Studying Lipid Organization in Biological Membranes Using Liposomes and EPR Spin Labeling

Electron paramagnetic resonance (EPR) spin-labeling methods provide a unique opportunity to determine the lateral organization of lipid bilayer membranes by discrimination of coexisting membrane domains or coexisting membrane phases. In some cases, coexisting membrane domains can be characterized without the need for their physical separation by profiles of alkyl chain order, fluidity, hydrophobicity, and oxygen diffusion-concentration product in situ. This chapter briefly explains how EPR spin-labeling methods can be used to obtain the above-mentioned profiles across lipid bilayer membranes (liposomes). These procedures will be illustrated by EPR measurements performed on multilamellar liposomes made of lipid extracts from cortical and nuclear fractions of the fiber cell plasma membrane of a cow-eye lens. To better elucidate the major factors that determine membrane properties, results for eye lens lipid membranes and simple model membranes that resemble the basic lipid composition of biological membranes will be compared.