Orientation and motion of amphiphilic spin labels in membranes.

The paramagnetic resonance spectra of N-oxyl-4','-dimethyloxazolidine derivatives of 12-ketostearic acid, 5-ketostearic acid, 5-ketotricosanoic acid, and 5alpha-androstan-3-one-17beta-ol have been observed in sonicated phospholipid dispersions, the walking-leg nerve fibers of Homarus americanus, and in erythrocytes oriented by hydrodynamic shear. The preferred orientation of these spin labels in the nerve and erythrocyte membranes is one in which the long amphiphilic axis is perpendicular to the membrane surface. The resonance spectra indicate that these labels in the nerve fiber are bound to a lipid bilayer very similar to that found in the sonicated phospholipid dispersion, whereas any lipid bilayer in the erythrocyte must be much more rigid.