Motion of steroid spin labels in membranes.

Steroid spin labels have been found to undergo rapid and sometimes anisotropic rotational motion in membranes and phospholipid vesicle preparations. N-oxyl-4',4'-dimethyloxazolidine derivatives of 5alpha-androstan-3-one-17beta-ol, 5beta-androstan-3-one-17beta-ol, and 5alpha-androstan-3-one were prepared according to the procedure described by Keana. The paramagnetic five-membered ring is rigidly attached to the steroid nucleus at the 3-position, and thus the resonance spectrum of the nitroxide group reflects the motion of the entire steroid nucleus. This rapid motion, with rotational diffusion frequencies of the order of 10(7) to 10(8) second(-1) is considered to lend plausibility to models of membrane transport involving rotations and/or translations of carriers within the hydrophobic region of the membrane. The steroid spin labels also form a convenient class of probes for studying conformational changes in membranes, since (a) their resonance spectra often fall in the category of "intermediate immobilization," which is most sensitive to small changes, and (b) these labels are quite soluble in a wide variety of membranes.