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

Monolayer membranes and bilayer vesicles characterized by α- and β-anomer of sulfoquinovosyldiacyglycerol (SQDG)

Physicochemical properties of 1,2-di-O-stearoyl-3-O-(6-deoxy-6-sulfo-alpha-D-glucopyranosyl)-sn-glycerol (alpha-SQDG-C(18:0)) and 1,2-di-O-stearoyl-3-O-(6-deoxy-6-sulfo-beta-D-glucopyranosyl)-sn-glycerol (beta-SQDG-C(18:0)) in monolayer and bilayer membranes were examined. Surface pressure measurements in monolayer membranes indicated the molecular area of beta-SQDG-C(18:0) to be slightly smaller than that of alpha-SQDG-C(18:0). In bilayer membranes, the phase transition temperature and the enthalpy of beta-SQDG-C(18:0) were higher than those of alpha-SQDG-C(18:0), while the trapping efficiency of beta-SQDG-C(18:0) vesicles was lower. The results suggested tighter packing with beta-SQDG-C(18:0) than alpha-SQDG-C(18:0), due to differences in the head group stereochemistry. High-performance liquid chromatography-electrospray ionization ion trap mass spectrometry (HPLC-ESI-MS) data and computational modeling studies provided supporting evidence for morphological differences. In both monolayer and bilayer membranes, the affinity of beta-SQDG-C(18:0) with cholesterol was greater than that of alpha-SQDG-C(18:0), again due to the differences in head group properties. Turbidity measurement and microscopic examination of alpha- and beta-SQDG-C(18:0)/cholesterol mixtures confirmed formation of large vesicles. The addition of cholesterol to SQDG-C(18:0) optimized membrane formation and stabilized its structure.

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.

Self-assembly of synthetic glycolipid/water systems

Glycolipids (amphiphiles that bear oligosaccharides as their hydrophilic headgroups) are of importance both scientifically and technically. This review describes recent advances in our understanding of the molecular correlations in phase behavior in aqueous glycolipids over the past several years. In the first part, we discuss how headgroup stereochemistry affects the phase behavior of glycolipids both in two- and three-dimensional systems. In the second part, we discuss the effects of alkyl chain structure and phase behavior of phytanyl-chained glycolipid/water systems. The physical properties of glycolipid/water systems depend strongly on the inter-headgroup interactions that are related to such factors as stereochemistry (conformation) and size of headgroups, type of sugar residues involved, alkyl chain structure, etc. Thus, apart from the conventional concept like 'hydrophilic/lipophilic balance', explicit accounts of headgroup interactions are crucial to control the particular glycolipid/water system concerned. This is in marked contrast to the conventional amphiphile/water systems where the inter-headgroup interactions are in most cases simply repulsive.

Lipids in total extracts from Acholeplasma laidlawii A pack more closely than the individual lipids. Monolayers studied at the air-water interface

Pressure-area curves were obtained at 25, 35 and 45 degrees C for total lipid extracts and four individual glucolipids isolated from Acholeplasma laidlawii strain A-EF22. The glucolipids are 1,2-diacyl-3-0-(alpha-D-glucopyranosyl)-sn-glycerol (MGlcDAG), 1,2 -diacyl-3-0-[alpha-D-glucopyranosyl-(1-->2)-0-alpha-D-glucopyranosyl] -sn-glycerol (DGlcDAG), 1,2-diacyl-3-0-[alpha-D-glucopyranosyl-(1-->2)-0-(6-0-acyl-alpha-D-gluco pyranosyl)]-sn-glycerol (MADGlcDAG), and 1,2-diacyl-3-0-[glycerophosphoryl-6-0-(alpha-D-glucopyranosyl-(1-- >)-0-alpha-D-glucopyranosyl)]-sn-glycerol (GPDGlcDAG). The total lipid extracts were obtained from A. laidlawii, grown at 37 degrees C with fatty acids of varying degrees of unsaturation and chain length. The mean surface area per molecule was obtained from these pressure-area curves at surface pressures equal to 10, 20, 30 and 40 mN/m. It was found that the interfacial area of the lipids increases with increasing degree of unsaturation, but is nearly independent of the acyl chain length at constant unsaturation. The surface charge density varied between 4.7 x 10(-3) e-/angstrom(2) and 9.4 x 10(-3) e-/angstrum(2) for the total lipid extracts studied, but did not exhibit any consistent dependence on variations in degree of unsaturation or acyl chain length. The mean area per molecule was found to be smaller for the total lipid extracts than for the individual lipids. It is concluded that the bacterium strives to regulate its lipid composition in such a way that the packing of the lipids in the membrane is appropriately tight, and/or to keep a slight negative spontaneous curvature of the lipid bilayer of the cell membrane ("optimal packing"). This is in accordance with the physico-chemical model for the regulation of the lipid composition in the membrane of A. laidlaiwii previously presented by us (see e.g. Andersson, A.-S., Riffors, L., Bergqvist, M., Persson, S. and Lindblom, G. (1996) Biochemistry 35, 11119-11130).

Grafted poly-(ethylene glycol) on lipid surfaces inhibits protein adsorption and cell adhesion

Monolayers of dipalmitoyl-phosphatidylethanolamine (DPPE) mixing with various mole percentages of distearoyl-phosphatidylethanolamine (DSPE)-conjugated poly-(ethylene glycol) (PEG m.w. 750-5000) were deposited on DPPE-coated glass surfaces by the Langmuir-Blodgett method. Increasing percentages of grafted PEG in these supported lipid surfaces increasingly inhibit the adsorption of bovine serum albumin (BSA), laminin, and fibronectin. Increasing percentages of grafted PEG also inhibit the adhesion of erythrocytes, lymphocytes, and macrophages to these supported lipid surfaces. The adsorption of proteins on lipid coated glass surfaces were assayed by the fluorescence of FITC-labelled proteins. Cell adhesion was measured mainly by microscopic counting. The concentration of PEG-grafted lipids required for the inhibition of erythrocyte adhesion decreases with increasing molecular weight of the grafted PEG. The inhibitory effects are strongly dependent on the graft density of PEG at low concentrations, but weakly dependent on graft density at higher concentrations. For DSPE-PEG5000, the change of graft density dependency occurs approximately at the complete coverage of the lipid surface by the grafted polymer in the mushroom conformation (0.7 mol%), and the transition to partial brush conformation. The change-overs become less distinctive for grafted PEG of lower molecular weights, probably due to the failure of strictly mushroom and brush models of the polymer. The relative inhibitory efficiency is protein or cell dependent. The implication on the function of stealth liposomes is discussed.

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.

Packing of a triacylglucolipid from the membrane of Acholeplasma laidlawii strain A at the air/water interface

Pressure-area curves were generated at 22 degrees C and 40 degrees C for three glucolipids isolated from Acholeplasma laidlawii strain A-EF22. The glucolipids are 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol (MGlcDAG), 1,2-diacyl-3-O-[alpha-D-glucopyranosyl-(1-->2)-O-alpha-D- glucopyranosyl]-sn-glycerol (DGlcDAG), and 1,2-diacyl-3-O-[3-O-acyl-(alpha-D-glucopyranosyl)]-sn- glycerol (MAMGlc-DAG). The curves for MGlcDAG and DGlcDAG are characteristic for monolayers in a liquid phase at both temperatures. MGlcDAG has a smaller molecular area at all surface pressures compared to DGlcDAG. At 22 degrees C MAMGlcDAG shows a phase transition at 13 mN/m. However, at 40 degrees C the pressure-area curve for this lipid is characteristic for a monolayer in a liquid state. Mixed MAMGlcDAG-DGlcDAG and MGlcDAG-DGlcDAG monolayers showed no significant deviation from the additivity rule at 40 degrees C. The area per acyl chain is nearly the same for MAMGlcDAG and MGlcDAG. Our study supports our previous results that aqueous dispersions of these lipids form non-lamellar, reversed aggregates.

Phases and phase transitions of the glycoglycerolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. The database is considered comprehensive for glycerophospholipids, glycoglycerolipids, sphingolipids and natural membrane extracts. Here, a review of the LIPIDAT data subset referring to glycoglycerolipids is presented together with an analysis of these data. The glycoglycerolipids subset represents 4% of all LIPIDAT records. It includes data collected over a 20-year period and consists of 419 records obtained from 37 articles in 13 journals. An analysis of the data in the subset has allowed us to identify trends in hydrated glycoglycerolipids phase behavior reflecting differences in hydrocarbon chain length, chain branching, chain-glycerol linkage type (ether vs. ester), sugar headgroup-glycerol linkage type (alpha vs. beta) and sugar headgroup identity. Included is a summary of the data concerning the effect of pH and of stereochemical purity on glycoglycerolipid phase behavior. Information on the mesomorphism of biologically derived and dry glycoglycerolipids is also presented. This review includes 92 references.

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.

Nonlamellar phases formed by membrane lipids

A brief review of membrane lipids forming cubic and reversed hexagonal phases is presented. An emphasis is made on anionic lipids and particular microbial lipids.

Methylation effects on the microdomain structures of phosphatidylethanolamine monolayers

Increasing methylation of the headgroup in DPPE results in an increase of minimum area per molecule in highly compressed monolayers at the air-water interface. The shape of solid domains, as observed by epifluorescence microscopy, also exhibits marked changes upon increasing headgroup methylation. Branching domains are observed in DPPE and DP(Me)PE, whereas U-shaped or round domains are observed in DP(Me)2PE and DPPC under our experimental conditions. The domain shape is determined more by the headgroup methylatin than by the corresponding shift in critical temperatures, as shown by the study of PCs of different acyl chain moieties. In mixed lipid monolayers, PC (phosphatidylcholine) and PE (phosphatidylethanolamine) do not mix ideally, as indicated by the non-linear variation of the average area per molecule with composition, and by distinct domain shapes in LE/LC (liquid expanded/liquid condensed) coexisting phases representing PE-enriched or PC-enriched domains in those mixed monolayers.