Cooperative Binding to Linear Biopolymers

Abstract

The binding of acridine orange to poly(α‐ l ‐glutamic acid) at pH 7.5 has been investigated by means of absorbance measurements at 492 nm. The equilibrium as well as chemical relaxation behavior is interpreted in the light of a basic model theory of cooperative binding. Binding and cooperativity are found to be stronger than in the previously studied case of proflavine. In principle, the static properties are analogous in both cases. The only distinct contrast is given by the fact that the degree of cooperativity is considerably affected by ionic strength and polymer to dye ratio. This is attributed to the effect of unfavorable electrostatic interactions which tend to prevent long aggregates of bound dye. Temperature jump measurements resulted in relaxation curves which agreed very well with the predictions of the theory. Mean relaxation times of cooperative binding could be determined and evaluated in terms of an effective rate constant for recombination of free dye with bound aggregates of dye. The effect of varying the concentration of added KCl can be attributed to blocking of reaction sites by bound K + ions. Extrapolation to zero KCl concentration yields a diffusion controlled rate constant for cooperative binding of monomer dye ( k R o is approx. 1.5 × 10 9 M –1 sec –1 ). The life time of terminal elements of bound aggregates of dye turns out to be about 400 μsec.