The role of co-operativity in determining the binding behaviour of hetero-charged polyanions with metachromatic dyes

The binding of acridine orange to carboxyl and sulfate containing polyanions has been investigated by fluorescence spectroscopy. Particular attention has been given to solutions containing both carboxyl and sulfate groups in two different situations, first where both types of site are present attached to the same polymer backbone in a regular repeating structure (heparin), and secondly where individual polymer chains carry exclusively one type of site (hyaluronic acid/K‐carrageenan mixtures). Greatly differing behaviour of the binding profiles as the concentrations of components were reduced have been explained in terms of the strong co‐operativity of the dye‐binding process. The carboxyl and sulfate sites of heparin appear to be equivalent in binding strength, while those of the polycarboxylate polysulfate mixture are no longer equivalent, binding profiles being dominated by the strong binding sulfate group.