COOPERATIVE BINDING OF QUINACRINE TO INORGANIC POLYPHOSPHATE

The cooperative binding of quinacrine to inorganic polyphosphate in aqueous solutions of different ionic strengths at pH 7 has been investigated by means of absorption and polarized fluorescence spectroscopy. In a wide range of molar phosphate-to-dye ratios, P/D, at a low ionic strength, the quinacrine molecules form aggregates by the chromophore stacking at the linear polyanion. The aggregation is characterized by a bathochromic shift of the visible absorption maxima accompanied by a hypsochromic effect and a nearly total quenching of the quinacrine fluorescence. On monomeric binding, which dominates at P/D greater than or similar to 10(4), the spectroscopic properties of the dye do not change. The parameters of cooperative binding were estimated by Schwarz's method: the number of binding sites per monomeric unity of the polyphosphate g = 0.45, the cooperativity parameter q similar or equal to 130, the cooperative binding constant is dependent on the ionic strength with d log K/d log [Na+]= -1.65.