The technique of long-range electronic energy transfer has been used to obtain structural data on nucleic acids in solution. Fluorescent chromophores have been covalently attached via an aminoethyl linker to the 5*-terminus of a synthetic oligonucleotide. Duplexes were formed with an energy donor, fluorescein, at one end and an acceptor, tetramethyl rhodamine, at the other. The rate of energy transfer has been determined from the fluorescence decay of the donor which was measured with a picosecond photon counting system. The effects of dye:DNA interactions cause additional quenching of the donor fluorescence and must be considered in the interpretation of the energy transfer experiments. An apparent donor:acceptor distance of 40.9 A was calculated for a 12 b.p. labelled duplex.
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