Modulation of the activity of Clostridium perfringens neuraminidase by the molecular organization of gangliosides in monolayers

The activity of Clostridium perfringens neuraminidase against gangliosides GM3, GD1a and GM1 was studied in lipid monolayers at the air-buffer solution interface. The enzyme activity assay against pure ganglioside monolayers is based on the markedly different molecular packing areas of the substrate gangliosides and the resulting product glycosphingolipids. This allows to control and monitor the surface pressure and the ganglioside intermolecular organization (cross-sectional packing areas and dipole potentials) in a continuous manner during the catalytic process. It was found that the rate and the extent of the enzymatic reaction depended markedly on the lateral surface pressure. In general, the activity of neuraminidase against GM3 and GD1a was higher at lower surface pressure. This corresponded to larger intermolecular spacings among the ganglioside molecules. Both the activity and the extent of the reaction against GM3 were higher than toward GD1a. GM1 could not be degraded by the enzyme, irrespective of the surface pressure but the enzyme could interact with this ganglioside. A latency period, longer for GM3 than for GD1a, was observed prior to the onset of rapid degradation; this indicates that pre-catalytic steps are occurring at the interface before effective ganglioside degradation takes place. The latency period, the total amount of ganglioside degraded, and the velocity of the reaction varied with the surface pressure in different manners. Our data indicate that the different steps of the catalytic reaction occurring at the surface (i.e., substrate recognition and interfacial adsorption, catalysis, maximum extent of substrate conversion) are independently regulated by the molecular organization of the substrate gangliosides.

3 alpha-hydroxysteroid dehydrogenase inhibitory activity of some N(3)-(1-R-4-carboxypyrazol-5-yl)-1,2,3-benzotriazin-4(3H)-one and quinazolin-4(3H)-one acids

Some carboxylic acids of N(3)-pyrazole substituted 1,2,3-benzotriazin-4-(3H)-ones and- quinazolin-4-(3H)-ones were prepared and tested for the inhibitory property of 3 alpha-hydroxysteroid dehydrogenase of rat liver cytosol. The results indicated that the degree of inhibition can be used to predict the antiinflammatory potency of the compounds described.

Modulation of phospholipases A2 and C activities against dilauroylphosphorylcholine in mixed monolayers with semisynthetic derivatives of ganglioside and sphingosine

Most of the semisynthetic ganglioside and sphingosine derivatives studied here decreased the rate as well as the extent of hydrolysis of monomolecular layers of dilauroylphosphorylcholine (dlPC) by both phospholipase A2 (PLA2) and C (PLC). For PLA2, the rate of enzymatic activity was inversely correlated (p < 0.001) with the duration of the latency period of the enzymatic reaction. The correlation between the rate of activity and the latency period was not statistically significant for PLC. The potency to inhibit PLA2 and PLC was not significantly correlated with the presence of specific chemical groups. Also, the inhibitory effect is not correlated to changes of the substrate intermolecular spacing or surface potential caused by the sphingolipids (SLs). Conversely, for PLA2 the variation of the kinetic parameters of the reaction with the molecular polarization vector of the SLs are statistically significant (p < 0.01). The rate of phospholipid degradation by PLA2 decreased, and the lag times tended to become longer, with increasing values of the SLs' polarization vector. The kinetic parameters of the reaction with PLC did not show statistically significant correlation with the polarization vector of the SLs. Our results suggest that the configuration of the electrostatic field across the interface is more important than are formal charges or specific chemical moieties in modulating the activity of PLA2. Inhibition of phospholipase activities of these types by specific SLs or their metabolites may be important as supramolecular regulatory steps at the membrane level of the amplification of lipid-mediated signaling pathways.

Regulation by gangliosides and sulfatides of phospholipase A2 activity against dipalmitoyl- and dilauroylphosphatidylcholine in small unilamellar bilayer vesicles and mixed monolayers

The modulation by gangliosides GM1 and GD1a, and sulfatide (Sulf) of the activity of porcine pancreatic phospholipase A2 was studied with small unilamellar vesicles of dipalmitoylphosphatidylcholine (L-dpPC) and lipid monolayers of dilauroylphosphatidylcholine (L-dlPC). The presence of Sulf always led to an increase of the maximum rate of the enzymatic reaction, irrespective on whether the vesicles were above, in the range of, or below the bilayer transition temperature. Sulf did not modify the latency period for the reaction that is observed at the bilayer transition temperature. Gangliosides inhibited the maximum rate of enzymatic activity bilayer vesicles in the gel phase but the effect was complex. When the reaction was carried out at a temperature within the range of the bilayer phase transition, the gangliosides inhibited the maximal rate of the reaction in proportion to their content in the bilayer. However, at the same time the latency period observed with vesicles of pure phospholipid at this temperature was shortened in proportion to the mole fraction of gangliosides in the bilayer. At temperatures above the bilayer phase transition, gangliosides stimulated the activity of PLA2. Preincubation of the enzyme with Sulf or gangliosides did not affect the activity against bilayer vesicles of pure substrate. These glycosphingolipids did not modify the rate or extent of desorption of the enzyme from the interface, nor the pre-catalytic steps for the interfacial activation of PLA2, or the enzyme affinity for the phospholipid substrate. Also, the activity of the enzyme was not altered irreversibly by glycosphingolipids. Our results indicate that Sulf and gangliosides modulate the catalytic activity of PLA2 at the interface itself, beyond the initial steps of enzyme adsorption and activation, probably through modifications of the intermolecular organization and surface electrostatics of the phospholipid substrate.

Molecular parameters of semisynthetic derivatives of gangliosides and sphingosine in monolayers at the air-water interface

The molecular parameters (molecular area, surface potential, collapse pressure, dipole moment contributions) of semisynthetic derivatives of ganglioside GM1 and of sphingosine were studied in lipid monolayers at the air-NaCl (145 mM, pH 5.6) interface at 22 +/- 0.3 degrees C. The chemical modifications included alterations of the fatty acyl chain moiety linked to the 2-amino position of the sphingosine (Sph) base. The compounds studied were PKS-1 (N-acetyl Sph), PKS-2 (N-chloroacetyl Sph), PKS-3 (N-dichloroacetyl Sph), PKS-4 (N-trichloroacetyl Sph), Lyso-GM1 (ganglioside GM1 lacking the N-linked fatty acyl chain and the N-acetyl group on the sialic acid), Liga-4 (N-acetyl, lyso[NeuAc]GM1) and Liga-20 (N-dichloroacetyl, lyso[NeuAc]GM1). Relatively small modifications of the chemical structure of sphingolipids introduce dramatic consequences on their surface molecular properties. The absence of the long chain fatty acyl moiety and of the N-acetyl group on the neuraminic acid in Lyso-GM1 leads to a more condensed behavior and to an increase of the collapse pressure compared with GM1. The acetylation or chloroacetylation at the 2-amino position in Liga-4 and Liga-20 induce an expansion of the surface pressure-area isotherm and a decrease of the collapse pressure. The limiting molecular areas of GM1 derivatives, taken at the collapse pressure point, are consistent with the oligosaccharide chain being oriented approximately perpendicularly to the interface. Sphingosine shows a liquid expanded isotherm. The acetylation and successive chlorination of the acetyl residue at the 2-amino position of Sph cause a progressive increase in the limiting molecular area. The variation of the resultant dipole moment under compression, calculated from the surface potential values, suggests the reorientation of selective groups within these molecules that depend on the degree of intermolecular packing. Thermodynamic-geometric correlations on the basis of the molecular parameters of these derivatives suggest that small alterations of the substituent group at the 2-amino position of Sph could have large and amplified consequences on the type, curvature and stability of the possible self-aggregated structure that these lipids may form in aqueous medium.

Surface behavior of axolemma monolayers: physico-chemical characterization and use as supported planar membranes for cultured Schwann cells

The axolemma membrane forms a stable and reproducible monomolecular layer at the air-aqueous interface. The major lipids and proteins are present in this monolayer in molar ratios similar to the original membrane. Acetylcholinesterase and Na-K-ATPase activities are preserved in the monolayer to levels of 64% and 25%, respectively. The total lipid fraction forms a homogeneously mixed phase. The presence of proteins in the monolayer introduces surface inhomogeneties. Among other features, this is revealed by the presence of two values of lateral pressure at which the monolayer shows partial or total collapse: a broad partial collapse at surface pressures between 13 to 30 mN/m and a sharp collapse point at 46 mN/m. The average molecular areas, the broad collapse point, and the variation of the surface potential per molecule suggest the relocation of protein components at surface pressures between 13 to 30 mN/m. The behavior is consistent with the extrusion and exposure of proteins toward the aqueous medium that depends on the lateral pressure. Schwann cells grown on coverslips coated with axolemma monolayers at 13 mN/m (beginning of the broad collapse) and 34 mN/m (above the broad collapse) recognize the difference in the surface organization of axolemma caused by the lateral pressure which affects their proliferation, morphology, and spatial pattern of organization. Our results show for the first time that response of Schwann cells depends on the intermolecular organization of the axolemma surface with which they interact. These results suggest that the local expression of putative surface molecules of axolemma that may mediate membrane recognition and the signalling of morphological and proliferative changes can be modulated by long range supramolecular properties.

Modulation by glycosphingolipids of membrane-membrane interactions induced by myelin basic protein and melittin

The effect of glycosphingolipids (GSLs) with oligosaccharide chains of different length and charge on membrane-membrane interactions induced by myelin basic protein (MBP) or melittin (Mel) was comparatively investigated with small unilamellar vesicles. MBP induces a fast vesicle aggregation and close membrane apposition. Merging of lipid bilayers and vesicle fusion induced by MBP are slower and less extensive processes compared to membrane apposition. The changes of membrane permeability concomitant to these phenomena are small. The Trp region of MBP remains in a rather polar environment when interacting with vesicles; its accessibility to NO3- or acrylamide quenching depends on the type of GSLs in the membrane. The Trp region of Mel is inserted more deeply into the lipid bilayer and its accessibility to the aqueous quenchers is less dependent on variations of the oligosaccharide chain of the GSLs. Mel induces a faster and more extensive membrane apposition and bilayer merging than does MBP. Extensive vesicle disruption occurs in the presence of Mel. Negatively charged GSLs facilitate membrane proximity and vesicle aggregation but an increase of the oligosaccharide chain length of either neutral or acidic GSLs decreases the interaction among vesicles that are induced by either protein. This effect is independent of the different mode of insertion of MBP and Mel into the membrane. Our results suggest that the modulation by the oligosaccharide chain on the protein-induced interactions between bilayers containing GSLs is probably exerted beyond the level of local molecular interactions between the basic proteins and the lipids.

Novel pyrazolo [4,3-e] [1,4] diazepin-5,8-diones

The synthesis of some pyrazolo [4,3-e] [1,4] diazepin-5,8-diones has been accomplished starting from the readily available 1-ethyl-3-methyl-4-nitro-5-pyrazolecarbonylchloride and the appropriate alpha-aminoesthers, 2a, b, c. The catalytic reduction of the obtained 3a, b, c gave the products 4a, b, c which were, in turn, cyclized into the title compounds 5a, b, c. The intermediates of type 3 and 4 and the pyrazolo-diazepin-diones 5 were tested in vitro for their antimicrobial activity.

Synthesis and evaluation of antimicrobial activity of new 4-nitroso and 4-diazopyrazole derivatives

Some N-(pyrazol-5-yl)-2-nitrobenzamides, variably substituted in the pyrazole nucleus as well as in the amidic group, were reacted in acetic acid media with potassium nitrite and hydrochloric acid. The different chemical behaviour of the reacted pyrazole derivatives in relation to the substitution pattern in both the pyrazole nucleus and the amidic group, was observed. All compounds isolated from the reaction mixtures (4-nitroso, 4-nitro, 4-diazo and 4-chloro derivatives) were evaluated by the agar diffusion method for their "in vitro" growth inhibitory activity against Candida albicans (our collection), Candida tropicalis ATCC 13803, Saccharomyces cerevisiae ATCC 36375, Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923 and Pseudomonas aeruginosa ATCC 27853. The 1-methylpyrazole derivatives showed larger inhibition zones than the 1-phenyl ones in the antimicrobial tests.

Antiinflammatory pyrazolones and pyrazolidones: a study of their inhibition of 5 beta-dihydrocortisone reduction in rat liver cytosol

Seven pyrazolone and pyrazolidone derivates, some of them widely used as analgesic and anti-inflammatory drugs, were tested for the inhibitory property of the 3 alpha-hydroxysteroid dehydrogenase of rat liver cytosol. The data obtained clearly show that, among pyrazolone and pyrazolidone derivates, the correlation between IC50 and therapeutic potency is not always verified.

Molecular parameters of gangliosides in monolayers: comparative evaluation of suitable purification procedures

The molecular area, collapse pressure, and surface potential of gangliosides obtained by different methods were systematically compared in monolayers at the air-water interface. Different values of these parameters are obtained depending on the purification procedure employed for the isolation of pure gangliosides. This is due to impurities (such as peptidaceous material) that remain in different amounts in the various preparations and that modify the ganglioside surface behavior. Routine purity checking by HPTLC analysis of gangliosides usually fails to reveal these impurities. On the other hand, even if the monolayer technique cannot identify the nature or amount of contaminants, it is extremely sensitive to reveal alterations of the surface molecular parameters caused by relatively small amounts of other components coextracted with the ganglioside or adventitiously introduced with the solvents or subphases employed. This is a serious problem for the obtention of correct and reproducible values of such important parameters as the molecular area of gangliosides, their electrostatic potential in oriented interfaces, and their interactions with other lipids and proteins. A procedure leading to consistent molecular parameters that remain reproducible after several repurification cycles is to perform an alkaline treatment on previously purified gangliosides species with NaOH, this is followed by dialysis against bidistilled water, rechromatography on DEAE-Sephadex A25, silicic acid or Iatrobeads, and Sephadex LH-20 columns; repurified gangliosides are stored in chloroform-methanol-0.01 M NaOH (60:30:4.5).

Degradation of dilauroylphosphatidylcholine by phospholipase A2 in monolayers containing glycosphingolipids

The ability of phospholipase A2 from porcine pancreas to degrade all of the available dilauroylphosphatidylcholine in mixed monolayers with galactocerebroside, sulfatide, or ganglioside GM1 was investigated at different constant surface pressures. Under the conditions used the interfacial glycosphingolipid composition was continuously enriched as the enzyme action proceeded. The total percentage of phospholipid degradation depends on the surface pressure and on the type of glycosphingolipid. The presence of sulfatide activates the enzyme while galactocerebroside and ganglioside GM1 are inhibitory. The extent of phospholipid hydrolysis is independent of the effect of glycosphingolipids on the enzyme velocity. This is so when the latter is measured either in conditions of constant glycosphingolipid composition and zero-order kinetics [Bianco, I.D., Fidelio, G.D., & Maggio, B. (1989) Biochem. J. 258, 95-99] or under variable surface composition as in the present work. The modulation of phospholipase A2 activity by glycosphingolipids operates at two independent levels. One controls the rate of enzyme activity, and the other modulates the total extent of substrate degradation. This depends on the initial interaction of the enzyme with the interface. The glycosphingolipid effect on the activity is different depending on whether the enzyme has access to the substrate from the subphase or is already adsorbed to the lipid interface.

Ganglioside GD3 lactones: polar head group mediated control of the intermolecular organization

The individual properties and intermolecular organization of ganglioside GD3 and of two of its lactone forms (GD3Lactone I and GD3Lactone II) were studied in lipid monolayers. The formation of the first lactone ring in GD3Lactone I eliminates one negative charge and leads to a decrease of the molecular area at all surface pressures. The intermolecular dispersion energy and collapse pressure are higher in GD3Lactone I compared to those in the parent GD3. The surface potential per unit of molecular surface density and the resultant molecular dipole moment are increased in GD3Lactone I with respect to those in GD3 at comparable values of molecular area. In GD3Lactone I the molecular parameters suggest an oligosaccharide chain oriented similarly to that of GD3. On the average, this is perpendicular to the surface, and the resultant polar head-group dipole moment points away from the interface. In GD3Lactone II the negative charges are eliminated, resulting in considerably larger molecular areas than for GD3 and GD3Lactone I at all pressures. The intermolecular dispersion energy of GD3Lactone II is also greatly diminished and the collapse pressure is further increased compared to those of GD3Lactone I. However, the surface potential per unit molecular surface density and the resultant molecular dipole moment of GD3Lactone II are higher than in GD3 Lactone I at similar values of molecular areas. This is probably due to a positive polar head-group dipole moment contribution induced by the additional lactone ring in GD3Lactone II. These changes result from a distorted conformation of the oligosaccharide chain owing to the presence of fused carbohydrate rings which require a greater intermolecular spacing compared to GD3 and GD3Lactone I.

Effect of sulfatide and gangliosides on phospholipase C and phospholipase A2 activity. A monolayer study

The effect of sulfatide and gangliosides GM1, GD1a and GT1b on the activity of phospholipase C from Clostridium perfringens on dilauroylphosphatidylcholine and of porcine pancreatic phospholipase A2 on dilauroylphosphatidic acid was studied in lipid monolayers containing different proportions of glycolipids under zero-order kinetics at various constant surface pressures. The presence of sulfatide in the monolayer increases the activity of phospholipase C at high surface pressures. Gangliosides shift the cut-off pressure to lower values and inhibit the action of phospholipase C. In mixed monolayers with dilauroylphosphatidic acid, sulfatide at a molar fraction of 0.5 increases the activity of phospholipase A2 at surface pressures below 18 mN/m and shows an inhibitory effect at higher pressures. Ganglioside GM1 at a molar fraction of 0.25 completely inhibits the enzyme above 20 mN/m and markedly reduces its activity at lower pressures. Gangliosides GD1a and GT1b abolish the enzyme activity at all pressures at molar fractions of 0.25 and 0.15, respectively. The modified velocity of the enzymatic reaction in the presence of glycosphingolipids is not due to an irreversible alteration of the catalytic activity.

Synthesis and evaluation of the analgesic and antiinflammatory activities of N,N'-bis(2-hydroxybenzoyl)-diaminoalkanes

The analgesic and antiinflammatory activities of some N,N'-bis(2-hydroxybenzoyl)-diaminoalkanes 3 a-l were studied. The compounds were prepared very conveniently by fusion of phenyl salicylate 2 and diaminoalkanes 1 a-l. The pharmacological activities were influenced by the number of carbon atoms in the polimethylenic chain. Some derivatives were more effective and less gastrolesive than salicylamide.

Interaction and fusion of unilamellar vesicles containing cerebrosides and sulfatides induced by myelin basic protein

The effects of myelin basic protein on the aggregation, lipid bilayer merging, intercommunication of aqueous compartments and leakage of small unilamellar vesicles of egg phosphatidylcholine containing different proportions of galactocerebroside and sulfatide were investigated. This was performed employing light scattering, absorbance changes and fluorescence assays (resonance energy transfer, Terbium/dipicolinic acid assay and carboxyfluorescein release). The apposition of membranes rapidly induced by myelin basic protein is enhanced by sulfatide but reduced by galactocerebroside compared to vesicles of egg phosphatidylcholine alone. On the other hand, the presence of either glycosphingolipid in the membrane interferes with the induction by myelin basic protein of lipid bilayer merging, subsequent fusion and changes of the membrane permeability. Our results support an important modulation by sulfatide and galactocerebroside on the interactions among membranes induced by myelin basic protein, depending on the relative proportions of the glycosphingolipids and phosphatidylcholine.

Modulation of phospholipase A2 activity by neutral and anionic glycosphingolipids in monolayers

The effect of neutral (galactocerebroside and asialo-ganglioside GM1) or anionic (sulphatide and gangliosides GM1, GD1a and GT1b) glycosphingolipids on the activity of phospholipase A2 from pig pancreas was studied in mixed monolayers of dilauroyl phosphatidylcholine with the glycosphingolipids in different molar fractions at various constant surface pressures. The activity of the enzyme depends on the proportion and type of glycosphingolipid in the interface. Sulphatide activates the enzyme at all proportions, whereas galactocerebroside shows inhibition or activation depending on its proportion in the film. Asialo-ganglioside GM1 and gangliosides GM1, GD1a and GT1b can strongly inhibit the enzyme at relatively low molar fractions in the film in the following order: asialo-ganglioside GM1 less than ganglioside GM1 less than ganglioside GT1b less than ganglioside GD1a. The changes of activity are not due to a direct action of the lipids on the active centre or interfacial recognition region of the enzyme.

Thermodynamic-geometric correlations for the morphology of self-assembled structures of glycosphingolipids and their mixtures with dipalmitoylphosphatidylcholine

The morphology of aqueous dispersions of five neutral glycosphingolipids (GalCer, GlcCer, LacCer, asialo-GM2, asialo-GM1), sulfatide, and five gangliosides (GM3, GM2, GM1, GD1a and GT1b) and their mixtures with dipalmitoylphosphatidylcholine was studied by negative staining electron microscopy. The morphological features are interpreted on the basis of thermodynamic and geometric constraints previously studied in these systems (Maggio, B (1985) Biochim. Biophys. Acta 815, 245-258). The correlation between the theoretical predictions and the experimental findings are in reasonable agreement. Small changes in the molecular parameters of the individual glycosphingolipids or in their proportion in mixtures with dipalmitoylphosphatidylcholine bring about remarkable variations on the type of structure formed, its radius of curvature and thermodynamic stability.

Interaction of 1-anilinonaphthalene 8-sulfonic acid with interfaces containing cerebrosides, sulfatides and gangliosides

The fluorescence lifetime, quantum yield and emission spectra of 1-anilinonaphthalene 8-sulfonic acid (ANS) associated with interfaces of pure dipalmitoylphosphatidylcholine or its mixtures with phosphatidylserine, galactosylceramide, sulfatide or gangliosides GM1 and GD1a were studied at low and high ionic strength. Modification of the molecular organization of the lipid interfaces in the presence of the probe was also studied with mixed lipid monolayers. ANS has little affect on the intermolecular packing of the lipids but influences their surface potential, consistent with a location of ANS in the polar head group region of the interface. ANS senses a more polar microenvironment when associated with interfaces containing anionic glycosphingolipids at low ionic strength but, except for interfaces containing phosphatidylserine, it detects approximately the same polarity for neutral or anionic interfaces in 0.25 M NaCl.

Effect of glycerol on the molecular properties of cerebrosides, sulphatides and gangliosides in monolayers

The presence of glycerol, free from surface-active impurities, modifies the molecular area, surface potential/molecule and thermodynamic parameters of compression of monolayers of galactosylceramide, sulphatide and gangliosides GM1, GD1a and GT1b. This may be due to changes of the composition and structural properties of the glycosphingolipid solvation shell with an influence on the intermolecular organization.

Surface topography of sulfatide and gangliosides in unilamellar vesicles of dipalmitoylphosphatidylcholine

The property of the dyes, acridine orange and methylene blue, to exhibit metachromatic changes upon binding to negatively charged groups that are within a defined spatial separation was employed to study the lateral and transverse topography of sulfatide and gangliosides GM1 and GD1a mixed with dipalmitoylphosphatidylcholine (DPPC) in unilamellar vesicles. The spectral changes of the dyes in the presence of liposomes containing anionic glycosphingolipids (GSLs) (hypochromism and frequency shift) are typical of polyanionic lattices while minor changes are found for neutral lipids. The metachromatic changes are abolished by the presence of Ca2+ in the external medium. The proportion of anionic GSLs accessible to the dyes on the external surface of the liposomes is greater as the GSLs are more complex (sulfatide less than GM1 less than GD1a) and as its proportion in the mixture decreases. The number of molecules of anionic GSLs that are laterally distributed on the external surface in a position favorable for the formation of dye dimers (at intermolecular distances not exceeding 1 nm) is greater for sulfatide than for ganglioside. This is correlated to the greater intermolecular distances and delocalization in ganglioside-, compared to sulfatide-containing interfaces. The experimental values indicate that the mixture with DPPC of any of the anionic GSLs studied behaves as if it was more enriched in the GSLs compared to the proportions of the whole mixture.

Effect of calcium ions on the thermotropic behaviour of neutral and anionic glycosphingolipids

In the concentration range of 10(-5) to 10(-1) M Ca2+ modulates the thermotropic properties of several neutral and anionic glycosphingolipids (galactosylceramide, asialo-GM1, sulfatide, GM1, GD1a, GT1b) and of their mixtures with dipalmitoylphosphatidylcholine. The transition temperature of gangliosides is not appreciably changed while the transition enthalpy increases by 20% in the presence of Ca2+. The more marked effect of Ca2+ is on the thermotropic behavior of systems containing sulfatide. Increasing concentrations of Ca2+ between 10(-5) and 10(-3) M (up to a molar ratio of Ca2+/sulfatide 1:2) induce a progressive increase of both the transition temperature and enthalpy. Further increases up to 10(-1) M Ca2+ induce a new phase transition at a lower temperature. No evidence is found for induction of phase separation of pure glycosphingolipid-Ca2+ domains in mixtures of any of the glycosphingolipids with dipalmitoylphosphatidylcholine. The modification of the phase behavior of anionic glycosphingolipids by Ca2+ does not involve detectable variations of the intermolecular packing but is accompanied by marked modifications of the dipolar properties of the polar head group region.