Time dependence of triplet-singlet excitation transfer from compact poly rA to bound dye at 77 K

The nonexponential phosphorescence decay of a highly folded form of poly-riboadenylic acid (poly rA) with noncovalently bound dye is explained by a novel application of a well-known theory of electronic excitation transfer based on the Förster mechanism. This theory, originally used to describe singlet-singlet energy transfer from donor molecules to an acceptor in a solution, is here applied to the transfer of triplet excitation from the adenine (in poly rA) to the singlet manifold of either of the bound dyes, ethidium bromide or proflavine. New experimental data are presented that allow straight-forward theoretical interpretation. These data fit the form predicted by the theory, U(t) exp(-Bt1/2), where U(t) is the decay of the poly rA phosphorescence in the absence of dye, for a range of relative concentrations of either dye. The self-consistency of these theoretical fits is demonstrated by the proportionality of B to the square root of the Förster triplet-singlet overlap integrals for transfer from poly rA to each of the dyes, as demanded by the theory. From these self-consistent values of B, the theory enables one to deduce the mean packing density of nucleotides in this folded poly rA, which we estimate to be approximately 1 nm-3. We conclude that some variations of the method described here may be useful for deducing packing densities of nucleotides in other compact nucleic acid structures.