A description of the phospholipid arrangement intermediate to the humidity produced Lα and HII phases in dioleoylphosphatidylcholine and its modification by dioleoylphosphatidylethanolamine as studied by X-ray diffraction

Neutron diffraction with an excess-water cell

As part of a study of the molecular basis of membrane fusion by enveloped viruses, we have used neutron diffraction to study the lamellar (L(α)) to inverse hexagonal (H(II)) phase transition in the phospholipid N-methylated dioleoylphosphatidylethanolamine. This lipid was chosen because its phase transitions are particularly sensitive to the presence of agents that have been demonstrated to promote or inhibit membrane fusion. Two different geometries of neutron diffraction were used: small angle scattering (SANS) and a membrane diffractometer. The SANS measurements were carried out on the SWAN instrument at KEK, Japan, using dispersions of multilamellar vesicles (MLVs). The diffractometer measurements used the V1 instrument at BeNSC-HMI, Germany, with a specially-constructed cell that holds a stack of lipid bilayers in an excess-water state. The two approaches are compared and discussed. Although the diffractometer takes considerably longer to collect the data, it records much higher resolution than the SANS instrument. The samples recorded in the excess-water cell were shown to be well aligned, despite the lipids being fully hydrated, allowing for the production of high-resolution data. Trial measurements performed have demonstrated that sample alignment is preserved throughout the L(α) to H(II) phase transition, thereby opening up possibilities for obtaining high-resolution data from non-lamellar phases.

Thermotropic phase behavior and headgroup interactions of the nonbilayer lipids phosphatidylethanolamine and monogalactosyldiacylglycerol in the dry state

BACKGROUND

Although biological membranes are organized as lipid bilayers, they contain a substantial fraction of lipids that have a strong tendency to adopt a nonlamellar, most often inverted hexagonal (HII) phase. The polymorphic phase behavior of such nonbilayer lipids has been studied previously with a variety of methods in the fully hydrated state or at different degrees of dehydration. Here, we present a study of the thermotropic phase behavior of the nonbilayer lipids egg phosphatidylethanolamine (EPE) and monogalactosyldiacylglycerol (MGDG) with a focus on interactions between the lipid molecules in the interfacial and headgroup regions.

RESULTS

Liposomes were investigated in the dry state by Fourier-transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). Dry EPE showed a gel to liquid-crystalline phase transition below 0°C and a liquid-crystalline to HII transition at 100°C. MGDG, on the other hand, was in the liquid-crystalline phase down to -30°C and showed a nonbilayer transition at about 85°C. Mixtures (1:1 by mass) with two different phosphatidylcholines (PC) formed bilayers with no evidence for nonbilayer transitions up to 120°C. FTIR spectroscopy revealed complex interactions between the nonbilayer lipids and PC. Strong H-bonding interactions occurred between the sugar headgroup of MGDG and the phosphate, carbonyl and choline groups of PC. Similarly, the ethanolamine moiety of EPE was H-bonded to the carbonyl and choline groups of PC and probably interacted through charge pairing with the phosphate group.

CONCLUSIONS

This study provides a comprehensive characterization of dry membranes containing the two most important nonbilayer lipids (PE and MGDG) in living cells. These data will be of particular relevance for the analysis of interactions between membranes and low molecular weight solutes or soluble proteins that are presumably involved in cellular protection during anhydrobiosis.

Effects of the acyl chain composition of phosphatidylcholines on the stability of freeze-dried small liposomes in the presence of maltose

The effects of the acyl chain composition of phosphatidylcholines (PCs) on the stability of small unilamellar vesicles during freeze-drying and rehydration in the presence of maltose were studied by monitoring the retention of a trapped marker, calcein, in the internal liposome compartment. In dipalmitoyl PC, beta-oleoyl-gamma-palmitoyl-PC and egg yolk PC liposomes, good or fair retentions (>50%) were observed in the presence of maltose, but maltose was ineffective in preserving retention in the dioleoyl PC (DOPC) liposomes (<10%). The extremely low retention in the DOPC liposome was ascribed to neither a formation of the inverted hexagonal phase of the liposomal membrane nor the fusion/aggregation of the liposomes in the drying-rehydration process. Differential scanning calorimetry measurements suggested that interactions of maltose with PC headgroups were essential to stabilizing the dry liposomes. These interactions were significant in the saturated or mixed chain liposomes but were markedly reduced in the DOPC liposomes.

Dehydration-induced lamellar-to-hexagonal-II phase transitions in DOPE/DOPC mixtures

Plasma membranes of protoplasts isolated from non-acclimated rye plants undergo a transition from the bilayer to the inverted hexagonal (HII) phase during freeze-induced dehydration at -10 degrees C. It has been suggested (Bryant, G. and Wolfe, J. (1989) Eur. Biophys. J. 16, 369-372) that the differential hydration of various membrane components may induce fluid-fluid demixing of highly hydrated (e.g., PC) from poorly hydrated (PE) components during dehydration. This could yield a PE-enriched domain more prone to form the HII phase. We have examined the lyotropic phase behavior of mixtures of DOPE and DOPC at 20 degrees C by freeze-fracture electron microscopy, differential scanning calorimetry, and X-ray diffraction. HII phase formation was favored by higher proportions of DOPE and lower water contents. Mixtures of 1:1 and 1:3 DOPE/DOPC had a hydration-dependent appearance of two L alpha phases at water contents just above those at which the HII phase occurred. The hydration-dependence of the lamellar repeat spacings suggested that the DOPE-enriched domains preferentially underwent the L alpha-to-HII phase transition. Mixtures of 3:1 DOPE/DOPC did not separate into two L alpha phases during dehydration. These data suggest that the differential hydration characteristics of various membrane components may induce their lateral fluid-fluid demixing during dehydration.