Fluorescence and Optical Activity of Acridine Dyes Bound to Poly‐A and DNA

Abstract

AbstractComplexes of acridine orange (AO) and 9‐aminoacridine (9‐AA) with poly‐A and DNA were studied by absorption and fluorescence spectroscopy, circular dichroism and circular polarization of fluorescence. The nucleotide:dye ratio (P/D) ranged from 20–2. The dye in the AO:poly‐A complex is predominantly in the dimeric form. The optical activity of the monomer dye in the excited state is very small, while that of the dimeric dye is relatively large and its dependence on wavelength and P/D indicates the presence of higher aggregates. At low P/D ratios some transfer of electronic excitation energy from dimer to monomer takes place. The spectral properties of the dye in AO:DNA complexes are different at the P/D ratios of 15 and 3, reflecting different modes of complexing. In contrast to the AO:poly‐A complexes, the optical activity of the AO:DNA complexes in the ground and in the excited states are of comparable magnitude; little change thus seems to take place in the interaction between dye and polymer upon electronic excitation of the dye in AO:DNA. In the 9‐AA:poly‐A complex the dye does not tend to dimerize when in the ground state. Excimer emission was, however, observed; dimers and higher aggregates of high optical activity form upon electronic excitation. In the complex of proflavine:DNA, the bound dye shows appreciable optical activity when in the ground state but is optically inactive when in the excited state. The mode of interaction between this dye and DNA thus changes appreciably upon electronic excitation. The occurrence of two electronic transitions in the last absorption band of acridine dyes and its implication on the interpretation of the corresponding CD spectra is discussed.