Cooperative Binding to Linear Biopolymers

Abstract

The binding of proflavine to poly(α‐ l ‐glutamic acid) at pH 7.5 has been investigated by means of absorbance measurements at 444 nm. Equilibrium properties were registered as a function of the total dye concentration and the polymer to dye ratio. The results agreed very well with the predictions of a basic model theory of binding to a linear lattice where cooperative interaction is restricted to nearest neighbor binding sites. The parameters g (number of binding sites per amino acid residue), K (cooperative binding constant), and q (factor measuring the strength of cooperativity) could be determined for various concentrations of added KCl electrolyte. While g of approx. 1.1 and q of approx. 200 were found to be little affected by increasing the ionic strength, a considerable decrease of K was observed. The latter is interpreted as the effect of competitive inhibition due to non‐cooperative binding of potassium ions ( K i = 160 M –1 ). Extrapolation to zero electrolyte concentration yields K → K o = 8 × 10 4 M –1 . It is concluded that the binding between the ligands and the polymer is of essentially electrostatic nature. Cooperative interaction, on the other hand, must be attributed to the stacking tendency of neighboring bound dye molecules.