Polyanions and their complexes. Part IV. A pulse radiolysis study of the interaction between methylene blue and heparin in aqueous solution

Abstract

A new method is described for investigating ion-binding between polyanions and their associated counterions. The reactions of hydrated electrons (e^–_aq) produced during pulse radiolysis of aqueous solutions are used to examine specifically the interactions between the cationic dye methylene blue (MB⁺) and anionic sites of heparin. The second-order rate constant for the reaction e^–_aq+ MB⁺→ MB has been measured (2·4 × 10¹⁰ mole^–1 sec.^–1). In the presence of heparin and D-glucose (to remove OH radicals), the fast reaction e^–_aq+ MB⁺ is replaced by a considerably slower reaction. Ion-binding at the anionic site of the polyanion, by short-range interaction forces, leads to charge neutralization of the counterion with subsequent unreactivity towards e^–_aq. The agents which destroy the complex and which reverse the metachromatic shift, namely addition of sodium chloride and methanol and increase in temperature, also induce a reversal of the e^–_aq rate, which therefore can be used as a specific criterion for site–dye interaction.