Steady-state polarization from cylindrically symmetric fluorophores undergoing rapid restricted motion

Steady-state fluorescence polarization measurements provide a relatively simple method for investigating the orientation of molecular components in ordered biological systems. However, the observed fluorescence polarization ratios also depend on any mobility of the fluorophores on the time scale of the fluorescence lifetime. Such mobility commonly arises from incomplete immobilization of a fluorescent probe on the macromolecule of interest. A theoretical formalism is presented for the interpretation of steady-state fluorescence polarization ratios in the presence of such rapid fluorophore motion. It is assumed that the fluorophores move freely within a cone between absorption and emission of a photon. Only one new parameter is introduced to describe fluorophore motion, the semi-angle of the cone, and this can be measured in separate experiments on an isotropic system. The fluorophore orientations are assumed to have cylindrical symmetry, but the symmetry axis need not be in the same plane as the center axes of the absorption and emission cones. This geometry applies to muscle and other biological fibers.