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 micellar systems have been studied with two different spectroscopic techniques, which provide complementary information (fluorescence correlation spectroscopy (FCS) and time-resolved fluorescence anisotropy (TRFA), respectively). One phospholipid had the fluorescent probe attached to the headgroup (abbreviated 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). From FCS experiments, the translational diffusion constant, aggregation number and hydrodynamic size of the micelles were determined, The micelles were larger in size than reported in the literature, which is due to the incorporation of the relatively large sized fluorescent probe. The FCS results were confirmed by the results of dynamic light scattering experiments on both empty micelles and micelles containing the fluorescent lipid probe. Both results showed that the probe has a very significant effect on the self-assembly behavior. The micelles loaded with the head-labeled phosholipid had a significantly larger radius compared to the micelles loaded with a tail-labeled probe. To obtain more insight into the intramicellar mobility and organization of the fluorescent lipid, TRFA experiments were performed. Analysis of the anisotropy decay showed that the lateral diffusion of the probe is the main, rapid contributing process in detergents such as Triton X-100, polyoxyethylene-9-lauryl ether (Thesit), cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). However, digitonin and deoxycholate micelles had significantly different properties, which could be explained by more rigidly packed micelles. The calculated values for the wobbling motion, cone angle and order parameters indicated that the tail-labeled probe undergoes less restricted motion than the head-labeled probe.