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Nanosecond fluorescence decay studies of the deoxyribonucleic acid-9-aminoacridine and deoxyribonucleic acid-9-amino-10-methylacridinium complexes.

The binding interaction of the mutagenic dye 9-aminoacridine (9AA) with DNAs of various base composition has been studied by steady-state and transient fluorescence measurements. It was found that the fluorescence decays of 9AA and 9AA bound to poly[d(A-T)] which contains only one type of binding site are single exponential but that those of 9AA bound to DNA can be well described as a sum of three-exponential functions. The fluorescence quantum yields confirm that the AT base pair is responsible for the fluorescence of the bound 9AA, while the GC base pair almost completely quenches it. Nanosecond time-resolved spectroscopy indicates that the structure of the DNA-9AA complex is not substantially altered during the lifetime of the excited singlet state of the dye. Furthermore, the fact that the shapes or the maxima of the absorption and fluorescence spectra exhibit no dependence on the GC content of DNA indicates that the radiative lifetime of the bound 9AA is constant under a variety of circumstances. On the basis of these results, it is concluded that the emitting sites of 9AA on DNA are composed of at least three classes having different quantum yields: (I) tau1 = 2.0 +/- 0.3 ns, phi1 = 0.06 +/- 0.01, (II) tau2 = 12.3 +/- 0.7 ns, phi2 = 0.35 +/- 0.02, and (III) tau3 = 28.3 +/- 0.5 ns, phi3 = 0.81 +/- 0.02. Fluorescence decay behavior is also reported for the nonmutagenic dye 9-amino-10-methylacridinium (10Me-9AA) bound to DNA. The results of 10Me-9AA were found to be very similar to those of 9AA.