Bilayer structure of ganglioside/cholesterol mixed system in the presence of Ca2+

Abstract

We employed small-angle X-ray scattering measurement using synchrotron radiation to study structural properties of monosialoganglioside (GM1)/cholesterol (chol) mixtures in aqueous dispersions, in the presence of 0–50 mM CaCl2. Various molar ratios of GM1/chol were examined from 1:0 to 1:5. Without CaCl2 the radius of gyration (Rg ) of the aggregate increased linearly with increasing cholesterol content up to GM1/chol = 1/1. At cholesterol content above GM1/chol = 1/1, Rg of the aggregate was almost constant, and a sharp diffraction peak at q = 0.18 Å−1, characteristic of cholesterol monohydrate crystals, was observed, suggesting the solubility limit of cholesterol in GM1 aggregates is about GM1/chol = 1/1. For the samples containing cholesterol of GM1/chol = 1/0.6 and above, the diffraction patterns showed a set of peaks that correspond to ~85 Å periodic lamellar structures in the presence of CaCl2 at concentrations higher than ~10 mM. The observed lamellar peak positions and the intensity ratio of the lamellar peaks (1st–4th) was not changed by the addition of CaCl2 up to 100 mM, demonstrating once formed multilayered structure of GM1/chol was not altered under the present conditions. Electron density profiles for this GM1/chol bilayer were obtained. The Ca2+-induced vesicle-to-lamellar phase separation of this system was found to be highly reversible by the Ca2+ chelation with ethylenediamine-tetraacetic acid.