Phosphatidylethanolamine Headgroups Research Articles

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.

Cholesterol dynamics in membranes

Time-resolved fluorescence anisotropy of the sterol analogue, cholestatrienol, and 13C nuclear magnetic resonance (NMR) spin lattice relaxation time (T1c) measurements of [13C4] labeled cholesterol were exploited to determine the correlation times characterizing the major modes of motion of cholesterol in unsonicated phospholipid multilamellar liposomes. Two modes of motion were found to be important: (a) rotational diffusion and (b) time dependence of the orientation of the director for axial diffusion, or "wobble." From the time-resolved fluorescence anisotropy decays of cholestatrienol in egg phosphatidylcholine (PC) bilayers, a value for tau perpendicular, the correlation time for wobble, of 0.9 x 10(-9) s and a value for S perpendicular, the order parameter characterizing the same motion, of 0.45 s were calculated. Both tau perpendicular and S perpendicular were relatively insensitive to temperature and cholesterol content of the membranes. The T1c measurements of [13C4] labeled cholesterol did not provide a quantitative determination of tau parallel, the correlation time for axial diffusion. T1c from the lipid hydrocarbon chains suggested a value for tau perpendicular similar to that for cholesterol. Steady-state anisotropy measurements and time-resolved anisotropy measurements of cholestatrienol were used to probe sterol behavior in a variety of pure and mixed lipid multilamellar liposomes. Both the lipid headgroups and the lipid hydrocarbons chains contributed to the determination of the sterol environment in the membrane, as revealed by these fluorescence measurements. In particular, effects of the phosphatidylethanolamine (PE) headgroup and of multiple unsaturation in the lipid hydrocarbon chains were observed. However, while the steady-state anisotropy was sensitive to these factors, the time-resolved fluorescence analysis indicated that tau perpendicular was not strongly affected by the lipid composition of the membrane. S perpendicular may be increased by the presence of PE. Both steady-state anisotropy measurements and time-resolved anisotropy measurements of cholestatrienol were used to probe sterol behavior in three biological membranes: bovine rod outer segment (ROS) disk membranes, human erythrocyte plasma membranes, and light rabbit muscle sarcoplasmic reticulum membranes. In the ROS disk membranes the value for S perpendicular was marginally higher than in the PC membranes, perhaps reflecting the influence of PE. The dramatic difference noted was in the value for tau perpendicular. In both the ROS disk membranes and the erythrocyte membranes, tau perpendicular was one-third to one-fifth of tau perpendicular in the phospholipid bilayers. This result may reveal an influence of membrane proteins on sterol behavior.

Analysis of the membrane-interacting domain of the sea urchin sperm adhesive protein bindin.

We have investigated the domain of the bindin polypeptide that selectively associates with gel-phase phospholipid vesicles. We found that small trypsin fragments of bindin retain the ability to selectively associate with gel-phase vesicles. The primary amino acid sequence of bindin suggests that these peptides are derived from the central portion of the polypeptide between residues 77 and 126, which is the most hydrophobic region of bindin. We have also employed 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and novel, radioiodinated, photoactivatable derivatives of the polar head group of phosphatidylethanolamine (ASD-PE and ASA-PE) to identify membrane-associated polypeptide segments after the transfer of radiolabel from the probe to the bindin polypeptide. After photolysis, bindin was selectively labeled only from probes incorporated in gel-phase vesicles. The labeling of bindin was much more efficient from the head group probes ASA-PE and ASD-PE (8 and 2% of the total label, respectively) in comparison to the hydrophobic probe TID (less than 0.02% of the total label), suggesting that bindin is localized within the polar part of the bilayer. Protease mapping experiments with V8 protease, trypsin, and endoprotease Lys-C suggest that some of the probe label is distributed along the amino-terminal portion of bindin between residues 1 and 76 and the rest of the label is restricted to the segments between residues 77 and 126 which also selectively bind to gel-phase vesicles. The carboxyl-terminal portion of bindin between residues 127 and 236 is not labeled.

A liposomal MRI contrast agent: phosphatidylethanolamine-DTPA.

The chelating agent, diethylenetriaminepentaacetic acid (DTPA), was attached via one -COOH group to the amino headgroup of phosphatidylethanolamine to produce a phospholipid which is also a powerful chelating agent. It readily assembles into the walls of lipid bilayer structures as a liposome-associated carrier of cations for MR contrast or radioisotope studies. Freeze-etch electron microscopy showed that phosphatidylethanolamine-DTPA formed satisfactory sonicated vesicles when mixed with natural phospholipids at up to 50 wt%. The resultant structures with bound gadolinium effectively shortened T1 and T2 of surrounding water protons. When sonicated liposomes bearing chelating agent with bound 111In3+ were injected intravenously into rats, uptake was primarily by liver and spleen. By 24 h postinjection there was biliary excretion of this material. Phosphatidylethanolamine-DTPA may have some general utility as an amphiphilic liposomal chelating agent for polyvalent cations.

A 2H-NMR study on the glycerol backbone of phospholipids extracted from Escherichia coli grown under high osmotic pressure: evidence for multiconformations of phosphatidylethanolamine

A glycerol-requiring auxotroph was isolated from mutagenized Escherichia coli K-12 UFA ts cells. This auxotroph was used for the specific deuteration of E. coli phospholipids. The cells were grown under high osmotic pressure (in the presence of 2.0% KCl). The membrane had a highly saturated fatty acid composition (76% phosphatidylethanolamine, 20% cardiolipin and 4% phosphatidylgycerol). The deuterium magnetic resonance spectra of coarse liposomes of the extracted phospholipids with perdeuterated glycerol incorporated into them were measured. To obtain well characterized information, phospholipid mixtures reconstituted from the deuterated and nondeuterated components at the same ratios as in the case of the total extract were used. On the analysis of the spectra, the following conclusions were drawn. (1) The whole polar region of cardiolipin is dynamically symmetric and quite rigid in the presence of phosphatidylethanolamine. (2) Although the quadrupole splittings of the deuterons at the C-2 and C-3 positions of the glycerol backbone were similar to each other, those at the C-1 position for phosphotidylethanolamine and cardiolipin are different, even in the same bilayer. (3) Furthermore, each C-1 deuteron of phosphatidylethanolamine gave rise to a doublet, suggesting the presence of two backbone conformations, between which there is slow exchange. (4) The polar head group of phosphatidylethanolamine interacts with cardiolipin and phosphatidylglycerol in different ways, which could be responsible for the different osmotic properties of the vesicles composed of them.

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.

The structure of human complement component C7 and the C5b-7 complex.

The molecular architecture of human complement component C7 was elucidated at several structural levels. The complete primary structure of C7 was derived from the cDNA sequence of clones isolated from a human liver library. C7 is a mosaic protein that consists of 821 amino acids. The amino-terminal two-thirds of C7 has 23-30% homology with complement components C8 and C9. In addition, the carboxyl-terminal third contains four cysteine-rich segments that have overlapping internal homology. The protein is a single polypeptide chain with 28 disulfide bonds and is glycosylated at two sites. Virtually all the cysteines are found in small units of 35-77 amino acids that exhibit homology with those of various proteins including the low density lipoprotein receptor, epidermal growth factor precursor, thrombospondin, and blood coagulation factors IX and X. The secondary structural analysis, estimated by circular dichroism, suggested a high content of beta-sheet (38%) and beta-turns (24%). The tertiary structure, visualized by transmission electron microscopy, indicated a flexible elongated molecule with dimensions of 151 X 59 X 43 A. The quaternary structure of the C5b-7 complex bound to lipid vesicles was observed to be in the form of monomers or dimers. The monomer C5b-7 consists of a leaflet and a long flexible stalk, and the dimer has two leaflets linked through a supercoiled stalk. Membrane binding is mediated by the stalk part of the complexes. Using a radioiodinated photoreactive cross-linking reagent bound to the polar head group of phosphatidylethanolamine, the stalk part of the C5b-7 complex could be labeled preferentially, and it was found to consist mainly of C6 and C7. Thus, C7 plays a major role in bringing about the hydrophilic-amphiphilic transition during the formation of the membrane attack complex, and it serves as a membrane anchor for the C5b-7 complex.

Purification of phosphatidylethanolamine N-methyltransferase from rat liver.

Phosphatidylethanolamine (PE) N-methyltransferase catalyzes the synthesis of phosphatidylcholine by the stepwise transfer of methyl groups from S-adenosylmethionine to the amino head group of PE. PE N-methyltransferase was solubilized from a microsomal membrane fraction of rat liver using the nonionic detergent Triton X-100 and purified to apparent homogeneity. Specific activities of PE N-methyltransferase with PE, phosphatidyl-N-monomethylethanolamine (PMME), and phosphatidyl-N,N-dimethylethanolamine (PDME) as substrates were 0.63, 8.59, and 3.75 mumol/min/mg protein, respectively. The purified enzyme was composed of a single subunit with a molecular mass of 18.3 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Methylation activities dependent on the presence of PE, PMME, and PDME and the 18.3-kDa protein co-eluted when purified PE N-methyltransferase was subjected to gel filtration on Sephacryl S-300 in the presence of 0.1% Triton X-100. All three methylation activities eluted with a Stokes radius 2.1 A greater than that determined for pure Triton micelles (molecular mass difference of 27.4 kDa). Two-dimensional analysis of PE N-methyltransferase employing nonequilibrium pH gradient gel electrophoresis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that the enzyme is composed of a single isoform. Analysis of enzyme activity using PE, PMME, and PDME at various Triton X-100 concentrations indicated the enzyme follows the "surface dilution" model proposed for other enzymes that act at the surface of mixed micelle substrates. Initial velocity data for all three lipid substrates (at fixed concentrations of Triton X-100) were highly cooperative in nature. Hill numbers for PMME and PDME ranged from 3 at 0.5 mM Triton to 6 at 2.0 mM Triton. All three methylation activities had a pH optimum of 10. These results provide evidence that a single membrane-bound enzyme catalyzes all three methylation steps for the conversion of PE to phosphatidylcholine.

Structure and dynamics of the phosphatidylcholine and the phosphatidylethanolamine head group in L-M fibroblasts as studied by deuterium nuclear magnetic resonance.

Mouse fibroblast L-M cells were grown in tissue culture medium containing selectively deuterated choline or ethanolamine. Both compounds were incorporated into the corresponding phospholipids at levels greater than 50% thus leading to a selective deuteration of these phospholipid head groups. Choline and ethanolamine were labeled at either the alpha- or the beta-carbon atom and well-resolved deuterium and phosphorus n.m.r. spectra were obtained from intact cells, crude plasma membranes and lipid extracts, leading to the following conclusions. (i) A large fraction, if not all, of the phospholipids in the intact L-M cell membranes were organized in a liquid crystalline bilayer. (ii) The phosphoethanolamine and the phosphocholine head group conformation were found to be remarkably similar in pure lipid bilayers and in intact L-M cell membranes with the head group dipoles being oriented parallel to the membrane surface. (iii) The deuterium T1 spin lattice relaxation times fell in the range of 7-25 ms and were similar in intact L-M cells and in pure lipid model membranes, suggesting that the two head groups are not involved in strong interactions with membrane proteins. The rotational diffusion rate of the two head groups was reduced by at least a factor of 10 compared to molecules of the same size in aqueous solution. (iv) The phosphocholine head group was sensitive to the size and sign of membrane surface charges as verified in mixing experiments with charged lipids. In L-M cell membranes the phosphocholine appeared to sense an electrically neutral environment in spite of the fact that L-M cell membranes contain 10-20% negatively charged lipids.

Lymphocyte and granulocyte phosphatidylethanolamine N-methyltransferase: properties and activity in cystic fibrosis.

Human lymphocyte and granulocyte membranes contain an enzyme, phosphatidylethanolamine N-methyltransferase (PEMT), which catalyzes the transfer of a methyl group from S-adenosylmethionine to the polar head group of phosphatidylethanolamine to form phosphatidylmonomethylethanolamine. This enzyme, in lymphocyte membranes, has Km for S-adenosylmethionine of 7.01 +/- 2.9 (SD) microM, and specific activity 0.57 +/- 0.31 pmol/mg protein/15 min, is inhibited by S-adenosylhomocysteine, displays optimal activity at pH 8.0-9.0, and is stimulated by isoproterenol in dose-dependent, propranolol-inhibitable fashion, to a lesser extent by epinephrine, but not by norepinephrine, prostaglandin E1, concanavalin A, or adenosine 3':5' cyclic monophosphate, with or without phosphodiesterase inhibitors. Granulocyte membrane PEMT has Km for S-adenosylmethionine of 4.4 microM and specific activity 0.54 +/- 0.51 pmol/mg protein/15 min, is inhibited by S-adenosylhomocysteine, displays optimal activity at pH 8.0-9.5, and is stimulated by isoproterenol greater than epinephrine greater than norepinephrine, but not by prostaglandin E1, serum-treated zymosan, formyl-methionyl-leucyl-phenylalanine, or adenosine 3':5' cyclic monophosphate. Because activation of PEMT reportedly contributes to several processes known to be abnormal in cystic fibrosis, including coupling of the beta-adrenergic receptor to adenylate cyclase, activity of PEMT was compared in lymphocyte and granulocyte membrane preparations from cystic fibrosis patients and healthy controls, in which abnormal coupling of beta-adrenergic receptor to adenylate cyclase had been demonstrated. For both cell types, the Km and specific activity of PEMT were comparable in normal and cystic fibrosis samples. Therefore, the hypothesis that reduced PEMT activity accounts for the impaired coupling of beta-adrenergic receptor to adenylate cyclase in lymphocytes and granulocytes in cystic fibrosis is rejected.

Relation between lipid polymorphism and transbilayer movement of lipid in rat liver microsomes

We have studied the effects of trinitrophenylation on the transbilayer movement of phosphatidylcholine and the macroscopic lipid structure in rat liver microsomal membranes. The transbilayer movement of phosphatidylcholine was investigated using the PC-specific transfer protein. 31P-NMR was employed to monitor the phospholipid organization in intact microsomal vesicles. The results indicate that modification of microsomes with trinitrobenzenesulfonic acid enhances the transbilayer movement of phosphatidylcholine at 4°C. Furthermore, phosphatidylethanolamine headgroup trinitrophenylation in microsomes increases the isotropic component in the 31P-NMR spectra even at 4°C, possibly representing the appearance of intermediate non-bilayer lipid structures. The observed parallel between these data suggests that phosphatidylethanolamine molecules in the microsomal membrane, probably in combination with a protein component, are able to destabilize the bilayer organization, thereby facilitating the transmembrane movement of phospholipids.

Behaviour of water at membrane surfaces - a molecular dynamics study

The molecular dynamics method was used to study the structure and dynamics of water above a layer of phosphatidylethanolamine headgroups, modelling the surface of a membrane. The simulation box contains one headgroup with sixteen ST2-water molecules. Planar periodic boundary conditions model a headgroup-water interface. The density distribution and the orientation profile along the layer normal were calculated. The water builds highly structured regions up to a ratio of ten water molecules per headgroup which compensate the dipole moments of the lipids. The velocity, angular velocity and direction autocorrelation functions show that the bound water molecules oscillate around minimum potential positions at preferred hydration sites.

Behaviour of water at membrane surfaces — a molecular dynamics study

The molecular dynamics method was used to study the structure and dynamics of water above a layer of phosphatidylethanolamine headgroups, modelling the surface of a membrane. The simulation box contains one headgroup with sixteen ST2-water molecules. Planar periodic boundary conditions model a headgroup—water interface. The density distribution and the orientation profile along the layer normal were calculated. The water builds highly structured regions up to a ratio of ten water molecules per headgroup which compensate the dipole moments of the lipids. The velocity, angular velocity and direction autocorrelation functions show that the bound water molecules oscillate around minimum potential positions at preferred hydration sites.

Evidence for conduction of protons along the interface between water and a polar lipid monolayer.

Movements of H+ along the polar heads of phospholipids spread in monolayers were compared to movements of H+ in the aqueous subphase. The probe for detecting H+ movement along the monolayer was a pH-sensitive fluorescein chromophore covalently bound to the head group of phosphatidylethanolamine. The behavior of this probe was not affected by the electrical properties of the lipid/water interface. Lateral diffusion of H+ along the phospholipid/water interface was then studied by acid-jump experiments in which advantage was taken of the large size of the monolayer. H+ was injected a few centimeters away from the probe observation area. The time needed for H+ diffusion to the probe was monitored by the change in the fluorescence signal, fluorescein being nonfluorescent in an acid medium. Diffusion of H+ in the bulk phase was monitored by the fluorescence change of water-soluble fluorescein isothiocyanate. Diffusion along the lipid monolayer was found to be 20 times faster than in the bulk water phase and required a structured monolayer in order to occur, as revealed by variation of the molecular area occupied by the lipid molecules. The molecular basis of rapid H+ transfer along the lipid monolayer may be the existence of a hydrogen-bond network along the polar heads, capable of supporting a rapid "hop and turn" of H+.

Synthesis of a photoactivable phospholipid containing an aromatic sulfonylazide and its interaction with proteins

The photoactivable compound, fluorobenzenesulfonylazide (FBSA) was prepared and the structure and stability verified by gas chromatographic-mass spectral analysis, infrared spectroscopy and elemental analysis. The FBSA was reacted at the amino headgroup of 3H-labeled phosphatidylethanolamine (PE) to produce the photoactivable phospholipid, 3H-labelled 1,2- diacylglycero-3-(N-4- sulfonylazidophenylaminoethyl) phosphate at a yeild of approx. 30%. Activation of the photoactivable phospholipid was accomplished with shortwave ultraviolet light as opposed to longwave ultraviolet light in which the sulfonylazide was unreactive. The [ 3H]PE-benzenesulfonylazide was partitioned at nmol levels into low-density lipoprotein, high-density lipoprotein and retinal rod disc membranes. Photolysis experiments coupled with SDS-polyacrylamide gel electrophoresis fractionation showed the [ 3H]PE-benzenesulfonylazide to crosslink to apolipoprotein B of low-density lipoprotein, apolipoprotein AI of high-density lipoprotein and rhodopsin of the disc membranes.

Covalent linkage of a synthetic peptide to a fluorescent phospholipid and its incorporation into supported phospholipid monolayers

A number of fluorescent peptide-lipid conjugates have been synthesized. Peptides with ten or eleven amino acids are linked through a single lysine residue to the headgroup of phosphatidylethanolamine, fluorescently labelled on one acyl chain, using homobifunctional disuccinimidyl crosslinking reagents. Peptide-lipids can be further derivatized with the hapten dinitrophenyl. Purified peptide-lipids have been incorporated into dimyristoylphosphatidylcholine monolayers at the interface of air and phosphate-buffered saline, at concentrations of up to 11 mol%. For equal average molecular areas, monolayers containing peptide-lipids have higher surface pressures than pure lipid monolayers; for equal surface pressures, peptide-lipid monolayers are have higher average molecular areas than pure lipid monolayers. When the peptide-lipid monolayers are transferred to hydrophobic glass slides, the fluorescence appears uniformly distributed. Fluorescence recovery after photobleaching measurements indicate that peptide-lipids diffuse in the monolayer with coefficient 1.5 · 10 −9 cm 2/s, which is much smaller than that of typical lipids in fluid membranes. In addition, the diffusion coefficient of peptide-lipids decreases with increasing peptide-lipid concentration. We conclude that the peptide portion of the peptide-lipid associates with the lipid monolayer and/or that peptide-lipids oligomerize.

Quantum-chemical and empirical calculations on Phospholipids VIII. The electrostatic potential from isolated molecules up to layer systems

Using 1-6-12 empirical functions with a solvent-averaged electrostatic contribution q Iq j ε(r Ij) × r Ij and electrostatic potentials from CNDO-type wavefunctions, the development of specific interactions of ions visualized by the molecular electrostatic potential of PO 4-group containing molecules was studied. Going from single molecules to monolayers made up of 37 head groups of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) for quantum-chemical calculations, or of 23 head groups for empirical calculations we found decreasing potential minima. Only the inclusion of the screening effect of water, simulated by a distance dependent dielectric constant, ε( r), gives an explanation of stereospecific interactions of model membranes with ions. This finding can be compared with results of simulation calculations on water structure above a PE head group layer.

Selective 31P{ 1H} nuclear overhauser effect study on the polar headgroup conformation of phospholipids in micelles in organic solvents

The polar headgroup structure of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS) in inverted micelles in chloroform or benzene was investigated by the selective 31P{ 1H} nuclear Overhauser effect (NOE). In the frequency dependence of the 31P{ 1H} NOE, PC micelles in CDCl 3 showed two maxima. The larger maximum was located at the resonance of the glycerol-CH 2OP protons and the smaller at the resonance of the N-methyl protons. In PC/PE mixed micelles in C 6D 6, both PC and PE showed three maxima which were located at the resonance of the CH 2OP protons, the N-methyl protons and the amino protons in the frequency dependence of the 31P-NOE. The N-methyl protons of PC and the amino protons of PE were closely spaced to the phosphate groups of neighboring lipid molecules. The polar headgroups of PC and PE in the mixed micelles were concluded to lie in the plane perpendicular to the molecular axes. The frequency dependence of the 31P{ 1H} NOE for PS micelles in C 6D 6 showed the maxima at the resonances of the amino protons and the CH 2OP protons. The polar headgroups of PS molecules were not extended parallel to the molecular axes in the inverted micelles.

Electrical interactions in phospholipid layers

The electrical interactions between the head groups of the phospholipids phosphatidyl choline and ethanolamine when packed in the natural ordered array are examined. It is shown that Langmuir films of these molecules are likely to have a domain structure in which the head group P --N + dipoles form arrays with a net dipole moment. Taking account of dipole-dipole interactions and of pH and counterion-dependent head group charge, it is possible to calculate the Langmuir pressure-area isotherms and to predict expanded-condensed film transitions. An electric field normal to the plane of the arrays induces repulsive dipole-dipole forces in the plane which, when sufficiently large, can rupture the structure. In bilayer form the effect of the field leads to a prediction of switching phenomena which, it is believed, have been observed experimentally.