Hydration and thermal reversibility of glycolipids depending on sugar chains.

To elucidate a relationship between the structural properties and hydration characteristic of gangliosides, time-resolved small-angle X-ray scattering measurements using synchrotron radiation have been performed on aqueous dispersions of various types of gangliosides (GM1, GD1a, GD1b and GM3) under a constant heating (5-65 degrees C) and cooling (65-5 degrees C) rate. In the case of GM3, they formed a vesicular aggregate with a high structural reversibility in the heating-and-cooling process. For the micelles of GM1, GD1a and GDlb, we found an evident thermal hysteresis in the structural changes of their headgroups and evaluated quantitatively the amounts of water molecules occluded in the micellar hydrophilic regions by using the shell modeling method reported previously. For all cases of GM1, GD1a and GD1b, the thickness of the hydrophilic region of the micelle shrunk after the heating process, and stayed mostly constant over the entire cooling range. On the other hand, the amounts of water molecules and the behavior of the GM1, GD1a and GD1b micelles in the heating-and-cooling process greatly depended on the number of sialic acid residues in the sugar chain, that is, the penetration of water molecules was much more reversible for the GM1 micelle compared with those for the GD1a and GD1b micelles. The observed clear hysteresis and the hydration characteristics of GD1 gangliosides would relate to their role in neuronal membranes, where GD1 gangliosides show the greatest concentrations.