Analysis of fluorescence energy transfer in duplex and branched DNA molecules.

Nonradiative fluorescence energy transfer (FET) is thought to be a highly sensitive measure of distance, occurring through a dipole coupling (Forster) mechanism in which the efficiency of FET depends on the inverse sixth power of the distance between fluorophores. The current work assesses the utility of FET for measuring distances in duplex and branched DNA molecules. The apparent efficiencies of FET between donor (fluorescein) and acceptor (eosin) fluorophores attached to opposite ends of oligonucleotide duplexes of varying length were determined; the results suggest that FET is a useful qualitative indicator of distance in DNA molecules. However, the apparent FET efficiency values cannot be fit to the Forster equation without the specification of highly extended DNA-to-fluorophore tethers and motionally restricted fluorophores, conditions that are unlikely to coexist. Three other lines of evidence further suggest that factors in addition to Forster transfer contribute to apparent FET in DNA: (1) The efficiency of FET appears to depend on the base sequence in some instances. (2) Donor fluorescence changes with the extent of thermally induced DNA melting in a sequence-dependent fashion, indicating dye-DNA interactions. (3) The distances between the ends of various pairwise combinations of arms of a DNA four-way junction do not vary as much as expected from previous work. Thus, the occurrence of any nondipolar effects on energy transfer in oligonucleotide systems must be defined before distances in DNA molecules can be quantified by using FET.