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 (G(M1)), disialogangliosides (G(D1a)) 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 degrees C. For every sample, the pH elevation from 3.6 to 8.0 at 25 degrees 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.