Dynamics of phospholipid molecules in micelles: Characterization with fluorescence correlation spectroscopy and time-resolved fluorescence anisotropy

The dynamic properties of two BODIPY labeled phospholipid molecules in mice llar systems have been studied with two different spectroscopic techniques, which provide complementary information (fluorescence correlation spectros copy (FCS) and time-resolved fluorescence anisotropy (TRFA), respectively). One phospholipid had the fluorescent probe attached to the headgroup (abbr eviated as head-labeled) and the other one had the probe at the acyl chain at the sn-2 position of the phospholipid (abbreviated as tail-labeled). Fro m FCS experiments, the translational diffusion constant, aggregation number and hydrodynamic size of the micelles were determined, The micelles were l arger in size than reported in the literature, which is due to the incorpor ation of the relatively large sized fluorescent probe. The FCS results were confirmed by the results of dynamic light scattering experiments on both e mpty micelles and micelles containing the fluorescent lipid probe. Both res ults showed that the probe has a very significant effect on the self-assemb ly behavior. The micelles loaded with the head-labeled phosholipid had a si gnificantly larger radius compared to the micelles loaded with a tail-label ed probe. To obtain more insight into the intramicellar mobility and organi zation of the fluorescent lipid, TRFA experiments were performed. Analysis of the anisotropy decay showed that the lateral diffusion of the probe is t he main, rapid contributing process in detergents such as Triton X-100, pol yoxyethylene-9-lauryl ether (Thesit), cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). However, digitonin and deoxycholate mice lles had significantly different properties, which could be explained by mo re rigidly packed micelles. The calculated values for the wobbling motion, cone angle and order parameters indicated that the tail-labeled probe under goes less restricted motion than the head-labeled probe.