Correlation between lipid plane curvature and lipid chain order.

The 1-palmitoyl-2-oleoyl-phosphatidylethanolamine: 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE:POPC) system has been investigated by measuring, in the inverted hexagonal (HII) phase, the intercylinder spacings (using x-ray diffraction) and orientational order of the acyl chains (using 2H nuclear magnetic resonance). The presence of 20 wt% dodecane leads to the formation of a HII phase for the composition range from 0 to 39 mol% of POPC in POPE, as ascertained by x-ray diffraction and 2H nuclear magnetic resonance. The addition of the alkane induces a small decrease in chain order, consistent with less stretched chains. An increase in temperature or in POPE proportion leads to a reduction in the intercylinder spacing, primarily due to a decrease in the water core radius. A temperature increase also leads to a reduction in the orientational order of the lipid acyl chains, whereas the POPE proportion has little effect on chain order. A correlation is proposed to relate the radius of curvature of the cylinders in the inverted hexagonal phase to the chain order of the lipids adopting the HII phase. A simple geometrical model is proposed, taking into account the area occupied by the polar headgroup at the interface and the orientational order of the acyl chains reflecting the contribution of the apolar core. From these parameters, intercylinder spacings are calculated that agree well with the values determined experimentally by x-ray diffraction, for the variations of both temperature and POPE:POPC proportion. This model suggests that temperature increases the curvature of lipid layers, mainly by increasing the area subtended by the hydrophobic core through chain conformation disorder, whereas POPC content affects primarily the headgroup interface contribution. The frustration of lipid layer curvature is also shown to be reflected in the acyl chain order measured in the L alpha phase, in the absence of dodecane; for a given temperature, increased order is observed when the curling tendencies of the lipid plane are more pronounced.     

Acyl chain orientational order in the hexagonal HII phase of phospholipid-water dispersions.

The deuterium nuclear magnetic resonance (2H NMR) spectrum of perdeuterated tetradecanol in a mixture of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) and water was used to compare the variation of the acyl chain orientational order parameter, S(n), with carbon position, n, in the liquid crystalline lamellar (L alpha) and hexagonal (HII) phases. The characteristics independence of S(n) with n (plateau) normally observed in the L alpha phase is replaced by a more rapid decrease of S(n) with n in the HII phase. It is suggested that as a consequence of the geometrical characteristics of the HII phase, there is an increase in conformational freedom available to different parts of the acyl chain.     

Lipid polymorphism and hydrocarbon order

The use of 2H nuclear magnetic resonance for the characterization of the polymorphic behavior of lipids is illustrated. Different lipid phase preferences may be expected to influence the orientational order and its variation along the acyl chains. Several results are presented to support that view. An increase of motional freedom and a redistribution of the order along the acyl chains are observed during the lamellar-to-hexagonal phase transition, showing that the order profile is sensitive to the lipid phase symmetry. In addition, if the preferences for nonlamellar phases are not expressed explicitly, the presence of "nonbilayer" lipids constrained in bilayer environment induces increased hydrocarbon order. This suggests that order parameters of the acyl chains and lipid polymorphic tendencies are intimately related.     

Effect of acyl chain composition on salt-induced lamellar to inverted hexagonal phase transitions in cardiolipin

Salt-induced fluid lamellar (L alpha) to inverted hexagonal (HII) phase transitions have been studied in diphosphatidylglycerols (cardiolipins) with different acyl chain compositions, using 31P nuclear magnetic resonance (NMR) spectroscopy. Cardiolipins with four myristoyl chains, tetramyristoyl cardiolipin (TMCL), and with four oleoyl chains, tetraoleoyl cardiolipin (TOCL), were synthesized chemically. TMCL was found to undergo a thermotropic lamellar gel to lamellar liquid-crystalline phase transition at 33-35 degrees C. This lipid exhibited an axially symmetric 31P-NMR spectrum corresponding to a lamellar phase at all NaCl concentrations between 0 and 6 M. In the case of TOCL, formation of an HII phase was induced by salt concentrations of 3.5 M NaCl or greater. These observations, taken together with earlier findings that bovine heart cardiolipin aqueous dispersions adopt an HII phase at salt concentrations of 1.5 M NaCl or greater, indicate that increasing unsaturation and length of the acyl chains favour formation of the HII phase in diphosphatidylglycerols.     

Activation energy and entropy for intramolecular excimer formation in a dipyrenylphosphatidylcholine probe in lamellar and hexagonal lipid phases.

Intramolecular excimer formation in pyrene-labeled phosphatidylcholine was used as a tool to determine thermodynamic characteristics of the lamellar to hexagonal phase transitions in a binary lipid system dilinoleoylphosphatidylethanolamine (DLPE)/palmitoyloleoylphosphatidylcholine (POPC). Upon an L alpha/HII phase transition, the activation energy Ea for excimer formation increased from 5.6 +/- 0.2 kcal/mol to 6.3 +/- 0.2 kcal/mol, while the activation entropy delta S decreased from -40.0 +/- 0.8 cal/K.mol to -38.4 +/- 0.8 cal/K.mol. The results are consistent with the idea of molecular splaying of the acyl chains in the hexagonal phase. It is estimated that the molecular area at the terminal carbon of the lipid acyl chains increases by a factor of 2.2 upon the L alpha HII transition in DLPE/POPC.     

A carbon-13 nuclear magnetic resonance spectroscopic study of inter-proton pair order parameters: a new approach to study order and dynamics in phospholipid membrane systems.

We report a simple new nuclear magnetic resonance (NMR) spectroscopic method to investigate order and dynamics in phospholipids in which inter-proton pair order parameters are derived by using high resolution 13C cross-polarization/magic angle spinning (CP/MAS) NMR combined with 1H dipolar echo preparation. The resulting two-dimensional NMR spectra permit determination of the motionally averaged interpair second moment for protons attached to each resolved 13C site, from which the corresponding interpair order parameters can be deducted. A spin-lock mixing pulse before cross-polarization enables the detection of spin diffusion amongst the different regions of the lipid molecules. The method was applied to a variety of model membrane systems, including 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/sterol and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/sterol model membranes. The results agree well with previous studies using specifically deuterium labeled or predeuterated phospholipid molecules. It was also found that efficient spin diffusion takes place within the phospholipid acyl chains, and between the glycerol backbone and choline headgroup of these molecules. The experiment was also applied to biosynthetically 13C-labeled ergosterol incorporated into phosphatidylcholine bilayers. These results indicate highly restricted motions of both the sterol nucleus and the aliphatic side chain, and efficient spin exchange between these structurally dissimilar regions of the sterol molecule. Finally, studies were carried out in the lamellar liquid crystalline (L alpha) and inverted hexagonal (HII) phases of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). These results indicated that phosphatidylethanolamine lamellar phases are more ordered than the equivalent phases of phosphatidylcholines. In the HII (inverted hexagonal) phase, despite the increased translational freedom, there is highly constrained packing of the lipid molecules, particularly in the acyl chain region.     

Fourier-transform infrared studies of CaATPase/phospholipid interaction: survey of lipid classes

CaATPase from rabbit skeletal muscle has been isolated, purified, delipidated, and reconstituted with retention of ATPase activity into lipid vesicles consisting respectively of 1,2-dipalmitoylphosphatidylethanolamine, 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE), 1-stearoyl-2-oleoylphosphatidylcholine (SOPC), and egg sphingomyelin. The effect of the enzyme on phospholipid order and melting characteristics were determined with Fourier-transform infrared spectroscopy. Taken together with prior data from this laboratory for 1,2-dipalmitoylphosphatidylcholine and 1,2-dioleoylphosphatidylcholine (DOPC), as well as for native sarcoplasmic reticulum (SR), three types of lipid response to protein incorporation have been observed: (1) Phospholipids with high levels of acyl chain unsaturation (DOPC or native SR) have their lipid acyl chains slightly ordered by CaATPase incorporation. The effect of protein on the gel-liquid crystal phase transition cannot be easily determined, since the cooperative melting even in these systems occurs at temperature well below 0 degrees C. (2) Phospholipids with saturated acyl chains show slightly lowered melting temperatures and reduced cooperativity of melting upon CaATPase insertion. In addition, protein induces (at most) slight disorder into the acyl chains at temperatures removed from the lipid melting point. (3) The strongest response is observed for phospholipids containing one saturated and one unsaturated chain (POPE or SOPC) or heterogeneous systems with low levels of unsaturation (egg sphingomyelin). In these cases, relatively low protein levels diminish the magnitude of or completely abolish the phospholipid phase transition. In addition, substantial disorder is introduced into the acyl chain compared with the pure lipid both above and below its transition temperature.(ABSTRACT TRUNCATED AT 250 WORDS)     

The influence of phase transitions in phosphatidylethanolamine models on the activity of violaxanthin de-epoxidase

In the present study, the influence of the phospholipid phase state on the activity of the xanthophyll cycle enzyme violaxanthin de-epoxidase (VDE) was analyzed using different phosphatidylethanolamine species as model lipids. By using (31)P NMR spectroscopy, differential scanning calorimetry and temperature dependent enzyme assays, VDE activity could directly be related to the lipid structures the protein is associated with. Our results show that the gel (L beta) to liquid-crystalline (L alpha) phase transition in these single lipid component systems strongly enhances both the solubilization of the xanthophyll cycle pigment violaxanthin in the membrane and the activity of the VDE. This phase transition has a significantly stronger impact on VDE activity than the transition from the L alpha to the inverted hexagonal (HII) phase. Especially at higher temperatures we found increased VDE reaction rates in the presence of the L alpha phase compared to those in the presence of HII phase forming lipids. Our data furthermore imply that the HII phase is better suited to maintain high VDE activities at lower temperatures.     

1H NMR dipolar echo decay spectroscopy: a sensitive probe of membrane structure

The structural and motional properties of membrane lipids in various physical states and macroscopic organizations are elucidated by 1H NMR dipolar echo decay spectroscopy (DECODE). Multilamellar lipid dispersions in the gel (L beta') and liquid-crystalline (L alpha) states and a nonbilayer, hexagonal (HII) phase are readily distinguished, a dynamic profile within these phases is identified, and dipolar order parameters are obtained in the fluid phases. The method is suitable for any pulsed NMR spectrometer. DECODE provides the first depth-dependent assay of lipid order that does not rely on isotopic labeling or exogenous probe.     

2H-nuclear magnetic resonance investigations on phospholipid acyl chain order and dynamics in the gramicidin-induced hexagonal HII phase.

The following results are reported in this paper: The interaction of gramicidin with [11,11-2H2]dioleoylphosphatidylcholine (DOPC) and [11,11-2H2]dioleoylphosphatidylethanolamine (DOPE) at different stages of hydration was studied by 2H- and 31P-nuclear magnetic resonance. In the L alpha phase in excess water the acyl chains of phosphatidylethanolamine (PE) are more ordered than phosphatidylcholine (PC) most likely as the result of the lower headgroup hydration of the former lipid. In excess water gramicidin incorporation above 5 mol % in DOPC causes a bilayer----hexagonal HII phase change. In the HII phase acyl chain order is virtually unaffected by gramicidin but the peptide restricts the fast chain motions. At low water content gramicidin cannot induce the HII phase but it markedly decreases chain order in the DOPC bilayer. Increasing water content results in separation between a gramicidin-poor and a gramicidin-rich L alpha phase with decreased order of the entire lipid molecule. Further increase in hydration reverts at low gramicidin contents the phase separation and at high gramicidin contents results in a direct change of the disordered lamellar to the hexagonal HII phase. Gramicidin also promotes HII phase formation in the PE system but interacts much less strongly with PE than with PC. The results support our hypothesis that gramicidin, by a combination of strong intermolecular attraction forces and its pronounced cone shape, both involving the four tryptophans at the COOH-terminus, has a strong tendency to organize, with the appropriate lipid, in intramembranous cylindrical structures such as is found in the HII phase.     

Fluorescence depolarization study of lamellar liquid crystalline to inverted cylindrical micellar phase transition of phosphatidylethanolamine.

The orientational order and rotational dynamics of 2-[3-(diphenyl-hexatrienyl) propanoyl]-3-palmitoyl-L-alpha- phosphatidylcholine (DPH-PC) embedded in dioleoylphosphatidyl-ethanolamine (DOPE) were studied by fluorescence depolarization technique. Upon increasing the temperature, the calculated wobbling diffusion constant D perpendicular of the fluorescent probe was found to decrease at the lamellar (L alpha) to inverted cylindrical (H II) phase transition (10 degrees C). This suggested that the increased gauche rotamers of the alkene chains in the HII phase imposes a constraint in the wobbling motion of the fluorophore. The calculated ratio of order parameter in the L alpha phase to that in the HII phase was 1.7 and different from the theoretical value of 2.0 as predicted from the change in packing symmetry. This result can be explained by a slightly higher local order parameter of the fluorophore or by the fast rotational diffusion motion of the fluorophore around the symmetry axis of the cylindrical tubes in the HII phase.     

Phase equilibria of membrane lipids from Acholeplasma laidlawii: importance of a single lipid forming nonlamellar phases

A basis for the reorganization of the bilayer structure in biological membranes is the different aggregate structures formed by lipids in water. The phase equilibria of all individual lipids and several in vivo polar lipid mixtures from acyl chain modified membranes of Acholeplasma laidlawii were investigated with different NMR techniques. All dioleoyl (DO) polar lipids, except monoglucosyldiglyceride (MGDG), form lamellar liquid crystalline (L alpha) phases only. The phase diagram of DOMGDG reveals reversed cubic (III), reversed hexagonal (HII), and L alpha phases. In mixtures of DOMGDG and dioleoyldiglycosyldiglyceride (DODGDG), the formation of an III (or HII) phase is enhanced by DOMGDG and low hydration or high temperatures. For in vivo mixtures of all polar DO lipids, a transition from an L alpha to an III phase is promoted by low hydration or high temperatures (50 degrees C). The phospholipids are incorporated in this III phase. Likewise, III and HII phases are formed at similar temperatures in a series of in vivo mixtures with different extents of acyl chain unsaturation. However, their melting temperatures (Tm) vary in an expected manner. All cubic and hexagonal phases, except the III phase with DOMGDG, exist in equilibrium with excess water. The maximum hydration of MGDG and DGDG is similar and increases with acyl chain unsaturation but is substantially lower than that for, e.g., phosphatidylcholine. The translational diffusion of the lipids in the cubic phases is rapid, implying bicontinuous structures. However, their appearances in freeze-fracture electron microscope pictures are different. The III phase of DOMGDG belongs to the Ia3d space group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of the orientational order profile of the lipid acyl chain in the Lα phase

The orientational order profile along the lipid acyl chain has been characterized under several different conditions of polar headgroup composition, temperature, and cholesterol content. Despite the different nature of these factors, the variation of the order is governed by two common trends. First, the relative change of order induced by the variation of these factors is always more pronounced towards the end of the chain than for the methylene groups near the interface. Second, there is, to a first approximation, a distinct correlation between the magnitude of the order parameters and the shape of the order profile. For example when the chain is highly ordered, the relative width of the order distribution is narrow indicating that the plateau region is longer. These conclusions suggest that the orientational order profile depends on only a small number of parameters and demonstrate clearly that the correlation length for changes in orientational order is much greater than one C-C bond length. Our results also show that the reduced temperature is not related in simple terms to orientational order and probably has little theoretical significance. The orientational order profiles of POPC and POPE bilayers are significantly different even when expressed in terms of reduced temperature. The behavior of POPC/cholesterol systems also indicates that the orientational order of the lipid chain and the gel-to-liquid crystalline phase transition temperature are not related in a straightforward manner.Abbreviations POPC 1-palmitoyl-2-oleoyl-phosphatidylcholine - POPE 1-palmitoyl-2-oleoyl-phosphatidylethanolamine - PC phosphatidylcholine - PE phosphatidylethanolamine - NMR nuclear magnetic resonance - EDTA ethylenediaminetetraacetic acid Offprint requests to: M. Bloom     

Deuteron nuclear magnetic resonance study of the dynamic organization of phospholipid/cholesterol bilayer membranes: molecular properties and viscoelastic behavior.

The influence of cholesterol on the dynamic organization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers was studied by deuteron nuclear magnetic resonance (2H NMR) using unoriented and macroscopically aligned samples. Analysis of the various temperature- and orientation-dependent experiments were performed using a comprehensive NMR model based on the stochastic Liouville equation. Computer simulations of the relaxation data obtained from phospholipids deuterated at the 6-, 13- and 14-position of the sn-2 chain and cholesterol labeled at the 3 alpha-position of the rigid steroid ring system allowed the unambiguous assignment of the various motional modes and types of molecular order present in the system. Above the phospholipid gel-to-liquid-crystalline phase transition, TM, 40 mol % cholesterol was found to significantly increase the orientational and conformational order of the phospholipid with substantially increased trans populations even at the terminal sn-2 acyl chain segments. Lowering the temperature continuously increases both inter- and intramolecular ordering, yet indicates less ordered chains than found for the pure phospholipid in its paracrystalline gel phase. Trans-gauche isomerization rates on all phospholipid alkyl chain segments are slowed down by incorporated cholesterol to values characteristic of gel-state lipid. However, intermolecular dynamics remain fast on the NMR time scale up to 30 K below TM, with rotational correlation times tau R parallel for DMPC ranging from 10 to 100 ns and an activation energy of ER = 35 kJ/mol. Below 273 K a continuous noncooperative condensation of both phospholipid and cholesterol is observed in the mixed membranes, and at about 253 K only a motionally restricted component is left, exhibiting slow fluctuations with correlation times of tau R perpendicular greater than 1 microsecond. In the high-temperature region (T greater than TM), order director fluctuations are found to constitute the dominant transverse relaxation process. Analysis of these collective lipid motions provides the viscoelastic parameters of the membranes. The results (T = 318 K) show that cholesterol significantly reduces the density of the cooperative motions by increasing the average elastic constant of the membrane from K = 1 x 10(-11) N for the pure phospholipid bilayers to K = 3.5 x 10(-11) N for the mixed system.     

Fusogenic Alzheimer's peptide fragment Abeta (29-42) in interaction with lipid bilayers: secondary structure, dynamics, and specific interaction with phosphatidyl ethanolamine polar heads as revealed by solid-state NMR

The interaction of the native Alzheimer's peptide C-terminal fragment Abeta (29-42), and two mutants (G33A and G37A) with neutral lipid bilayers made of POPC and POPE in a 9:1 molar ratio was investigated by solid-state NMR. This fragment and the lipid composition were selected because they represent the minimum requirement for the fusogenic activity of the Alzheimer's peptide. The chemical shifts of alanine methyl isotropic carbon were determined by MAS NMR, and they clearly demonstrated that the major form of the peptide equilibrated in membrane is not in a helical conformation. (2)H NMR, performed with acyl chain deuterated POPC, demonstrated that there is no perturbation of the acyl chain's dynamics and of the lipid phase transition temperature. (2)H NMR, performed with alanine methyl-deuterated peptide demonstrated that the peptide itself has a limited mobility below and above the lipid phase transition temperature (molecular order parameter equal to 0.94). MAS (31)P NMR revealed a specific interaction with POPE polar head as seen by the enhancement of POPE phosphorus nuclei T(2) relaxation. All these results are in favor of a beta-sheet oligomeric association of the peptide at the bilayer interface, preferentially recruiting phosphatidyl ethanolamine polar heads.     

13C NMR studies of the thermal properties of a model high density lipoprotein. Apolipoprotein A-I-dimyristoylphosphatidylcholine complex

The most abundant lipid and protein components of human plasma high density lipoproteins are phosphatidylcholine and apolipoprotein A-I (A-I). Under appropriate conditions, A-I spontaneously associates with dimyristoylphosphatidylcholine (DMPC) to quantitatively form a lipid-protein complex with a DMPC/A-I molar ratio of 100:1. Differential scanning calorimetry of this complex reveals two broad thermal transitions centered at approximately 27 and 72 degrees C. 13C NMR spectra of the complex have been obtained above, at, and below the lower transition temperature. The 13C resonance arising from the 3' carbon of the fatty acyl chains is a doublet, split by approximately 0.2 ppm, suggesting that the 3' carbon nuclei occupy two magnetically inequivalent sites. By replacing the sn-2 fatty acyl chain with myristate selectively 13C-enriched at carbon 3', we have shown that the splitting is, in fact, a result of magnetic inequivalence of the two sites and have assigned the lower field resonance to the 3' carbon nucleus of the sn-2 chain. The temperature dependence of the NMR relaxation rates indicates that the endothermic transition at 27 degrees C is associated with increased motional freedom for the phospholipids within this complex. The temperature dependence of the fatty acyl chain methylene 13C chemical shifts suggests that the population of gauche conformers increases above the transition temperature. These dynamic and conformational changes are characteristic of gel----liquid crystalline phase transitions observed in pure phospholipid systems. For the DMPC-A-I complex at 37 degrees C, the chemical shifts of the fatty acyl C 4'- 11' methylene envelope and of the C 7' and C 13' resonances occur significantly downfield from the corresponding chemical shifts for the DMPC vesicle. These results suggest that the apoprotein rigidifies the acyl chains by increasing their number of trans conformers.     

Effect of doxorubicin on the order of the acyl chains of anionic and zwitterionic phospholipids in liquid-crystalline mixed model membranes: absence of drug-induced segregation of lipids into extended domains.

We investigated the effect of the antineoplastic drug doxorubicin on the order of the acyl chains in liquid-crystalline mixed bilayers consisting of dioleoylphosphatidylserine (DOPS) or -phosphatidic acid (DOPA), and dioleoylphosphatidylcholine (DOPC) or -phosphatidylethanolamine (DOPE). Previous 2H-NMR studies on bilayers consisting of a single species of di[11,11-2H2]oleoyl-labeled phospholipid showed that doxorubicin does not affect the acyl chain order of pure zwitterionic phospholipid but dramatically decreases the order of anionic phospholipid [de Wolf, F. A., et al. (1991) Biochim. Biophys. Acta 1096, 67-80]. In the present work, we studied mixed bilayers in which alternatively the anionic or the zwitterionic phospholipid component was 2H-labeled so as to monitor its individual acyl chain order. Doxorubicin decreased the order parameter of the mixed anionic and zwitterionic lipids by approximately the same amount and did not induce a clear segregation of the lipid components into extended, separate domains. The drug had a comparable disordering effect on mixed bilayers of unlabeled cardiolipin and 2H-labeled zwitterionic phospholipid, indicating the absence of extensive segregation also in that case. Upon addition of doxorubicin to bilayers consisting of 67 mol% DOPE and 33 mol% anionic phospholipid, a significant part of the lipid adopted the inverted hexagonal (HII) phase at 25 degrees C. This bilayer destabilization, which occurred only in mixtures of anionic phospholipid and sufficient amounts of DOPE, might be of physiological importance. Even upon formation of extended HII-phase domains, lipid segregation was not clearly detectable, since the relative distribution of 2H-labeled anionic phospholipid and [2H]DOPE between the bilayer phase and HII phase was very similar. Our findings argue against a role of extensive anionic/zwitterionic lipid segregation in the mechanism of action and toxicity of doxorubicin.     

Motional narrowing of the 2H NMR spectra near the chain melting transition of phospholipid/D2O mixtures

The reduction in spectral splitting, or motional narrowing, of the deuterium spectra of D₂O/phos-pholipid mixtures near the main chain melting phase transition was studied for palmitoyloleoylphosphatidylcholine (POPC), palmitoyloleoylphosphatidylethanolamine (POPE) and equimolar mixtures of the two at 10% hydration. For POPC the splitting was about 1700 Hz in both the fluid and gel phases, dropping to zero near the phase transition (as reported previously). For POPE the splitting remained approximately constant above the phase transition. Below the phase transition the spectrum showed a single broad line whose linewidth varied between 100 Hz and 800 Hz. This was interpreted as being due to small domains of water within a weakly hydrated crystal. POPC:POPE (1:1) samples exhibited motional narrowing behaviour similar to that for POPC except that the splitting above the phase transition was approximately twice that below the transition. The relatively broad temperature range (20 K) of the transition is explained using a simple physical model involving lipid fluctuations near the phase transition.Abbreviations NMR Nuclear Magnetic Resonance - PC phosphatidylcholine - PE phosphatidylethanolamine - POPC Palmitoyloleoylphosphatidylcholine - POPE Palmitoyloleoylphosphatidylethanolamine - H_II Inverse hexagonal phase     

Order and dynamics in the lamellar L alpha and in the hexagonal HII phase. Dioleoylphosphatidylethanolamine studied with angle-resolved fluorescence depolarization.

Fluorescence depolarization techniques are used to determine the molecular order and reorientational dynamics of the probe molecule TMA-DPH embedded in the lamellar L alpha and the hexagonal HII phases of lipid/water mixtures. The thermotropically induced L alpha----HII phase transition of the lipid DOPE is used to obtain macroscopically aligned samples in the hexagonal HII phase at 45 degrees C from samples prepared in the lamellar L alpha phase at 7 degrees C. The interpretation of angle-resolved fluorescence depolarization experiments on these phases, within the framework of the rotational diffusion model, yields the order parameters (P2) and (P4), and the diffusion constants for the reorientational motions. The reorientational motion rates of the TMA-DPH molecules in the hexagonal HII phase are comparable with those in the lamellar L alpha phase. Furthermore, the lateral diffusion of the probe molecule on the surface of the lipid/water cylinder in the hexagonal phase is found to be considerably slower than the reorientational motion.