Lipid polymorphism of mixtures of dioleoylphosphatidylethanolamine and saturated and monounsaturated phosphatidylcholines of various chain lengths

The L alpha-HII phase transition behavior of many lipid-water liquid crystals is dominated by the competition between the tendency to curl the lipid layers to an intrinsic radius of curvature and opposing hydrocarbon packing constraints. In particular, packing constraints can increase the free energy of the inverted hexagonal (HII) phase as compared to that of the lamellar (L alpha) phase. This is especially true where the lipid molecule is not long enough to reach into the corners of the lattice in large hexagonal structures necessitated by a large intrinsic radius of curvature. In this paper it is shown that the addition of a minor fraction long-chain lipid to a system of otherwise uniform chain composition can also relax packing constraints, thereby lowering the lamellar to hexagonal transition temperature. For the specific systems used, dioleoylphosphatidylethanolamine (di-18:1c-PE) with minor fractions of 1,2-diacyl-sn-glycero-3-phosphocholines [di-n:1c-PC (n = 14, 18, 22, and 24)], the observed HII lattices systematically increased in size with increasing chain length, suggesting that the chain length also may affect the intrinsic curvature of the mixture. These experiments demonstrate that the lipid "shape concept", which is a qualitative expression of the concept quantitatively described by the intrinsic radius of curvature, is insufficient to understand the L alpha-HII transition. It is necessary to, at least, consider the competition between curvature and packing.     

Curvature properties of novel forms of phosphatidylcholine with branched acyl chains.

We studied the properties of a series of phosphatidylcholine molecules with branched acyl chains. These lipids have previously been shown to have marked stimulatory effects on the side-chain cleavage activity of cytochrome P450SCC (CYP11A1), an enzyme of the inner mitochondrial membrane. The synthetic lipids used were diacyl phosphatidylcholines with the decanoyl, dodecanoyl or tetradecanoyl chain having a hexyl, octyl or decyl straight chain aliphatic branch at the 2-position. All three lipids lowered the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine, the lipids with longer acyl chains being more effective in this regard. As pure lipids all of the forms were found by X-ray diffraction to be predominantly in the hexagonal phase (HII) over the entire temperature range of 7-75 degrees C. The properties of the HII phase were unusual with regard to the small size of the lattice spacings and the small temperature dependence of the spacings. We used tetradecane to relieve hydrocarbon packing constraints to determine the intrinsic radius of curvature of the lipid monolayer. The elastic bending modulus was measured in the presence of tetradecane by introducing an osmotic gradient across the hexagonal phase cylinders with aqueous solutions of poly(ethylene glycol). The elastic bending modulus was found to be higher than that observed with other lipids and to increase with temperature. Both the small intrinsic radius of curvature and the high elastic bending modulus indicate that the presence of these lipids in bilayer membranes will impose a high degree of negative curvature strain.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 1. Differential scanning calorimetric studies

The thermotropic phase behavior of aqueous dispersions of phosphatidylcholines containing one of a series of methyl iso-branched fatty acyl chains was studied by differential scanning calorimetry. These compounds exhibit a complex phase behavior on heating which includes two endothermic events, a gel/gel transition, involving a molecular packing rearrangement between two gel-state forms, and a gel/liquid-crystalline phase transition, involving the melting of the hydrocarbon chains. The gel to liquid-crystalline transition is a relatively fast, highly cooperative process which exhibits a lower transition temperature and enthalpy than do the chain-melting transitions of saturated straight-chain phosphatidylcholines of similar acyl chain length. In addition, the gel to liquid-crystalline phase transition temperature is relatively insensitive to the composition of the aqueous phase. In contrast, the gel/gel transition is a slow process of lower cooperativity than the gel/liquid-crystalline phase transition and is sensitive to the composition of the bulk aqueous phase. The gel/gel transitions of the methyl iso-branched phosphatidylcholines have very different thermodynamic properties and depend in a different way on hydrocarbon chain length than do either the "subtransitions" or the "pretransitions" observed with linear saturated phosphatidylcholines. The gel/gel and gel/liquid-crystalline transitions are apparently concomitant for the shorter chain iso-branched phosphatidylcholines but diverge on the temperature scale with increasing chain length, with a pronounced odd/even alternation of the characteristic temperatures of the gel/gel transition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intrinsic curvature in normal and inverted lipid structures and in membranes.

The intrinsic or spontaneous radius of curvature, R(o), of lipid monolayer assemblies is expressed in terms of a lipid molecular packing parameter, V/AI, for various geometries. It is shown that the equivalent lipid length, 1, in inverted hexagonal (HII) phases, defined by a cylindrical shell of equal total lipid volume, yields an expression for R o identical to that for inverted cylindrical micelles (or, equivalently, HII phases in the presence of excess hydrocarbon). This identity is used to obtain values of the effective packing parameter for various phosphatidylethanolamines. The temperature dependence of the intrinsic radius of curvature is predicted to be negative and to be considerably greater than that for the lipid length in nearly all cases. The thermal expansion coefficient is not constant but is found to vary, depending on the value of the lipid packing parameter. A possible addition rule is constructed for the intrinsic radius of curvature of lipid mixtures, based on the linear additivity of the effective molecular volumes, V, and molecular areas, A. This relation is found to hold for mixtures of dioleoyl phosphatidylcholine (DOPC) with dioleoyl phosphatidylethanolamine, and a value of R(o) of > or = 9 A (V/AI = 1.08) is obtained for DOPC. The energetics of the intrinsic curvature and lamellar-nonlamellar transitions are also discussed within the framework of the model.     

X-ray diffraction study of the polymorphic behavior of N-methylated dioleoylphosphatidylethanolamine

The polymorphic phase behavior of aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and its N-methylated analogues, DOPE-Me, DOPE-Me2, and DOPC, has been investigated by X-ray diffraction. In the fully hydrated lamellar (L alpha) phase at 2 degrees C, the major structural difference is a large increase in the interlamellar water width from DOPE to DOPE-Me, with minor increases with successive methylation. Consistent with earlier reports, inverted hexagonal (HII) phases are observed upon heating at 5-10 degrees C in DOPE and at 65-75 degrees C in DOPE-Me and are not observed to at least 85 degrees C in DOPE-Me2 or DOPC. In DOPE, the L alpha-HII transition is facile and is characterized by a relatively narrow temperature range of coexistence of L alpha and HII domains, each with long-range order. DOPE-Me exhibits complex nonequilibrium behavior below the occurrence of the HII phase: Upon heating, the L alpha lattice spontaneously disorders on a time scale of days; on cooling from the HII phase, the disorder rises on a time scale of minutes. It is shown that, in copious water, the disordered state transforms very slowly into phases with cubic symmetry. This process is assisted by the generation of small amounts of lipid degradation products. The relative magnitudes of the monolayer spontaneous radius of curvature, R0 [Kirk, G. L., Gruner, S. M., & Stein, D. L. (1984) Biochemistry 23, 1093; Gruner, S. M. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 3665], are inferred from the HII lattice spacings vs temperature and are shown to increase with increasing methylation. The relative magnitudes of R0 are categorized as small for DOPE, intermediate for DOPE-Me, and large for DOPC. It is suggested, and examples are used to illustrate, that small R0 lipid systems exhibit facile, low-temperature L alpha-HII transitions, intermediate R0 systems exhibit complex nonequilibrium transition behavior and are likely to form cubic phases, and large R0 systems are stable as L alpha phases. The relationship between the cubic phases and minimal periodic surfaces is discussed. It is suggested that minimal periodic surfaces represent geometries in which near constant, intermediate R0 values can be obtained concomitantly with monolayers of near constant thickness, thereby leading to equilibrium cubic phases. Thus, the relative magnitude of the spontaneous radius of curvature may be used to predict mesomorphic behavior.(ABSTRACT TRUNCATED AT 400 WORDS)     

Headgroup hydration and motional order of lipids in lamellar liquid crystalline and inverted hexagonal phases of unsaturated phosphatidylethanolamine--a time-resolved fluorescence study

By the use of frequency domain cross-correlation fluorometry, the fluorescence lifetime of the water soluble probe 8,1-anilinonapthalene sulfonic acid (ANS) in aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and phosphatidylethanolamine transphosphatidylated from egg phosphatidylcholine (TPE) was measured. The orientational order parameter and rotational diffusion constant of the lipophilic probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) were also determined in TPE dispersions. In agreement with a previous study on DOPE (Cheng (1989) Biophys. J. 55, 1025-1031), abrupt changes in both the order packing and rotational diffusion constant were found at the lamellar liquid crystalline (L alpha) to inverted hexagonal (HII) phase transition of TPE. Owing to the subnanosecond resolution capability of this frequency domain fluorometric technique, the heterogeneous fluorescence decay of ANS was resolved into three distinct components with different decay lifetimes (tau's). They were 0 less than tau less than 0.5 ns, 2 less than tau less than 9 ns and tau greater than 15 ns. These lifetime regions were attributed to the partitioning of ANS into the bulk aqueous medium, the lipid/water interface and the lipid hydrocarbon region, respectively. These classifications of lifetime regions were further supported by the sensitivity of those lifetime components with the solvent isotopic shift of D2O. Similar to the changes of orientational order and rotational diffusion of lipophilic probe, the lifetime and intensity fraction of ANS associated with the lipid/water interfacial region declined abruptly at the L alpha-HII transition of both DOPE and TPE. This observation suggested that a dehydration of the lipid headgroup surface occurs at the L alpha-HII transition. This study provided evidence that both the lipid headgroup surface hydration and the lipid dynamics change drastically as a result of the macroscopic rearrangement of lipids at the L alpha-HII transition.     

Infrared and time-resolved fluorescence spectroscopic studies of the polymorphic phase behavior of phosphatidylethanolamine/diacylglycerol lipid mixtures

Fourier transform infrared (FTIR) and time-resolved fluorescence spectroscopy have been employed to examine the structural dynamics of lipid fatty acyl chains and lipid/water interfacial region of a binary lipid mixture containing unsaturated phosphatidylethanolamine (PE) and diacylglycerol (DG). Infrared vibrational frequencies of the CH2 symmetric stretching and the C = O stretching bands of the lipids were measured at different lipid compositions and temperatures. For 0% DG, the lamellar gel to lamellar liquid crystalline (L beta-L alpha) and the L alpha to inverted hexagonal (L alpha-HII) phase transitions were observed at approximately 15 degrees and 55 degrees C, respectively. As the DG content increased gradually from 0% to 15%, the L alpha-HII phase transition temperature decreased drastically while the L beta-L alpha phase transition temperature decreased only slightly. At 10% DG, a merge of these two phase transitions was noticed at approximately 10 degrees C. For the composition study at 23 degrees C, the L alpha-HII transition occurred at approximately 6-10% DG as indicated by abrupt increases in both the CH2 and C = O stretching frequencies at those DG contents. Using time-resolved fluorescence spectroscopy, abrupt decreases in both the normalized long time residual and the initial slope of the anisotropy decay function of lipid probes, 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5- hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine, in these PE/DG mixtures were observed at the L alpha-HII phase transition. These changes in the anisotropy decay parameters suggested that the rotational dynamics and orientational packing of the lipids were altered at the composition-induced L alpha-HII transition, and agreed with a previous temperature-induced L alpha-HII transition study on pure unsaturated PE (Cheng (1989) Biophys. J. 55, 1025-1031). The fluorescence lifetime of water soluble probes, 8,1-anilinonapthalenes sulfonate acid, in PE/DG mixtures increased abruptly at the L alpha-HII phase transition, suggesting that the conformation and hydration of the lipid/water interfacial region also undergo significant changes at the L alpha-HII transition.     

Inverted micellar intermediates and the transitions between lamellar, cubic, and inverted hexagonal lipid phases. II. Implications for membrane-membrane interactions and membrane fusion.

Results of a kinetic model of thermotropic L alpha----HII phase transitions are used to predict the types and order-of-magnitude rates of interactions between unilamellar vesicles that can occur by intermediates in the L alpha----HII phase transition. These interactions are: outer monolayer lipid exchange between vesicles; vesicle leakage subsequent to aggregation; and (only in systems with ratios of L alpha and HII phase structural dimensions in a certain range or with unusually large bilayer lateral compressibilities) vesicle fusion with retention of contents. It was previously proposed that inverted micellar structures mediate membrane fusion. These inverted micellar structures are thought to form in all systems with such transitions. However, I show that membrane fusion probably occurs via structures that form from these inverted micellar intermediates, and that fusion should occur in only a sub-set of lipid systems that can adopt the HII phase. For single-component phosphatidylethanolamine (PE) systems with thermotropic L alpha----HII transitions, lipid exchange should be observed starting at temperatures several degrees below TH and at all higher temperatures, where TH is the L alpha----HII transition temperature. At temperatures above TH, the HII phase forms between apposed vesicles, and eventually ruptures them (leakage). In most single-component PE systems, fusion via L alpha----HII transition intermediates should not occur. This is the behavior observed by Bentz, Ellens, Lai, Szoka, et al. in PE vesicle systems. Fusion is likely to occur under circumstances in which multilamellar samples of lipid form the so-called "inverted cubic" or "isotropic" phase. This is as observed in the mono-methyl DOPE system (Ellens, H., J. Bentz, and F. C. Szoka. 1986. Fusion of phosphatidylethanolamine containing liposomes and the mechanism of the L alpha-HII phase transition. Biochemistry. In press.) In lipid systems with L alpha----HII transitions driven by cation binding (e.g., Ca2+-cardiolipin), fusion should be more frequent than in thermotropic systems.     

Distribution of decane within the unit cell of the inverted hexagonal (HII) phase of lipid-water-decane systems determined by neutron diffraction.

The addition of a free alkane such as decane to lipid-water systems is known to promote the formation of a low-temperature inverted hexagonal (HII) phase [Kirk, G. L., & Gruner, S. M. (1985) J. Phys. (Paris) 46, 761]. Kirk et al. [Kirk, G. L., Gruner, S. M., & Stein, D. E. (1984) Biochemistry 23, 1093] have discussed the hydrocarbon packing anisotropy in the HII unit cell and have suggested that free alkane will distribute in a way that reduces this packing anisotropy by allowing the lipid chain environment to become more uniform. By combining neutron and X-ray diffraction data to do a Fourier reconstruction of the HII phase of dioleoylphosphatidylethanolamine (DOPE) + water + deuterated decane, it was found that the decane preferentially partitions into the interstitial regions of the HII unit cell where it should be the most effective in alleviating the hydrocarbon chain packing stress, supporting the suggestion of Kirk et al. Using the distribution of decane within the unit cell, we have calculated the lipid length distribution for the situations with and without added alkane. With a suitable molecular model, this lipid length distribution may eventually be used to calculate the free energy change upon the addition of alkane. Such a measurement is important for a more realistic understanding of the interactions which lead to the formation of the HII phase.     

Influences of membrane curvature in lipid hexagonal phases studied by deuterium NMR spectroscopy

The presence of reversed hexagonal phase, HII, favoring lipids in membranes has been proposed to be significant in various biological processes. Therefore an understanding of the HII phase and the transition from the lamellar to hexagonal phase is of importance. We have applied deuterium NMR spectroscopy to study the bilayer and reversed hexagonal phases of 1-perdeuteriopalmitoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamin e. The difference in packing between the HII and L alpha phases leads to smaller segmental order parameters in the former case. Since the order profiles are sensitive to the geometry of the aggregates, they can be used to extract structural information about the phases. We present a new means of calculating the radius of curvature, R1, for the HII phase from 2H NMR data. This method gives a value of R1 = 18.1 A, which is in agreement with current understanding of the structure of the HII phase and with x-ray diffraction data.     

Pretransitions in the hydrocarbon chains of phosphatidylethanolamines. A wide angle X-ray diffraction study

The hydrocarbon chain packing of fully hydrated phosphatidylethanolamine multilayers is investigated by X-ray diffraction. An analysis of the wide angle reflections (short spacings) as a function of temperature indicates that, apart from the well-known ordered-disordered lipid phase transition, a second transition takes place at lower temperatures. This transition, which is in the present paper referred to as the pretransition, is characterized by a transformation of the hydrocarbon chain packing. A first model for the chain lattice is presented, which gives rise to the expectation that similar pretransitions might be found with other phospholipids.     

Energetics of a hexagonal-lamellar-hexagonal-phase transition sequence in dioleoylphosphatidylethanolamine membranes.

The phase diagram of DOPE/water dispersions was investigated by NMR and X-ray diffraction in the water concentration range from 2 to 20 water molecules per lipid and in the temperature range from -5 to +50 degrees C. At temperatures above 22 degrees C, the dispersions form an inverse (HII) phase at all water concentrations. Below 25 degrees C, an HII phase occurs at high water concentrations, an L alpha phase is formed at intermediate water concentrations, and finally the system switches back to an HII phase at low water concentrations. The enthalpy of the L alpha-HII-phase transition is +0.3 kcal/mol as measured by differential scanning calorimetry. Using 31P and 2H NMR and X-ray diffraction, we measured the trapped water volumes in HII and L alpha phases as a function of osmotic pressure. The change of the HII-phase free energy as a function of hydration was calculated by integrating the osmotic pressure vs trapped water volume curve. The phase diagram calculated on the basis of the known enthalpy of transition and the osmotic pressure vs water volume curves is in good agreement with the measured one. The HII-L alpha-HII double-phase transition at temperatures below 22 degrees C can be shown to be a consequence of (i) the greater degree of hydration of the HII phase in excess water and (ii) the relative sensitivities with which the lamellar and hexagonal phases dehydrate with increasing osmotic pressure. These results demonstrate the usefulness of osmotic stress measurements to understand lipid-phase diagrams.     

Subgel phases of n-saturated diacylphosphatidylcholines: a Fourier-transform infrared spectroscopic study

The subgel phases of a homologous series of saturated straight-chain diacylphosphatidylcholines with hydrocarbon chains consisting of 10-18 carbon atoms were studied by Fourier-transform infrared spectroscopy. All of these lipids initially form a subgel phase which is spectroscopically similar to that obtained when fully hydrated multilamellar dispersions of dipalmitoylphosphatidylcholine are incubated at 0-4 degrees C for 2-4 days. However, further low-temperature incubation of those phosphatidylcholines with acyl chains of 16 or fewer carbon atoms results in the sequential formation of 1 or more additional, spectroscopically distinct subgel phases, with the number of such phases increasing as hydrocarbon chain length decreases. Our data indicate that the formation of all of these subgel phases involves both reorientation of the acyl chains and major changes in hydration and/or hydrogen-bonding interactions at the polar/apolar interfacial region of the lipid bilayer. We suggest that the driving force behind the formation of these Lc phases is the formation of an extended hydrogen-bonding network in the interfacial region of the bilayer and that the optimization of this network probably requires some distortion of the optimal packing of the acyl chains. As a result, an increase in acyl chain length makes the formation of these Lc phases less favorable and eventually prevents optimization of the hydrogen-bonding network at the bilayer polar/apolar interface.     

Correlation between lipid plane curvature and lipid chain order.

The 1-palmitoyl-2-oleoyl-phosphatidylethanolamine: 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE:POPC) system has been investigated by measuring, in the inverted hexagonal (HII) phase, the intercylinder spacings (using x-ray diffraction) and orientational order of the acyl chains (using 2H nuclear magnetic resonance). The presence of 20 wt% dodecane leads to the formation of a HII phase for the composition range from 0 to 39 mol% of POPC in POPE, as ascertained by x-ray diffraction and 2H nuclear magnetic resonance. The addition of the alkane induces a small decrease in chain order, consistent with less stretched chains. An increase in temperature or in POPE proportion leads to a reduction in the intercylinder spacing, primarily due to a decrease in the water core radius. A temperature increase also leads to a reduction in the orientational order of the lipid acyl chains, whereas the POPE proportion has little effect on chain order. A correlation is proposed to relate the radius of curvature of the cylinders in the inverted hexagonal phase to the chain order of the lipids adopting the HII phase. A simple geometrical model is proposed, taking into account the area occupied by the polar headgroup at the interface and the orientational order of the acyl chains reflecting the contribution of the apolar core. From these parameters, intercylinder spacings are calculated that agree well with the values determined experimentally by x-ray diffraction, for the variations of both temperature and POPE:POPC proportion. This model suggests that temperature increases the curvature of lipid layers, mainly by increasing the area subtended by the hydrophobic core through chain conformation disorder, whereas POPC content affects primarily the headgroup interface contribution. The frustration of lipid layer curvature is also shown to be reflected in the acyl chain order measured in the L alpha phase, in the absence of dodecane; for a given temperature, increased order is observed when the curling tendencies of the lipid plane are more pronounced.     

The effect of the phase transition on the hydration and electrical conductivity of phospholipids.

Adsorption isotherms for various saturated phosphatidylcholines have been obtained. Lipids above and below their phase transition temperature differ only in the amount of water adsorbed and not in the nature of their adsorption isotherms. Cholesterol has an effect similar to that of increasing unsaturation in the hydrocarbon chains. Decreasing the length of the hydrocarbon chains for lipids below their phase transition temperature has no effect on the isotherms. If the chain length is short enough so that the lipids are above their transition temperature, however, a large increase in water adsorption occurs. All of the phospholipids exhibit a rapid increase of electrical conductivity for a few water molecules adsorbed per lipid molecule. All of the phospholipids show a saturation in conductivity at greater amounts of adsorbed water; the shape of the saturation region depends on whether the lipids are above or below their phase transition temperature. The activation energy for the electrical conductivity process depends on whether the hydrated lipids are in the "liquid-like" of the crystalline state, being lower for phospholipids in the liquid-like state. If the lipids are hydrated above their phase transition temperatures, their activation energies are lower than if they are hydrated below the transition temperature. Cholesterol lowers the activation energy. The phosphatidylcholines can be characterized by different activation energies, depending both upon their physical state and the presence of unsaturation in their hydrocarbon chains.     

Interactions of a Rhodococcus sp. biosurfactant trehalose lipid with phosphatidylethanolamine membranes

Trehalose lipids are an important group of glycolipid biosurfasctants mainly produced by rhodococci. Beside their known industrial applications, there is an increasing interest in the use of these biosurfactants as therapeutic agents. We have purified a trehalose lipid from Rhodococcus sp. and made a detailed study of the effect of the glycolipid on the thermotropic and structural properties of phosphatidylethanolamine membranes of different chain length and saturation, using differential scanning calorimetry, small and wide angle X-ray diffraction and infrared spectroscopy. It has been found that trehalose lipid affects the gel to liquid crystalline phase transition of phosphatidylethanolamines, broadening and shifting the transition to lower temperatures. Trehalose lipid does not modify the macroscopic bilayer organization of saturated phosphatidylethanolamines and presents good miscibility both in the gel and the liquid crystalline phases. Infrared experiments evidenced an increase of the hydrocarbon chain conformational disorder and an important dehydrating effect of the interfacial region of the saturated phosphatidylethanolamines. Trehalose lipid, when incorporated into dielaidoylphosphatidylethanolamine, greatly promotes the formation of the inverted hexagonal HII phase. These results support the idea that trehalose lipid incorporates into the phosphatidylethanolamine bilayers and produces structural perturbations which might affect the function of the membrane.     

Hydrophobic lipid additives affect membrane stability and phase behavior of N-monomethyldioleoylphosphatidylethanolamine.

The rate of formation of high-curvature intermediates or disordered cubic phases in N-methyldioleoylphosphatidylethanolamine (N-methyl-DOPE) dispersions with or without additives was studied by 31P NMR spectroscopy. In N-methyl-DOPE dispersions, both the L alpha liquid-crystalline phase and the hexagonal HII phase convert into phases of high curvature giving rise to isotropic 31P NMR resonances. Addition of the bilayer destabilizers 1,2-diolein, 1,3-diolein, or eicosane lowers the threshold temperature of the isotropic phase. The isotropic threshold temperature is strongly correlated with the L alpha-HII phase transition temperature (TH). The addition of hexagonal phase promoters does not change the rate of formation of the isotropic phase at a temperature shifted by a fixed amount below TH. However, the formation of "isotropic" phases from the additive-stabilized hexagonal phase is slow compared to that observed in pure N-methyl-DOPE lipid dispersions. Membrane leakage and fusion are promoted by the dioleins and well as by eicosane, but changes in the rates of these processes do not correlate well with the extent of formation of isotropic phases. All three additives have similar effects on phase behavior and on vesicle leakage and fusion. These similarities occur despite the fact that eicosane is believed to partition differently into the membrane than diolein. In addition to the general similarities in the effects of the two diolein isomers, 1,2-diolein is somewhat more potent in promoting the hexagonal phase and in increasing rates of leakage and fusion than is 1,3-diolein.     

Spin-label characterisation of the lamellar-to-hexagonal (HII) phase transition in egg phosphatidylethanolamine aqueous dispersions

The thermotropic behaviour of egg yolk phosphatidylethanolamine dispersions in excess aqueous phase has been investigated by spin label electron spin resonance spectroscopy and differential thermal analysis. Phosphatidylethanolamine isomers spin-labelled at six different positions along the acyl chain, and steroid spin labels, indicate both gel-fluid lamellar and lamellar-reverse hexagonal (HII) phase transitions, in agreement with complementary calorimetric studies. Analysis of spin label data shows that the transition to the HII phase is accompanied by an increase in conformational freedom of the acyl chain, more pronounced towards the methyl terminus, and representing an increase in the population of gauche isomers which can only be accommodated by a transition to the non-bilayer phase. Raising the bulk pH to, and above, pH 8.5 results in stabilisation of the bilayer phase and no transition to the HII phase is observed. The phosphatidylethanolamine spin labels also indicate a polarity profile which is characteristic of each phase.     

The physical properties of glycosyldiacylglycerols. Calorimetric studies of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols

The polymorphic phase behavior of aqueous dispersions of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols was studied by differential scanning calorimetry. At fast heating rates, unannealed samples of these lipids exhibit a strongly energetic, lower temperature transition, which is followed by a weakly energetic, higher temperature transition. X-ray diffraction studies have enabled the assignments of these events to a lamellar gel/liquid crystalline (chain-melting) phase transition and a bilayer/nonbilayer phase transition, respectively. Whereas the values for both the temperature and enthalpy of the chain-melting phase transition increase with increasing acyl chain length, those of the bilayer/nonbilayer phase transition show almost no chain-length dependence. However, the nature of the bilayer/nonbilayer transition is affected by the length of the acyl chain. The shorter chain compounds form a nonbilayer 2-D monoclinic phase at high temperature whereas the longer chain compounds from a true inverted hexagonal (HII) phase. Our studies also show that the gel phase that is initially formed on cooling of these lipids is metastable with respect to a more stable gel phase and that prolonged annealing results in a slow conversion to the more stable phase after initial nucleation by incubation at appropriate low temperatures. The formation of these stable gel phases is shown to be markedly dependent upon the length of the acyl chains and whether they contain an odd or an even number of carbon atoms. There is also evidence to suggest that, in the case of the shorter chain compounds at least, the process may proceed via another gel-phase intermediate. In annealed samples of the shorter chain compounds, the stable gel phase converts directly to the L alpha phase upon heating, whereas annealed samples of the longer chain glycolipids convert to a metastable gel phase prior the chain melging.(ABSTRACT TRUNCATED AT 250 WORDS)