Nanosecond fluorescence anisotropy decays of n-(9-anthroyloxy) fatty acids in dipalmitoylphosphatidylcholine vesicles with regard to isotropic solvents

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Abstract
A set of n-(9-anthroyloxy) fatty acids [2-, 7-, 9-, and 12-(9-anthroyloxy)stearic acid (AS) and 16-(9-anthroyloxy)palmitic acid (16-AP)] was studied by time-resolved and steady-state fluorescence anisotropy measurements in isotropic media (i.e., propylene glycol and a liquid paraffinic oil, Primol 342) and in vesicles of dipalmitoylphosphatidylcholine. The two modes of rotation, in-plane and out-of-plane, of the anthroyl ring can be detected by varying the excitation wavelength. In both isotropic solvents, the value of the in-plane rotational rate is of the same order of magnitude as the out-of-plane rate for each one of the n-(9-anthroyloxy) fatty acids. In propylene glycol, the anthroyl ring motions are similar for all derivatives except for the 16-AP for which the fluorophore rotates at a higher rate. In the liquid paraffinic oil, identical motions of the fluorophore are observed for the 7-, 9-, and 12-AS; the motion for the 16-AP is again faster, while that for the 2-AS is slower. Moreover, the fluorophore motion for each probe is faster in this solvent than in propylene glycol in conditions of identical viscosity. When embedded in phospholipid bilayers, these probes report the microenvironment at a graded series of depths fromt surface to the center of the bilayer. Studies in dipalmitoylphosphatidylcholine vesicles were performed at 3 temperatures (21, 37, and 47.degree. C) corresponding to different lipid phases. The out-of-plane mode of rotation is unhindered as demonstrated by an anisotropy decay profile asymptotic to zero. Thus, evaluation of a membrane fluidity parameter at different depths of the bilayer is possible, even in the steady-state mode of observation. When the in-plane mode of rotation contributes to the anisotropy decay, a hindrance to the motion is observed below the gel to liquid-crystalline transition. Then information about lipid order can be obtained from the plateau value of the fluorescence anisotropy decay. In the pretransition temperature range (37.degree. C), the results evidence the existence of structural lipid changes mainly localized in the hydrophobic core of the bilayer. The main transition leads to a complete disappearance of the hindrances onthe in-plane rotation.

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