Cubic phases of lipid-containing systems. The structure of phase Q223 (space group Pm3n). An X-ray scattering study.

The hexagonal (H) and the cubic (Q223) phases of the systems dodecyltrimethylammonium chloride-water and palmitoyllysophosphatidy choline-water have been studied by X-ray scattering techniques. The signs of the reflections of phase H were determined by a systematic study as a function of the water content, those of phase Q223 were assessed using a pattern recognition approach based upon the axiom that the histograms of the electron density maps of phases Q223 and H, extrapolated to the same concentration and properly normalized in scale and shape, are very similar to each other. In the case of phase Q223, all the sign combinations (the phi-sets) compatible with the observed reflections were generated, and each of the corresponding histograms was compared with the histogram of the map of phase H. One novelty of this work is the use of a highly sensitive criterion to estimate the similarity of the histograms, namely the distance in the six-dimensional space of the moments [mean value of (delta rho)n]1/n, for 3 greater than or equal to n greater than or equal to 8. In the two systems, the use of this criterion has led to the unambiguous choice of one electron density map. The maps show that the structure of phase Q223 consists of disjointed micelles (of type I), belonging to two different classes: those of one class are quasi-spherical in shape and are centered at the points a, those of the other class are disc-shaped and are centred at the points c. The results of this work rule out a structure formed by a cage-like distribution of rods enclosing a set of quasi-spherical micelles and is consistent with previous proposals. This is the second example, after that of phase Q227, of a micellar cubic phases in lipid-containing systems; all the known examples of phase Q223 are of type I, those of phase Q227 of type II.     

Phase transition from a gel to a fluid phase of cubic symmetry in dimyristoylphosphatidylcholine/myristic acid (1:2, mol/mol) bilayers

Aqueous dispersions (pH 4.0) of a 2:1 (mol/mol) mixture of myristic acid with dimyristoylphosphatidylcholine undergo a sharp transition at 45-47 degrees C from a lamellar gel phase to a fluid phase which is optically isotropic. This fluid phase gives rise to 31P-NMR spectra, and 2H-NMR spectra of the chain-deuterated components, which are also isotropic. X-ray diffraction studies of the fluid phase at 49 degrees C, reveal reflections with spacings in the ratio square root of 2: (square root of 3): square root of 4: square root of 6: square root of 8, accompanied by a strong diffuse scatter. These reflections index on a cubic lattice of primitive space group Pn3 or Pn3m, or possibly the body-centered group Im3m, with a lattice constant of 21.2 nm. The dimensions of the phase are consistent with a structure composed of two systems of tetrahedrally (octahedrally) oriented inverted lipid cylinders, found for other cubic lipid phases with Pn3m (Im3m) symmetry. At higher temperatures the cubic phase gradually converts, with increasing temperature, to a coexisting inverted hexagonal phase.     

Polymorphism of a lipid extract from Pseudomonas fluorescens: structure analysis of a hexagonal phase and of a novel cubic phase of extinction symbol Fd--

The phase diagram of the Pseudomonas fluorescens lipid extract is unusual, in the sense that it displays a cubic phase straddled by a hexagonal phase. The hexagonal phase was studied over an extended concentration range, and the reflections were phased on the assumption that the structure contains circular cylinders of known radius. The cubic phase, whose extinction symbol is Fd--, was analyzed by reference to space group No. 227 (Fd3m). The phases of the reflections were determined by using a novel pattern recognition approach, based upon the notion that the average fourth power of the electron density contrast mean value of (delta r)4 is dependent on chemical composition but not on physical structure, provided that the function delta r(r) satisfies the constraints mean value of (delta r) = 0 and mean value of (delta r)2 = 1. As a further constraint, a shape normalization is used, in the form of a Gaussian apodization of the intensities, which has the effect of normalizing the curvature of the autocorrelation function at the origin. We analyzed two cubic samples of different composition: for each of them we generated all the phase combinations compatible with the X-ray scattering data and we searched for those whose mean value of (delta r)4 best agrees with the hexagonal phase. Taking advantage of the favorable properties of the phase diagram, we carefully explored the effects of various parameters; we concluded that the chemical composition of the phases being compared must be identical, that the X-ray scattering data should not be truncated artificially, and that the apodization must be mild so that the curvature takes a value intermediate between those corresponding to the raw data of the two phases. When all these precautions were taken, mean value of (delta r)4 was found to be remarkably invariant; this conclusion is important in view of the possible usefulness of the novel technique in tackling ab initio--and at very low resolution--structural problems of more general interest. The structure of the cubic phase consists of a 3D network of rods joined tetrahedrally 4 by 4 according to a diamond lattice and of a family of quasi-spherical disjointed micelles; the core of the rods and of the micelles is polar, and the interstices are filled by the hydrocarbon chains (structure of type II). All the dimensions (diameter of rods and micelles, area per chain at the polar/apolar interface) are consistent with the chemical properties of the system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Phase equilibria in four lysophosphatidylcholine/water systems. Exceptional behaviour of 1-palmitoyl-glycerophosphocholine

The phase equilibria in four lysophosphatidylcholine/water systems were investigated at different temperatures. Each of the 1-palmitoyl-, 1-stearoyl-, 1-oleoyl- and 1-linoleoyl-sn-glycero-3-phosphocholines was dispersed in heavy water at different concentrations. The phase structures were determined by 2H-, 14N- and 31P-NMR, polarization microscopy and low-angle X-ray diffraction. The phase diagrams of the oleoyl and linoleoyl systems were quite similar. At room temperature and with decreasing water content the isotropic micellar solution was followed by a hexagonal phase and then a cubic phase. Finally the lamellar phase appeared before the region of hydrated crystals. The same sequence of phases was observed in the stearoyl system at elevated temperatures. The palmitoyl system differed from the others: here a cubic phase followed after the micellar solution, then came a hexagonal phase and after this a lamellar phase. In general the lysophosphatidylcholines seem to behave similarly to the many soaps and detergents as they show the same sequence of isotropic micellar solution, hexagonal phase, lamellar phase with interspersed cubic phases. The presently established phase diagrams demonstrate that the major lysophosphatidylcholines which may be generated by phospholipase A2 in mammalian cell membranes, viz. 1-palmitoyl- and 1-stearoyl-glycerophosphocholines differ greatly in their packing properties. The extraordinary ability of 1-palmitoyl-glycerophosphocholine to form a cubic phase in equilibrium with a micellar solution is of particular interest with regard to the possible occurrence of cubic structures in biomembranes during the process of fusion.     

Cubic phases in hydrated 1:1 and 1:2 dipalmitoylphosphatidylcholine-dipalmitoylglycerol mixtures.

The structures of fully hydrated 1:1 and 1:2 (mol/mol) dipalmitoylphosphatidylcholine (DPPC)-dipalmitoylglycerol (DPG) mixtures were studied by means of small-angle x-ray diffraction. The x-ray diffraction pattern of the 1:1 (mol/mol) DPPC-DPG mixture at 65 degrees C contains three reflections with spacings in the ratio of 1:1/ square root of 2:1/ square root of 3 in addition to reflections of an inverted hexagonal (H11) phase. A possible interpretation of this result is that a cubic phase of the body-centered space group lm3m, with a lattice constant of 23.1 +/- 0.6 nm, is formed. This cubic phase appears at intermediate temperatures between the lamellar and the H11 phases. The 1:2 (mol/mol) DPPC-DPG mixture gives an x-ray diffraction pattern at temperatures higher than the lamellar-to-H11 transition containing a number of reflections that index a cubic phase structure. The space group of the cubic phase was assigned a face-centered group Fd3m with a lattice constant of 16.3 +/- 0.1 nm at 82 degrees C. The possible role of cubic phases in membrane phenomena such as transmembrane signal transduction and fusion is discussed.     

Nuclear magnetic resonance studies of lipid hydration in monomethyldioleoylphosphatidylethanolamine dispersions.

Solid-state proton nuclear magnetic resonance has been used to examine surface hydration in suspensions of monomethyldioleoylphosphatidylethanolamine (MeDOPE). The magic-angle spinning (MAS) 1H spectra for aqueous suspensions of MeDOPE in the L alpha phase exhibited two resonances of roughly equal intensity that could be ascribed to water protons, but both their spin-lattice relaxation times and chemical shifts converged upon conversion to the hexagonal phase. Only a single water peak was observed for analogous samples of dioleoylphosphatidylcholine (DOPC). MAS-assisted two-dimensional nuclear Overhauser effect spectroscopy (NOESY) was conducted for multibilayers of both MeDOPE and DOPC. Through-space interactions were identified between pairs of lipid protons, as expected from their chemical structure. For lamellar suspensions of MeDOPE, positive NOESY cross-peaks were observed between the downfield-shifted water resonance (only) and both CH2N and NH2CH3+ protons of the lipid headgroup. These cross-peaks were not observed in the NOESY spectra of MeDOPE in its hexagonal or cubic phases or for lamellar DOPC reference samples. Taken together, the observation of two water peaks, spin-lattice relaxation behavior, and NOESY connectivities in MeDOPE suspensions support the interpretation that the low-field water peak corresponds to hydrogen-bonded interlamellar water interacting strongly with the lipid. Such a population of water molecules exists in association with MeDOPE in the lamellar phase but not for its inverted phases or for lamellar dispersions of DOPC.     

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.     

Phase diagram of soybean phosphatidylcholine-diacylglycerol-water studied by x-ray diffraction and 31P- and pulsed field gradient 1H-NMR: evidence for reversed micelles in the cubic phase.

The phase equilibria of the system soybean phosphatidylcholine, diacylglycerol, and water has been determined using a combination of classical methods together with x-ray diffraction and NMR techniques. In particular, the extent of the phase regions of the lamellar, the reversed hexagonal, and the cubic phases have been determined. By pulsed field gradient 1H-NMR, the diffusion coefficients of all three components in a cubic phase composed of soybean phosphatidylcholine, diacylglycerol, and heavy water have been determined at 25 and 59 degrees C and also for the corresponding cubic phase composed of the chemically more well defined synthetic components 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoylglycerol (DOG), and heavy water. The extension of the phase region of the cubic phase did not seem to change appreciably for the two ternary systems studied. The translational diffusion coefficient of DOPC in this cubic phase is more than an order of magnitude smaller (3 x 10(-13) m2 s-1, 59 degrees C) than the lateral diffusion coefficient of DOPC in an oriented lipid bilayer (5 x 10(-12) m2 s-1, 35 degrees C), whereas the diffusion coefficients of water and DOG were found to be about two orders of magnitude larger than DOPC at 59 degrees C. It is concluded that the cubic phase is built built up of closed reversed micelles in accordance with the suggestion from previous x-ray diffraction studies.     

Observation of inverted cubic phase in hydrated dioleoylphosphatidylethanolamine membranes

We report the observation of an inverted cubic phase in aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) by small-angle X-ray diffraction. DOPE is a paradigm in the study of nonlamellar phases in biological systems: it exhibits a well-known phase transition from the lamellar (L alpha) to the inverted hexagonal phase (HII) as the temperature is raised. The transition is observed to occur rapidly when a DOPE dispersion is heated from 2 degrees C, where the L alpha phase is stable, to 15 degrees C, where the HII phase is stable. We report on the induction of a crystallographically well-defined cubic lattice that is slowly formed when the lipid dispersion is rapidly cycled between -5 and 15 degrees C hundreds of times. Once formed, the cubic lattice is stable at 4 degrees C for several weeks and exhibits the same remarkable metastability that characterizes other cubic phases in lipid-water systems. X-ray diffraction indicates that the cubic lattice is most consistent with either the Pn3m or Pn3 space group. Tests of lipid purity after induction of the cubic indicate the lipid is at least 98% pure. The cubic lattice can be destroyed and the system reset by cycling the specimen several times between -30 and 2 degrees C. The kinetics of the formation of the cubic are dependent on the thermal history of the sample, overall water concentration, and the extreme temperatures of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.     

Molecular dynamics simulation of spontaneous membrane fusion during a cubic-hexagonal phase transition

We report a molecular dynamics simulation of the phase transition of monoolein from an inverted cubic phase to an inverted hexagonal phase. The transition proceeds via an intermediate structure consisting of water channels in a cubic geometry, in agreement with the predictions of the modified stalk theory (Siegel, 1999). Two mechanisms are identified by which the topology changes during the transition. Bilayer fusion proceeds via the formation of trans-monolayer contacts, whereas bilayer rupture is observed as a gradual thinning of each monolayer.     

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)     

Interaction of the peptide antibiotic alamethicin with bilayer- and non-bilayer-forming lipids: influence of increasing alamethicin concentration on the lipids supramolecular structures

Incorporation of the helical antimicrobial peptide alamethicin from aqueous phase into hydrated phases of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) was investigated within a range of peptide concentrations and temperatures by time-resolved synchrotron X-ray diffraction. It was found that alamethicin influences the organizations of the non-bilayer-forming (DOPE) and the bilayer-forming (DOPC) lipids in different ways. In DOPC, only the bilayer thickness was affected, while in DOPE new phases were induced. At low peptide concentrations (<1.10(-4) M), an inverted hexagonal (H(II)) phase was observed as with DOPE dispersions in pure buffer solution. A coexistence of two cubic structures was found at the critical peptide concentration for induction of new lipid/peptide phases. The first one Q224 (space group Pn3m) was identified within the entire temperature region studied (from 1 to 45 degrees C) and was found in coexistence with H(II)-phase domains. The second lipid/peptide cubic structure was present only at temperatures below 16 degrees C and its X-ray reflections were better fitted by a Q212 (P4(3)32) space group, rather than by the expected Q229 (Im3m) space group. At alamethicin concentrations of 1 mM and higher, a nonlamellar phase transition from a Q224 cubic phase into an H(II) phase was observed. Within the investigated range of peptide concentrations, lamellar structures of two different bilayer periods were established with the bilayer-forming lipid DOPC. They correspond to lipid domains of associated and nonassociated helical peptide. The obtained X-ray results suggest that the amphiphilic alamethicin molecules adsorb from the aqueous phase at the lipid head group/water interface of the DOPE and DOPC membranes. At sufficiently high (>1.10(-4) M) solution concentrations, the peptide is probably accommodated in the head group region of the lipids thus inducing structural features of mixed lipid/peptide phases.     

Catalysis by enzymes entrapped into hydrated surfactant aggregates having lamellar or cylindrical (hexagonal) or ball-shaped (cubic) structure in organic solvents

Instead of aqueous solutions, universally recognized in enzymology, ternary systems of the water/organic solvent/surfactant type are suggested as liquid-crystalline media for enzymatic reactions. Two systems, water/octane/Aerosol OT and water/cyclohexane/Brij 96, have been used to solubilize acid and alkaline phosphatases and peroxidase. The enzymes under study do function in liquid-crystalline mesophases having lamellar, cylindrical (reversed hexagonal) and ball-shaped (reversed cubic) packing of the surfactant molecules. A significant result is that the phase transition from one liquid-crystalline structure to another entails, as a rule, a reversible change in the catalytic activity of the solubilized enzyme.     

Structural relationships between lamellar,cubic and hexagonal phases in monoglyceride-water systems. possibility of cubic structures in biological systems

The transitions lamellar → cubic → hexagonal in the aqueous system of sunflower oil monoglycerides are analysed. X-Ray diffraction data show linear relationships between the lattices of the three phases, which are discussed on the basis of structures formed by lipid bilayer units. The cubic structure is related to ‘Schwarz's primitive cubic minimal surface’ and consists of a three-dimensional continuous bilayer system separating two separate water channel systems.It is also pointed out that the three-dimensional membrane system in plant plastids, the prolamellar body, which is involved in the formation of thylakoid membranes of chloroplasts, has a structure which is closely related to or identical with that of the cubic phase of monoglyceride-water systems.     

Effect of bile salts on monolayer curvature of a phosphatidylethanolamine/water model membrane system.

A partial phase diagram of the ternary system dioleoylphosphatidylethanolamine (DOPE)/sodium cholate/water has been determined using 31P Nuclear Magnetic Resonance (NMR) spectroscopy. In the absence of cholate, it is well known that the DOPE/water system forms a reversed hexagonal (HII) phase. We have found that addition of even small amounts of cholate to the DOPE/water system leads to a transition to a lamellar (L alpha) phase. At higher cholate concentrations, a cubic (I) phase (low water content) or a micellar solution (L1) phase (high water content) is present. Thus, cholate molecules have a strong tendency to alter the lipid monolayer curvature. Increasing the concentration of cholate changes the curvature of DOPE from negative (HII phase), through zero (L alpha phase), and finally to a phase of positive curvature (micellar solution). This observation can be rationalized in terms of the molecular structure of cholate, which is amphipathic and has one hydrophobic and one hydrophilic side of the steroid ring system. The cholate molecules have a tendency to lie flat on the lipid aggregate surface, thereby increasing the effective interfacial area of the polar head groups, and altering the curvature free energy of the system.     

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.     

Three-dimensional crystals of the light-harvesting chlorophyll a/b protein complex from pea chloroplasts

Two forms of three-dimensional crystals of the light-harvesting chlorophyll a/b protein complex from pea have been obtained. Crystals of one form grew as hexagonal plates measuring up to 150 micron across and 2 to 3 micron in thickness. Electron diffraction patterns of thin hexagonal plates showed sharp reflections to a resolution of 3.7 A on a hexagonal reciprocal lattice. The unit cell in projection (a = 127.0 A) and the symmetry of the diffraction pattern (6 mm) suggested that the hexagonal plates were highly ordered stacks of two-dimensional crystals suitable for structure analysis by electron microscopy and image processing. Crystals of a second form grew as dark green octahedra measuring roughly 0.5 mm across. Low-resolution X-ray diffraction patterns suggested a large cubic unit cell (a = 390 A). SDS/polyacrylamide gel electrophoresis of single octahedral crystals showed the same polypeptide composition as the starting solution, one major band at 24,000 apparent molecular weight and two satellite bands of 23,000 and 23,500 apparent molecular weight.     

Molecular arrangements in glycosphingolipids

A number of homogeneous glycosphingolipids have been prepared and their structural behaviour studied in the solid state as well as in lipid-water systems and in surface films. Mainly X-ray diffraction techniques have been used in the phase analyses. A very complex phase pattern is usually found — e.g. cerebroside containing 2-hydroxy fatty acids has 5 crystalline phases and 2 thermotropic mesophases. This is also the case in the water systems, where hexagonal, lamellar and cubic mesophases are observed. Whereas in earlier surface film studies of complex lipids, such as phospholipids, only one liquid expanded phase usually has been found, cerebrosides also exhibit numerous condensed phases. Comparisons with corresponding natural lipids have shown a close relationship both in the phase behaviour and structure of the different polymorphs.