X-ray diffraction structures of some phosphatidylethanolamine lamellar and inverted hexagonal phases.

X-ray diffraction is used to solve the low-resolution structures of fully hydrated aqueous dispersions of seven different diacyl phosphatidylethanolamines (PEs) whose hydrocarbon chains have the same effective chain length but whose structures vary widely. Both the lower-temperature, liquid-crystalline lamellar (L(alpha)) and the higher-temperature, inverted hexagonal (H(II)) phase structures are solved, and the resultant internal dimensions (d-spacing, water layer thickness, average lipid length, and headgroup area at the lipid-water interface) of each phase are determined as a function of temperature. The magnitude of the L(alpha) and H(II) phase d-spacings on either side of the L(alpha)/H(II) phase transition temperature (T(h)) depends significantly on the structure of the PE hydrocarbon chains. The L(alpha) phase d-spacings range from 51.2 to 56.4 A, whereas those of the H(II) phase range from 74.9 to 82.7 A. These new results differ from our earlier measurements of these PEs (Lewis et al., Biochemistry, 28:541-548, 1989), which found near constant d-spacings of 52.5 and 77.0-78.0 A for the L(alpha) and H(II) phases, respectively. In both phases, the d-spacings decrease with increasing temperature independent of chain structure, but, in both phases, the rate of decrease in the L(alpha) phase is smaller than that in the H(II) phase. A detailed molecular description of the L(alpha)/H(II) phase transition in these PEs is also presented.

The physical properties of glycosyl diacylglycerols. Calorimetric, X-ray diffraction and Fourier transform spectroscopic studies of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols.

We have synthesized a homologous series of saturated 1,2-di-O-n-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols with odd- and even-numbered hydrocarbon chains ranging in length from 10 to 20 carbon atoms, and have investigated their physical properties using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. The DSC results show a complex pattern of phase behaviour, which in a typical preheated sample consists of a lower temperature, moderately energetic lamellar gel/lamellar liquid-crystalline (L(beta)/L(alpha)) phase transition and a higher temperature, weakly energetic lamellar/nonlamellar phase transition. On annealing at a suitable temperature below the L(beta)/L(alpha) phase transition, the L(beta) phase converts to a lamellar crystalline (L(c1)) phase which may undergo a highly energetic L(c1)/L(alpha) or L(c1)/inverted hexagonal (H(II)) phase transition at very high temperatures on subsequent heating or convert to a second L(c2) phase in certain long chain compounds on storage at or below 4 degrees C. The transition temperatures and phase assignments for these galactolipids are supported by our XRD and FTIR spectroscopic measurements. The phase transition temperatures of all of these events are higher than those of the comparable phase transitions exhibited by the corresponding diacyl alpha- and beta-D-glucosyl glycerols. In contrast, the L(beta)/L(alpha) and lamellar/nonlamellar phase transition temperatures of the beta-D-galactosyl glycerols are lower than those of the corresponding diacyl phosphatidylethanolamines (PEs) and these glycolipids form inverted cubic phases at temperatures between the lamellar and H(II) phase regions. Our FTIR measurements indicate that in the L(beta) phase, the hydrocarbon chains form a hexagonally packed structure in which the headgroup and interfacial region are undergoing rapid motion, whereas the L(c) phase consists of a more highly ordered, hydrogen-bonded phase, in which the chains are packed in an orthorhombic subcell similar to that reported for the diacyl-beta-D-glucosyl-sn-glycerols. A comparison of the DSC data presented here with our earlier studies of other diacyl glycolipids shows that the rate of conversion from the L(beta) to the L(c) phase in the beta-D-galactosyl glycerols is slightly faster than that seen in the alpha-D-glucosyl glycerols and much faster than that seen in the corresponding beta-D-glucosyl glycerols. The similarities between the FTIR spectra and the first-order spacings for the lamellar phases in both the beta-D-glucosyl and galactosyl glycerols suggest that the headgroup orientations may be similar in both beta-anomers in all of their lamellar phases. Thus, the differences in their L(beta)/L(c) conversion kinetics and the lamellar/nonlamellar phase properties of these lipids probably arise from subtly different hydration and H-bonding interactions in the headgroup and interfacial regions of these phases. In the latter case, such differences would be expected to alter the ability of the polar headgroup to counterbalance the volume of the hydrocarbon chains. This perspective is discussed in the context of the mechanism for the L(alpha)/H(II) phase transition which we recently proposed, based on our X-ray diffraction measurements of a series of PEs.

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-alpha-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.

Synthesis and thermotropic characterization of a homologous series of racemic beta-D-glucosyl dialkylglycerols.

The phase behaviour of aqueous dispersions of a series of synthetic 1,2-di-O-alkyl-3-O-(beta-D-glucosyl)-rac-glycerols with both odd and even hydrocarbon chain lengths was studied by differential scanning calorimetry and low angle X-ray diffraction (XRD). Thermograms of these lipids show a single, strongly energetic phase transition, which was shown to correspond to either a lamellar gel/liquid crystalline (L(beta)/L(alpha)) phase transition (short chain compounds, n < or =14 carbon atoms) or a lamellar gel/inverted hexagonal (L(beta)/H(II)) phase transition (longer chain compounds, n > or =15 carbon atoms) by XRD. The shorter chain compounds may exhibit additional transitions at higher temperatures, which have been identified as lamellar/nonlamellar phase transitions by XRD. The nature of these nonlamellar phases and the number of associated intermediate transitions can be seen to vary with chain length. The thermotropic phase properties of these lipids are generally similar to those reported for the corresponding 1,2-sn-diacyl alpha- and beta-D-glucosyl counterparts, as well as the recently published 1, 2-dialkyl-3-O-(beta-D-glycosyl)-sn-glycerols. However, the racemic lipids studied here show no evidence of the complex patterns of gel phase polymorphism exhibited by the above mentioned compounds. This suggests that the chirality of the glycerol molecule, by virtue of its position in the interfacial region, may significantly alter the phase properties of a lipid, perhaps by controlling the relative positions of hydrogen bond donors and acceptors in the polar region of the membrane.

Efficient modulation of glucolipid enzyme activities in membranes of Acholeplasma laidlawii by the type of lipids in the bilayer matrix.

It is generally anticipated, but so far not fully shown, that the physical properties of membrane lipid bilayers are governed by the concerted actions of the lipid-synthesizing enzymes. In the membrane of Acholeplasma laidlawii a constant surface charge density, similar phase equilibria, and a nearly constant spontaneous curvature are maintained for the polar lipids. Important for these properties are monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), forming mainly reversed nonlamellar and lamellar phases, respectively. The syntheses of these lipids (from 1,2-DAG and MGlcDAG) by two consecutively acting, membrane-bound glucosyltransferases have been analyzed in synthetic lipid bilayers of selected physical properties. Both enzymes demanded the presence of activator lipids; for MGlcDAG synthesis a critical fraction of anionic lipids was important, whereas for the DGlcDAG synthesis substantial amounts of a liquid-crystalline phosphatidylglycerol (PG) with a certain chain length were essential. The rates of the syntheses for the two glucolipids increased with decreasing chain length of the DAG and MGlcDAG substrates. The enzymatic formation of DGlcDAG (bilayer-forming) was influenced in a dose-dependent manner by the nonbilayer (curvature) propensities of several amphiphilic and hydrophobic lipids in two different bilayer matrixes. However, the preceding synthesis of the nonlamellar MGlcDAG was only affected to a minor extent by such additives. The mechanism for modulation involved an enhancement of the activating potencies of PG in a cooperative fashion at physiological concentrations for PG.(ABSTRACT TRUNCATED AT 250 WORDS)

The specificity of glycolipid-preferredoxin interaction: requirements for membrane binding.

Preferredoxin (prefd) is a precursor protein that is imported into chloroplasts. Monolayer experiments have shown that prefd has a high affinity for monogalactosyldiglyceride (MGaIDG) isolated from chloroplasts, which contains polyunsaturated fatty acid constituents and is therefore in a liquid-expanded state, but has been found to interact also with MGaIDG with long-chain saturated fatty acids, which exist in a gel state. For an optimal interaction, the fatty acid chain length and the extent of unsaturation are also important parameters, whereas the conformation of the sugar moiety, the sugar-glycerol or glycerol-hydrocarbon chain linkages are of little influence on the pressure changes measured in monomolecular layers. Conversely, steric hindrance of a methyl group at position 3 of the sugar largely inhibits the interaction. Quantification of the interaction with radiolabelled prefd shows that only a small part of the molecule is able to penetrate MGaIDG in the gel state, whereas a nearly four-times larger part is able to penetrate MGaIDG isolated from chloroplasts. It is likely that interactions of the transit sequence of prefd with the glycolytic head group MGaIDG are involved in targeting and binding to the chloroplast membrane.

Thermotropic phase properties of 1,2-di-O-tetradecyl-3-O-(3-O-methyl- beta-D-glucopyranosyl)-sn-glycerol.

The hydration properties and the phase structure of 1,2-di-O-tetradecyl-3-O(3-O-methyl-beta-D-glucopyranosyl)-sn-glycerol (3-O-Me-beta-D-GlcDAIG) in water have been studied via differential scanning calorimetry, 1H-NMR and 2H-NMR spectroscopy, and x-ray diffraction. Results indicate that this lipid forms a crystalline (Lc) phase up to temperatures of 60-70 degrees C, where a transition through a metastable reversed hexagonal (Hll) phase to a reversed micellar solution (L2) phase occurs. Experiments were carried out at water concentrations in a range from 0 to 35 wt%, which indicate that all phases are poorly hydrated, taking up < 5 mol water/mol lipid. The absence of a lamellar liquid crystalline (L alpha) phase and the low levels of hydration measured in the discernible phases suggest that the methylation of the saccharide moiety alters the hydrogen bonding properties of the headgroup in such a way that the 3-O-Me-beta-D-GlcDAIG headgroup cannot achieve the same level of hydration as the unmethylated form. Thus, in spite of the small increase in steric bulk resulting from methylation, there is an increase in the tendency of 3-O-Me-beta-D-GlcDAIG to form nonlamellar structures. A similar phase behavior has previously been observed for the Acholeplasma laidlawii A membrane lipid 1,2-diacyl-3-O-(6-O-acyl-alpha-D-glucopyranosyl)-sn-glycerol in water (Lindblom et al. 1993. J. Biol. Chem. 268:16198-16207). The phase behavior of the two lipids suggests that hydrophobic substitution of a hydroxyl group in the sugar ring of the glucopyranosylglycerols has a very strong effect on their physicochemical properties, i.e., headgroup hydration and the formation of different lipid aggregate structures.

Differential scanning calorimetry and X-ray diffraction studies of the thermotropic phase behavior of the diastereomeric di-tetradecyl-beta-D-galactosyl glycerols and their mixture.

We have investigated the thermotropic phase behavior of aqueous dispersions of the 1,2- and 2,3-di-O-tetradecyl-1(3)-O-(beta-D-galactopyranosyl)-sn- glycerols and their diastereomeric mixture using differential scanning calorimetry and low-angle and wide-angle x-ray diffraction. Upon heating, unannealed aqueous dispersions of these compounds all exhibit a lower temperature, moderately energetic phase transition at approximately 52 degrees C and a higher temperature, weakly energetic phase transition at approximately 63 degrees C, both of which are reversible on cooling. X-ray diffraction measurements identify these events as the L beta (or L' beta)/L alpha and L alpha/HII phase transitions, respectively. The structures of the L beta, L alpha, and HII phases of these lipids, as determined by x-ray diffraction measurements, are identical within the error bars for all of these lipids. On annealing below the L beta/L alpha phase transition temperature, the L beta phase converts to an Lc phase at a rate which is strongly dependent on the chirality of the glycerol backbone (1,2-sn > 1,2-rac > 2,3-sn). The temperature of the phase transition from the Lc phase seen on reheating is also dependent on the glycerol chirality. In addition, the nature of the Lc phase changes on subsequent heating in the 1,2-sn and 1,2-rac lipids, but we have not been able to detect this Lc1/Lc2 phase transition by calorimetry. However, wide-angle x-ray diffraction measurements indicate that these Lc phases differ mostly in their hydrocarbon chain packing modes. The Lc2 phase does not appear to be present in the 2,3-sn compound, suggesting that its formation is not favored in this diastereomeric isomer. These observations are discussed in relation to the effect of glycerol chirality on the molecular packing of these glycolipids, particularly on hydrogen bonding and hydration in the interfacial region of the bilayer.

Effect of the chirality of the glycerol backbone on the bilayer and nonbilayer phase transitions in the diastereomers of di-dodecyl-beta-D-glucopyranosyl glycerol.

We have studied the physical properties of aqueous dispersions of 1,2-sn- and 2,3-sn-didodecyl-beta-D-glucopyranosyl glycerols, as well as their diastereomeric mixture, using differential scanning calorimetry and low angle x-ray diffraction. Upon heating, both the chiral lipids and the diastereomeric mixture exhibit characteristically energetic L beta/L alpha phase transitions at 31.7-32.8 degrees C and two or three weakly energetic thermal events between 49 degrees C and 89 degrees C. In the diastereomeric mixture and the 1,2-sn glycerol derivative, these higher temperature endotherms correspond to the formation of, and interconversions between, several nonlamellar structures and have been assigned to L alpha/QIIa, QIIa/QIIb, and QIIb/HII phase transitions, respectively. The cubic phases QIIa and QIIb, whose cell lattice parameters are strongly temperature dependent, can be identified as belonging to space groups Ia3d and Pn3m/Pn3, respectively. In the equivalent 2,3-sn glucolipid, the QIIa phase is not observed and only two transitions are seen at 49 degrees C and 77 degrees C, which are identified as L alpha/QIIb and QIIb/HII phase transitions, respectively. These phase transitions temperatures are some 10 degrees C lower than those of the corresponding phase transitions observed in the diastereomeric mixture and the 1,2-sn glycerol derivative. On cooling, all three lipids exhibit a minor higher temperature exothermic event, which can be assigned to a HII/QIIb phase transition. An exothermic L alpha/L beta phase transition is observed at 30-31 degrees C. A shoulder is sometimes discernible on the high temperature side of the L alpha/L beta event, which may originate from a QIIb/L alpha phase transition prior to the freezing of the hydrocarbon chains. None of the lipids show evidence of a QIIa phase on cooling. No additional exothermic transitions are observed on further cooling to -3 degrees C. However, after nucleation at 0 degrees C followed by a short period of annealing at 22 degrees C, the 1,2-sn glucolipid forms an Lc phase that converts to an L alpha phase at 39.5 degrees C on heating. Neither the diastereomeric mixture nor the 2,3-sn glycerol derivative shows such behavior even after extended periods of annealing. Our results suggest that the differences in the phase behavior of these glycolipid isomers may not be attributable to headgroup size per se, but rather to differences in the stereochemistry of the lipid polar/apolar interfacial region, which consequently effects hydrogen-bonding, hydration, and the hydrophilic/hydrophobic balance.

Differential scanning calorimetry and X-ray diffraction studies of a series of synthetic beta-D-galactosyl diacylglycerols.

We have investigated the physical properties of a homologous series of synthetic, saturated 1,2-di-O-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols using calorimetry and X-ray diffraction. Unannealed aqueous dispersions of these compounds exhibit a lower temperature, moderately energetic, chain-melting (L beta/L alpha) phase transition and a higher temperature, weakly energetic, bilayer/nonbilayer phase transition. On annealing below the L beta/L alpha phase transition, the L beta phase converts to an LC phase, which may undergo a highly energetic LC/L alpha or LC/HII phase transition at very high temperatures on reheating. The temperatures of these phase transitions are higher than those seen in the corresponding alpha- and beta-D-glucosyl diacylglycerols. However, the L beta/L alpha and bilayer/nonbilayer phase transition temperatures of the beta-D-galactosyl diacylglycerols are lower than those of the corresponding diacyl phosphatidylethanolamines. These observations are discussed in terms of the hydration and hydrogen bonding properties of their respective headgroups.

2,3-di-O-tetradecyl-1-O-(beta-D-glucopyranosyl)-sn-glycerol is a substrate for human glucocerebrosidase.

Glucocerebrosidase, the lysosomal enzyme that is deficient in patients with Gaucher's disease, hydrolyses non-physiological aryl beta-D-glucosides and glucocerebroside, its substrate in vivo. We document that 2,3,-di-O-tetradecyl-1-O-(beta-D-glucopyranosyl)-sn-glycerol (2,3,-di-14:0-beta-Glc-DAG) inhibits human placental glucocerebrosidase activity in vitro (Ki 0.18 mM), and the nature of inhibition is typical of a mixed-type pattern. Furthermore, 2,3-di-14:0-beta-Glc-DAG was shown to be an excellent substrate for the lysosomal beta-glucosidase (Km 0.15 mM; Vmax. 19.8 units/mg) when compared with the natural substrate glucocerebroside (Km 0.080 mM; Vmax. 10.4 units/mg). The observations that (i) glucocerebrosidase-catalysed hydrolysis of 2,3-di-14:0-beta-Glc-DAG is inhibited by conduritol B epoxide and glucosylsphingosine, and (ii) spleen and brain extracts from patients with Gaucher's disease are unable to hydrolyse 2,3-di-14:O-beta-Glc-DAG demonstrate that the same active site on the enzyme is responsible for catalysing the hydrolysis of 4-methylumbelliferyl beta-D-glucopyranoside, glucocerebroside and 2,3-di-14:O-beta-Glc-DAG. With the aid of computer modelling we have established that the oxygen atoms in 2,3-DAG-Glc at the C-1, C-4*, C-5* (the ring oxygen in glucose) and C-2 positions correspond topologically to the oxygens at C-1, C-4* and C-5* and the nitrogen atom attached to C-2 respectively in glucocerebroside (* signifies a carbon atom in glucose); furthermore, all of the distances with respect to overlap of corresponding heteroatoms range from 0.02 A to 0.77 A (0.002-0.077 nm). A root-mean-square deviation of 0.31 A (0.031 nm) was obtained when the energy-minimized structures of 2,3-di-14:O-beta-Glc-DAG and glucocerebroside were compared using the latter four heteroatom co-ordinates.

Physical properties of glycosyldiacylglycerols: an infrared spectroscopic study of the gel-phase polymorphism of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols.

The thermotropic and barotropic gel-phase polymorphism of a homologous series of saturated, straight-chain beta-D-glucosyldiacylglycerols was studied by Fourier transform infrared spectroscopy. Three spectroscopically distinct lamellar gel phases were detected thermotropically. Upon cooling to temperatures below the gel/liquid-crystalline phase transition temperature, all of these lipids form a metastable L beta gel phase characterized by orientationally disordered all-trans acyl chains. The transformation of the metastable L beta phase to a stable crystalline (Lc2) phase first involves the formation of an intermediate which itself is an ordered crystal-like (Lc1) phase. In the intermediate Lc1 phase, the zigzag planes of the polymethylene chains are nearly perpendicular to one another, and one of the ester carbonyl oxygens is engaged in a strong hydrogen bond, probably to the 2-hydroxyl of the sugar headgroup. The transformation of the Lc1 phase to the Lc2 phase involves a reorientation of the all-trans hydrocarbon chains and is probably driven by the strengthening of the hydrogen bond between the carbonyl ester oxygen and its proton donors. Since a "solid-state" reorganization of the acyl chains is an integral part of that process, it tends to become more sluggish as the chain length increases and is not observed with the longer chain homologues (N greater than 16). The spectroscopic characteristics of the most stable gel phases of the odd- and even-numbered members of this homologous series of compounds exhibit only minor differences, indicating that the structures of these phases are generally similar. The barotropic phase behavior of the shorter and longer chain beta-D-glucosyldiacylglycerols is also different. Compression of the L beta phase of the shorter chain compounds results in immediate conversion to their stable lc phases, whereas compression of the L beta phase of the longer chains does not. Furthermore, compression of the longer chain compounds may result in the formation of chain-interdigitated bilayers, whereas this is not the case for the shorter chain homologues. We suggest that the gel phase formed by any given homologue at a given temperature or pressure is that which maximizes the sometimes competing requirements for the optimal packing of the sugar headgroups and the hydrocarbon chains.

Physical properties of glycosyl diacylglycerols. 1. Calorimetric studies of a homologous series of 1,2-di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols.

The polymorphic phase behavior of aqueous dispersions of a homologous series of 1,2-di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols was studied by differential scanning calorimetry. At fast heating rates unannealed samples of these lipids exhibit a strongly energetic transition, which has been identified as a lamellar gel/liquid crystalline (L beta/L alpha) phase transition (short- and medium-chain compounds) or a lamellar gel to inverted hexagonal (L beta/HII) phase transition (long-chain compounds) by X-ray diffraction studies (Sen et al., 1990). At still higher temperatures, some of the lipids that form lamellar liquid-crystalline phases exhibit an additional transition, which has been identified as a transition to an inverted nonbilayer phase by X-ray diffraction studies. The lamellar gel phase formed on initial cooling of these lipids is a metastable structure, which, when annealed under appropriate conditions, transforms to a more stable lamellar gel phase, which has been identified as a poorly hydrated crystal-like phase with tilted acyl chains by X-ray diffraction measurements (Sen et al., 1990). With the exception of the di-19:0 homologue, the crystalline phases of these lipids are stable to temperatures higher than those at which their L beta phases melt and, as a result, they convert directly to L alpha or HII phases on heating. Our results indicate that the length of the acyl chain affects both the kinetic and thermodynamic properties of the crystalline phases of these lipids as well as the type of nonbilayer phase that they form. Moreover, when compared with the beta-anomers, these alpha-D-glucosyl diacylglycerols are more prone to form ordered crystalline gel phases at low temperatures and are somewhat less prone to form nonbilayer phases at elevated temperatures. Thus the physical properties of glucolipids (and possibly all glycolipids) are very sensitive to the nature of the anomeric linkage between the sugar headgroup and the glycerol backbone of the lipid molecule. We suggest that this is, in part, due to a change in orientation of the glucopyranosyl ring relative to the bilayer surface, which in turn affects the way(s) in which the sugar headgroups interact with each other and with water.

Physical properties of glycosyl diacylglycerols. 2. X-ray diffraction studies of a homologous series of 1,2-Di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols.

X-ray diffraction methods were used to characterize the thermotropic polymorphism exhibited by aqueous dispersions of a homologous series of 1,2-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols. Upon cooling from temperatures at which the acyl chains of these lipids are melted, all of these compounds form structures that exhibit both low-angle and wide-angle diffraction patterns consistent with the formation of lamellar L beta gel phases. After a suitable protocol of low-temperature annealing, complex diffraction patterns consistent with the formation of highly ordered, lamellar, crystal-like phases are obtained. These patterns are similar for all of the compounds studied, suggesting that the unit cell structure is invariant. The assumption that the unit cell structure is invariant permits the assignment of phases to the diffraction orders, thereby making possible the construction of electron density profiles. These electron density profiles indicate that the crystal-like phases of these lipids are poorly hydrated structures with the hydrocarbon chains inclined at 35 degrees to the bilayer normal. The diffraction patterns of the crystal-like phases of these lipids changed abruptly at the calorimetrically determined phase transition temperatures to those characteristic of either lamellar liquid crystalline phases (N less than or equal to 17) or inverted nonbilayer phases. With these X-ray diffraction data we demonstrate that, at elevated temperatures, the shorter chain homologues (N less than or equal to 16) form cubic phases of the Pn3m space group, whereas the longer chain compounds form inverted hexagonal phases.

A comparative monomolecular film study of 1,2-di-O-palmitoyl-3-O-(alpha- and beta-D-glucopyranosyl)-sn-glycerols.

The polar headgroup contribution to monolayer behavior of dipalmitoylglucosylglycerol has been examined through studies of 1,2-di-O-palmitoyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol (di-16:0-alpha GlcDG) and 1,2-di-O-palmitoyl-3-O-(beta-D-glucopyranosyl)-sn-glycerol (di-16:0-beta GlcDG) in which the sugar headgroup is linked via an alpha or beta linkage to the diacylglycerol moiety. The results indicate that the limiting areas per molecule of the resultant condensed states are smaller than those of the corresponding phosphatidylcholine (DPPC) but larger than those of dipalmitoylphosphatidylethanolmine (DPPE). In the expanded state, while the areas per molecule are similar to those of DPPC at low pressures, both glycolipids occupy smaller areas at higher pressures. The expanded-state areas of the glucolipids are also slightly greater than those of DPPE. The initial compressional phase transition pressure of the glucolipid liquid-expanded/liquid-condensed transition (pi t) is, however, less sensitive to temperature than are the pi t values of phospholipids. Both of these effects must relate to strong headgroup/water interactions, which, in turn, result in a stabilization of the liquid-expanded states. In the expanded states the alpha anomers are slightly less tightly packed than the beta anomers, as is indicated by the somewhat higher areas per molecule of the expanded states and the lower transition temperatures. These differences in chain-melting temperatures are slightly smaller than those observed in bilayers. While the areas per molecule of the dipalmitoyl glucolipids are greater than those of dipalmitoylphosphatidylethanolamine, they nevertheless exhibit a greater tendency to form nonbilayer structures. Such observations indicate that other factors besides geometric shape play a role in bilayer/nonbilayer transitions.

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)

The physical properties of glycosyldiacylglycerols. Calorimetric studies of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols.

The polymorphic phase behavior of aqueous dispersions of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols was studied by differential scanning calorimetry. At fast heating rates, unannealed samples of these lipids exhibit a strongly energetic, lower temperature transition, which is followed by a weakly energetic, higher temperature transition. X-ray diffraction studies have enabled the assignments of these events to a lamellar gel/liquid crystalline (chain-melting) phase transition and a bilayer/nonbilayer phase transition, respectively. Whereas the values for both the temperature and enthalpy of the chain-melting phase transition increase with increasing acyl chain length, those of the bilayer/nonbilayer phase transition show almost no chain-length dependence. However, the nature of the bilayer/nonbilayer transition is affected by the length of the acyl chain. The shorter chain compounds form a nonbilayer 2-D monoclinic phase at high temperature whereas the longer chain compounds from a true inverted hexagonal (HII) phase. Our studies also show that the gel phase that is initially formed on cooling of these lipids is metastable with respect to a more stable gel phase and that prolonged annealing results in a slow conversion to the more stable phase after initial nucleation by incubation at appropriate low temperatures. The formation of these stable gel phases is shown to be markedly dependent upon the length of the acyl chains and whether they contain an odd or an even number of carbon atoms. There is also evidence to suggest that, in the case of the shorter chain compounds at least, the process may proceed via another gel-phase intermediate. In annealed samples of the shorter chain compounds, the stable gel phase converts directly to the L alpha phase upon heating, whereas annealed samples of the longer chain glycolipids convert to a metastable gel phase prior the chain melging.(ABSTRACT TRUNCATED AT 250 WORDS)