A Spin Label Study of the Organization and Fluidity of Hydrated Phospholipid Multibilayers – a Model Membrane System

Abstract

The essential theory of the spin label method is presented and applied to the case of a rigid spin label localized in an oriented phospholipid smectic mesophase (hydrated phospholipid multibilayers). The fatty acid chains of the phospholipids in a model membrane of egg lecithin are shown to make an average angle of 24° with respect to a perpendicular to the bilayer plane. Addition of cholesterol stiffens the bilayers and decreases this angle to 10° at 55% cholesterol. The various requirements of a sterol for this condensing effect are explored, and the thesis is developed that one of the roles of sterols in membranes is to increase organization and rigidity of fatty acid chains, thus decreasing passive permeability and regulating lipid-lipid and lipid-protein interactions. Ions, proteins, anesthetics, antibiotics, and decouplers of oxidative phosphorylation are shown to have profound effects on the structure of lipid bilayers. Some extensions of the technique to real biological membranes are described, and the future for this application is predicted to be excellent.