Permeability and Stability of Lipid Bilayers

Abstract

I. INTRODUCTION The lipid bilayers of natural membranes generally exist in a fluid state which occurs above the gel-toliquid-crystalline phase transition temperature.1 Knowledge of the structure of such “fluid” bilayers, which is of obvious importance for understanding the permeability and stability of membranes, can be obtained from X-ray and neutron diffraction measurements.2,3 However, fluid bilayers present special problems to the structural biologist. We are accustomed to viewing highly detailed images of protein crystallographic structures in which the mean relative positions of small groups of atoms are well defined and measurable. Diffraction studies of phospholipid crystals at low hydrations can provide a similar view of phospholipid molecules.4,5 Such three-dimensional images are not possible for fluid bilayers because of their inherent thermal motion and disorder. However, it is reasonable to consider the average transbilayer distribution of multiatom submolecular groups comprising the lipids and proteins.6 The “image” of the membrane in this case consists of the average spatial distribution of the submolecular groups projected onto a line normal to the plane of the membrane, from which the relative intergroup distances can be determined. It is this image, called a bilayer profile, which we take as “the structure” of the fluid bilayer. Even though it does not provide direct information about membrane structure in the other two dimensions (bilayer plane), this one-dimensional image lets us understand how lipid composition and proteins affect the transbilayer distributions of the submolecular groups.7.