Phase behavior of ethanolamine plasmalogen

In the present study the phase behavior of multilamellar dispersions of 1-O-(1′-alkenyl)-2-oleoyl-glycerophosphoethanolamine (ethanolamine plasmalogen), 1-O-alkyl-2-oleoyl-glycerophosphoethanolamine and 1-acyl-2-oleoyl-glycerophosphoethanolamine was compared using differential scanning calorimetry (DSC) and ³¹P-NMR. The three compounds differed only in the type of bonding (vinyl ether, alkyl ether or acyl ester) linking the aliphatic moiety to position 1 of sn-glycerol.The gel to liquid-crystalline phase transition temperature as determined by DSC was lowest for ethanolamine plasmalogen (26°C) and was similar for the alkylacyl and diacyl analogs (29.5° and 30°C, respectively). Enthalpies of the G → L phase transition were not significantly different for the three phospholipids tested.Ethanolamine plasmalogen undergoes the lamellar to hexagonal phase transition at 30°C, the analogous alkylacyl-glycerophosphoethanolamine(alkylacyl-GPE) and diacyl-GPE at 53°C and 69°C, respectively. Thus, an alkenyl ether bond in position 1 of sn-glycerol, the structural characteristic of plasmalogens, effectively stabilizes the hexagonal H_II arrangement of ethanolamine glycerophospholipids, while it has relatively little effect on destabilization of the lamellar gel state.     

Role of head group structure in the phase behavior of amino phospholipids. 2. Lamellar and nonlamellar phases of unsaturated phosphatidylethanolamine analogues

Three types of analogues of unsaturated phosphatidylethanolamines (PE) have been prepared: phosphatidyl-omega-amino-1-alkanols, N-alkyl-PE's, and C2-alkyl-PE's, with alkyl substitution of carbon-2 of the ethanolamine head group. The physical properties of dioleoyl, dielaidoyl, and 1-palmitoyl-2-oleoyl phospholipids with these head groups have been examined by calorimetry, 31P NMR, freeze-fracture electron microscopy, and X-ray diffraction. N-Alkylation of PE, or substitution of the ethanolamine moiety by 3-amino-1-propanol or 4-amino-1-butanol, decreases the transition temperature of the hydrated gel phase (Tc) and considerably increases the temperature of the lamellar to hexagonal II transition (TH). The pattern of these effects for various PE analogues suggests that head group size and hydrophobicity as well as hydrogen bonding are important determinants of the phase behavior of these lipids. C2-Alkylated PE analogues exhibit several rather surprising properties, notably the ready formation of a quasi-crystalline "high-melting" solid phase even for di-cis-unsaturated species and substantially lower TH values than are observed for the parent PE species. The behavior of these compounds suggests that "hydration forces" can be more important than considerations of lipid "dynamic shape" in predicting the relative stabilities of lamellar vs. nonlamellar phases for at least some zwitterionic phospholipids.     

Bilayer phase transitions of N-methylated dioleoylphosphatidylethanolamines under high pressure

The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)-pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T-p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127-0.145 K MPa-1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lalpha) phase for DOPC bilayer (0.131 K MPa-1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lbeta) phase to the Lalpha phase for DOPC bilayer (0.233 K MPa-1) was significantly different from that of the Lc/Lalpha transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lalpha transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lalpha phase although they have been regarded as the main transition in the previous studies. The Lbeta phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.     

Stabilization of non-bilayer structures by the etherlipid ethanolamine plasmalogen

The thermotropic phase behavior of mixtures between diradylphosphatidylethanolamines and diacylphosphatidylcholine was studied using polarized light microscopy, 31P-NMR spectroscopy and synchrotron X-ray diffraction. Multilamellar liposomes composed of alkenylacylphosphatidylethanolamine (ethanolamine plasmalogen) undergo a phase transition from a lamellar to an inverse hexagonal lipid structure at 30 degrees C, which is about 20 degrees C and 30 degrees C lower as compared to its alkylacyl- and diacyl-analog, respectively. These results indicate a higher affinity to non-bilayer structures for the ether lipids. In the presence of the bilayer stabilizing phospholipid, palmitoyloleoylphosphatidylcholine, the transition is shifted to higher temperature without any significant changes in the overall structural parameters as revealed by X-ray diffraction experiments. Again, ethanolamine plasmalogen stabilizes the inverted hexagonal phase to the highest extent, i.e. even in the presence of 40 mol% palmitoyloleoylphosphatidylcholine a pure inverse hexagonal phase is formed at 60 degrees C. Such a result was not reported so far for a diacylphosphatidylethanolamine. This property of ethanolamine plasmalogen might be predominantly explained by an optimized packing of the hydrocarbon chains in the corners and interface region of the hexagonal tubes, owing to a different conformation of the sn-2 chain, which was deduced from 2H-NMR experiments (Malthaner, M., Hermetter, A., Paltauf, F. and Seelig, J. (1987) Biochim. Biophys. Acta 900, 191-197). Data obtained by time resolved X-ray diffraction show a coexistence of lamellar and inverse hexagonal structures in the phase transition region, but do not indicate the existence of non-lamellar intermediates or disorder within the sensitivity limits of the method.     

Water binding and phase structures for different Acholeplasma laidlawii membrane lipids studied by deuteron nuclear magnetic resonance and x-ray diffraction

Water binding capability and phase structures for different lipid species extracted from Acholeplasma laidlawii A membranes have been studied using deuteron nuclear magnetic resonance and low-angle X-ray diffraction.The dominating membrane lipids are monoglucosyldiglyceride and diglucosyldiglyceride and each of them takes up limited amounts of water (bound plus trapped), i.e., up to 13% (w/w), whereas the phospholipids and phosphoglycolipids have larger hydration capacities.Addition of magnesium and calcium ions, but not sodium ions, to the diglucosyldiglyceride increases the hydration capability. This increase is accompanied by the formation of a metastable liquid crystalline phase and a hysteresis effect for the transition temperature.Large differences in water deuteron quadrupole splitting were observed between mono- and diglucosyldiglyceride. Both ²H nuclear magnetic resonance and low-angle X-ray diffraction studies on lipids containing biosynthetically incorporated ω-d₃-palmitic acid clearly indicate the existence of a reverse hexagonal phase structure for the monoglucosyldiglyceride and lamellar structures for the diglucosyldiglyceride and the other membrane lipids.The low hydration capability of the large diglucosyldiglyceride polar head is discussed in terms of polar head configuration.Both mono- and diglucosyldiglyceride have several physical properties similar to those of phosphatidylethanolamine.     

Light scattering measurement and Avrami analysis of the lamellar to inverse hexagonal phase transition kinetics of the lipid DEPE

The lamellar to inverse hexagonal phase transition of lipids is much studied as a model for understanding cellular processes such as membrane fusion and pore formation. Much remains unknown, including a theoretical understanding and a definitive value of the phase transition temperature for DEPE, as literature values vary over 10°C. Avrami theory has been commonly used to analyze phase transition kinetics. However, to the best of our knowledge, Avrami theory has not been used to analyze the lamellar to inverse hexagonal transition in lipids until now. We used laser light scattering to measure phase transition temperature of the lipid DEPE (1,2-dielaidoyl-sn-phosphatidylethanolamine) and found it to be 61.0 ± 0.5°C. We found the hysteresis, |T(measured)-T(equilibrium)|, scaled as r(β), where r is the ramp rate and β=0.29 ± 0.02. This is the same power law behavior found by others for an isomer of DEPE known as DOPE (1,2-dioleoyl-sn-glycero-3 ethanolamine); however, DEPE exhibits roughly half the hysteresis of DOPE. An analysis of DEPE kinetics yields Avrami exponents ranging from 1 to 7, suggesting the transition propagates one dimensionally and is initiated by a widely varying nucleation rate.     

The magnesium-ion-dependent cleavage of the vinyl ether linkage of brain ethanolamine plasmalogen

1. There was a significant decrease in the amount of endogenous ethanolamine phospholipids when preparations of whole brain were incubated in bicarbonate–Ringer solutions, leading in particular to the hydrolysis of vinyl ether groups. 2. The hydrolysis of ethanolamine phospholipids in such preparations was abolished in the absence of bivalent cations. 3. An enzyme present in extracts of acetone-dried brain powders that cleaved the vinyl ether linkage in ethanolamine plasmalogen maximally at pH 7·4 required Mg²⁺ for activity. 4. The cleavage of the vinyl ether linkage of an ethanolamine lysoplasmalogen was enhanced in the presence of Mg²⁺ but the requirement was not absolute.     

Phase equilibria of model milk membrane lipid systems

The phase behaviour of mixtures of recombined milk membrane lipids dioleoylphosphatidylcholine (DOPC), sphingomyelin (SM), dioleoylphosphatidylethanolamine (DOPE), phosphatidylinositol (PI) and dioleoylphosphatidylserine (DOPS) in 60% water was examined as a function of temperature between 5 and 90 degrees C. The aim was to examine under which lipid composition the average properties turn from balanced over to hydrophobic. The phase boundaries were determined by small angle X-ray diffraction (SAXD) and differential scanning calorimetry (DSC). The lamellar phase was dominating in the DOPC/SM/DOPE system. The phase boundary for the reversed hexagonal phase was only observed at high DOPE content within the examined temperature interval. The anionic phospholipids PI and DOPS induced a swollen lamellar phase, but no significant change of the transition between the lamellar phase and the reversed hexagonal phase was observed.     

The interaction of coenzyme Q with phosphatidylethanolamine membranes.

Coenzyme Q (CoQ) is a component of the mitochondrial respiratory chain which carries out additional membrane functions, such as acting as an antioxidant. The location of CoQ in the membrane and the interaction with the phospholipid bilayer is still a subject of debate. The interaction of CoQ in the oxidized (ubiquinone-10) and reduced (ubiquinol-10) state with membrane model systems of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (Ela2Gro-P-Etn) has been studied by means of differential scanning calorimetry (DSC), 31P-nuclear magnetic resonance (31P-NMR) and small angle X-ray diffraction (SAXD). Ubiquinone-10 did not visibly affect the lamellar gel to lamellar liquid-crystalline phase transition of Ela2Gro-P-Etn, but it clearly perturbed the multicomponent lamellar liquid-crystalline to lamellar gel phase transition of the phospholipid. The perturbation of both transitions was more effective in the presence of ubiquinol-10. A location of CoQ forming head to head aggregates in the center of the Ela2Gro-P-Etn bilayer with the polar rings protruding toward the phospholipid acyl chains is suggested. The formation of such aggregates are compatible with the strong hexagonal HII phase promotion ability found for CoQ. This ability was evidenced by the shifting of the lamellar to hexagonal HII phase transition to lower temperatures and by the appearance of the characteristic hexagonal HII 31P-NMR resonance and SAXD pattern at temperatures at which the pure Ela2Gro-P-Etn is still organized in extended bilayer structures. The influence of CoQ on the thermotropic properties and phase behavior of Ela2Gro-P-Etn is discussed in relation to the role of CoQ in the membrane.     

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)     

Thermotropic and barotropic phase transitions of N-methylated dipalmitoylphosphatidylethanolamine bilayers

In order to understand the effect of polar head group modification on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidyl-N-methylethanolamine (DPMePE), dipalmitoylphosphatidyl-N,N-dimethylethanolamine (DPMe2PE) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were observed by differential scanning calorimetry and high-pressure optical methods. The temperatures of the so-called main transition from the gel (L(beta)) or ripple gel (P(beta)') phase to the liquid crystalline (L(alpha)) phase were almost linearly elevated by applying pressure. The slope of the temperature-pressure boundary, dT/dp, was in the range of 0.220-0.264 K MPa(-1) depending on the number of methyl groups in the head group of lipids. The main-transition temperatures of N-methylated DPPEs decreased with increasing size of head group by stepwise N-methylation. On the other hand, there was no significant difference in thermodynamic quantities of the main transition between the phospholipids. With respect to the transition from the subgel (L(c)) phase to the lamellar gel (L(beta) or L(beta)') phase, the transition temperatures were also elevated by applying pressure. In the case of DPPE bilayer the L(c)/L(beta) transition appeared at a pressure higher than 21.8 MPa. At a pressure below 21.8 MPa the L(c)/L(alpha) transition was observed at a temperature higher than the main-transition temperature. The main (L(beta)/L(alpha)) transition can be recognized as the transformation between metastable phases in the range from ambient pressure to 21.8 MPa. Polymorphism in the gel phase is characteristic of DPPC bilayer membrane unlike other lipid bilayers used in this study: the L(beta)', P(beta)' and pressure-induced interdigitated gel (L(beta)I) phases were observed only in the DPPC bilayer. Regarding the bilayers of DPPE, DPMePE and DPMe2PE, the interdigitation of acyl chain did not appear even at pressures as high as 200 MPa.     

Lipid-dependent differential effects of stereoisomers of anesthetic alcohols

The cis- and trans-alkenols are equally potent general anesthetics but, respectively, lower and raise the gel-to-liquid crystalline phase transition temperature of saturated phosphatidylcholines (Pringle, M.J. and Miller, K.W. (1978) Biochem. Biophys. Res. Commun. 85, 1191-1198). Here we show that although this differential effect is somewhat reduced when a double bond is introduced into the sn-2 position of phosphatidylcholine, it is abolished when the ethanolamine head group is substituted for the choline head group in dimyristoyl lipids at neutral pH. At high pH, however, dimyristoylphosphatidylethanolamine assumes a negative charge, and its phase transition temperature drops to a value close to that for the corresponding phosphatidylcholine. Under these conditions the differential effect of the alkenol isomers is restored; the cis-alkenol lowers, while the trans-alkenol raises, the phase transition temperature of deprotonated dimyristoylphosphatidylethanolamine. Thus, the differential effects of cis- and trans-alkenols on the gel-to-liquid crystalline phase transition are dependent on the physical chemical characteristics of the polar region of the perturbed lipid species, but only weakly on that of the acyl region.     

Salt tolerance of archaeal extremely halophilic lipid membranes

The membranes of extremely halophilic Archaea are characterized by the abundance of a diacidic phospholipid, archaetidylglycerol methylphosphate (PGP-Me), which accounts for 50-80 mol% of the polar lipids, and by the absence of phospholipids with choline, ethanolamine, inositol, and serine head groups. These membranes are stable in concentrated 3-5 m NaCl solutions, whereas membranes of non-halophilic Archaea, which do not contain PGP-Me, are unstable and leaky under such conditions. By x-ray diffraction and vesicle permeability measurements, we demonstrate that PGP-Me contributes in an essential way to membrane stability in hypersaline environments. Large unilamellar vesicles (LUV) prepared from the polar lipids of extreme halophiles, Halobacterium halobium and Halobacterium salinarum, retain entrapped carboxyfluorescein and resist aggregation in the whole range 0-4 m NaCl, similarly to LUV prepared from purified PGP-Me. By contrast, LUV made of polar lipid extracts from moderately halophilic and non-halophilic Archaea (Methanococcus jannaschii, Methanosarcina mazei, Methanobrevibacter smithii) are leaky and aggregate at high salt concentrations. However, adding PGP-Me to M. mazei lipids results in gradual enhancement of LUV stability, correlating with the PGP-Me content. The LUV data are substantiated by the x-ray results, which show that H. halobium and M. mazei lipids have dissimilar phase behavior and form different structures at high NaCl concentrations. H. halobium lipids maintain an expanded lamellar structure with spacing of 8.5-9 nm, which is stable up to at least 100 degrees C in 2 m NaCl and up to approximately 60 degrees C in 4 m NaCl. However, M. mazei lipids form non-lamellar structures, represented by the Pn3m cubic phase and the inverted hexagonal H(II) phase. From these data, the forces preventing membrane aggregation in halophilic Archaea appear to be steric repulsion, because of the large head group of PGP-Me, or possibly out-of-plane bilayer undulations, rather than electrostatic repulsion attributed to the doubly charged PGP-Me head group.     

Effects of replacement of a double bond by a cyclopropane ring in phosphatidylethanolamines: a 2H NMR study of phase transitions and molecular organization

The thermotropic behavior and molecular properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-dihydrosterculoyl-sn-glycero-3-phosphoethanolamine (PDSPE) have been investigated by 2H NMR spectroscopy using samples selectively labeled at the 5'-, 9'-, 10'-, and 16'-positions of the sn-2 chains. Comparison with the corresponding phosphocholine analogues (POPC and PDSPC), obtained as intermediate synthetic products, was used to monitor the role of the polar head group. Replacement of the choline moiety by ethanolamine increased the gel to liquid-crystal transition temperature by 10-32 degrees C and led to a significantly higher ordering of the fatty acyl chains in the liquid-crystalline bilayer state. The lateral compression effect, due to the smaller area per polar head group in PE, results in a bilayer to hexagonal phase transition at elevated temperatures. The effects on both PC and PE due to replacement of the olefinic group by a cyclopropane unit are similar. A decrease in the temperature of the gel to liquid-crystal phase transition, Tc, is observed upon introduction of a cyclopropane ring; it goes from 26 degrees C in POPE to approximately 10 degrees C in PDSPE. In addition, a very significant broadening of the transition profile is observed. These observations are consistent with the poor packing ability of mixed saturated and cyclopropane-containing chains due to the bulky substituent effect. The temperature of the bilayer-hexagonal phase transition of PE samples was decreased by 15-20 degrees C on replacement of oleoyl chains by dihydrosterculoyl chains at the sn-2 position.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ion permeability of bilayer membranes formed from synthetic phospholipids in the phase transition region

Ion permeability of black lipid membranes formed from synthetic phospholipids has been studied. The resistance of BLM formed from phosphatidylcholine, tiophosphatidylcholine, threealkylphosphate and threealkyltiophosphate was 10(7)--10(8) Ohm.cm2. It was shown that the membrane potential of the 10--30 mV arised in KCl gradient indicating the preference cation conductance in synthetic lipid membranes. A sharp decrease of the membrane conductance near to the phase transition temperature was discovered. The change of conductance by phase transition temperature was sensitive to chemical nature of the polar head of phospholipids used.     

Hydration properties of N-(alpha-hydroxyacyl)-sphingosine: X-ray powder diffraction and FT-Raman spectroscopic studies

The thermotropic properties of N-(alpha-hydroxyacyl)-sphingosine (CER[AS]) in dry and hydrated state were studied by means of X-ray powder diffraction and FT-Raman spectroscopy. The polymorphic states of the CER[AS]/water mixture (lamellar crystalline, lamellar hexagonal gel, liquid crystalline) depend on the thermal pre-treatment of the sample. Only by heating the CER[AS]/water mixture above the melting chain transition can the system be hydrated. At room temperature, both dry and hydrated states form lamellar structures, which differ in their repeat distance and packing of hydrocarbon chains. Above the melting chain transition, hydrated CER[AS] forms a liquid crystalline hexagonal phase, whereas anhydrous CER[AS] forms an isotropic liquid phase. The various phases of hydrated CER[AS] are distinguished on the basis of the corresponding Raman spectra.     

Hydration and the lamellar to hexagonal II phase transition of phosphatidylethanolamine

The effects of chaotropic agents on the lamellar to hexagonal II phase transition of soy phosphatidylethanolamine were examined. Guanidine hydrochloride, urea, and NaSCN were used as chaotropic agents. In each case, the lamellar phase was stabilized by the presence of the chaotropic agent. In the case of NaSCN, the temperature of the lamellar to hexagonal phase transition of soy phosphatidylethanolamine was increased by more than 60 degrees C. Guanidine hydrochloride was capable of substantially reducing the aggregation of phosphatidylethanolamine vesicles. These data lead to a thermodynamic understanding of the lamellar to hexagonal phase transition.     

Thermotropic changes in the conductivity of black bilayers from egg phosphatidylethanolamine

Current fluctuations in black bilayers from phosphatidyl ethanolamine obtained from egg lecithin were registered in the temperature region of the main phase transition of this phospholipid and the bilayer--hexagonal phase transition about 35 degrees; they correspond to the conductivity changes of hundreds of pSm. This transition takes place in the same temperature region as shown by 31P-NMR and depolarization of light-scattering method in phosphatidyl ethanolamine multilamellar liposomes. The scheme of bilayer transformation into hexagonal phase in the temperature region of lipid polymorphic transition is discussed.     

Small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) studies of amide phospholipids

Varying chemically the structure of phospholipids in the region between hydrophobic and hydrophilic segments is expected to have a strong influence on the interaction with water and the phase behavior. This is studied in this work with the motivation to investigate these lipids as potential inhibitors of phospholipase A2. Thus the amide phospholipids L-ether-amide-PC (1-O-hexadecyl-2-N-palmitoyl-2-amino-2-deoxy-sn-glycero-3-phosphocholine), L-ester-amide-PC (1-palmitoyl-2-N-palmitoyl-2-amino-2-deoxy-sn-glycero-3-phosphocholine) and L-ether-amide-PE (1-O-hexadecyl-2-N-palmitoyl-2-deoxy-sn-glycero-3-phosphoethanolamine) have been synthesized and characterized. The phase behavior and thermal transitions in buffer dispersions are examined by a combination of high-sensitivity differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) experiments between 10 and 80 degrees C at pH 8.9. The onset temperatures determined from DSC measurements agree well with the starting temperatures of changes in the repeat distance obtained by SAXS measurements. The phases observed are lamellar both below and above the main phase transition. The phase transition temperatures and enthalpies depend strongly on the substitutions in sn-1 position and head group structure. The lamellar repeat distance in gel and liquid-crystalline phases increases with increasing temperature for L-ester-amide-PC and L-ether-amide-PC, whereas the temperature dependence is opposite for the L-ether-amide-PE. The observed behavior is discussed and compared with that of DPPC and DPPE, indicating the strong dependence of hydration and phase behavior on head group structure.     

The inverted hexagonal phase is more sensitive to hydroperoxidation than the multilamellar phase in phosphatidylcholine and phosphatidylethanolamine aqueous dispersions.

The effect of phase behaviour (hexagonal II phase and lamellar phase) on the peroxidation of membrane phospholipids has been investigated in dilinoleoyl phosphatidylcholine (DLPC)/dilinoleoyl phosphatidylethanolamine (DLPE) aqueous dispersions. Peroxidation was initiated with a water-soluble radical inducer 2,2'-azobis (2-amidino-propane) dihydrochloride (AAPN). The phospholipid morphology was monitored by 31P-nuclear magnetic resonance (NMR). Phospholipid hydroperoxides (PCOOH and PEOOH) were determined by chemiluminescence high-performance liquid chromatography (CL-HPLC). In pH-induced phase transition systems, DLPE in the bilayer state was much less oxidized than in the hexagonal II state. In composition-induced phase transition systems, the formation of total hydroperoxides and the consumption of alpha-tocopherol in the hexagonal II phase were greater than in the bilayer phase. These data suggest that the hexagonal II phase is more sensitive to hydroperoxidation than the bilayer phase in phospholipid aqueous dispersions.