Ethidium bromide electron transfer quenching

The enhanced fluorescence of the ethidium-DNA complex can be quenched partially by the addition of a second ligand. Two mechanisms have been proposed to account for quenching, namely displacement of ethidium bromide by melting of the helix or electron transfer from external quencher molecules. The most efficient quenching occurs of course when both the quencher and ethidium bromide molecules are bound to DNA. This nondisplacement quenching is correlated with DNA-enhanced electron transfer, either from excited ethidium to an acceptor (methylviologen, copper (II) counterions) or from an donor to an excited ethidium acceptor. The DNA double helix works as a well-defined matrix for an organized electron transfer. It enhances its yield by a factor K. (DNA) K, (H,0), which is often in the order of 5 x 10. ... 

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