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 interpreted within the frame of Israelachvili's concept of packing constraints yielding the critical packing parameter, f, as a measure of the asymmetry of the molecular shape of the membrane constituents. The asymmetry of the molecular shape of the detergent increases with the ethylene oxide 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 molecule 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 fluorescence lifetime data are discussed to give an indication of the accessibility of the probe environment to water molecules.