Photophysical processes in the complexes of DNA with ethidium bromide and acridine orange: A femtosecond study

Ethidium bromide (EB) and acridine orange (AO) in the complexes with DNA in aqueous solution were studied using variety of spectroscopy techniques: em ission up-conversion, absorption pump-probe, flash-photolysis, steady-state luminescence, and circular dichroism (CD). Combination of the methods allo wed to detect oxidized form of ethidium and to associate this with a fast 1 0 ps component observed in the upconversion experiments. Thus, a photoinduc ed electron transfer from a small fraction of the dyes to the DNA bases was concluded. It suggests that those DNA sites have a redox potential higher than -0.9 V (an upper limit for the excited-state oxidation potential of et hidium), while the redox potential of the most affinic DNA bases, thymine a nd cytosine in water, is -1.09 V. It is proposed that certain conformations of the stacked bases have low-lying electronic orbitals. The involvement o f such low-lying electronic levels in the processes of electron transfer th rough DNA stack is suggested. The results obtained by the pump-probe experi ments for the AO-DNA system and by the CD measurements for the EB-DNA compl ex point at a delocalization of the electronic excitation over the neighbor ing dyes separated by two base pairs, i.e., 10 Angstrom, in the DNA stack.