Fluorescence Polarization and Rotational Mobility

Abstract

Dynamic information on the behavior of a protein molecule in solution can be obtained with the help of another fluorescent parameter — the degree of polarization. If characteristic times of Brownian rotation are comparable in magnitude with fluorescence lifetime, the size and, sometimes, the shape of the molecule or its mobile segment can be estimated by measuring the fluorescence polarization. The first variant of this method, suggested by Weber (1953), is based on measurements of steady state polarization of fluorescent labels which are covalently attached to protein. Recently, signs of keen interest to polarization of intrinsic fluorescence of proteins have appeared. The depolarization of intrinsic fluorescence of the protein in solution is accounted for by three factors: the rotations of the whole protein and sometimes of its large fragment, the rotations or rotational vibrations of the chromophore itself, and the transfer of excitation energy during its lifetime. This causes substantial complications in the interpretation of experimental data. This chapter, apart from setting forth the principles of the method and some essential results, presents a discussion of spectroscopic and temporal features of polarization associated with the process of dipole-reorientational relaxation in the electronically excited state. The effect of relaxation is not confined to the mere influence on fluorescence decay kinetics which, in turn, must influence the time dependence of polarization. Along with the Brownian rotation, the induced turns of molecules are to be observed directly, associated with dipolar relaxation.