The Liquid-Crystallographic Structure of Fluid Lipid Bilayer Membranes

Abstract

Knowledge of the structure of lipid bilayers in the fluid liquid-crystalline state is important for understanding the permeability and stability of membranes and the insertion and folding of membrane proteins. Quantitative structural models are especially important at the present time for validation of Monte Carlo and molecular dynamics simulations of lipid bilayers (Pastor, 1994). Diffraction studies of phospholipid crystals at low hydrations can provide atomic-resolution images of the phospholipid molecules of membranes (Pascher et al, 1992) but such images are of marginal value for understanding membrane bilayers for the obvious reason that the phospholipids are in a noncrystalline state. Fluid bilayers present special problems to the structural biologist because their inherent thermal motion and disorder exclude entirely the possibility of obtaining three-dimensional structural information. The only kind of structural image that can be obtained by diffraction methods is a one-dimensional one consisting of the time-averaged transbilayer distributions of the multiatom submolecular groups comprising the lipids such as the phosphate, carbonyl groups, double-bonds, etc. (Figure 1). Such projections have become a standard method for describing the results of bilayer simulations (Damodaran and Merz, 1994; Egberts et al, 1994; Fattal and Ben-Shaul, 1994; Heller et al, 1993).