Dependence of the bilayer to hexagonal phase transition on amphiphile chain length

Several series of amphiphiles of increasing chain length were tested for their abilities to modify the L alpha-HII transition of dielaidoylphosphatidylethanolamine using differential scanning calorimetry. Acylcarnitines, alkyl sulfates, alkylsulfobetaines, and phosphatidylcholines, with chain lengths between about 6 and 12 carbon atoms, show an increasing capacity to raise the L alpha-HII phase transition temperature of phosphatidylethanolamine. This is ascribed to increased partitioning of the added amphiphile from water into the membrane as the chain length increases. Alkyl sulfates and alkyltrimethylammonium bromides have diminished capacities to raise the L alpha-HII transition temperature as the chain length is increased from 12 to 16. This is caused by an increase in the hydrophobic portion of the amphiphile leading to a change in the intrinsic radius of curvature and a decrease in the hydrocarbon packing constraints in the HII phase relative to the shorter chain amphiphiles. The L alpha-HII transition temperature of phosphatidylethanolamine with acylcarnitines of chain length 14-20 carbon atoms, alkylsulfobetaines above 14 carbon atoms, and phosphatidylcholines with acyl groups having above 10 carbon atoms is relatively insensitive to chain length. We suggest that this is caused by a balance between increasing hydrocarbon volume promoting the HII phase through decreased intrinsic radius of curvature and greater relief of hydrocarbon packing constraints vs greater intermolecular interactions favoring the more condensed L alpha phase. This latter effect is more important for amphiphiles with large headgroups which can pack more efficiently in the L alpha phase. The phosphatidylcholines show a gradual decrease in bilayer stabilization between 10 and 22 carbon atoms.(ABSTRACT TRUNCATED AT 250 WORDS)     

An analysis of the relationship between fatty acid composition and the lamellar gel to liquid-crystalline and the lamellar to inverted nonlamellar phase transition temperatures of phosphatidylethanolamines and diacyl-α-D-glucosyl glycerols

The lamellar gel to lamellar liquid-crystalline (Lbeta/Lalpha) and lamellar liquid-crystalline to inverted hexagonal (Lalpha/H(II)) phase transitions of a number of phosphatidylethanolamines (PEs) and diacyl-alpha-D-glucosyl-sn-glycerols (alpha-D-GlcDAGs) containing linear saturated, linear unsaturated, branched or alicyclic hydrocarbon chains of various lengths were examined by differential scanning calorimetry and low-angle X-ray diffraction. As reported previously, for each homologous series of PEs or alpha-D-GlcDAGs, the Lbeta/Lalpha phase transition temperatures (Tm) increase and the Lalpha/H(II) phase transition temperatures (Th) decrease with increases in hydrocarbon chain length. The Tm and the especially the Th values for the PEs are higher than those of the corresponding alpha-D-GlcDAGs. For PEs having the same effective hydrocarbon chain length but different chain configurations, the Tm and Th values vary markedly but with an almost constant temperature interval (deltaT(L/NL)) between the two phase transitions. Moreover, although the Tm and Th values of the PEs and alpha-D-GlcDAGs are equally sensitive on the temperature scale to variations in the length and chemical configuration of the hydrocarbon chains, the deltaT(L/NL) values are generally larger in the PEs and vary less with the hydrocarbon chain structure. This suggests that the PE headgroup has a greater ability to counteract variations in the packing properties of different hydrocarbon chain structures than does the alpha-D-GlcDAG headgroup. With decreasing chain length, this ability of the PE headgroup to counteract the hydrocarbon chain packing properties increases, significantly expanding the temperature interval over which the Lalpha phase is stable relative to the corresponding regions in the alpha-D-GlcDAGs. Overall, these findings indicate that the PEs have a smaller propensity to form the H(II) phase than do the alpha-D-GlcDAGs with an identical fatty acid composition. In contrast to our previous report, there is some variation in the d-spacings of these various PEs (and alpha-D-GlcDAGs) in both the Lalpha and H(II) phases when the hydrocarbon chain structure is changed while the effective chain length is kept constant. These hydrocarbon chain structural modifications produce different d-spacings in the Lalpha and H(II) phases, but those changes are consistent between the PEs and alpha-D-GlcDAGs, probably reflecting differences in the hydrocarbon chain packing constraints in these two phases. Overall, our experimental observations can be rationalized to a first approximation by a simple lateral stress model in which the primary bilayer strain results from a mismatch between the actual and optimal headgroup areas and the primary strain in the H(II) phase arises from a simple hydrocarbon chain packing term.     

Depolarized light-scattering studies of bilayer structures in phospholipid vesicles

Depolarized light scattering has been used to investigate the hydrocarbon chain packing of phospholipids in vesicles below the phase transition and ordering of their chains above the phase transition. The chain packing and ordering have been demonstrated for vesicles of l-α-dipalmitoylphosphatidylethanolamine and some phosphatidylcholines of different hydrocarbon chain lenghts. Anisotropy ratios for phospholipid vesicles could be determined by measuring depolarization ratios for several vesicle sizes at low concentrations of the lipids. The following results were obtained. Hydrocarbon chains of l-α-dimyristoyl and distearoylphosphatidylcholines below their phase transitions pack at tilting angles in good agreement with X-ray diffraction data. On the other hand, hydrocarbon chains of dipalmitoylphosphatidylethanolamine pack perpendicular to the bilayer surface. Values of the averaged order parameter for dimyristoyl, dipalmitoyl and distearoylphosphatidylcholines at 2.5°C above their phase transition are all the same and the value for dipalmitoylphosphatidylcholine is in agreement with results from ²H-NMR experiments. The value of the order parameter for dipalmitoylphosphatidylethanolamine is slightly larger than that for dipalmitoylphosphatidylcholine.     

Lipid polymorphism of mixtures of dioleoylphosphatidylethanolamine and saturated and monounsaturated phosphatidylcholines of various chain lengths

The L alpha-HII phase transition behavior of many lipid-water liquid crystals is dominated by the competition between the tendency to curl the lipid layers to an intrinsic radius of curvature and opposing hydrocarbon packing constraints. In particular, packing constraints can increase the free energy of the inverted hexagonal (HII) phase as compared to that of the lamellar (L alpha) phase. This is especially true where the lipid molecule is not long enough to reach into the corners of the lattice in large hexagonal structures necessitated by a large intrinsic radius of curvature. In this paper it is shown that the addition of a minor fraction long-chain lipid to a system of otherwise uniform chain composition can also relax packing constraints, thereby lowering the lamellar to hexagonal transition temperature. For the specific systems used, dioleoylphosphatidylethanolamine (di-18:1c-PE) with minor fractions of 1,2-diacyl-sn-glycero-3-phosphocholines [di-n:1c-PC (n = 14, 18, 22, and 24)], the observed HII lattices systematically increased in size with increasing chain length, suggesting that the chain length also may affect the intrinsic curvature of the mixture. These experiments demonstrate that the lipid "shape concept", which is a qualitative expression of the concept quantitatively described by the intrinsic radius of curvature, is insufficient to understand the L alpha-HII transition. It is necessary to, at least, consider the competition between curvature and packing.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 1. Differential scanning calorimetric studies

The thermotropic phase behavior of aqueous dispersions of phosphatidylcholines containing one of a series of methyl iso-branched fatty acyl chains was studied by differential scanning calorimetry. These compounds exhibit a complex phase behavior on heating which includes two endothermic events, a gel/gel transition, involving a molecular packing rearrangement between two gel-state forms, and a gel/liquid-crystalline phase transition, involving the melting of the hydrocarbon chains. The gel to liquid-crystalline transition is a relatively fast, highly cooperative process which exhibits a lower transition temperature and enthalpy than do the chain-melting transitions of saturated straight-chain phosphatidylcholines of similar acyl chain length. In addition, the gel to liquid-crystalline phase transition temperature is relatively insensitive to the composition of the aqueous phase. In contrast, the gel/gel transition is a slow process of lower cooperativity than the gel/liquid-crystalline phase transition and is sensitive to the composition of the bulk aqueous phase. The gel/gel transitions of the methyl iso-branched phosphatidylcholines have very different thermodynamic properties and depend in a different way on hydrocarbon chain length than do either the "subtransitions" or the "pretransitions" observed with linear saturated phosphatidylcholines. The gel/gel and gel/liquid-crystalline transitions are apparently concomitant for the shorter chain iso-branched phosphatidylcholines but diverge on the temperature scale with increasing chain length, with a pronounced odd/even alternation of the characteristic temperatures of the gel/gel transition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Iodide penetration into lipid bilayers as a probe of membrane lipid organization.

The quenching efficiency of iodide as a penetrating fluorescence quencher for a membrane-associated fluorophore was utilized to measure the molecular packing of lipid bilayers. The KI quenching efficiency of tryptophan-fluorescence from melittin incorporated in DMPC bilayer vesicles peaks at the phase transition temperature (24 degrees C) of DMPC, whereas acrylamide quenching efficiency does not depend on temperature. The ability of iodide to penetrate the hydrocarbon region of the bilayer was examined by measuring the fluorescence quenching of the pyrene-phosphatidylcholine incorporated into DMPC vesicles (pyrene was attached to the 10th carbon of the sn-2 chain). The quenching efficiency of pyrene by iodide again shows a maximum at the lipid phase transition. We conclude that iodide penetrates the membrane hydrocarbon region at phase transition through an increased number of bilayer defects. The magnitude of change in quenching efficiency of iodide during lipid phase transition provides a sensitive technique to probe the lipid organization in membranes.     

How membrane chain-melting phase-transition temperature is affected by the lipid chain asymmetry and degree of unsaturation: an effective chain-length model.

Hydrocarbon effects on the lipid chain-melting phase-transition temperature are analyzed. The membrane fluidization temperature is shown to increase with the effective chain length, which is proportional to the thickness of the well-packed hydrocarbon region. The latter, as a rule, increases with the length of the longest ordered and aligned segment on each chain. This conclusion is independent of the cause for the reduced chain packing in membrane interior: chain unsaturation (which effectively decouples the two hydrocarbon segments disjoined by a double bond) or chain asymmetry (which causes the terminal hydrocarbon segments to lose close contact) both affect the bilayer chain-melting phase-transition temperature comparably on the effective chain-length scale. Thermodynamic consequences of the trans unsaturation are approximately 50% smaller than the effects of the double bonds in the cis conformation, owing to the smaller membrane perturbation by the former double bonds. A simple quantitative model is introduced for the analysis of the phospholipid chain-melting phase behavior. This new model permits quantitative predictions of the chain-melting transition temperature solely on the basis of the known lipid chemical composition. It also explains lipid sensitivity to the hydrocarbon type and attachment. The model agreement with the experimental data is usually better than to within 99% and thus comparable to experimental scatter, even when only a few or no adjustable parameters are used. The membrane fluidization temperature is calculated for a number of potentially interesting, also as yet unexplored, phospholipids, and the biological significance of the effective chain-length concept is discussed.     

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.     

X-ray diffraction and spectroscopic studies of oleic acid-sodium oleate

The sodium oleate-oleic acid (1:1) complex (NaHOl(2)) is characterized using X-ray diffraction, FT-IR photoacoustic spectroscopy, FT-Raman spectroscopy, and DSC. The special arrangement of hydrogen-bonded pairs of carboxylic acid and carboxylate groups into unique "head-group" is supported by frequency shifts and partial or total disappearance of characteristic vibrations of carboxylic acid dimer and of carboxylate groups. The well-ordered state of hydrocarbon chains is demonstrated by the existence of sharp Raman bands in the C-C stretching region (1000-1150 cm-1) and other conformationally sensitive modes. The FT-Raman results suggest that the transition at about 32 degrees C involves the cooperative melting of methyl- and carboxyl/carboxylate-sided hydrocarbon chains. From the X-ray diffraction data it is clear that this transition is associated with the disintegration of the hydrogen-bonded carboxylate-carboxylic acid complex, followed by the separate formation of oleic acid and sodium oleate. The packing of hydrocarbon chain in the acid-soap complex is different from the parent oleic acid or sodium oleate. The hydrocarbon chains in the NaHOl(2) form more stable packing (O subcell) in comparison to that of oleic acid. A temperature composition phase diagram is presented.     

The crystal structure of cholesteryl dodecanoate: co-packing of steroid skeleta and hydrocarbon chains

The crystal structure of cholesteryl dodecanoate has been determined. The compound shows a co-packing of cholesterol skeleta and hydrocarbon chains. There are two molecules in the asymmetric unit both almost fully extended. The hydrocarbon chain axes are however somewhat bent in order to get a good close-packing side by side with the rigid cholesterol skeleta. The two non-symmetry related skeleta show different packing surroundings. One skeleton packs with both hydrocarbon chains and other skeleta while the other skeleton is completely surrounded by hydrocarbon chains. The latter packing is of particular interest as it is considered to indicate important packing principles in biological lipid bilayers.     

Accommodation of hydroxyl groups and their hydrogen bond system in a hydrocarbon matrix

From data of sisingle crystal analysis of 12-D-hydroxyoctadecanoic acid methyl ester principles for the incorporation of hydroxyl groups into a hydrocarbon chain matrix can be deduced. In the crystalline compound infinite hydrogen bond systems are accommodated in an orthorhombic perpendicular (O) chain arrangement. The O hydrocarbon subcell is expanded towards a hexagonal packing pattern, allowing more space and optimal geometry for the hydrogen bond system. The arrangement of the bond system in the O subcell requires that hydrogen bonded carbon chains carry alternatingly hydroxyl roups with opposite configuration. For the enantiomeric compound this requirement is met by a head to tail packing of molecules in a single layer arrangement. The corresponding racemates on the other hand pack head to head in double layers as confirmed by X-ray powder and IR studies. In monolayers both enantiomers and racemates behave identicalyy. The hydrogen bonding of the hydroxyl groups apparently leads to the formation of lipid clusters, in which the geometric conditions for both a close packing of hydrocarbon chains and the formation of an extensive hydrogen bond system do not exist.     

Characterization of membrane lipids of a general fatty acid auxotrophic bacterium by electron spin resonance spectroscopy and differential scanning calorimetry

Lipids in the plasma membrane of the general fatty acid auxotroph Butyrivibrio S2 pack as a bilayer that is characterized by a high order and high motional anisotropy and a low membrane fluidity compared to mammalian plasma membranes. Lipid packing as determined by the electron spin resonance (ESR) order parameter and membrane fluidity as measured by ESR correlation times are, however, comparable to those of other bacterial membranes. Membranes of the organism grown with saturated fatty acids of well-defined hydrocarbon chain length undergo a broad reversible endothermic phase transition, the peak temperature of which is well below the growth temperature; the end-point temperature of this thermal transition approximately coincides with the minimum temperature supporting significant growth of the organism. The lipid phase transition is also reflected in the temperature dependence of various ESR parameters, whereby the transition temperature thus derived is higher than the peak temperature of the endothermic transition but still lower than the growth temperature. ESR and calorimetry evidence taken together suggest that the endothermic transition is a gel to liquid-crystal transition and that, at the growth temperature, the plasma membrane of Butyrivibrio S2 is in the liquid-crystalline state. Similar values were measured for the order parameter of cell membranes of Butyrivibrio S2 regardless of whether the organism was grown on myristic, palmitic, or stearic acid. Butyrivibrio S2 has a mechanism enabling it to maintain membrane packing and fluidity at a fairly constant level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 2. Infrared and 31P NMR spectroscopic studies

The polymorphic phase behavior of aqueous dispersions of a number of representative phosphatidylcholines with methyl iso-branched fatty acyl chains was investigated by Fourier transform infrared (FT-IR) and phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. For the longer chain phosphatidylcholines, where two transitions are resolved on the temperature scale, the higher temperature event can unequivocally be assigned to the melting of the acyl chains (i.e., a gel/liquid-crystalline phase transition), whereas the lower temperature event is shown to involve a change in the packing mode of the methylene and carbonyl groups of the hydrocarbon chains in the gel state (i.e., a gel/gel transition). The infrared spectroscopic data suggest that the methyl iso-branched phosphatidylcholines assume a partially dehydrated, highly ordered state at low temperatures, resembling the Lc phase recently described for the long-chain n-saturated phosphatidylcholines. At higher temperatures, some branched-chain phosphatidylcholines appear to assume a fully hydrated, loosely packed gel phase similar to but not identical with the P beta, phase of their linear saturated analogues. Thus, the iso-branched phosphatidylcholine gel/gel transition corresponds, at least approximately, to a summation of the structural changes accompanying both the subtransition and the pretransition characteristic of the longer chain n-saturated phosphatidylcholines. The infrared spectroscopic data also show that, in the low-temperature gel state, there are significant differences between the odd- and even-numbered isoacylphosphatidylcholines with respect to their hydrocarbon chain packing modes as well as to their head group and interfacial hydration states.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure and thermotropic properties of phosphatidylethanolamine and its N-methyl derivatives

The structure and thermotropic properties of hydrated bilayers of 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine (DMPE) and its N-monomethyl (mmDMPE) and N,N-dimethyl (dmDMPE) derivatives have been investigated by differential scanning calorimetry and X-ray diffraction. For DMPE, mmDMPE, and dmDMPE, multilamellar dispersions (approximately 50 wt % water) show chain melting bilayer gel----bilayer liquid-crystal transitions (onset) at 49.2, 42.3, and 30.7 degrees C, respectively, with the corresponding value for 1,2-dimyristoyl-sn-glycero-3-phosphocholine occurring at 23 degrees C. Thus, the bilayer chain melting transition decreases with increasing N-methylation, as originally reported for the corresponding palmitoyl series [Vaughan, D.J., & Keough, K.M. (1974) FEBS Lett. 47, 158-161]. This transition is reversible on cooling, and DMPE, mmDMPE, and dmDMPE form the original bilayer gel phase with the rotationally disordered hydrocarbon chains packed in a hexagonal lattice. Following prolonged incubation at -4 degrees C, the bilayer gel phase is shown to be metastable, and conversion to a low-temperature "crystalline" phase occurs with the hydrocarbon chains adopting a specific packing mode. For DMPE, mmDMPE, and dmDMPE, either a single or a double endothermic transition occurs as the "crystal" bilayer phase converts to the bilayer gel phase. A similar pattern of behavior is observed for the palmitoyl series. The relatively slow kinetic conversion of the metastable bilayer gel phase with hexagonally packed hydrocarbon chains to a bilayer phase in which the chains have "crystallized" appears to be a general property of membrane phospholipids and sphingolipids.     

Phase transitions of phospholipid vesicles under osmotic stress and in the presence of ethylene glycol.

The effects of poly(ethylene glycol) (PEG) on the phase transition of phospholipid multilamellar vesicles (MLVs) were investigated by using differential scanning calorimetry (DSC). Main transition temperature (Tm) and the pre-transition temperature (Tp) of neutral phospholipid-, DMPC-1, DPPC- and DSPC-MLVs increased with an increase in PEG concentration. The subtransition temperature of DPPC-MLV also increased with an increase in PEG concentration. These results could be qualitatively explained by enhancement of the lateral packing on the basis of the osmoelastic coupling theory. The pretransition temperature increased faster than the main transition temperature did with an increase in PEG concentration. The increment of Tm depended on the hydrocarbon chain length, the shorter the hydrocarbon chain length was, the larger the increment was. The transition width in the DSC peak was broadened with an increase in PEG concentration. These three above-mentioned effects are the main differences between the effects of the osmotic stress on the phase transition of MLVs and those of hydrostatic pressure. On the other hand, ethylene glycol (EG), which is the monomer of PEG, had a biphasic effect on transition temperature of DPPC-, DSPC-, and DMPC-MLV, reducing Tm and Tp at low concentrations, but increasing Tm and extinguishing pretransition at high concentrations. This is explained by the induction of an interdigitated gel phase at high concentrations of EG, which indicates that EG can easily penetrate into the head group region of the lipid, in contrast with PEG 6K, because EG is small. Temperature-EG concentration phase diagrams for the various PC-MLVs were determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hydrocarbon chain packing modes in lipids: effect of altered sub-cell dimensions and chain rotation

The lateral hydrocarbon chain packing modes of lipids have been described in terms of specific hydrocarbon sub-cells as deduced from single crystal structural studies. To understand the changes in hydrocarbon chain packing in lipid bilayers induced by variations in temperature, hydration, ion-binding, etc., we have examined the effect on the calculated X-ray diffraction pattern of (a) systematic variations in the dimensions of the hydrocarbon sub-cell and (b) the effect of chain rotation at fixed lattice sites. For the O perpendicular (orthorhombic) sub-cell, the a and b sub-cell parameters were varied from as = 4.96 to 4.85 A and bs = 7.42 to 8.40 A in six steps and the positions (s = 2 sin theta/lambda) and intensities (Icalc = F2) of the strong sub-cell reflections calculated. In this way, the conversion of the O perpendicular sub-cell (with either fixed chain orientations or simulated chain rotation) to the hexagonal (H) sub-cell (with chain rotation) was followed. Notably, the two strong reflections characteristic of the O perpendicular sub-cell at 4.12 A (110) and 3.71 A (020) show progressive shifts in position and intensity, finally merging to give the strong (O1O) reflection at 4.2 A characteristic of the hexagonal sub-cell. Similar calculations were performed for the orthorhombic (O' perpendicular) and monoclinic (M parallel) sub-cells. This approach can be used to analyze changes in the X-ray diffraction data due to modifications of the hydrocarbon chain packing modes characteristic of simple and complex lipids.     

Differences in hydrocarbon chain tilt between hydrated phosphatidylethanolamine and phosphatidylcholine bilayers. A molecular packing model.

Both wide-angle and lamellar x-ray diffraction data are interpreted in terms of a difference in hydrocarbon chain tilt between fully hydrated dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE). Although the hydrocarbon chains of multilayers of DPPC tilt ty approximately 30 degrees relative to the normal to the plane of the bilayer, as previously reported by others, the hydrocarbon chains of DPPE appear to be oriented approximately normal to the plane of the bilayer. It is found that the chain tilt in DPPC bilayers can be reduced by either: (a) adding an n-alkane to the bilayer interiors or (b) adding lanthanum ions to the fluid layers between bilayers. A molecular packing model is presented which accounts for these data. According to this model, DPPC chains tilt because of the size and conformation of the PC polar head group.     

Thermodynamic and structural characterization of amino acid-linked dialkyl lipids

Using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), we determined some thermodynamic and structural parameters for a series of amino acid-linked dialkyl lipids containing a glutamic acid-succinate headgroup and di-alkyl chains: C12, C14, C16 and C18 in CHES buffer, pH 10. Upon heating, DSC shows that the C12, C14 and annealed C16 lipids undergo a single transition which XRD shows is from a lamellar, chain ordered subgel phase to a fluid phase. This single transition splits into two transitions for C18, and FTIR shows that the upper main transition is predominantly the melting of the hydrocarbon chains whereas the lower transition involves changes in the headgroup ordering as well as changes in the lateral packing of the chains. For short incubation times at low temperature, the C16 lipid appears to behave like the C18 lipid, but appropriate annealing at low temperatures indicates that its true equilibrium behavior is like the shorter chain lipids. XRD shows that the C12 lipid readily converts into a highly ordered subgel phase upon cooling and suggests a model with untilted, interdigitated chains and an area of 77.2A(2)/4 chains, with a distorted orthorhombic unit subcell, a=9.0A, b=4.3A and beta=92.7 degrees . As the chain length n increases, subgel formation is slowed, but untilted, interdigitated chains prevail.     

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.     

Conformation and packing properties of membrane lipids: the crystal structure of sodium dimyristoylphosphatidylglycerol

The conformation and molecular packing of sodium 1,2-dimyristoyl-sn-glycero-phospho-rac-glycerol (DMPG) have been determined by single crystal analysis (R = 0.098). The lipid crystallizes in the monoclinic spacegroup P2(1) with the unit cell dimensions a = 10.4, b = 8.5, c = 45.5 A and beta = 95.2 degrees. There are two independent molecules (A and B) in the asymmetric unit which with respect to configuration and conformation of their glycerol headgroup are mirror images. The molecules pack tail to tail in a bilayer structure. The phosphoglycerol headgroups have a layer-parallel orientation giving the molecules an L-shape. At the bilayer surface the (-) phosphoglycerol groups are arranged in rows which are separated by rows of (+) sodium ions. Laterally the polar groups interact by an extensive network of hydrogen, ionic and coordination bonds. The packing cross-section per molecule is 44.0 A2. The hydrocarbon chains are tilted (29 degrees) and have opposite inclination in the two bilayer halves. In the chain matrix the chain planes are arranged according to a so far unknown hybride packing mode which combines the features of T parallel and O perpendicular subcells. The two fatty acid substituted glycerol oxygens have mutually a - synclinal rather than the more common + synclinal conformation. The conformation of the diacylglycerol part of molecule A and B is distinguished by an axial displacement of the two hydrocarbon chains by four methylene units. This results in a reorientation of the glycerol back bone and a change in the conformation and stacking of the hydrocarbon chains. In molecule A the beta-chain is straight and the gamma-chain is bent while in molecule B the chain conformation is reversed.