Change of the Binding Mode of the DNA/Proflavine System Induced by Ethanol

The equilibria and kinetics of the binding of proflavine to poly(dG-dC)poly(dG-dC) and poly(dA-dT)poly(dA-dT) were investigated in ethanol/water mixtures using spectrophotometric, circular dichroism, viscometric, and T-jump methods. All methods concur in showing that two modes of interaction are operative: intercalation and surface binding. The latter mode is favored by increasing ethanol and/or the proflavine content. Both static and kinetic experiments show that, concerning the poly(dG-dC)poly(dG-dC)/proflavine system, intercalation largely prevails up to 20% EtOH. For higher EtOH levels surface binding becomes dominant. Concerning the poly(dA-dT)poly(dA-dT)/proflavine system, melting experiments show that addition of proflavine stabilizes the double stranded structure, but the effect is reduced in the presence of EtOH. The Delta H degrees and Delta S degrees values of the melting process, measured at different concentrations of added proflavine, are linearly correlated, revealing the presence of the enthalpy-entropy compensation phenomenon (EEC). The nonmonotctnicity of the "entropic term" of the EEC reveals the transition between the two binding modes. T-jump experiments show two relaxation effects, but at the highest levels of Etat (>25%) the kinetic curves become monophasic, confirming the prevalence of the surface complex. A branched mechanism is proposed where diffusion controlled formation of a precursor complex occurs in the early stage of the binding process. This evolves toward the surface and/or the intercalated complex according to two rate-determining parallel steps. CD spectra suggest that, in the surface complex, proflavine is bound to DNA in the form of an aggregate.