Energy Migration and Fluorescence Depolarization: Structural Studies of Ethidium Bromide-Nucleic Acid Complexes

Abstract

Excitation energy transfer may occur between like fluorescent molecules in solutions of sufficiently high concentration. The excitation energy of a molecule which absorbs a photon at time zero may jump from molecule to molecule until emission occurs at some later time t. Thus a fluorescence photon, which in dilute solution is emitted by the absorbing molecule, may in concentrated solution be emitted by one of the molecules which surround the initially excited one. This process broadens the angular distribution of the transition moments of the emitting molecules and consequently gives rise to a depolarization of the emission. The rate of a transfer step depends on the mutual distance and orientation between the donor and acceptor molecules. Therefore the measurement of fluorescence anisotropy decay due to energy transfer should provide information on the geometrical arrangement of an array of chromophores. In the following, an application of this principle to the study of ethidium-nucleic acid and ethidium-chromatin complexes is described.