EFFECTS OF ENERGY TRANSFER AND ROTATIONAL DIFFUSION UPON FLUORESCENCE POLARIZATION OF MACROMOLECULES

The rotational relaxation time of a macromolecule computed from data of fluorescence polarization is an average of the principal relaxation times of the associated hydrodynamic ellipsoid weighted according to the orientation of the transition moments in absorption and emission with respect to the ellipsoid’s axes. Memming’s equations, describing the fluorescence polarization of these cases, are transformed so as to express the polarization as an explicit function of the angle θ between the transition moments, and the angle γ between the plane containing them and the equator of the ellipsoid. This transformation is particularly suitable to enumerate the cases arising when an aromatic ligand with π → π* electronic transitions in absorption and emission is bound to a macromolecule with preferential orientation. The effects of energy transfer among bound ligands may also be easily visualized and calculated. We have made calculations for a typical prolate ellipsoid of axial ratio 4, varying systematically θ, γ, and the fraction transferred when more than one ligand is involved, in the following cases: (a) bound single ligand, (b) two ligands bound in the same plane in fixed relative orientations, and (c) coplanar ligands having no further fixed orientation with respect to each other. These considerations open up the possibility of demonstrating the preferential orientation of multiple bound ligands by the effects of energy transfer upon the apparent rotational relaxation time. A practical case of this type is described in the following paper (Anderson, S. R., and Weber, G. (1969), Biochemistry 8,371). © 1969, American Chemical Society. All rights reserved.