An X-ray diffraction study of the effect of alpha-tocopherol on the structure and phase behaviour of bilayers of dimyristoylphosphatidylethanolamine.

The effect of alpha-tocopherol on the thermotropic phase transition behaviour of aqueous dispersions of dimyristoylphosphatidylethanolamine was examined using synchrotron X-ray diffraction methods. The temperature of gel to liquid-crystalline (Lbeta-->Lalpha) phase transition decreases from 49.5 to 44.5 degrees C and temperature range where gel and liquid-crystalline phases coexist increases from 4 to 8 degrees C with increasing concentration of alpha-tocopherol up to 20 mol%. Codispersion of dimyristoylphosphatidylethanolamine containing 2.5 mol% alpha-tocopherol gives similar lamellar diffraction patterns as those of the pure phospholipid both in heating and cooling scans. With 5 mol% alpha-tocopherol in the phospholipid, however, an inverted hexagonal phase is induced which coexists with the lamellar gel phase at temperatures just before transition to liquid-crystalline lamellar phase. The presence of 10 mol% alpha-tocopherol shows a more pronounced inverted hexagonal phase in the lamellar gel phase but, in addition, another non-lamellar phase appears with the lamellar liquid-crystalline phase at higher temperature. This non-lamellar phase coexists with the lamellar liquid-crystalline phase of the pure phospholipid and can be indexed by six diffraction orders to a cubic phase of Pn3m or Pn3 space groups and with a lattice constant of 12.52+/-0.01 nm at 84 degrees C. In mixed aqueous dispersions containing 20 mol% alpha-tocopherol, only inverted hexagonal phase and lamellar phase were observed. The only change seen in the wide-angle scattering region was a transition from sharp symmetrical diffraction peak at 0.43 nm, typical of gel phases, to broad peaks centred at 0.47 nm signifying disordered hydrocarbon chains in all the mixtures examined. Electron density calculations through the lamellar repeat of the gel phase using six orders of reflection indicated no difference in bilayer thickness due to the presence of 10 mol% alpha-tocopherol. The results were interpreted to indicate that alpha-tocopherol is not randomly distributed throughout the phospholipid molecules oriented in bilayer configuration, but it exists either as domains coexisting with gel phase bilayers of pure phospholipid at temperatures lower than Tm or, at higher temperatures, as inverted hexagonal phase consisting of a defined stoichiometry of phospholipid and alpha-tocopherol molecules.     

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.     

The distribution of alpha-tocopherol in mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine

The effect of alpha-tocopherol on the structure and phase behaviour of mixed aqueous dispersions of phosphatidylcholine and phosphatidylethanolamine has been examined by synchrotron X-ray diffraction. Equimolar mixtures of dioleoylphosphatidylethanolamine:dioleoylphosphatidylcholine and dimyristoylphosphatidylcholine:dioleoylphosphatidylethanolamine did not show evidence of phase separation of an inverted hexagonal structure typical of alpha-tocopherol and phosphatidylethanolamine from lamellar phase. Mixed dispersions of dioleoyl derivatives of phosphatidylethanolamine:phosphatidylcholine (3:1) form a typical miscible gel phase at low temperatures but which phase separates into lamellar liquid-crystal and inverted hexagonal phases at temperatures greater than 65 degrees C. The presence of 1, 2 or 5 mol% alpha-tocopherol caused a decrease in the temperature at which the inverted hexagonal phase appears. Phase separation of non-lamellar phase from lamellar gel phase can be detected in the presence of 7.5 and 10 mol% alpha-tocopherol, indicating a limited capacity of the phosphatidylcholine to incorporate alpha-tocopherol into the lamellar domain. A partial phase diagram of the ternary mixture has been constructed from the X-ray scattering data. It was concluded that there is no preferential interaction of alpha-tocopherol with phosphatidylethanolamine in mixed aqueous dispersions containing phosphatidylcholines.     

The interaction of alpha-tocopherol with bilayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine

The effect of alpha-tocopherol on the structure and phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylcholine was examined by real-time synchrotron X-ray diffraction and freeze-fracture electron microscopic methods. X-ray scattering intensity was recorded from mixed aqueous dispersions of phospholipid with 2.5, 5, 10 and 20 mol% alpha-tocopherol during temperature scans at 3 degrees /min between -25 and 10 degrees C. A ripple structure is induced by the presence of alpha-tocopherol that coexists with the ripple phase characteristic of the pure phospholipid in mixtures containing 2.5 mol% alpha-tocopherol but completely replaces it in mixtures containing greater proportions of alpha-tocopherol. Freeze-fracture replicas of dispersions containing 5 mol% alpha-tocopherol indicate a ripple phase with a periodicity of about 9 nm. Increasing amounts of alpha-tocopherol result in a progressive reduction in temperature of the gel to liquid-crystal phase transition and broadening of the transition. Two lamellar phases coexist in the liquid-crystal state, one with a spacing of 6.4 nm assigned to an alpha-tocopherol-enriched lamellar structure and the other with a lamellar repeat of 6.1 nm corresponding to bilayers of pure phospholipid.     

Modulation of the phase heterogeneity of aminoglycerophospholipid mixtures by sphingomyelin and monovalent cations: maintenance of the lamellar arrangement in the biological membranes

The phase behaviour of mixed molecular species of phosphatidylethanolamine, phosphatidylserine and sphingomyelin of biological origin were examined in aqueous co-dispersions using synchrotron X-ray diffraction. The co-dispersions of phospholipids studied were aimed to model the mixing of lipids populating the cytoplasmic and outer leaflets in the resting or scrambled activated cell membrane. Mixtures enriched with phosphatidylethanolamine and phosphatidylserine were characterized by a phase separation of non-lamellar phases (cubic and inverted hexagonal) with a lamellar gel phase comprising the most saturated molecular species. Inclusion of sphingomyelin in the mixture resulted in a suppression of the hexagonal-II phase in favour of lamellar phases at temperatures where a proportion of the phospholipid was fluid. The effect was also dependent on the total amount of sphingomyelin in ternary mixtures, and the lamellar phase dominated in mixtures containing more than 30 mol%, irrespective of the relative proportions of phosphatidylserine/sphingomyelin. A transition from gel to liquid-crystal phase was detected by wide-angle scattering during heating scans of ternary mixtures enriched in sphingomyelin and was shown by thermal cycling experiments to be coupled with a hexagonal-II phase to lamellar transition. In such samples there was evidence of a coexistence of non-lamellar phases with a lamellar gel phase. A transition of the gel phase to the fluid state on heating from 35 to 41 °C was evidenced by a progressive increase in the lamellar d-spacing. The presence of calcium enhanced the phase separation of a lamellar gel phase from a hexagonal-II phase in mixtures enriched in phosphatidylserine. This effect was counteracted by charge screening with 150 mM NaCl. The effect of sphingomyelin on stabilizing the lamellar phase is discussed in the context of an altered composition in the cytoplasmic/outer leaflets of the plasma membrane resulting from scrambling of the phospholipid distribution. The results suggest that a lamellar structure can be retained by the inward translocation of sphingomyelin in biological membranes. The presence of monovalent cations serves also to stabilize the bilayer in activated cells where a translocation of aminoglycerophospholipids and an influx of calcium occur simultaneously.Abbreviations PC phosphatidylcholine - PE phosphatidylethanolamine - PS phosphatidylserine - SAXS small-angle X-ray scattering - SM sphingomyelin - WAXS wide-angle X-ray scattering - XRD X-ray diffraction     

Structure and interactions of ether- and ester-linked phosphatidylethanolamines

The ether-linked phospholipid 1,2-dihexadecylphosphatidylethanolamine (DHPE) was studied as a function of hydration and in fully hydrated mixed phospholipid systems with its ester-linked analogue 1,2-dipalmitoylphosphatidylethanolamine (DPPE). A combination of differential scanning calorimetry (DSC) and X-ray diffraction was used to examine the phase behavior of these lipids. By DSC, from 0 to 10 wt % H2O, DHPE displayed a single reversible transition that decreased from 95.2 to 78.8 degrees C and which was shown by X-ray diffraction data to be a direct bilayer gel to inverted hexagonal conversion, L beta----HII. Above 15% H2O, two reversible transitions were observed which stabilized at 67.1 and 92.3 degrees C above 19% H2O. X-ray diffraction data of fully hydrated DHPE confirmed the lower temperature transition to be a bilayer gel to bilayer liquid-crystalline (L beta----L alpha) phase transition and the higher temperature transition to be a bilayer liquid-crystalline to inverted hexagonal (L alpha----HII) phase transition. The lamellar repeat distance of gel-state DHPE increased as a function of hydration to a limiting value of 62.5 A at 19% H2O (8.6 mol of water/mol of DHPE), which corresponds to the hydration at which the transition temperatures are seen to stabilize by DSC. Electron density profiles of DHPE, in addition to calculations of the lipid layer thickness, confirmed that DHPE in the gel state forms a noninterdigitated bilayer at all hydrations. Fully hydrated mixed phospholipid systems of DHPE and DPPE exhibited two reversible transitions by DSC.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Stable cubic phases in codispersions of glucocerebroside and palmitoyloleoylphosphatidylethanolamine

The effect of glucocerebroside (GlcCer) on the structure and thermotropic phase behavior of aqueous dispersions of palmitoyloleoylphosphatidylethanolamine (POPE) has been examined using simultaneous small-angle and wide-angle X-ray diffraction methods. Binary mixtures of GlcCer:POPE in molar ratios of 2:100, 5:100, 10:100, 20:100, 30:100, and 40:100 were examined in the temperature range 20-90 degrees C. Cubic phase has been observed in binary mixtures comprised of molar ratios greater than 5:100 in the temperature range of 60-90 degrees C upon heating at a rate of 2 degrees C/min. The cubic phase is relatively stable and coexists with inverted hexagonal or lamellar phases. It persists in the codispersions throughout subsequent cooling scans to 30 degrees C. The space group of the cubic phase is determined to be Pn3m or Pn3. The lattice constant of the Pn3m cubic phase was found to be almost constant when it coexists with lamellar liquid-crystal phase. Marked temperature-dependent changes were observed when cubic phase coexists with hexagonal phase or lamellar-gel phases. This is the first report of cubic phases formed by codispersions of glycosphingolipids and phospholipids. The mechanism of cubic phase formation and the interaction between GlcCer and POPE is discussed in terms of the putative biological functions of glycolipids.     

Characterization of a quasicrystalline phase in codispersions of phosphatidylethanolamine and glucocerebroside

Synchrotron x-ray diffraction, differential scanning calorimetry, and electron spin resonance spectroscopy have been employed to characterize a quasicrystalline phase formed in aqueous dispersions of binary mixtures of glucocerebroside and palmitoyloleoylphosphatidylethanolamine. Small- and wide-angle x-ray scattering intensity patterns were recorded during temperature scans between 20 degrees and 90 degrees C from mixtures of composition 2, 5, 10, 20, 30, and 40 mol glucocerebroside per 100 mol phospholipid. The quasicrystalline phase was characterized by a broad lamellar d-spacing of 6.06 nm at 40 degrees C and a broad wide-angle x-ray scattering band centered at approximately 0.438 nm, close to the gel phase centered at approximately 0.425 nm and distinct from a broad peak centered at 0.457 nm observed for a liquid-crystal phase at 80 degrees C. The quasicrystalline phase coexisted with gel and fluid phase of the pure phospholipid. An analysis of the small-angle x-ray scattering intensity profiles indicated a stoichiometry of one glucosphingolipid per two phospholipid molecules in the complex. Structural transitions monitored in cooling scans by synchrotron x-ray diffraction indicated that a cubic phase transforms initially into a lamellar gel. Thermal studies showed that the gel phase subsequently relaxes into the quasicrystalline phase in an exothermic transition. Electron spin resonance spectroscopy using spin labels located at positions 7, 12, and 16 carbons of phospholipid hydrocarbon chains indicated that order and motional constraints at the 7 and 12 positions were indistinguishable between gel and quasicrystalline phases but there was a significant decrease in order and increase in rate of motion at the 16 position on transformation to the quasicrystalline phase. The results are interpreted as an arrangement of polar groups of the complex in a crystalline array and a quasicrystalline packing of the hydrocarbon chains predicated by packing problems in the bilayer core requiring disordering of the highly asymmetric chains. The possible involvement of quasicrystalline phases in formation of membrane rafts is considered.     

Influence of the physical state of the membrane on the enzymatic activity and energy of activation of protein kinase C alpha.

The activation of protein kinase C alpha was studied by using a lipid system consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) (molar ratio 4:1) and different proportions of 1-palmitoyl-2-oleoyl-sn-glycerol (POG). The phase behavior of the lipidic system was characterized by using differential scanning calorimetry and 31P NMR, and a phase diagram was elaborated. The results suggested the formation of two diacylglycerol/phospholipid complexes, one at 15 mol % of POG and the second at 30 mol % of POG. These two complexes would define the three regions of the phase diagram: in the first region (concentrations of POG lower than 15 mol %) there is gel-gel immiscibility at temperatures below that of the phase transition between C1 and pure phospholipid, and a fluid lamellar phase above of the phase transition. In the second region (between 15 and 30 mol % of POG), gel-gel immiscibility between C1 and C2 with fluid-fluid immiscibility was observed, while inverted hexagonal HII and isotropic phases were detected by 31P NMR. In the third region (concentrations of POG higher than 30 mol %), gel-gel immiscibility seemed to occur between C2 and pure POG along with fluid-fluid immiscibility, while an isotropic phase was detected by 31P NMR. When PKC alpha activity was measured, as a function of POG concentration, maximum activity was found at POG concentrations as low as 5-10 mol %; the activity slightly decreased as POG concentration was increased to 45 mol % at 32 degrees C (above Tc) whereas activity did not change with increasing concentrations of POG at 5 degrees C (below Tc). When the activity was studied as a function of temperature, at different POG concentrations, and depicted as Arrhenius plots, it was found that the activity increased with increasing temperatures, showing a discontinuity at a temperature very close to the phase transition of the system and a lower activation energy at the upper slope of the graph, indicating that the physical state of the membrane affected the interaction of PKC alpha with the membrane.     

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.     

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)     

Cubic phase is induced by cholesterol in the dispersion of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine

The effect of cholesterol, a major constituent of eukaryotic cell membranes, on the structure and thermotropic phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) dispersed in excess water was examined by synchrotron X-ray diffraction methods. Temperature scans over the range 10-75 degrees C showed that the gel to liquid-crystalline phase transition decreased from 25 to 10 degrees C in the presence of 20 mol% cholesterol, and no gel phase could be detected in the wide-angle X-ray scattering (WAXS) intensity profile of mixtures containing 35 mol% cholesterol. The small-angle X-ray scattering (SAXS) intensity profiles showed that the lamellar to nonlamellar phase transition temperature was also decreased in mixtures containing up to 30 mol% cholesterol but the trend was reversed in mixtures containing a higher proportion of cholesterol. There was evidence that the transition of the lamellar liquid-crystal phase is to cubic phases in mixtures containing less than 30 mol% cholesterol. The space group of one of these cubic phases was assigned as Pn3m. This effect of cholesterol on non-bilayer-forming phospholipids is considered in the context of the role of cholesterol in membrane organization and function.     

Influence of vitamin E on phosphatidylethanolamine lipid polymorphism

The effect of vitamin E, in its major form alpha-tocopherol and its synthetic analog alpha-tocopheryl acetate, on phosphatidylethanolamine lipid polymorphism has been studied by mean of differential scanning calorimetry and 31P-nuclear magnetic resonance techniques. From the interaction of these tocopherols with dielaidoylphosphatidylethanolamine it is concluded that both molecules promote the formation of the hexagonal HII phase at temperatures lower than those of the pure phospholipid. When the tocopherols were incorporated in the saturated dimiristoylphosphatidylethanolamine, which has been shown not to undergo bilayer to hexagonal HII phase transition, up to 90 degrees C, they induce the phospholipid to partially organize in hexagonal HII phase. From our experiments it is shown that alpha-tocopherol is more effective than its analog in promoting HII phase in these systems. It is also shown that, while alpha-tocopheryl acetate does not significantly perturb the gel to liquid-crystalline phase transition of dimirystoylphosphatidylethanolamine, alpha-tocopherol does so and more than one peak appears in the calorimetric profile, indicating that lateral phase separations are taking place.     

The structure and phase behaviour of alpha-tocopherol-rich domains in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine dispersed in excess water was examined by synchrotron X-ray diffraction and differential scanning calorimetry. Small- and wide-angle X-ray scattering intensity profiles were recorded from mixed dispersions containing up to 20 mol% alpha-tocopherol during temperature scans over the range 10-75 degrees C. These showed that a domain enriched in alpha-tocopherol phase separated from pure phospholipid in the mixture. This domain tends to have inverted hexagonal structure which coexists with phospholipid bilayers depleted of alpha-tocopherol. The scattering intensity and dimensions of the phase are dependent on the temperature and proportion of alpha-tocopherol in the mixture. Phase separations were also manifest in calorimetric scans of the mixed dispersions evidenced from the appearance of multiple peaks at temperatures corresponding to transitions observed in the X-ray scattering experiments. The effect of alpha-tocopherol in the range 0-20 mol% on the phase behaviour and structure of the phospholipid as observed from the X-ray scattering and calorimetric results have been used to construct a partial phase diagram of the mixture in the temperature range 10-75 degrees C. This shows that alpha-tocopherol has a marked tendency to partition from bilayers of the phospholipid to form an enriched domain in which the phospholipid assumes a hexagonal-II structure.     

The effect of dolichol on the structure and phase behaviour of phospholipid model membranes

The effect of dolichol C(95) on the structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine and stearoyloleoylphosphatidylethanolamine has been examined by synchrotron X-ray diffraction and differential scanning calorimetry. The presence of dolichol C(95) had no detectable effects on the temperature of either the gel to ripple or the ripple to liquid-crystal phase transition of dipalmitoylphosphatidylcholine. A proportionate increase of a few degrees in the temperature of the gel to lamellar liquid-crystal phase transition is observed in dispersions of dipalmitoylphosphatidylethanolamine and significantly there is a decrease in the temperature of the lamellar to non-lamellar phase transition of stearoyloleoylphosphatidylethanolamine. There was no significant change in the bilayer repeat spacing of all three mixed dispersions in gel phase in the presence of up to 20 mol% dolichol C(95). Electron density calculations showed that there was no change of bilayer thickness of dipalmitoylphosphatidylcholine with incorporation of up to 7.5 mol% dolichol C(95). These data suggest that effect of dolichol on the phospholipid model membranes depend on both the head group and the hydrocarbon chains of the phospholipid molecules. The presence of dolichol in phosphatidylcholine bilayers conforms to a model in which the polyisoprene compound is phase separated into a central domain sandwiched between the two monolayers in gel phase. In bilayers of phosphatidylethanolamines dolichol tends to stabilize the bilayers in gel phase at low temperatures and destabilize the bilayers in lamellar disordered structure at high temperatures. Non-lamellar structures coexist with lamellar disordered phase over a wide temperature range suggesting that dolichol is enriched in domains of non-lamellar structure and depleted from lamellar phase. These findings are useful to understand the function of dolichol in cell membranes.     

Different effects of long- and short-chain ceramides on the gel-fluid and lamellar-hexagonal transitions of phospholipids: a calorimetric, NMR, and x-ray diffraction study

The effects on dielaidoylphosphatidylethanolamine (DEPE) bilayers of ceramides containing different N-acyl chains have been studied by differential scanning calorimetry small angle x-ray diffraction and (31)P-NMR spectroscopy. N-palmitoyl (Cer16), N-hexanoyl (Cer6), and N-acetyl (Cer2) sphingosines have been used. Both the gel-fluid and the lamellar-inverted hexagonal transitions of DEPE have been examined in the presence of the various ceramides in the 0-25 mol % concentration range. Pure hydrated ceramides exhibit cooperative endothermic order-disorder transitions at 93 degrees C (Cer16), 60 degrees C (Cer6), and 54 degrees C (Cer2). In DEPE bilayers, Cer16 does not mix with the phospholipid in the gel phase, giving rise to high-melting ceramide-rich domains. Cer16 favors the lamellar-hexagonal transition of DEPE, decreasing the transition temperature. Cer2, on the other hand, is soluble in the gel phase of DEPE, decreasing the gel-fluid and increasing the lamellar-hexagonal transition temperatures, thus effectively stabilizing the lamellar fluid phase. In addition, Cer2 was peculiar in that no equilibrium could be reached for the Cer2-DEPE mixture above 60 degrees C, the lamellar-hexagonal transition shifting with time to temperatures beyond the instrumental range. The properties of Cer6 are intermediate between those of the other two, this ceramide decreasing both the gel-fluid and lamellar-hexagonal transition temperatures. Temperature-composition diagrams have been constructed for the mixtures of DEPE with each of the three ceramides. The different behavior of the long- and short-chain ceramides can be rationalized in terms of their different molecular geometries, Cer16 favoring negative curvature in the monolayers, thus inverted phases, and the opposite being true of the micelle-forming Cer2. These differences may be at the origin of the different physiological effects that are sometimes observed for the long- and short-chain ceramides.     

Effect of fatty acyl chain length and structure on the lamellar gel to liquid-crystalline and lamellar to reversed hexagonal phase transitions of aqueous phosphatidylethanolamine dispersions

The lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transitions of aqueous dispersions of a number of synthetic phosphatidylethanolamines containing linear saturated, branched chain, and alicyclic fatty acyl chains of varying length were studied by differential scanning calorimetry, 31P nuclear magnetic resonance spectroscopy, and X-ray diffraction. For any given homologous series of phosphatidylethanolamines containing a single chemical class of fatty acids, the lamellar gel/liquid-crystalline phase transition temperature increases and the lamellar liquid-crystalline/reversed hexagonal phase transition temperature decreases with increases in hydrocarbon chain length. For a series of phosphatidylethanolamines of the same hydrocarbon chain length but with different chemical structures, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary markedly and in the same direction. In particular, at comparable effective hydrocarbon chain lengths, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary in parallel, such that the temperature difference between these two phase transitions is nearly constant. Moreover, at comparable effective acyl chain lengths, the d spacings of the lamellar liquid-crystalline phases and of the inverted hexagonal phases are all similar, implying that the thickness of the phosphatidylethanolamine bilayers at the onset of the lamellar liquid-crystalline/reversed hexagonal phase transition and the diameter of the water-filled cylinders formed at the completion of this phase transition are comparable and independent of the chemical structure of the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Galactocerebroside-phospholipid interactions in bilayer membranes.

Differential scanning calorimetry (DSC) and x-ray diffraction have been used to study the interaction of hydrated N-palmitoylgalactosylsphingosine (NPGS) and dipalmitoylphosphatidylcholine (DPPC). For mixtures containing less than 23 mol% NPGS, complete miscibility of NPGS into hydrated DPPC bilayers is observed in both the bilayer gel and liquid-crystal phases. X-ray diffraction data demonstrate insignificant differences in the DPPC-bilayer gel-phase parameters on incorporation of up to 23 mol% NPGS. At greater than 23 mol% NPGS, additional high-temperature transitions occur, indicating phase separation of cerebroside. For these cerebroside concentrations, at 20 degrees C, x-ray diffraction shows two lamellar phases, hydrated DPPC-NPGS gel bilayers (d = 64 A) containing 23 mol% NPGS, and NPGS "crystal" bilayers (d = 55 A). On heating to temperatures greater than 45 degrees C, the mixed DPPC-NPGS bilayer phase undergoes chain melting, and on further increasing the temperature progressively more NPGS is incorporated into the liquid-crystal DPPC-NPGS bilayer phase. At temperatures greater than 82 degrees C (the transition temperature of hydrated NPGS), complete lipid miscibility is observed at all DPPC/NPGS molar ratios.