The role carbohydrate moieties play in determining the structure and energetics of glycolipid model membranes has been investigated by small- and wide-angle X-ray scattering, differential scanning densitometry (DSD), and differential scanning microcalorimetry (DSC). The dependence of a variety of thermodynamic and structural parameters on the stereochemistry of the OH groups in the pyranose ring and on the size of the sugar head group has been studied by using an homologous series of synthetic stereochemically uniform glyceroglycolipids having glucose, galactose, mannose, maltose, or trimaltose head groups and saturated ether-linked alkyl chains with 10, 12, 14, 16, or 18 carbon atoms per chain. The combined structural and thermodynamic data indicate that stereochemical changes of a single OH group in the pyranose ring can cause dramatic alterations in the stability and in the nature of the phase transitions of the membranes. The second equally important determinant of lipid interactions in the membrane is the size of the head group. A comparison of lipids with glucose, maltose, or trimaltose head groups and identical hydrophobic moieties has shown that increasing the size of the neutral carbohydrate head group strongly favors the bilayer-forming tendency of the glycolipids. These experimental results provide a verification of the geometric model advanced by Israelachvili et al. (1980) [Israelachvili, J. N., Marcelja, S., & Horn, R. G. (1980) Q. Rev. Biophys. 13, 121-200] to explain the preferences lipids exhibit for certain structures. Generally galactose head groups confer highest stability on the multilamellar model membranes as judged on the basis of the chain-melting transition. This is an interesting aspect in view of the fact that galactose moieties are frequently observed in membranes of thermophilic organisms. Glucose head groups provide lower stability but increase the number of stable intermediate structures that the corresponding lipids can adopt. Galactolipids do not even assume a stable intermediate L alpha phase for lipids with short chain length but perform only Lc----HII transitions in the first heating. The C2 isomer, mannose, modifies the phase preference in such a manner that only L beta----HII changes can occur. Maltose and trimaltose head groups prevent the adoption of the HII phase and permit only L beta----L alpha phase changes. The DSD studies resulted in a quantitative estimate for the volume change associated with the L alpha----HII transition of 14-Glc. The value of delta v = 0.005 mL/g supports the view that the volume difference between L alpha and HII is minute.(ABSTRACT TRUNCATED AT 400 WORDS)

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Phase diagrams of binary mixtures of the glycoglycerolipids 1,2-di-O-tetradecyl-3-O-beta-D-galactosyl-sn-glycerol (14-Gal) and 1,2-di-O-tetradecyl-3-O-beta-D-glucosyl-sn-glycerol (14-Glc) with the phospholipids L-dimyristoylphosphatidylcholine (DMPC) and L-dimyristoylphosphatidylethanolamine (DMPE) were recorded by high-sensitivity differential scanning calorimetry and used for determination of the glycolipid-phospholipid miscibility in solid and liquid-crystalline states. As a consequence of a metastable behavior of both glycolipids and DMPE, the solid-state glycolipid/phospholipid miscibility was strongly dependent on the temperature prehistory of the samples. While DMPC and 14-Glc mix continuously, the other three binaries display extended regions of solid-solid-phase separation in the equilibrium low-temperature states. The DMPE/glycolipid phase diagrams were of clearly expressed eutectic type. Continuous solutions were formed in the liquid-crystalline and in the metastable solid phases of the mixtures. Simulations of the shape of the phase diagrams using the Bragg-Williams approximation showed certain deviations from ideal mixing in the liquid-crystalline continuous solutions. Since both glycolipids and phospholipids contain fully saturated fatty acids of equal chain length, their mixing properties were predominantly determined by the interactions between the lipid polar moieties, assuming the influence of ester or either linkages of the alkyl chains on the mixing parameters to be negligible. The clearly expressed differences in the mixing of 14-Glc and 14-Gal with phospholipids are most probably due to different hydrogen-bond networks formed by the glucosyl and galactosyl residues.

In order to investigate the effects of a net surface charge on the properties of glycolipid membranes, we have synthesized a glyceroglycolipid, 1,2-O-dialkyl-3-O-beta-D-glucuronosyl- sn-glycerol (GlcUA lipid), with saturated alkyl chains of varying length (14, 16, and 18 carbon atoms; 14-, 16-, and 18-GlcUA, respectively) and glucuronic acid with an ionizable 6-carboxyl group as polar residue. Aqueous dispersions of GlcUA lipids have been characterized by differential scanning calorimetry, densitometry, and X-ray diffraction methods as a function of pH. The carboxyl group deprotonation of apparent pK about 5.5 leads to a decrease of the melting temperatures by about 7 degrees C for all three compounds and to a chain-length-dependent reduction of the transition enthalpies by 0, 7, and 14% for 14-, 16-, and 18-GlcUA, respectively. The decrease of the transition temperature is consistent with current electrostatic concepts and models of charged membrane interfaces, but the chain-length-specific dependence of the enthalpy decrease with an increase of pH shows that the pH effects in GlcUA lipids are not of purely electrostatic origin. However, these effects appear to be simpler in some instances than corresponding effects in phospholipids with multiply ionizable head groups. For this reason, the lipids with the glucuronic acid head group appear to represent an appropriate model system for studies of net electric charge effects on the membrane properties.(ABSTRACT TRUNCATED AT 250 WORDS)

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