Abstract
(Na+ + K+)‐ATPase membranes from Torpedo marmorata electric organ were labeled with high concentrations of paramagnetic phospholipid analogs (up to 5 mol of spin labels for 100 mol of endogeneous phospholipids). The mobile lipid bilayer component of the complex resonance spectra obtained showed low‐field linewidth broadening due to spin—spin interactions; this was used as a measure of probe concentration in this compartment. Our results show that in the lipid shell surrounding the protein, there is a considerable enrichment in negatively charged phospholipids over neutral ones.