SURFACE-AREAS AND PACKING CONSTRAINTS IN POPC C(12)EO(N) MEMBRANES - A TIME-RESOLVED FLUORESCENCE STUDY/

The surface area occupied by nonionic detergents of the type C(12)EO(n ) (n = 1-8) in POPC/C(12)EO(n) mixed membranes was studied by means of time-resolved resonance energy transfer (RET) between the fluorescent probe molecules NBD-PE and rhodamine-PE. The area data were interpret ed within the frame of Israelachvili's concept of packing constraints yielding the critical packing parameter, f, as a measure of the asymme try of the molecular shape of the membrane constituents. The asymmetry of the molecular shape of the detergent increases with the ethylene o xide chain length and correlates with the potency of the detergent to solubilize the bilayers and the reduction of the DPH order parameter. For n = 1-3, the membrane surface was found to expand by 0.25-0.30 nm( 2) per incorporated C(12)EO(n) molecule. This value corresponds to the cross section of one hydrocarbon chain in liquid-crystalline phases. On increasing n from n = 4 to n = 8 the net area per detergent molecul e increases from 0.43 nm(2) to 1.16 nm(2). These surface requirements are consistent with a disordered, coiled conformation of the EO-chains hydrated with up to two water molecules per ethylene oxide unit. For n > 5 the limiting mole fraction of the bilayer saturation was deduced from the f-data in the two-component bilayer. DPH and NBD-PE fluoresc ence lifetime data are discussed to give an indication of the accessib ility of the probe environment to water molecules.