Motional state of spin-labeled stearates in lecithin-cholesterol liposomes and their incorporation capability.

Abstract

Electron spin resonance spectra of nitroxide-labeled stearates were investigated in the presence of dipalmitoyl or dimyristoyl lecithin-cholesterol liposomes. Beside the broad resonance lines, originated from the incorporated spin probes, the spectra were characterized with the sharp three resonance lines, which were attributed to the spin probes not incorporated in the liposomal membrane. The superposed spectra were analyzed in terms of the peak height ratio of the sharp and broad lines, and also of the order parameter, derived from hyperfine splittings of the broad lines. The two quantities changed depending on the cholesterol content in the liposomes, but appeared to behave entirely different from each other : in the dipalmitoyl lecithin-cholesterol system, the spin probes were more incorporated in the membrane of higher motional state, while, in the dimyristoyl lecithin-cholesterol, they were less incorporated. The relation was discussed and compared with permeability property of the liposomal membranes.