Interactions of acridine orange with double stranded nucleic acids. Spectral and affinity studies

Spectral properties of acridine orange (AO) alone or in complexes with natural and synthetic nucleic acids of various base composition have been studied in aqueous solutions by absorption and fluorescence spectroscopy. The dimerization constant and absorption spectra of the dye in monomeric and dimeric form were established; dimerization of AO resulted in quenching of its fluorescence. Complexes of the dye with synthetic nucleic acids differed in the degree of enhancement of fluorescence quantum yield, varying between 1.42 to 2.38 fold as compared to AO monomer; these differences, however, were not base-dependent. Affinity of the dye to natural and synthetic polymers was studied and analyzed using McGhee-von Hippel model of polymer-ligand interactions. Because the sterical requirement for intercalative binding assumes interaction of dye monomer, the correction for AO dimerization was made in all calculations. All studied DNAs (natural and synthetic ones, the latter being homopolymer pairs or alternating copolymers of A,T or G,C or I,C base composition) had similar intrinsic association constants (KI = 5 X 10(4) - 1 X 10(5), M-1) and binding site size (n = 2.0-2.4 b.p.). The exception was poly(dA).poly(dT), having KI = 1.2 X 10(4) and n = 19.3 b.p. The results of KI measurement for calf thymus DNA and AO in different sodium ion concentration were in good agreement with predictions of the counterion condensation theory. The intercalation of AO into DNA is discussed in view of recent theoretical models of DNA-ligand interactions.