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: emission up-conversion, absorption pump−probe, flash-photolysis, steady-state luminescence, and circular dichroism (CD). Combination of the methods allowed to detect oxidized form of ethidium and to associate this with a fast 10 ps component observed in the up-conversion experiments. Thus, a photoinduced 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 ethidium), while the redox potential of the most affinic DNA bases, thymine and cytosine in water, is −1.09 V. It is proposed that certain conformations of the stacked bases have low-lying electronic orbitals. The involvement of such low-lying electronic levels in the processes of electron transfer through DNA stack is suggested. The results obtained by the pump−probe experiments for the AO−DNA system and by the CD measurements for the EB−DNA complex point at a delocalization of the electronic excitation over the neighboring dyes separated by two base pairs, i.e., 10 Å, in the DNA stack.