Thermotropic and lyotropic properties of long chain alkyl glycopyranosides. Part II. Disaccharide headgroups

We have investigated the thermotropic and lyotropic properties of some long chain alkyl glycosides with disaccharide headgroups. The thermotropism was measured with polarising microscopy and additionally the lyotropism with the contact preparation method, Fourier-transform Infrared (FTIR) spectroscopy, X-ray diffraction and small angle neutron scattering. A broad thermotropic as well as lyotropic polymorphism was found. The compounds displayed thermotropic S(A) (lamellar) and cubic phases, and the investigation of the lyotropic phase behaviour led to the observation of inverted bicontinuous cubic V(II) phases, lamellar L(alpha) phases, normal bicontinuous cubic V(I) phases, normal columnar H(I) phases, normal discontinuous cubic I(I) phases and lyotropic cholesteric phases. The phases are discussed with respect to the chemical structures that have been varied systematically to derive structure-property relationships.     

Hydrophobic Surfactant Proteins Induce a Phosphatidylethanolamine to Form Cubic Phases

The hydrophobic surfactant proteins SP-B and SP-C promote rapid adsorption of pulmonary surfactant to an air/water interface. Previous evidence suggests that they achieve this effect by facilitating the formation of a rate-limiting negatively curved stalk between the vesicular bilayer and the interface. To determine whether the proteins can alter the curvature of lipid leaflets, we used x-ray diffraction to investigate how the physiological mixture of these proteins affects structures formed by 1-palmitoyl-2-oleoyl phosphatidylethanolamine, which by itself undergoes the lamellar-to-inverse hexagonal phase transition at 71°C. In amounts as low as 0.03% (w:w) and at temperatures as low as 57°C, the proteins induce formation of bicontinuous inverse cubic phases. The proteins produce a dose-related shift of diffracted intensity to the cubic phases, with minimal evidence of other structures above 0.1% and 62°C, but no change in the lattice-constants of the lamellar or cubic phases. The induction of the bicontinuous cubic phases, in which the individual lipid leaflets have the same saddle-shaped curvature as the hypothetical stalk-intermediate, supports the proposed model of how the surfactant proteins promote adsorption.     

An inverse face-centered cubic phase formed by diacylglycerol-phosphatidylcholine mixtures

Fully hydrated unsaturated diacylglycerol-phosphatidylcholine mixtures are found to adopt an inverse face-centered cubic phase, of crystallographic cubic aspect 15. The same behavior is observed for either the 1,2- or 1,3-isomer of the diacylglycerol. This Q15 cubic phase, of probable space group Fd3m (Q227), occurs between an inverse hexagonal (HII) phase and an inverse micellar (L2) solution, with increasing diacylglycerol concentration, which implies that the mean curvature of the interface is more negative than that of the HII phase. This behavior is quite different from that of the more usual bicontinuous inverse cubic phases Pn3m (Q224), Im3m (Q229), and Ia3d (Q230), which normally occur between the lamellar L alpha and the HII phases. One possible structure for the Fd3m cubic phase has recently been proposed (Mariani, P., Luzzati, V., & Delacroix, H. (1988) J. Mol. Biol. 204, 165-189), consisting of tetrahedrally arranged clusters of inverse micelles surrounded by a continuous cage of tetrahedrally connected water/lipid (inverse) channels.     

Phase equilibria of mixtures of plant galactolipids. The formation of a bicontinuous cubic phase

The chloroplast galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were isolated from wheat leaves. The phase equilibria of galactolipid-water systems with MGDG / DGDG molar ratios equal to 0:1, 1:2, 1.2:1, 2:1 and 1:0 were investigated, using nuclear magnetic resonance (NMR) methods. MGDG and DGDG form reversed hexagonal and lamellar phases, respectively, at temperatures between 10 and 40°C at all water contents studied (up to about 14 mol ²H₂O per mol lipid). The galactolipid mixtures show a complex phase forming reversed hexagonal, lamellar and reversed cubic phases, depending on water content and temperature. It was found that the water hydration is similar for the lamellar and hexagonal phases formed by DGDG and MGDG, respectively. The non-lamellar phase areas increase with increasing content of MGDG. Small-angle X-ray measurements show that the cubic phase belongs to the Ia3d space group. From translational diffusion studies by NMR it is concluded that the structure of this cubic phase is bicontinuous.     

Temperature- and pressure-dependent phase behavior of monoacylglycerides monoolein and monoelaidin.

We used x-ray and neutron diffraction to study the temperature- and pressure-dependent structure and phase behavior of the monoacylglycerides 1-monoelaidin (ME) and 1-monoolein (MO) in excess water. The monoacylglycerides were chosen for investigation of their phase behavior because they exhibit mesomorphic phases with one-, two-, and three-dimensional periodicity, such as lamellar, an inverted hexagonal and bicontinuous cubic phases, in a rather easily accessible temperature and pressure range. We studied the structure, stability, and transformations of the different phases over a wide temperature and pressure range, explored the epitaxial relations that exist between different phases, and established a relationship between the chemical structure of the lipid molecules and their phase behavior. For both systems, a temperature-pressure phase diagram has been determined in the temperature range from 0 to 100 degrees C at pressures from ambient up to 1400 bar, and drastic differences in phase behavior are found for the two systems. In MO-water dispersions, the cubic phase Pn3m extends over a large phase field in the T,p-plane. At temperatures above 95 degrees C, the inverted hexagonal phase is found. In the lower temperature region, a crystalline lamellar phase is induced at higher pressures. The phases found in ME-water include the lamellar crystalline Lc phase, the L beta gel phase, the L alpha liquid-crystalline phase, and two cubic phases belonging to the crystallographic space groups Im3m and Pn3m. In addition, the existence of metastable phases has been exploited. Between coexisting metastable cubic structures, a metric relationship has been found that is predicted theoretically on the basis of the curvature elastic energy approximation only.     

Phase separations of alpha-tocopherol in aqueous dispersions of distearoylphosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of distearoylphosphatidylethanolamine was examined by using synchrotron X-ray diffraction methods. There was evidence that alpha-tocopherol does not distribute randomly in the dispersed phospholipid but instead phospholipid phases enriched in alpha-tocopherol are formed. Heating codispersions from lamellar gel phase induced formation of hexagonal-II phase at temperatures below the main transition of the pure phospholipid and which were enriched in alpha-tocopherol. Codispersions containing 5 or 10 mol% alpha-tocopherol were induced to form a cubic phase at temperatures above the lamellar to hexagonal-II phase transition. Such phases were not observed in codispersions containing 2.5 or 20 mol% alpha-tocopherol in which only lamellar and hexagonal-II phases were formed. The space group of the cubic phases were tentatively assigned as Pn3m. Equilibration of codispersions at 4 degrees C results in the formation of lamellar crystalline phases enriched in alpha-tocopherol and phase separated domains of pure phospholipid. Two lamellar crystalline phases were characterized on the basis of their particular wide-angle X-ray scattering patterns. The lamellar crystalline phases were also distinguished from other lamellar phases of the pure phospholipid by the lamellar repeat. Partitioning of alpha-tocopherol into phosphatidylethanolamine domains in membranes may introduce instability into the structure.     

Stabilization of the monoolein Pn3m cubic structure on trehalose glasses

Trehalose is known to protect some organisms from various stresses due to drought and high temperature. To explore the molecular mechanism of the protective function, the mesomorphic properties of the monoolein-water system, dried in the presence of trehalose, were studied by X-ray diffraction. While, in pure water, two bicontinuous inverse cubic structures (the Pn3m and Ia3d phases) and a lamellar Lα phase exist as a function of concentration, only the Pn3m cubic phase has been detected in concentrated trehalose solutions or in trehalose glasses, even under extremely dry conditions. Depending on the sugar concentration, or after glass dehydration, the Pn3m cubic unit cell decreases to very low values, much below the smaller one observed in pure water. However, as no phase transitions occur, a simple osmotic mechanism can be excluded. An additional stabilization of the lipid phase, arising from interfacial free energy changes due to trehalose-water-lipid direct interactions, and large enough to affect the energetic balance between the Pn3m and the Ia3d cubic phases, evidently occurs. Moreover, no differences in the Pn3m cubic structure were observed when the sugar platelets convert to the glassy state; no apparent structural modifications that can be related to mechanical pressure exerted on the lipid phase have been detected. Received: 5 October 1998 / Accepted: 13 November 1998     

A comparative study of the effects of dimethylsulfoxide and glycerol on the bicontinuous cubic structure of hydrated monoolein and its phase behavior

Both dimethylsulfoxide (DMSO) and glycerol act cryoprotectants for biological systems and materials. Knowledge of molecular interactions of DMSO and glycerol with biological lipids is important for understanding of their cryoprotecitive mechanisms. In this study, the phase behavior and structures of hydrated monoolein were investigated in the presence of DMSO or glycerol, using differential scanning calorimetry (DSC) and simultaneous X-ray diffraction/DSC measurements. Based on the results obtained by this study, partial phase diagrams were constructed as a function of DMSO or glycerol concentrations and temperature. DMSO and glycerol hardly affect the enthalpy value for melting temperature of lamellar crystal phase of monoolein and the structure. On the other hand, DMSO and glycerol greatly affect the phase transformations associated with bicontinuous cubic phases of monoolein and the cubic phase structures. DMSO expands Im3m/Pn3m cubic phase co-existence region in the phase diagram and increases the lattice constant of the Pn3m monoolein cubic phase. Glycerol shows opposite effects. The present study suggests that different mechanisms act in the cryopreservation by DMSO and glycerol.     

Cubic phases in phosphatidylcholine-cholesterol mixtures: cholesterol as membrane "fusogen"

X-ray diffraction reveals that mixtures of some unsaturated phosphatidylcholines (PCs) with cholesterol (Chol) readily form inverted bicontinuous cubic phases that are stable under physiological conditions. This effect was studied in most detail for dioleoyl PC/Chol mixtures with molar ratios of 1:1 and 3:7. Facile formation of Im3m and Pn3m phases with lattice constants of 30-50 nm and 25-30 nm, respectively, took place in phosphate-buffered saline, in sucrose solution, and in water near the temperature of the Lalpha-HII transition of the mixtures, as well as during cooling of the HII phase. Once formed, the cubic phases displayed an ability to supercool and replace the initial Lalpha phase over a broad range of physiological temperatures. Conversion into stable cubic phases was also observed for mixtures of Chol with dilinoleoyl PC but not for mixtures with palmitoyl-linoleoyl PC or palmitoyl-oleoyl PC, for which only transient cubic traces were recorded at elevated temperatures. A saturated, branched-chain PC, diphytanoyl PC, also displayed a cubic phase in mixture with Chol. Unlike the PEs, the membrane PCs are intrinsically nonfusogenic lipids: in excess water they only form lamellar phases and not any of the inverted phases on their own. Thus, the finding that Chol induces cubic phases in mixtures with unsaturated PCs may have important implications for its role in fusion. In ternary mixtures, saturated PCs and sphingomyelin are known to separate into liquid-ordered domains along with Chol. Our results thus suggest that unsaturated PCs, which are excluded from these domains, could form fusogenic domains with Chol. Such a dual role of Chol may explain the seemingly paradoxical ability of cell membranes to simultaneously form rigid, low-curvature raft-like patches while still being able to undergo facile membrane fusion.     

Phase equilibria of membrane lipids from Acholeplasma laidlawii: importance of a single lipid forming nonlamellar phases

A basis for the reorganization of the bilayer structure in biological membranes is the different aggregate structures formed by lipids in water. The phase equilibria of all individual lipids and several in vivo polar lipid mixtures from acyl chain modified membranes of Acholeplasma laidlawii were investigated with different NMR techniques. All dioleoyl (DO) polar lipids, except monoglucosyldiglyceride (MGDG), form lamellar liquid crystalline (L alpha) phases only. The phase diagram of DOMGDG reveals reversed cubic (III), reversed hexagonal (HII), and L alpha phases. In mixtures of DOMGDG and dioleoyldiglycosyldiglyceride (DODGDG), the formation of an III (or HII) phase is enhanced by DOMGDG and low hydration or high temperatures. For in vivo mixtures of all polar DO lipids, a transition from an L alpha to an III phase is promoted by low hydration or high temperatures (50 degrees C). The phospholipids are incorporated in this III phase. Likewise, III and HII phases are formed at similar temperatures in a series of in vivo mixtures with different extents of acyl chain unsaturation. However, their melting temperatures (Tm) vary in an expected manner. All cubic and hexagonal phases, except the III phase with DOMGDG, exist in equilibrium with excess water. The maximum hydration of MGDG and DGDG is similar and increases with acyl chain unsaturation but is substantially lower than that for, e.g., phosphatidylcholine. The translational diffusion of the lipids in the cubic phases is rapid, implying bicontinuous structures. However, their appearances in freeze-fracture electron microscope pictures are different. The III phase of DOMGDG belongs to the Ia3d space group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inverted micellar intermediates and the transitions between lamellar, cubic, and inverted hexagonal amphiphile phases. III. Isotropic and inverted cubic state formation via intermediates in transitions between L alpha and HII phases

Inverted cubic and isotropic phases have been observed in phospholipid and glycolipid systems. These phases exhibit characteristic morphologies in freeze-fracture electron micrographs, isotropic 31P-NMR resonances and (in some cases) cubic X-ray diffraction patterns. It is proposed here that these phases may form from the same intermediates that are involved in lamellar/inverted hexagonal (L alpha/HII) phase transitions, and that it is possible that these cubic and isotropic phases are metastable. According to a kinetic theory of L alpha/HII phase transitions, intermediates in such transitions can form structures known as interlamellar attachments (ILAs). It is shown that ILAs should form in large numbers during L alpha/HII transitions in systems like those reported to form inverted cubic or isotropic structures. ILAs cannot readily assemble into either the HII phase or well-ordered arrays of L alpha phase bilayers, and represent a kinetic trap for intermediates in L alpha/HII transitions (although it is possible that they are marginally more stable in a thermodynamic sense than the L alpha phase in a small temperature range below TH). It is also shown that arrays of ILAs should form metastable arrays with the same morphology and isotropic 31P-NMR resonances that are observed in isotropic and inverted cubic states. In particular, under some circumstances ILAs will assemble into a structure identical to the bicontinuous inverted cubic phase previously described in monoglycerides and very similar in morphology to structures observed in phospholipid systems. Finally, since isotropic and cubic states form from ILAs, which also can mediate fusion of unilamellar vesicles, unilamellar vesicles should fuse to at least some extent under the same conditions in which multilamellar samples of the same lipid form isotropic or inverted cubic states. This correlation has been observed.     

Relationship between three-dimensional arrays of "lipidic particles" and bicontinuous cubic lipid phases

Several lipid-water mixtures form phases that give rise to freeze-fracture replicas exhibiting three-dimensional regular arrays of closely packed globular elements, often called "lipidic particles". These phases have often been poorly classified with respect to long-range organization and symmetry and have in most cases been asserted to be built up by closed lipid aggregates, such as reversed micelles. However, studies of phases giving rise to the above-mentioned freeze-fracture replicas, with X-ray diffraction and the nuclear magnetic resonance pulsed field gradient diffusion technique, have revealed that they are cubic liquid-crystalline phases and with one exception bicontinuous phases, i.e., cubic phases in which both the hydrocarbon and the water regions are continuous. Up to now the only known exception is a cubic phase composed of closed rod-shaped micelles of the normal type. Thus it is not possible to decide from a freeze-fracture image of a cubic phase, showing three-dimensional arrays of "lipidic particles", if the phase is bicontinuous or composed of closed lipid aggregates. Hitherto, it has not been shown that a biological membrane lipid-water system is able to form a cubic liquid-crystalline phase consisting of reversed micelles. The existence of such a phase is also improbable considering the location in the phase diagrams of cubic phases formed by biological membrane lipid-water systems.     

Relating structure and translational dynamics in aqueous dispersions of monoolein

The temperature dependence of the molecular diffusion in monoolein/water systems is investigated at several levels of hydration. Using the proton/deuteron selectivity, field gradient NMR allows the simultaneous determination of the diffusion constants of both, lipid and water molecules in the various lamellar and non-lamellar phases. Due to the mesoscopic structure of the monoolein/water phases, the diffusion coefficients are interpreted as 'reduced' or 'effective' diffusion coefficients, and are related to the microscopic molecular displacements by a so-called 'obstruction factor'. Changes in the microscopic structure at the phase transition from the bicontinuous cubic phases to the inverse hexagonal phase are reflected in the obstruction factor of the monoolein diffusion coefficients. The reduction of the water diffusion coefficients is too high to be explained by an obstruction factor only, implying a mechanism of molecular motion, which strongly differs from that of bulk water. Experiments on samples prepared with isotopic labeled water (2H(2)O and H(2)(17)O) indicate a chemical exchange of protons between the water molecules and the lipid headgroups on a millisecond timescale.     

Solubilization of ubiquinone-10 in the lamellar and bicontinuous cubic phases of aqueous monoolein

Using X-ray diffraction measurements and polarizing microscopy, the solubilization of ubiquinone-10 (UQ10) was investigated in the lamellar and reversed bicontinuous cubic phases of aqueous monoolein (MO, 86 wt% of monooleoylglycerol). At 25 degrees C and UQ10 content below 0.5 wt%, a partial phase diagram of the MO/UQ10/H2O system indicated the same sequence of hydration-induced phases as found in the MO/H2O system. This low amount of coenzyme had no effect on the MO bilayer thickness and swelling behavior of phases, but it promoted thermotropic Q230-->HII phase transition. We suggested that the effect was determined by the UQ10 partitioning into the HII phase regions where the MO chains must be stressed upon the phase transition. At UQ10 contents above 0.5 wt%, a solid 'UQ10-rich' phase appeared inside the initially homogeneous phases within a few days. It was proposed that this process was driven by the coenzyme lateral diffusion in the MO bilayer.     

Electrostatic control of phospholipid polymorphism

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

Interaction of a new anticancer prodrug, gemcitabine-squalene, with a model membrane : Coupled DSC and XRD study

Gemcitabine is an anticancer nucleoside analogue active against a wide variety of solid tumors. However it is rapidly deaminated to an inactive metabolite, leading to short biological half-life and induction of resistance. A new prodrug of gemcitabine, coupling squalene to gemcitabine (GemSq), has been designed to overcome the above drawbacks. It has been previously shown that this prodrug displays significantly higher anticancer activity than gemcitabine against leukemia. In the present study the structural modifications of dipalmitoylphosphatidylcholine (DPPC) model membranes induced by increasing concentrations of GemSQ have been investigated using small and wide angle X-ray scattering (SWAXS) and differential scanning calorimetry (DSC). At room temperature an unusual inverse bicontinuous cubic phase formed over a broad composition range. The basic bilayer structure displayed an intermediate order between those of the gel and fluid phases of DPPC. A reversible transition to a fluid lamellar phase occurred upon heating. The transitions between these two phases were governed by different mechanisms depending on the GemSq concentration in the membrane. Finally, the biological relevance of these observations for the cytotoxic activity of GemSq has been discussed.     

Crystallization of transmembrane proteins in cubo: mechanisms of crystal growth and defect formation

Crystallization of membrane proteins is a major stumbling block en route to elucidating their structure and understanding their function. The novel concept of membrane protein crystallization from lipidic cubic phases, "in cubo", has yielded well-ordered crystals and high-resolution structures of several membrane proteins, yet progress has been slow due to the lack of understanding of the molecular mechanisms of protein transport, crystal nucleation, growth, and defect formation in cubo. Here, we examine at molecular and mesoscopic resolution with atomic force microscopy the morphology of in cubo grown bacteriorhodopsin crystals in inert buffers and during etching by detergent. The results reveal that crystal nucleation occurs following local rearrangement of the highly curved lipidic cubic phase into a lamellar structure, which is akin to that of the native membrane. Crystals grow within the bulk cubic phase surrounded by such lamellar structures, whereby transport towards a growing crystalline layer is constrained to within an individual lamella. This mechanism leads to lack of dislocations, generation of new crystalline layers at numerous locations, and to voids and block boundaries. The characteristic macroscopic lengthscale of these defects suggests that the crystals grow by attachment of single molecules to the nuclei. These insights into the mechanisms of nucleation, growth and transport in cubo provide guidance en route to a rational design of membrane protein crystallization, and promise to further advance the field.     

Formation of monolayers and bilayer foam films from lamellar,inverted hexagonal and cubic lipid phases

This study revealed large distinctions between the lamellar and non-lamellar liquid crystalline lipid phases in their spreading at the air/water interface and propensity to form bilayer foam films. Comparative measurements were made for the lamellar L(alpha), the inverted hexagonal H(II) and the bicontinuous cubic Pn3m phases of the phospholipid dipalmitoleoylphosphatidylethanolamine (DPoPE). With regard to monolayer formation, followed as the decrease of surface tension with time, the best spreading (lowest surface tension) was observed for the L(alpha) phase, and poorest spreading (highest surface tension) was recorded for the H(II) phase. The cubic Pn3m phase of DPoPE, induced by temperature cycling, retained an intermediate position between the L(alpha) and H(II) phases. According to their ability to lower surface tension and disintegrate at the air/water interface, the three phases thus order as L(alpha)>Pn3m>H(II). Clearly expressed threshold (minimum) bulk lipid concentrations, C(t), required for formation of stable foam bilayers from these phases, were determined and their values were found to correlate well with the bulk lipid phase behaviour. The C(t) values for L(alpha) and H(II) substantially increase with the temperature. Their Arrhenius plots, ln C(t) versus 1/ T, are linear and intersect at approximately 36-37 degrees C, coinciding with the onset of the bulk L(alpha)-->H(II) phase transition, as determined by differential scanning calorimetry. However, the C(t) value for the Pn3m phase, equal to 30 micro g/mL, was found to be constant over the whole range investigated between 20 degrees C and 50 degrees C. The horizontal C(t) versus T plot for the Pn3m phase crosses the respective plot for the L(alpha) phase at the temperature bounding from below the hysteretic loop of the L(alpha)<-->H(II) transition (approximately 26 degrees C), thus providing a certain insight about the thermodynamic stability of the Pn3m phase relative to the L(alpha) phase. The established strong effect of the particular lipid phase on the formation of monolayers and stable black foam films should be of importance in various in vitro and in vivo systems, where lipid structures are in contact with interfaces and disintegrate there to different extents.     

Thermal behaviour of cubic phases rich in 1-monooleoyl-rac-glycerol in the ternary system. 1-monooleoyl-rac-glycerol/n-octyl-beta-D-glucoside/water.

Using synchrotron X-ray diffraction the thermal behaviour was studied of the cubic phases in the 1-monooleoyl-rac-glycerol (MO)/n-octyl-beta-d-glucopyranoside (OG)/2H2O system with 58 or 45 wt % MO concentration and varying OG/2H2O contents. These MO contents correspond to a Pn3m cubic single-phase or a Pn3m cubic phase in excess water on the binary MO/water axis of the ternary phase diagram. The cubic liquid crystalline phases are stable with small fractions of OG, while higher OG concentrations trigger a cubic-to-lamellar phase transition. Moreover, with increasing OG concentration the initial Pn3m structure is completely converted to an Ia3d structure prior to the Lalpha phase being formed. Upon heating this effect is reversed, resulting in an Ia3d-to-Pn3m phase transition. For some samples additional peaks were observed in the diffractograms upon heating, resulting from the metastability notoriously shown by bicontinuous cubic phases. This judgement is supported by the fact that upon cooling these peaks were absent. Remarkably, both the Ia3d and the Pn3m cubic structures could be in equilibrium with excess water in this ternary system. A comparison is made with previous results on n-dodecyl-beta-d-maltoside (DM), showing that cubic phases with OG have higher thermal and compositional stability than with DM.     

Lipid and water diffusion in bicontinuous cubic phases measured by NMR.

Lipid and water diffusion coefficients in bicontinuous cubic liquid crystalline phases have been determined with the NMR pulsed magnetic field gradient technique. In the monoolein-water system, a discontinuity in the variation of the water diffusion coefficient with water content is observed, which coincides with the two-phase region between the two cubic phases in this system. The degree of water association to the lipid has been determined, considering the obstruction factor for diffusion in the cubic phases. The lipid diffusion coefficient increases with increased unsaturation of the lipid, and decreases when larger amphiphile molecules like cholesterol, gramicidin-A, and lyso-oleoyl-phosphatidylcholine are solubilized in the cubic phase. In a cubic liquid crystal of monoolein (MO), dioleoylphosphatidylcholine (DOPC), and water, the individual lipid diffusion coefficients have been determined simultaneously in the same sample. The diffusion coefficients of MO and DOPC differ by a factor of two, and both decrease with increasing DOPC content. The results are discussed in relation to probe techniques for measurements of lipid diffusion.