Mitsuhiro Hirai

Thermotropic phase behavior and stability of monosialoganglioside micelles in aqueous solution

The thermotropic phase behavior of monosialoganglioside in a dilute aqueous dispersion at pH 6.8 was measured by using synchrotron radiation small-angle x-ray scattering and was analyzed by a shell-modeling method. Previous calorimetric studies on ganglioside systems have shown quite different thermotropic behaviors from other biological lipid systems, however, the details have still been ambiguous. Because of high statistical data and a shell-modeling analysis, we could elucidate the internal structural change of monosialoganglioside micelle induced by the elevation of temperature from 6 to 60 degrees C, that is, the shrinkage of the hydrophilic region and the slight expansion of the hydrophobic region occurring simultaneously, accompanying the elongation of the axial ratios of the ellipsoidal micelles. The model structures obtained explain the changes in the experimental scattering curves, the distance distribution functions, and the gyration radii. In addition we have also found an evident thermal hysteresis in the scattering curves and in the structural parameters. The present result suggests that the thickness of the hydrophilic region, namely, the conformation of oligosaccharide chains, is sensitive to a change of temperature.

INTERMICELLAR INTERACTION OF GANGLIOSIDE AGGREGATES AND STRUCTURAL STABILITY ON PH VARIATION

We have studied the effect of concentration and pH variations on micellar structures of sialoglycosphingolipids (gangliosides) by using synchrotron radiation small-angle X-ray scattering. We have treated four different aqueous dispersions of gangliosides containing monosialogangliosides (GM1), disialogangliosides (GD1a) and two different types of the ganglioside mixtures. To discuss the change in the scattering curve with concentration in the range of 0.1–10% w/v, we employed a rescaled mean spherical approximation method (RMSA) for charged particle dispersions combined with a shell-modelling method. The present modelling method well describes the characteristics of the whole experimental scattering curve, namely an intermicellar correlation peak at a very small angle and a rounded peak at a medium angle reflecting an intramicellar scattering density fluctuation. We determined simultaneously the inter- and intra-micellar structures of the ganglioside aggregates under the repulsive screened Coulomb potential between them. The estimated micellar surface charge is ca. 57 e in 0.01 M citrate buffer at pH 6.7, 25 °C. For every sample, the pH elevation from 3.6 to 8.0 at 25 °C caused similar changes in the experimental scattering curve, gyration radius and distance distribution function, suggesting a shrinkage of the micellar dimension and an intramicellar structural change. The modelling analyses can explain the above changes mostly resulting from the change of the oligosaccharide chain portions in the micelles. The structural differences depending on the specimens, containing different ganglioside components are also clarified.

Characteristics of thermotropic phase transition of glycosphingolipid (Ganglioside) aggregates in aqueous solution

Abstract In this report, by using synchrotron radiation X-ray small-angle scattering, we show the structural characteristics of the ganglioside aggregates depending on both the temperature and oligosaccharide chain. The experimental results of the aqueous solutions containing G M1 , G D1 , or crude mixture gangliosides show that the elevation of temperature induces the change of the micellar structure accompanied by the change of the internal scattering density distribution. These changes are well described by applying a shell-modeling method to the observed scattering curves since the shell-modeling method can determine the intramicellar structures. A noticeable common feature found in the thermotropic transition of the ganglioside micelles is the contraction of the micellar dimension resulting from the shrinkage of the hydrophilic region of the micelle. The shrinkage amounts to 20–30% of the thickness of the hydrophilic region, suggesting the structural change of the oligosaccharide chains of the gangliosides from an extended conformation to a compact one. The differences of the thermotropic stability depending on the number of the sialic acids of the oligosaccharide chain were also observed. The present results which suggest a thermal perturbation in the physiological range 20–50°C induces a conformational change of the oligosaccharide chains of ganglioside molecules very sensitively.

Solvent contrast variation study of w/o AOT microemulsions using small-angle neutron scattering

Abstract Recently, we showed that the hydrolysis of some esters catalyzed by α-chymotrypsin entrapped in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) microemulsions is enhanced at low water-surfactant molar ratio w 0 ( = [H 2 O]/[AOT]) and that the internal interface structure and the oligomeric phase of the AOT microemulsions below this w 0 range would play an important role for the acceleration of the metabolic turnover. To clarify the relation between the appearance of superactivity and the internal structures of the AOT microemulsions, we have carried out small-angle neutron-scattering experiments. In this report, we treat the AOT/water/hexane or heptane system by using solvent-contrast variation method, and we discuss a change of the excluded volume of AOT molecule at low w 0 value.

Wide-angle neutron scattering study of thermal denaturation of lysozyme

Abstract Many studies of folding-unfolding transition of protein have been carried out for the clarification of the fundamental biological problem of protein folding, namely, how a polypeptide chain attains its native conformation. Although one of the approaches to solve protein-folding mechanisms is evidently to follow denaturation processes of protein, there is little direct evidence showing intramolecular structural changes under thermal perturbation. By using a wide-angle neutron-scattering spectrometer, we have monitored the thermal denaturation process of hen egg-white lysozyme at different pH under the unaggregative condition. The present results show that the temperature elevation gradually induces an intramolecular structural fluctuation prior to a main transition of the tertiary structure without accompanying significant heat absorption, and the multiplicity in the thermal transition observed suggests the presence of hierarchic stability depending on the structural hierarchy which would characterize an energetic landscape of folding-unfolding transition of protein.