Increase of fluorescence anisotropy upon self-assembly in headgroup-labeled surfactants

The change in fluorescence anisotropy upon micellization in headgroup-labeled surfactants is investigated. After eliminating the likelihood of depolarizing RET, anisotropy is shown to increase upon self-assembly due to increased rotational correlation times of the fluorophore. This is shown using two surfactant-fluorophore systems. Anisotropy in NBD-labeled phospholipids is studied both in chloroform (unaggregated) and in water (unilamellar vesicles), while in tryptophan-containing peptide-amphiphiles, the variation of anisotropy with concentration leads to a reasonable measurement of CAC. Anisotropy increase is shown to be largely the product of increased rotational correlation times for the fluorophore, relative to its tau. These results serve as a basis for future work that measures the amount of depolarizing energy transfer, characterizing distances between similar fluorescent headgroups on mixed micelles.