31P NMR and the Phospholipid Headgroups of Membranes
Biological membranes are central elements of cell structure and function. For example, cell plasma membranes define the boundary of the cell, control communication between the inside and outside of the cell, function in cellular metabolism, and mediate cell-cell interactions, among many roles. The sensitivity of spin labels permits their use at probe concentrations in membranes, thereby avoiding severe perturbations of the overall lipid behavior and membrane structure. However, their bulk, relative to the molecules on which they reside, produces local perturbations that may dominate the environment of the spin label. 31P nuclear magnetic resonance (NMR) uniquely reveals membrane structure without labeling or isotopic enrichment, and without perturbing the membrane. 31P NMR performs equally well in sonicated vesicles, unsonicated liposomes, and biological membranes and is therefore superior to the techniques that are tied to high resolution measurements. 31P NMR studies of membranes began over 10 years ago, in a variety of investigators laboratories.