Interaction of phenosafranine with nucleic acids and model polyphosphates. I. Self-aggregation and complex formation...

Aggregation of phenosafranine in concentrated aqueous solutions and interaction with polyphosphates was studied by absorption and fluorescence spectroscopy. At concentrations greater than 10(-3)M phenosafranine forms dimers (Kd = 3.8 x 10(2) 1.mole-1), which are characterized by a hypsochromic shift of the visible and near ultraviolet absorption maxima accompanied by a hypochromic effect. No fluorescence could be detected from phenosafranine dimers. Analogues spectra changes were observed when a polyphosphate was titrated with phenosafranine, which indicated that with increasing saturation of the polyphosphate binding sites phenosafranine gradually became bound in the aggregated form. Full saturation of the polyphosphate binding sites with phenosafranine was reached only when an excess of free dye was present. The cooperative binding of phenosafranine to a polyphosphate could be evaluated by means of a theory proposed by Schwarz et al. At the zero ionic strength and at 25 degrees C the binding was characterized by cooperative binding constant K = 6.2 x 10(5) 1.mole-1, number of binding sites per monomeric phosphate residue g = 0.4, and cooperativity parameter q-30. Spectroscopic properties of phenosafranine in the aggregated and polyphosphate-bound states were compared with those of ethidium bromide.