Interaction of acridine orange and polyanions: fluorimetric determination of binding strengths and the influence of simple electrolytes

Abstract

Binding affinities of Acridine Orange and six polyanions have been determined by measuring association constants in essentially salt-free solution, and also by observing competition between the dye and a simple salt for the polyanion site. Differences were found in the dye-binding order for the six polyanions using the two techniques. These differences were related to the breakdown in the uniform binding behaviour of carboxy and sulphate groups bound to the same polymer backbone as the salt concentration is increased. A mechanism is proposed which fits the thermodynamic parameters calculated by the application of Schwarz treatment and the Langmuir isotherm. The difference in the free-energy of binding between the strongest binding polymer, polystyrene sulphonate (ΔG°=–41 kJ mol–1) and the weakest binding, hyaluronic acid (ΔG°=–29 kJ mol–1) could be accounted for by considering coulombic site-dye interactions which destabilise the dye aggregates. The main influence promoting dye binding appears to be the free-energy of dye aggregation.The degree of co-operativity of dye binding, q, has also been determined. An attempt has been made to relate this parameter to the physical properties of the polyanion chain. It appears unlikely that q has the significance initially attached to it.