Deuterium NMR study of head-group deuterated phosphatidylserine in pure and binary phospholipid bilayers. Interactions with monovalent cations Na+ and Li+

Transmembrane potential of physiologically relevant model membranes: Effects of membrane asymmetry

Transmembrane potential difference (V_m) plays important roles in regulating various biological processes. At the macro level, V_m can be experimentally measured or calculated using the Nernst or Goldman-Hodgkin-Katz equation. However, the atomic details responsible for its generation and impact on protein and lipid dynamics still need to be further elucidated. In this work, we performed a series of all-atom molecular dynamics (MD) simulations of symmetric model membranes of various lipid compositions and cation contents to evaluate the relationship between membrane asymmetry and V_m. Specifically, we studied the impact of the asymmetric distribution of POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine), PIP2 (phosphatidylinositol 4,5-bisphosphate), as well as Na⁺ and K⁺ on V_m using atomically detailed MD simulations of symmetric model membranes. The results suggest that, for an asymmetric POPC-POPC/POPS bilayer in the presence of NaCl, the presence of the monovalent anionic lipid POPS in the inner leaflet polarizes the membrane (ΔV_m < 0). Intriguingly, replacing a third of the POPS lipids by the polyvalent anionic signaling lipid PIP2 counteracts this effect, resulting in a smaller negative membrane potential. We also found that replacing Na⁺ ions in the inner region by K⁺ depolarizes the membrane (ΔV_m > 0). These divergent effects arise from variations in the strength of cation-lipid interactions and are correlated with changes in lipid chain order and head-group orientation.

Association of alkali metal cations with phosphatidylcholine liposomal membrane surface

Interactions of alkali metal cations (Li⁺, Na⁺, K⁺, Cs⁺) with phosphatidylcholine (PC) liposomal membranes were investigated through experimental studies and theoretical considerations. Using a microelectrophoresis technique, charge densities of experimental membrane surfaces were measured as a function of the pH of electrolyte solutions. Equilibria between the PC liposomal membranes and monovalent ions were mathematically analyzed and described quantitatively through a previously proposed theoretical model. Association constants between functional groups of PC and the studied ions were determined and used to define theoretical curves of membrane surface charge density versus pH. Theoretical and experimental data were compared to verify the model. The PC membrane was found to have the highest affinity for lithium ions, among the ions tested.

The effect of pH on the electrical capacitance of phosphatidylcholine-phosphatidylserine system in bilayer lipid membrane

This paper reports measurements on the pH dependence of the electrical capacitance of lipid membranes formed by 1:1 phosphatidylcholine-phosphatidylserine mixtures. A theoretical model was developed to describe this dependence, in which the contributions of functional groups (as the active centers of adsorption of the hydrogen and hydroxide ions) to the overall membrane capacitance were assumed to be additive. The proposed model was verified experimentally using electrochemical impedance spectroscopy. The theoretical predictions agreed with the experimental results over the measured pH range. A minimum corresponding to the isoelectric point appeared in both the theoretical equation and the experimental data.

Synthesis and thermotropic phase behavior of four glycoglycerolipids

Four glycoglycerolipids with different head groups have been synthesized and their physicochemical properties studied. The lengths of the head groups from a mono-saccharide to a trisaccharide, in addition to the anomeric stereochemistry for the smaller glycoglycerolipids, have been modified. The synthesis has been optimized to avoid glycerol epimerization and to allow up-scaling. The physicochemical properties of the glycoglycerolipids were studied and a strong de-mixing of the gel-phase, depending on the head-group, was observed.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Calorimetric and spectroscopic studies of the thermotropic phase behavior of lipid bilayer model membranes composed of a homologous series of linear saturated phosphatidylserines

The thermotropic phase behavior of lipid bilayer model membranes composed of the even-numbered, N-saturated 1,2-diacyl phosphatidylserines was studied by differential scanning calorimetry and by Fourier-transform infrared and (31)P-nuclear magnetic resonance spectroscopy. At pH 7.0, 0.1 M NaCl and in the absence of divalent cations, aqueous dispersions of these lipids, which have not been incubated at low temperature, exhibit a single calorimetrically detectable phase transition that is fully reversible, highly cooperative, and relatively energetic, and the transition temperatures and enthalpies increase progressively with increases in hydrocarbon chain length. Our spectroscopic observations confirm that this thermal event is a lamellar gel (L(beta))-to-lamellar liquid crystalline (L(alpha)) phase transition. However, after low temperature incubation, the L(beta)/L(alpha) phase transition of dilauroyl phosphatidylserine is replaced by a higher temperature, more enthalpic, and less cooperative phase transition, and an additional lower temperature, less enthalpic, and less cooperative phase transition appears in the longer chain phosphatidylserines. Our spectroscopic results indicate that this change in thermotropic phase behavior when incubated at low temperatures results from the conversion of the L(beta) phase to a highly ordered lamellar crystalline (L(c)) phase. Upon heating, the L(c) phase of dilauroyl phosphatidylserine converts directly to the L(alpha) phase at a temperature slightly higher than that of its original L(beta)/L(alpha) phase transition. Calorimetrically, this process is manifested by a less cooperative but considerably more energetic, higher-temperature phase transition, which replaces the weaker L(beta)/L(alpha) phase transition alluded to above. However, with the longer chain compounds, the L(c) phase first converts to the L(beta) phase at temperatures some 10-25 degrees C below that at which the L(beta) phase converts to the L(alpha) phase. Our results also suggest that shorter chain homologues form L(c) phases that are structurally related to, but more ordered than, those formed by the longer chain homologues, but that these L(c) phases are less ordered than those formed by other phospholipids. These studies also suggest that polar/apolar interfaces of the phosphatidylserine bilayers are more hydrated than those of other glycerolipid bilayers, possibly because of interactions between the polar headgroup and carbonyl groups of the fatty acyl chains.

An electron spin resonance study of interactions between phosphatidylcholine and phosphatidylserine in oriented membranes

A detailed electron spin resonance (ESR) study of mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and phosphatidylserine (POPS) in oriented multilayers in the liquid crystalline phase is reported with the purpose of characterizing the effects of headgroup mixing on the structural and dynamical properties of the acyl chains. These studies were performed over a range of blends of POPC and POPS and temperatures, utilizing the spin-labeled lipids 16-phosphatidylcholine and 5-phosphatidylcholine as well as cholestane (CSL). The ESR spectra were analyzed by nonlinear least-squares fitting using detailed spectral simulations. Whereas CSL shows almost no variation in ordering and rotational dynamics versus mole fraction POPS, (i.e. XPS), and 5-PC shows small effects, the weakly ordered end-chain labeled 16-PC shows large relative effects, such that the orientational order parameter, S is at a minimum for XPS = 0.5 where it is about one-third the value observed for XPS = 0 and 1. This is directly reflected in the ESR spectrum as a substantial variation in the hyperfine splitting with XPS. The least-squares analysis also shows a reduction in rotational diffusion coefficient, R perpendicular by a fractor of 2 for XPS = 0.5 and permits the estimation of S2, the ordering parameter representing deviations from cylindrically symmetric alignment. These results are contrasted with 2H NMR studies which were insensitive to effects of mixing headgroups on the acyl chains. The ESR results are consistent with a somewhat increased disorder in the end-chain region as well as a small amount of chain tilting upon mixing POPC and POPS. They demonstrate the high sensitivity of ESR to subtle effects in chain ordering and dynamics.

Competition between Li+ and Mg2+ for the phosphate groups in the human erythrocyte membrane and ATP: an NMR and fluorescence study

We investigated the mechanism of competition between Li+ and Mg2+ in Li(+)-loaded human red blood cells (RBCs) by making 7Li and 31P NMR and fluorescence measurements. We used 7Li NMR relaxation times to probe Li+ binding to the human RBC membrane and ATP; an increase in Mg2+ concentration caused an increase in both 7Li T1 and T2 values in packed Li(+)-loaded RBCs, in suspensions of Li(+)-loaded RBC ghosts, in suspensions of Li(+)-containing RBC membrane, and in aqueous solutions of ATP, indicating competition between Li+ and Mg2+ for binding sites in the membrane and ATP. We found that increasing concentrations of either Li+ or Mg2+ in the presence of human RBC membrane caused an increase in the 31P NMR chemical shift anisotropy parameter, which describes the observed axially symmetric powder pattern, indicating metal ion binding to the phosphate groups in the membrane. Competition between Li+ and Mg2+ for phosphate groups in ATP and in the RBC membrane was also observed by both fluorescence measurements and 31P NMR spectroscopy at low temperature. The ratio of the stoichiometric binding constants of Mg2+ to Li+ to the RBC membrane was approximately 20; the ratio of the conditional binding constants in the presence of a free intracellular ATP concentration of 0.2 mM was approximately 4, indicating that Li+ competes for approximately 20% of the Mg(2+)-binding sites in the RBC membrane. Our results indicate that, regardless of the spectroscopic method used, Li+ competes with Mg2+ for phosphate groups in both ATP and the RBC membrane; the extent of metal ion competition for the phosphate head groups of the phospholipids in the RBC membrane is enhanced by the presence of ATP. Competition between Li+ and Mg2+ for anionic phospholipids or Mg(2+)-activated proteins present in cell membranes may constitute the basis of a general molecular mechanism for Li+ action in human tissues.

23Na-nuclear magnetic resonance study of ionophore-mediated cation exchange between two populations of liposomes

A model system to observe and investigate the transfer of Na+ ions between different internal compartments in suspension of liposomes was developed, and the exchange was followed by nuclear magnetic resonance spectroscopy. The experiments were performed under conditions of a Donnan equilibrium. Quantitative analysis of this three-site transmembrane exchange system allowed us to distinguish between direct and indirect exchange between liposomes. It also disclosed a "confining" effect on the exchange between the two populations of liposomes. This confining effect may have been due to an electrostatic field in the presence of a membrane potential. Donnan potentials and ionic compositions at equilibrium for the three-compartment system were calculated numerically. The model system may be used to explore further the effects of membrane potentials, surface potentials, and ionic mobilities on ion transport in biological (model) systems in general.

Calcium binding by phosphatidylserine headgroups. Deuterium NMR study

The binding of calcium to headgroup deuterated 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphoserine (POPS) was investigated by using deuterium magnetic resonance in pure POPS membranes and in mixed 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC)/POPS 5:1 (m:m) bilayers. Addition of CaCl2 to pure POPS bilayers led to two component spectra attributed, respectively, to liquid-crystallin POPS (less than 15 kHz) and POPS molecules in the calcium-induced dehydrated phase (cochleate) (approximately 120 kHz). The liquid-crystalline component has nearly disappeared at a Ca2+ to POPS ratio of 0.5, indicating that, under such conditions, most of the POPS molecules are in the precipitated cochleate phase. After dilution of the POPS molecules in zwitterionic POPC membranes (POPC/POPS 5:1 m:m), single component spectra characteristic of POPS in the liquid-crystalline state were observed in the presence of Molar concentrations of calcium ions (Ca2+ to POPS ratio greater than 50), showing that the amount of dehydrated cochleate PS-Ca2+ phase, if any, was low (less than 5%) under such conditions. Deuterium NMR data obtained in the 15-50 degrees C temperature range with the mixed PC/PS membranes, either in the absence or the presence of Ca2+ ions, indicate that the serine headgroup undergoes a temperature-induced conformational change, independent of the presence of Ca2+. This is discussed in relation to other headgroup perturbations such as that observed upon change of the membrane surface charge density.

Membrane electrostatics

In conclusion, charged membrane together with their adjacent electrolyte solution form a thermodynamic and physico-chemical entity. Their surfaces represent an exceptionally complicated interfacial system owing to intrinsic membrane complexity, as well as to the polarity and often large thickness of the interfacial region. Despite this, charged membranes can be described reasonably accurately within the framework of available theoretical models, provided that the latter are chosen on the basis of suitable criteria, which are briefly discussed in Section A. Interion correlations are likely to be important for the regular and/or rigid, thin membrane-solution interfaces. Lateral distribution of the structural membrane charge is seldom and charge distribution perpendicular to the membranes is nearly always electrostatically important. So is the interfacial hydration, which to a large extent determines the properties of the innermost part of the interfacial region, with a thickness of 2-3 nm. Fine structure of the ion double-layer and the interfacial smearing of the structural membrane charge decrease whilst the surface hydration increases the calculated value of the electrostatic membrane potential relative to the result of common Gouy-Chapman approximation. In some cases these effects partly cancel-out; simple electrostatic models are then fairly accurate. Notwithstanding this, it is at present difficult to draw detailed molecular conclusions from a large part of the published data, mainly owing to the lack of really stringent controls or calibrations. Ion binding to the membrane surface is a complicated process which involves charge-charge as well as charge-solvent interactions. Its efficiency normally increases with the ion valency and with the membrane charge density, but it is also strongly dependent on the physico-chemical and thermodynamic state of the membrane. Except in the case of the stereospecific ion binding to a membrane, the relatively easily accessible phosphate and carboxylic groups on lipids and integral membrane proteins are the main cation binding sites. Anions bind preferentially to the amine groups, even on zwitterionic molecules. Membrane structure is apt to change upon ion binding but not always in the same direction: membranes with bound ions can either expand or become more condensed, depending on the final hydrophilicity (polarity) of the membrane surface. The more polar membranes, as a rule, are less tightly packed and more fluid. Diffusive ion flow across a membrane depends on the transmembrane potential and concentration gradients, but also on the coulombic and hydration potentials at the membrane surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Coupled changes between lipid order and polypeptide conformation at the membrane surface. A 2H NMR and Raman study of polylysine-phosphatidic acid systems

Thermotropism and segmental chain order parameters of sn-2-perdeuteriated dimyristoyl-phosphatidic acid (DMPA)-water dispersions, with and without poly(L-lysine) (PLL) of different molecular weights, have been investigated by solid-state deuterium NMR spectroscopy. The segmental chain order parameter profile of this negatively charged lipid is similar to that already found for other lipids. Addition of long PLL (MW = 200,000) increases the temperature, Tc, of the lipid gel-to-fluid phase transition, whereas short PLL (MW = 4000) has practically no effect on Tc. In the fluid phase both varieties of PLL increase the "plateau" character of segmental order parameters up to carbon position 10. At the same reduced temperature, long PLL more significantly increases the segmental ordering, especially at the methyl terminal position. This leads to the conclusion that polar head-group capping and charge neutralization by PLL induce severe changes in lipid chain ordering, even down to the bilayer core. The structure of PLL bound to the lipid bilayer surface was monitored by Raman spectroscopy, following the amide I bands. Results show that the lipid gel-to-fluid phase transition triggers a conformational transition from ordered beta-sheet to random structure of short PLL, while it does not affect the strongly stabilized beta-sheet structure of long PLL. It is concluded that both short and long PLL can efficiently cap and neutralize lipid head groups, whatever their structure, and that peptide length is a key parameter in whether lipids or peptides are the driving force in conformationally coupled changes of both partners in the membrane.

The pH dependence of headgroup and acyl chain structure and dynamics of phosphatidylserine, studied by 2H-NMR

By varying the pH, the influence of the ionization degree on the structure and dynamics of aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) was studied, using 2H-NMR methods. For this purpose DOPS was synthesized with deuterium labels incorporated either stereospecifically at the beta-position of the serine headgroup ([2-2H]DOPS) or at the 11-position of both acyl chains ([11,11-2H2]DOPS), allowing the effects of pH on headgroup and acyl chains to be measured in parallel. A large scale synthesis procedure of stereospecific 1,2-dioleoyl-sn-glycero-3-phospho-[2-2H]-L- serine is described. The quadrupolar splitting (delta nu q) of [2-2H]DOPS is shown to be a sensitive sensor for the degree of protonation of the molecule. Whereas the delta nu q of [2-2H]DOPS decreases upon lowering the pH, that of [11,11-2H2]DOPS gradually increases, indicating an increase in acyl chain ordering. In the pH range below the pKa value, DOPS exhibits a temperature-dependent bilayer to hexagonal HII phase transition, apparent from the 31P-NMR spectra and the occurrence of a second component in the [11,11-2H2]DOPS 2H-NMR spectrum, with a much smaller delta nu q. The HII phase component in spectra from [2-2H]DOPS coincides with the isotropic position and has no defined delta nu q. In the bilayer organization delta nu q and spin-lattice relaxation time (T1) values for the acyl chain deuterated DOPS are similar to those obtained for other lipid systems. In contrast the PS headgroup region displays a relatively rigid structure as evidenced by a large delta nu q and very small T1 values. Upon adopting the HII phase the T1 values of the acyl chain deuterons are hardly affected. The uniqueness of the PS headgroup with respect to structure and motional properties is reinforced by the occurrence of a T1 minimum at 45 degrees C in the measurement of the temperature dependence of T1 for [2-2H]DOPS in the hexagonal HII configuration. Quantitative analysis yields a correlation time (tau c) for the motions determining T1 under these conditions, of 3.45 ns.

Weak interaction of spectrin with phosphatidylcholine-phosphatidylserine multilayers: a 2H and 31P NMR study

Spectrin from human erythrocytes binds to bilayer dispersions of both DMPC and DMPS:DMPC (1:1, w/w). However, no effect of bound spectrin on the conformation of the lipid head groups, as measured from the deuterium quadrupolar splittings of DMPC or DMPS specifically deuterated in the polar head groups, was detected in 1:1 mixtures of the two lipids containing either deuterated DMPC or DMPS. Neither the phase transition of the DMPS:DMPC mixtures, nor the spin-lattice relaxation time (T1) of the deuterated DMPS head group, was affected by spectrin. These results argue against any strong interaction of spectrin with phosphatidylserine and rule out the possibility that spectrin is responsible for the maintenance of PS in the inner monolayer of the erythrocyte membrane during the whole life-span of this cell.

Surface charge effects upon membrane transport processes: the effects of surface charge on the monensin-mediated transport of lithium ions through phospholipid bilayers studied by 7Li-NMR spectroscopy

Addition of monensin to preparations of large unilamellar vesicles prepared from egg-yolk phosphatidylcholine (egg PC) or egg PC containing 5% phosphatidylserine (PS-) or cetylpyridinium (CP+) ions in lithium chloride solution allows the transport of Li+ ions to be monitored by an NMR magnetisation transfer technique. The kinetics of the transport are followed as a function of the metal ion and monensin concentrations and are compatible with a model in which one monensin molecule transports one Li+ ion. The data allow the extraction of the rate constants for the association and dissociation of the monensin-Li+ complex in the water/membrane interfaces and the evaluation of the stability constants for complex formation in the interfaces. Placing positive charge (CP+) on the membrane surface reduces the formation rate by a factor of about three but hardly alters the dissociation rate. Placing negative charge (PS-) on the membrane surface hardly alters the formation rate but speeds the dissociation rate by about a factor of three. Data from relaxation times of 7Li+ inside the vesicles and from the total enclosed volumes as the vesicles are formed, point to appreciable Li+ surface interactions that increase as the charge on the surface is made more negative. The size of the vesicles formed by the dialytic detergent removal technique increases with the surface charge. The results support a view that enzyme-phospholipid or substrate-phospholipid interactions could play an important role in determining the efficacity of action of membrane bound enzymes. The relevance of the results in the role of Li+ in the control of manic depression is also discussed.

Inhibition of adenylate cyclase from luteinized rat ovary by monovalent cations: roles of the stimulatory guanine nucleotide-binding regulatory component and stimulatory hormone receptor

Sodium and other monovalent cations (added as chloride salts) inhibited adenylate cyclase of luteinized rat ovary. Sodium chloride (150 mM) inhibited basal enzyme activity by 20%. Sodium chloride inhibition was enhanced to 34-54% under conditions of enzyme stimulation by guanine nucleotides (GTP and its nonhydrolyzable analog 5'-guanylyl imidodiphosphate), fluoride anion, and agonists (ovine luteinizing hormone (oLH) and the beta-adrenergic catecholamine isoproterenol) acting at stimulatory receptors linked to adenylate cyclase. Sodium chloride inhibition was dependent on salt concentration over a wide range (25-800 mM) as well as the concentrations of GTP and oLH. Inhibition by NaCl was of rapid onset and appeared to be reversible. The order of inhibitory potency of monovalent cations was Li+ greater than Na+ greater than K+. The role of individual components of adenylate cyclase in the inhibitory action of monovalent cations was examined. Exotoxins of Vibrio cholerae and Bordetella pertussis were used to determine respectively the involvement of the stimulatory and inhibitory guanine nucleotide-binding regulatory components (Ns and Ni) in NaCl inhibition. Sodium chloride inhibited cholera toxin-activated adenylate cyclase activity by 29%. Ni did not appear to mediate cation inhibition of adenylate cyclase because pertussis toxin did not attenuate inhibition by NaCl. Enzyme stimulation by agents (forskolin and Mn2+) thought to activate the catalytic component directly was not inhibited by NaCl but was instead significantly enhanced. Sodium chloride (150 mM) increased both the Kd for high-affinity binding of oLH to 125I-human chorionic gonadotropin binding sites and the Kact for oLH stimulation of adenylate cyclase by sevenfold. In contrast, NaCl had no appreciable effect on either isoproterenol binding to (-)-[125I]iodopindolol binding sites or the Kact for isoproterenol stimulation of adenylate cyclase. The results suggest that in luteinized rat ovary monovalent cations uncouple, or dissociate, Ns from the catalytic component and, in a distinct action, reduce gonadotropin receptor affinity for hormone. Dissociation of the inhibitory influence of Ni from direct catalytic activation could account for NaCl enhancement of forskolin- and Mn2+-associated activities. On the basis of these results, the spectrum of divergent stimulatory and inhibitory effects of monovalent cations on adenylate cyclase activities in a variety of tissues may be interpreted in terms of differential enzyme susceptibilities to cation-induced uncoupling of N and catalytic component functions.

Effect of electrostatic repulsion on the morphology and thermotropic transitions of anionic phospholipids

Samples of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylserine which exhibit highly cooperative phase transitions in suspension with 0.5 M NaCl, 10 mM sodium phosphate, pH 7.0, demonstrate a marked broadening of the phase transition when suspended in distilled water at pH 7. The cooperativity of the thermal transition of dimyristoylphosphatidic acid at pH 7.0, in contrast, was little affected by the presence or absence of 0.5 M NaCl. The most dramatic changes in phase transition properties were observed with phosphatidylglycerol. The morphology of phosphatidylglycerol was also altered by NaCl. At high salt concentrations, typical multilamellar structures were observed by freeze-fracture electron microscopy. However, in distilled water the large multilamellar structures are disrupted, with opened shells and smaller particles of 20-30 nm diameter being observed, in addition to a few larger vesicles. Structures of differing morphology could be partially separated from aqueous 'solutions' of phosphatidylglycerol by ultracentrifugation. The supernate contained only small particles. The results are discussed in terms of the large effective head group volume resulting from strong electrostatic repulsion at low ionic strengths. This allows the anionic lipids, particularly phosphatidylglycerol, to form hydrophilic surfaces with high curvature so as to seal the bilayer edges of discs or shells. Thus, certain anionic lipids in distilled water destabilize bilayers by a mechanism which is opposite to that found with lipids which destabilize bilayers by forming the hexagonal phase.