% molar ratio. The ratio of the phosphatidylcholine peake heights (acyl methylene: choline N-methyl) was used as a criterion of the relative condensing activity for the different sterols. This ratio was inversely proportional to the molar volume of the incorporated sterol, as measured by the parachor of the compound. Small sterols had little condensing effect, and the larger sterols such as cholesterol and ergosterol had maximum condensing effects. The study confirmed the importance of the sterol side-chain at C-17 as a requirement for sterol-phospholipid interaction.

A structural model for the cholesterol-phosphatidylcholine complexes in bilayer membranes

Based on the structural properties of phospholipid and cholesterol molecules, and making use of the known structural and motional effects of cholesterol and its analogs on phospholipid bilayers, a model for the cholesterol-phosphatidylcholine complex is proposed. In this model, the 3beta-hydroxyl group of cholesterol is assumed to engage in hydrogen bonding with the carbonyl oxygen of the fatty acyl groups in phospholipids. Some specific configurations of the saturated and unsaturated fatty acyl chains of the phospholipid are suggested to participate in van der Waals attractive interactions with the apha and beta surface of the steroid nucleus.