NMR Studies of Membrane Structure and Dynamics

Over the past decade, there has been considerable interest in the motional state of the phospholipid bilayer membrane. The motivation underlying these efforts has been the contention that the phospholipid bilayer is the basic matrix in which membrane proteins are embedded to form the biological membrane, and that the permeability and mechanical properties of the membrane, as well as the enzymatic activity of membrane proteins, are dependent upon the fluidity of the bilayer, especially the motional state of the hydrocarbon chains. For many years, much of our knowledge about the motional state of the bilayer has come from Electron Spin Resonance (ESR) measurements on spin-labeled lipids incorporated into the bilayer (1, 2). Recent efforts have been directed toward developing spectroscopic probes that would circumvent the possible perturbations in the bilayer produced by the bulky nitroxide radical. Nuclear Magnetic Resonance (NMR) spectroscopy is a natural tool for this purpose and there has been considerable effort using IH, 2H, 13C, and 31p probes which are either endogenous or incorporated into the lipid molecule (3-11). These efforts constitute the subject of the present review. The application of NMR spectroscopy to the study of the lipid bilayer is not without some difficulties. In particular, the interpretation of NMR spectral parameters for a partially ordered system undergoing complex local motion is not straightforward, hence the conclusions deduced are often controversial because of the ambiguous way in which the data can be interpreted. These controversies are only now being resolved. Because of the extensive coverage in the chemical literature given to NMR studies of phospholipid systems, no attempt is made to completely review this area of research. Instead, we concern ourselves with those investigations most pertinent to