An assessment of the fluidity gradient of the lipid bilayer as determined by a set of n-(9-anthroyloxy) fatty acids (n = 2, 6, 9, 12, 16).

The rotational behavior of a set of n-(9-anthroyloxy) fatty acid fluorescent probes is examined in two liquid paraffins and in liposomes composed of dipalmitoyl phosphatidylcholine. As has been observed with other membrane fluorescent probes (Hare, F., and Lussan, C. (1977) Biochim. Biophys. Acta 467, 262-272), the degree of fluorescence depolarization for a given solvent viscosity is dependent on the solvent standard employed. In addition, when the anthroyloxy group is in the terminal position of the acyl chain, it has more rotational freedom than when it is conjugated to positions 6, 9, or 12 where the rotational motion of the fluorophore is similar. When incorporated into lipid bilayers, values of fluorescence polarization reflect the gradient of "fluidity" which extends from the surface to the center of the membrane. The nature of this polarization gradient is discussed in relation to the intrinsic differences between the probes and the anisotropic rotations responsible for depolarization.