Dynamics of the binding of acridine dyes to DNA investigated by triplet excited state probe techniques

The binding of the polynuclear aromatic dyes acridine orange (AO) and proflavin (PF) to DNA in aqueous phosphate buffer solution at 25 ± 1°C has been studied by measuring the properties of the triplet excited states of these dyes. The triplet lifetimes can be measured either by triplet-triplet absorption flash photolysis techniques or by delayed fluorescence methods. The triplet lifetimes of AO vary from about 0.5 ms with no DNA present to 20-35 ms at DNA concentration above 10^-3 M expressed in concentration of DNA phosphate [P], or at [P]/[D] ratios above 1000 ([D] is the dye concentration). At all DNA concentrations the decay profiles are exponential, except at high excitation intensities where nonexponentialities, attributed to triplet-triplet annihilation, become apparent. Similar results are observed with PF-DNA solutions. The exponentiality of the triplet decay at all DNA concentrations is attributed to rapid association and dissociation of the dye-DNA complexes on the time scales of the triplet lifetimes. A simplified one-step binding model is utilized to describe this effect. A dissociation rate of AO-DNA complexes ≳ 10³ s^-1 has been estimated from these results. It is shown that a detailed study of the triplet lifetime vs. DNA concentration provides a novel method for the estimation of the apparent equilibrium association constant K* for dye molecules in the triplet excited state and DNA. For AO, K* ≈ 10⁵ M^-1, while for PF it is ≈ 3 × 10⁴ M^-1. These values are of the same order of magnitude as the ground-state dye-DNA equilibrium association constants measured by others.