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-G_M1, sulfatide, G_M1, G_D1a, G_T1b) 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.