BENDING ELASTICITIES OF MODEL MEMBRANES - INFLUENCES OF TEMPERATURE AND STEROL CONTENT

Giant liposomes obtained by electroformation and observed by phase-con trast video microscopy show spontaneous deformations originating from Brownian motion that are characterized, in the case of quasispherical vesicles, by two parameters only, the membrane tension sigma and the b ending elasticity k(c). For liposomes containing dimyristoyl phosphati dylcholine (DMPC) or a 10 mol% cholesterol/DMPC mixture, the mechanica l property of the membrane, k(c), is shown to be temperature dependent on approaching the main (thermotropic) phase transition temperature T -m. In the case of DMPC/cholesterol bilayers, we also obtained evidenc e for a relation between the bending elasticity and the corresponding temperature/cholesterol molecular ratio phase diagram. Comparison of D MPC/cholesterol with DMPC/cholesterol sulfate bilayers at 30 degrees C containing 30% sterol ratio shows that k(c) is independent of the sur face charge density of the bilayer, Finally, bending elasticities of r ed blood cell (RBC) total lipid extracts lead to a very low k(c) at 37 degrees C if we refer to DMPC/cholesterol bilayers. At 25 degrees C t heory low bending elasticity of a cholesterol-free RBC lipid extract s eems to be related to a phase coexistence, as it can be observed by so lid-stale P-31-NMR. At the same temperature, the cholesterol-containin g RBC lipid extract membrane shows an increase in the bending constant comparable to the one observed for a high cholesterol ratio in DMPC m embranes.