A comparison of a spin-label and a fluorescent cell membrane probe using pure and mixed monomolecular films.

Monocular films studies of 12-nitroxide stearic acid and 12-(9-anthroyl) stearic acid reveal that deviations from the behavior of the parent molecule (stearic acid) are as much dictated by the polar, or nonpolar, nature of the probe group as by its size. In mixed films under membrane-like conditions, the spin label probe, 12-nitroxide stearic acid, exhibits positive deviations from ideality and should read too high a fluidity. The picture is, however, complicated by a tendency of this probe molecule to adopt a bent conformation, a tendency apparently enhanced by specific interactions with the lecithin zwitterion. 12-(9-anthroyl) stearic acid, in contrast, shows only negative deviations from ideality in mixed dipalmitoyl lecithin films and should read too low a fluidity.