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

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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.

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