Universal Behavior in Phospholipid Multimembrane Systems

Abstract

Stacked multimembrane structures of the phospholipid DMPC show two interesting universal behaviors due to geometric concerns. X-ray scattering studies were done to elucidate the structure of the ordered phases where the Lß’ phase was shown to be in fact 3 distinct phases characterized by the direction of chain tilt with respect to the nearest neighbor positions. Two of these phases are lyotropic analogues to the smectic-F and -I phases, while the phase of intermediate symmetry (-L) had not been seen in other systems. Transitions from structures with smectic-F symmetry to those with smectic-I symmetry can be expected in systems where one can reduce the problem to contours on a hexagonal lattice which are effectively repulsive or would like to be straight. In the LA, phase of this system, the layers only dilute with water to ~ 25Å. By adding cosurfactant (pentanol) to the system, the thickness of the bilayer is reduced, thus reducing the bending modulus kc from 25k_BT to ~ k_BT driving the system into a fluctuation-induced universal regime dominated by the repulsive Helfrich interaction which allows the bilayers to dilute to separations >200Å.