Coexistence of two lyotropic lamellar phases induced by a polymer in a phospholipid-water system

The effect of a hydrophobically modified polysaccharide, cholesteryl-pullulan (CHP), on the swelling of the DMPC L-alpha lamellar phase has been investigated by small angle neutron scattering. The CHP derivative can be introduced in the aqueous layers of the lamellar phase by anchoring lateral cholesterol groups into the bilayers. The resulting lamellar phase (L-p) is stabilized at large membrane separations by the introduction of a new repulsive and long range contribution in the force balance of the system. We emphasize here the temperature dependence of two coexisting lamellar phases (L-alpha + L-p) differing in their polymer content and in their periodicities. At low polymer content (DMPC:CHP=99:1 by weight), the two lamellar phases at thermodynamic equilibrium change into a single phase on heating from room temperature to 50 degrees C. The new phase (L-p') is characterized by a very large correlation peak whose position is consistent with a lamellar structure following an ideal dilution law. The transition L-alpha+L-p-->L-p' is reversible on cooling, indicating that the observed coexistence of the two lamellar phases at room temperature is a true thermodynamic equilibrium. At higher polymer content (DMPC:CHP = 95:5 by weight) the critical behaviour has not been observed. The periodicity of the L-p phase slightly decreases on heating indicating a reduction in the miscibility gap and a possible critical point at temperatures higher than 50 degrees C, However, in the investigated temperature range, the thermodynamic coexistence of the two lamellar phases is not affected in this case. (C) 1997 Elsevier Science B.V.