COEXISTENCE OF 2 LYOTROPIC LAMELLAR PHASES INDUCED BY A POLYMER IN A PHOSPHOLIPID-WATER SYSTEM

The effect of a hydrophobically modified polysaccharide, cholesteryl-p ullulan (CHP), on the swelling of the DMPC L-alpha lamellar phase has been investigated by small angle neutron scattering. The CHP derivativ e can be introduced in the aqueous layers of the lamellar phase by anc horing lateral cholesterol groups into the bilayers. The resulting lam ellar phase (L-p) is stabilized at large membrane separations by the i ntroduction of a new repulsive and long range contribution in the forc e balance of the system. We emphasize here the temperature dependence of two coexisting lamellar phases (L-alpha + L-p) differing in their p olymer content and in their periodicities. At low polymer content (DMP C:CHP=99:1 by weight), the two lamellar phases at thermodynamic equili brium change into a single phase on heating from room temperature to 5 0 degrees C. The new phase (L-p') is characterized by a very large cor relation peak whose position is consistent with a lamellar structure f ollowing an ideal dilution law. The transition L-alpha+L-p-->L-p' is r eversible on cooling, indicating that the observed coexistence of the two lamellar phases at room temperature is a true thermodynamic equili brium. At higher polymer content (DMPC:CHP = 95:5 by weight) the criti cal behaviour has not been observed. The periodicity of the L-p phase slightly decreases on heating indicating a reduction in the miscibilit y gap and a possible critical point at temperatures higher than 50 deg rees C, However, in the investigated temperature range, the thermodyna mic coexistence of the two lamellar phases is not affected in this cas e. (C) 1997 Elsevier Science B.V.