Micellar organization and dynamics: A wavelength-selective fluorescence approach

Wavelength-selective fluorescence comprises a set of approaches based on the red edge effect in fluorescence spectroscopy, which can be used to monitor directly the environment and dynamics around a fluorophore in a complex biological system. A shift in the wavelength of maximum fluorescence emission toward higher wavelengths, caused by a shift in the excitation wavelength toward the red edge of the absorption band, is termed the red edge excitation shift (REES). This effect is mostly observed with polar fluorophores in motionally restricted media such as very viscous solutions or condensed phases. We have previously shown that REES and related techniques (wavelength-selective fluorescence approach) offer a novel way to monitor organization and dynamics of membrane-bound probes and peptides, In this paper, we report REES of NBD-PE, a phospholipid whose headgroup is covalently labeled with the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) moiety, when incorporated into micelles formed by a variety of detergents (SDS, Triton X-100, CTAB and CHAPS) which differ in their charge and organization. In addition, fluorescence polarization of NBD-PE in these micelles shows both excitation and emission wavelength dependence. The lifetime of NBD-PE was found to be dependent on both excitation and emission wavelengths, These wavelength-dependent lifetimes are correlated to the reorientation of solvent dipoles around the excited-state dipole of the NBD group in micellar environments. Taken together, these observations are indicative of the motional restriction experienced by the fluorophore when bound to micelles. Wavelength-selective fluorescence promises to be a powerful tool for studying micellar organization and dynamics.