Characterization of complexes formed in fully hydrated dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol.

The phase diagram of fully hydrated binary mixtures of dipalmitoylphosphatidylcholine (DPPC) with 1,2-dipalmitoylglycerol (DPG) published recently by López-García et al. identifies regions where stoichiometric complexes of 1:1 and 1:2 DPPC:DPG, respectively, are formed. In this study, the structural parameters of the 1:1 complex in the presence of pure DPPC was characterized by synchrotron low angle and static x-ray diffraction methods. Structural changes upon transitions through phase boundaries were correlated with enthalpy changes observed by differential scanning calorimetry in mixtures of DPPC with 5, 7.5, 10, and 20 mol% DPG dispersed in excess water. Phase separation of a complex in gel phase could be detected by calorimetry in the mixture containing 5 mol% DPG but was not detectable by synchrotron low angle x-ray diffraction. Static x-ray measurements show evidence of phase separation, particularly in the reflections indexing chain packing. In the mixture containing 7.5 mol% DPG, two distinct lamellar repeat spacings could be seen in the temperature range from 25 to 34 degrees C. The lamellar spacing of about 6.6 nm was assigned to pure gel phase DPPC because the change in the spacing corresponds with thermal transition of the pure phospholipid, and a longer repeat spacing of about 7.2 nm was assigned to domains of the 1:1 complex of DPPC-DPG.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Structure and phase behavior of hydrated mixtures of L-dipalmitoylphosphatidylcholine and palmitic acid. Correlations between structural rearrangements, specific volume changes and endothermic events

Several new features of the phase diagram of L-dipalmitoylphosphatidylcholine (DPPC)/palmitic acid mixtures in excess water were established by means of static and time-resolved X-ray diffraction, densitometry and differential scanning calorimetry (DSC). At low temperatures, palmitic acid has a biphasic effect on the lamellar subgel phases: at concentrations below 5-6 mol%, it prevents formation of the DPPC subgel phase (Lc), while at higher contents (between about 40 and 90 mol%) another subgel phase (Lccom) is formed as a result of lipid co-crystallization at 1 DPPC: 2 palmitic acid stoichiometry. A crystalline palmitic acid phase separates from Lccom above 70-80 mol% of fatty acid. The Lccomphase transforms into a lamellar gel phase (L beta) in an endothermic transition centered at 38 degrees C. At high temperatures, the mixtures form hexagonal liquid-crystalline phase (HII) in the region of 60-70 mol% and an isotropic phase (I) at 90-100 mol% of palmitic acid. No coexistence of HII phase with the fluid lamellar phase of DPPC was observed at intermediate compositions (20 and 50 mol% of palmitic acid) but rather formation of a complex phase with non-periodic geometry characterized by molten chains and a broad, continuous small-angle scattering band. No evidence for fluid phase coexistence was found also at compositions between HII and I phases. The L beta--HII transition at 60-70 mol% of palmitic acids is readily reversible and two-state in both heating and cooling modes. It is characterized by the coexistence of initial and final phases with no detectable intermediates by time-resolved and static X-ray diffraction. The crystalline-isotropic transition in palmitic acid is two-state only in heating direction. On cooling, it is characterized by strong undercooling and gradually relaxing lamellar crystalline structures. The slowly reversible Lccom--L beta transition proceeds continuously through intermediate states. Although clearly discernible by both DSC and X-ray diffraction, it is not accompanied by specific volume changes.     

The influence of low amounts of cholesterol on the interdigitated gel phase of hydrated dihexadecylphosphatidylcholine.

Dihexadecylphosphatidylcholine (DHPC)/cholesterol binary mixtures in excess of water have been characterized by small-angle X-ray diffraction and differential scanning calorimetry and a temperature-composition phase diagram for this binary has been constructed. The property of cholesterol to perturb the hydrocarbon chain interdigitation in the lamellar gel phase of DHPC and to convert it into a non-interdigitated state has been observed by small- angle X-ray diffraction at cholesterol concentrations as low as 0.1 mol%. The interdigitated and non-interdigitated lamellar gel phases coexist in the range up to 5 mol% cholesterol. At this and higher cholesterol concentrations only non-interdigitated phases have been found in the phase diagram of the mixture. It is suggested that the ability of cholesterol in low concentration to eliminate the hydrocarbon chain interdigitation is related to the free energy increase due to unfavourable line boundaries between the interdigitated and non-interdigitated lipid domains.     

Effect of 2,4-dichlorophenol on DPPC/water liposomes studied by X-ray and freeze-fracture electron microscopy

The effect of 2,4-dichlorophenol (DCP) was studied on the fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)--water liposomes. The structure and the thermotropic phase behaviour of the liposomes was examined in the presence of DCP (DCP/DPPC molar ratio, varied from 2x10(-2) up to 1) using small- and wide-angle X-ray scattering (SAXS, WAXS) and freeze-fracture electron microscopy. The structural behaviour of the DPPC/DCP/water system was strongly dependent on the concentration of the DCP. In the pretransition range the DCP molecules (at 2x10(-2) DCP/DPPC molar ratio) induced the interdigitated phase beside the parent (gel and rippled gel) phases, locally which can be form at higher DCP concentration. When the DCP/DPPC molar ratio was increased the pretransition disappeared and the main transition was shifted to lower temperatures. In the molar ratio range from 2x10(-1) up to 5x10(-1), a coexistence of different phases was observed in the wide temperature range from 20 up to 40 degrees C. With a further increase of the DCP/DPPC molar ratio (6x10(-1) to 1) only the interdigitated gel phase occurred below 25 degrees C. A schematic phase diagram of DPPC/DCP/water system was constructed to summarise the results.     

Properties of ganglioside GM1 in phosphatidylcholine bilayer membranes.

Gangliosides have been shown to function as cell surface receptors, as well as participating in cell growth, differentiation, and transformation. In spite of their multiple biological functions, relatively little is known about their structure and physical properties in membrane systems. The thermotropic and structural properties of ganglioside GM1 alone and in a binary system with 1,2-dipalmitoyl phosphatidylcholine (DPPC) have been investigated by differential scanning calorimetry (DSC) and x-ray diffraction. By DSC hydrated GM1 undergoes a broad endothermic transition TM = 26 degrees C (delta H = 1.7 kcal/mol GM1). X-ray diffraction below (-2 degrees C) and above (51 degrees C) this transition indicates a micellar structure with changes occurring only in the wide angle region of the diffraction pattern (relatively sharp reflection at 1/4.12 A-1 at -2 degrees C; more diffuse reflection at 1/4.41 A-1 at 51 degrees C). In hydrated binary mixtures with DPPC, incorporation of GM1 (0-30 mol%; zone 1) decreases the enthalpy of the DPPC pretransition at low molar compositions while increasing the TM of both the pre- and main transitions (limiting values, 39 and 44 degrees C, respectively). X-ray diffraction studies indicate the presence of a single bilayer gel phase in zone 1 that can undergo chain melting to an L alpha bilayer phase. A detailed hydration study of GM1 (5.7 mol %)/DPPC indicated a conversion of the DPPC bilayer gel phase to an infinite swelling system in zone 1 due to the presence of the negatively charged sialic acid moiety of GM1. At 30-61 mol % GM1 (zone 2), two calorimetric transitions are observed at 44 and 47 degrees C, suggesting the presence of two phases. The lower transition reflects the bilayer gel --> L alpha transition (zone 1), whereas the upper transition appears to be a consequence of the formation of a nonbilayer, micellar or hexagonal phase, although the structure of this phase has not been defined by x-ray diffraction. At > 61 mol % GM1 (zone 3) the calorimetric and phase behavior is dominated by the micelle-forming properties of GM1; the presence of mixed GM1/DPPC micellar phases is predicted.     

The effect of cholesterol on measured interaction and compressibility of dipalmitoylphosphatidylcholine bilayers

We have examined the phase diagram of dipalmitoylphosphatidylcholine (DPPC)--cholesterol-water mixtures at low cholesterol content, and report phase separation between 3 and 10 mol% cholesterol. The two lamellar phases at equilibrium in this region appear to be pure DPPC and 11 mol% cholesterol in DPPC. For these two lamellar phases, which are made up of alternating layers of water and bimolecular lipid leaflets, we have measured the forces of interaction between leaflets and the lateral pressure and compressibility of the leaflets. Both bilayers experience a strong repulsive force when forced together only a few ?ngstr?ms (1 A = 0.1 nm) closer than their maximum separation in excess water. However, the presence of 11 mol% cholesterol causes the bilayers to move apart of 35-A separation from the 19-A characteristic of pure DPPC in excess water. This swelling may result from a decrease in van der Waals attraction between bilayers or from an increase in bilayer repulsion. Differences in bilayer interaction can be a cause for phase separation. More importantly these differences can cause changes in the composition of regions of membranes approaching contact. At 11 mol%, cholesterol substantially increases the lateral compressibility of DPPC bilayers leading to higher lateral density fluctuations and potentially higher bilayer permeability.     

Packing characteristics of two-component bilayers composed of ester- and ether-linked phospholipids.

The miscibility properties of ether- and ester-linked phospholipids in two-component, fully hydrated bilayers have been studied by differential scanning calorimetry (DSC) and Raman spectroscopy. Mixtures of 1,2-di-O-hexadecyl-rac-glycero-3-phosphocholine (DHPC) with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DHPE) and of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) with 1,2-di-O-hexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) have been investigated. The phase diagram for the DPPC/DHPE mixtures indicates that these two phospholipids are miscible in all proportions in the nonrippled bilayer gel phase. In contrast, the DHPC/DPPE mixtures display two regions of gel phase immiscibility between 10 and 30 mol% DPPE. Raman spectroscopic measurements of DHPC/DPPE mixtures in the C-H stretching mode region suggest that this immiscibility arises from the formation of DHPC-rich interdigitated gel phase domains with strong lateral chain packing interactions at temperatures below 27 degrees C. However, in the absence of interdigitation, our findings, and those of others, lead to the conclusion that the miscibility properties of mixtures of ether- and ester-linked phospholipids are determined by the nature of the phospholipid headgroups and are independent of the character of the hydrocarbon chain linkages. Thus it seems unlikely that the ether linkage has any significant effect on the miscibility properties of phospholipids in biological membranes.     

Effect of cholesterol on the formation of an interdigitated gel phase in lysophosphatidylcholine and phosphatidylcholine binary mixtures

We previously reported that 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) forms an interdigitated gel phase in the presence of 1-palmitoyl-sn-glycero-3-phosphocholine (16:0LPC) at concentrations below 30 mol%. In the present investigation, fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH), X-ray diffraction, and differential scanning calorimetry (DSC) were used to investigate the effect of cholesterol on the phase behavior of 16:0LPC/DPPC binary mixtures. At 25 degrees C, 30 mol% 16:0LPC significantly decreases the DPH fluorescence intensity during the transition of DPPC from the L(beta') phase to the L(betaI) phase. However, the addition of cholesterol to 16:0LPC/DPPC mixtures results in a substantial increase in fluorescence intensity. The changes in DPH fluorescence intensity reflect the probe's redistribution from an orientation parallel to the acyl chain to the center of the bilayer, suggesting a bilayer structure transition from interdigitation to noninterdigitation. The normal repeat period of small angle X-ray diffraction patterns can be restored and a reflection appears at 0.42 nm with a broad shoulder around 0.41 nm in wide angle X-ray diffraction patterns when 10 mol% cholesterol is incorporated into 30 mol% 16:0LPC/DPPC vesicles, indicating that the mixtures are in the gel phase (L(beta')). Moreover, DSC results demonstrate that 10 mol% cholesterol is sufficient to significantly decrease the main enthalpy, cooperativity and lipid chain melting of 30 mol% 16:0LPC/DPPC binary mixtures, which are L(betaI), indicating that the transition of the interdigitated phase is more sensitive to cholesterol than that of the noninterdigitated phase. Our data imply that the interdigitated gel phase induced by 16:0LPC is prevented in the presence of 10 mol% cholesterol, but unlike ethanol, an increasing concentration of 16:0LPC is not able to restore the interdigitation structure of the lipid mixtures.     

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.     

Comparative study of the gel phases of ether- and ester-linked phosphatidylcholines

Calorimetric, X-ray diffraction, and 31P nuclear magnetic resonance (NMR) studies of aqueous dispersions of 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) gel phases at low temperatures (-60 to 22 degrees C) show thermal, structural, and dynamic differences when compared to aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) gel phases at corresponding temperatures. Differential scanning calorimetry of DHPC dispersions demonstrates a reversible, low-enthalpy "subtransition" at 4 degrees C in contrast to the conditionally reversible, high-enthalpy subtransition observed at 17 degrees C for annealed DPPC bilayers. X-ray diffraction studies indicate that DHPC dispersions form a lamellar gel phase with dav congruent to 46 A both above and below the "subtransition". It is suggested that the reduced dav observed for DHPC (46 A as compared to 64 A in DPPC) is due to an interdigitated lamellar gel phase which exists at all temperatures below the pretransition at 35 degrees C. 31P NMR spectra of DHPC gel-phase bilayers show an axially symmetric chemical shift anisotropy powder pattern which remains sharp down to -20 degrees C, suggesting the presence of fast axial diffusion. In contrast, 31P spectra of DPPC bilayers indicate this type of motion is frozen out at approximately 0 degrees C.     

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.     

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.     

Effect of lysophosphatidylcholine on behavior and structure of phosphatidylcholine liposomes

Differential scanning calorimetry (DSC), fluorescence polarization and X-ray diffraction were per-formed to investigate the kinetics of the micellar to the lamellar phase transition of dipalmitoylphosphatidylcholine/1-palmitoylphosphatidylcholine (16:0 LPC/DPPC) liposomes at gel phase. With a 16:0 LPC concentration up to 27 mol% only the sharp main transition with relatively high enthalpy (△H) values of DPPC was observed. Increasing 16 : 0 LPC concentration, the phase transition was broadened and the transition enthalpy was decreased and finally totally disappeared. The fluorescence probes of 3AS, 9AS, 12AS, and 16AP were employed, respectively, to detect the mo-bility of various sites of carbon chains of DPPC or 16:0 LPC/DPPC liposomes. It was shown that DPPC liposomes formed in the absence of 16:0 LPC always had a fluidity gradient in both gel and liquid-crystalline phase, while in the presence of 14.1 mol% and 27.0 mol% 16:0 LPC in the mixtures, the fluidity gradient tended to disappear below 40℃:     

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.     

Phase diagram of stigmasterol-dipalmitoylphosphatidylcholine mixtures dispersed in excess water

As a simple model of rafts in plant cells, the effect of stigmasterol, one of the predominant sterols in plant plasma membranes, on the phase behavior of dipalmitoylphosphatidylcholine (DPPC) multilayers has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and freeze-fracture electron microscopy (FFEM) techniques. A partial phase diagram of the binary system has been constructed. Particularly, the stigmasterol concentrations of the "left endpoint" and "right endpoint" of the three-phase line have been determined using the newly developed linear and nonlinear fitting method. They are 6.2 and 23.7 mol%, respectively. Furthermore, the resemblance and difference of phase diagrams of DPPC/stigmasterol, DPPC/cholesterol, and DPPC/ergosterol have been compared and the efficiency of these sterols in promoting the formation of the liquid-ordered domains (rafts) have also been discussed.     

The phase diagram of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/sucrose in the dry state. Sucrose substitution for water in lamellar mesophases

The phase diagram of the binary system, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/sucrose, was determined by DSC. In contrast to dry DPPC, which exhibits chain melting at 342.5 K, the main feature of the DPPC/sucrose system is eutectic melting at 320 K. This was supported earlier by Crowe, J.H., Crowe, L.M. and Chapman, D. (Science 223 (1984) 701-703), who reported a drastic decrease in the chain-melting temperature of the dry lipid in the presence of some mono- and disaccharides. Electron microscopy suggests that the phase structures on either side of the phase transition are of the lamellar type. Definite sugar saturation concentrations can be derived from this phase diagram. Up to about 17 mol% sucrose, i.e., 1 mol of sucrose per 5 mol of lipid is adopted by DPPC in the low-temperature phase Lc. In the high-temperature phase Lm the saturation concentration is well above 90 mol% sucrose at 320 K (eutectic point) but decreases with increasing temperature. The lower limit of 50 mol% sucrose is reached at 455 K. At this temperature, peritectic melting of sucrose occurs. Because of some similarities in the phase diagrams of DPPC/sucrose and DPPC/water, it is possible to understand the sucrose substitution for water in dry lamellar mesophases.     

Thermal phase behaviour and structure of hydrated mixtures between dipalmitoyl- and dihexadecylphosphatidylcholine

Mixtures of 1,2-dipalmitoyl- and 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine (DPPC and DHPC) in dispersion with excess water were studied by differential scanning calorimetry (DSC) and X-ray diffraction techniques. The transition parameters of the main gel-to-liquid crystalline transition show a monotonous dependence on the composition, indicating ideal miscibility of the two lipids, in keeping with the closely similar structures of the pure, hydrated lipids in the P beta' and L alpha states. The pre-transition shows a depression to a minimum temperature of 23 degrees C occurring around equimolar mixtures. Below the pre-transition temperatures, the L beta' gel phase of DPPC maintains bimolecular structure up to DHPC admixtures of 50 mol%, with adaptations in hydrocarbon chain packing and multilayer periodicity. On the side of DHPC, the interdigitated gel structure shows full solubility for DPPC up to equimolarity without major structural changes. The crystalline Lc-phase of DPPC exhibits immiscibility with DHPC, demonstrated by the fact that the subtransition is abolished already at less than 15 mol% DHPC. DHPC, below its subtransition, can accommodate up to 50 mol% DPPC within an interdigitated layer structure with unperturbed, crystalline hydrocarbon chain packing.     

Studies of the structure and organization of cationic lipid bilayer membranes: calorimetric, spectroscopic, and x-ray diffraction studies of linear saturated P-O-ethyl phosphatidylcholines

Differential scanning calorimetry, x-ray diffraction, and infrared and (31)P-nuclear magnetic resonance ((31)P-NMR) spectroscopy were used to examine the thermotropic phase behavior and organization of cationic model membranes composed of the P-O-ethyl esters of a homologous series of n-saturated 1,2-diacyl phosphatidylcholines (Et-PCs). Differential scanning calorimetry studies indicate that on heating, these lipids exhibit single highly energetic and cooperative endothermic transitions whose temperatures and enthalpies are higher than those of the corresponding phosphatidylcholines (PCs). Upon cooling, these Et-PCs exhibit two exothermic transitions at temperatures slightly below the single endotherm observed upon heating. These cooling exotherms have both been assigned to transitions between the liquid-crystalline and gel phases of these lipids by x-ray diffraction. The x-ray diffraction data also show that unlike the parent PCs, the chain-melting phase transition of these Et-PCs involves a direct transformation of a chain-interdigitated gel phase to the lamellar liquid-crystalline phase for the homologous series of n > or = 14. Our (31)P-NMR spectroscopic studies indicate that the rates of phosphate headgroup reorientation in both gel and liquid-crystalline phases of these lipids are comparable to those of the corresponding PC bilayers. However, the shape of the (31)P-NMR spectra observed in the interdigitated gel phase indicates that phosphate headgroup reorientation is subject to constraints that are not encountered in the non-interdigitated gel phases of parent PCs. The infrared spectroscopic data indicate that the Et-PCs adopt a very compact form of hydrocarbon chain packing in the interdigitated gel phase and that the polar/apolar interfacial regions of these bilayers are less hydrated than those of corresponding PC bilayers in both the gel and liquid-crystalline phases. Our results indicate that esterification of PC phosphate headgroups results in many alterations of bilayer physical properties aside from the endowment of a positively charged surface. This fact should be considered in assessing the interactions of these compounds with naturally occurring lipids and with other biological materials.     

Phase diagram of stigmasterol-dipalmitoylphosphatidylcholine mixtures dispersed in excess water

As a simple model of rafts in plant cells, the effect of stigmasterol, one of the predominant sterols in plant plasma membranes, on the phase behavior of dipalmitoylphosphatidylcholine (DPPC) multilayers has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and freeze-fracture electron microscopy (FFEM) techniques. A partial phase diagram of the binary system has been constructed. Particularly, the stigmasterol concentrations of the “left endpoint” and “right endpoint” of the three-phase line have been determined using the newly developed linear and nonlinear fitting method. They are 6.2 and 23.7 mol%, respectively. Furthermore, the resemblance and difference of phase diagrams of DPPC/stigmasterol, DPPC/cholesterol, and DPPC/ergosterol have been compared and the efficiency of these sterols in promoting the formation of the liquid-ordered domains (rafts) have also been discussed.