Membranes of palmitoyloleoylphosphatidylcholine and C12E4 - A lattice model simulation

The structure of the lamellar phase of the ternary system of palmitoyloleoy lphosphatidylcholine (POPC)/tetra(ethylene oxide) monododecyl ether (C12E4) has been characterized in detail by a X-ray and neutron scattering and by H-2 NMR spectroscopy. The addition of small amounts of surfactant to the PO PC bilayers causes the bilayer to become more rigid, while the addition of large amounts of surfactants induces a fluidization of the structure. Here, we report a Monte Carlo study of the underlying mechanisms yielding these changes in the orientational order of POPC/C12E4 bilayers. The study employ s a lattice Monte Carlo dynamics algorithm that rests on the assumption tha t the conformational dynamics of chain molecules can be described as the su perposition of local structural rearrangements involving short-chain segmen ts. This approach is particularly useful for following structural and confo rmational changes on time scales longer than those associated with the gauc he/trans isomerization of hydrocarbon chains. The simulations reproduce the principal features of the order parameter profiles of the surfactant molec ules found in pure bilayers and bilayers of POPC/C12E4 mixtures and provide a physical framework for understanding the experimental observations.