Interaction of surfactant-solubilized monomeric hemin with imidazole in water, cyclohexane, and benzene: effects of aqueous and reversed micelles

Abstract

Hemin is present as monomers in aqueous micellar hexadecyltrimethylammonium bromide (htab), and sodium dodecyl sulphate, Na(ds), as well as in polar solvent pools in cyclohexane and in benzene, solubilized by polyoxyethylene (6) nonylphenol, lgepal CO-530. Interaction of imidazole (im) with thesurfactant-solubilized monomeric hemin results in the equilibrium formation of bis(imidazole)hemin. Equilibrium constants for the formation of this complex, K_app., have been determined in aqueous 0.05 mol dm^–3 htab and 0.50 mol dm^–3 Na(ds) as functions of pH. These data have been rationalized in terms of two kinetically indistinguishable mechanisms: hemin(OH^–, H₂O)₂(S)+ imH++ im [graphic omitted] hemin(im)₂+ 2H₂O; and hemin(H₂O)₂(S)+ 2im [graphic omitted] hemin(im)₂+ 2H₂O. Values of the pH-independent equilibrium constants, K′ and K″, have been calculated in 0.05 mol dm^–3 htab as 1.2 × 10⁵ and 1.0 × 10⁷ and in 0.50 Na(ds) as 1.0 × 10⁶ and 5.0 × 10⁹ dm⁶ mol^–2. These should be compared with values of K′= 8.0 × 10⁶ dm⁶ mol^–2 and K″= 4.0 × 1 0⁷dm⁶ mol^–2 reported in ethanol–water (8.9: 11.1w/w). Equilibrium constants for the formation of bis(imidazofe)hemin have also been determined in methanol and water pools, solubilized by lgepal CO-530 in cyclohexane and in benzene, together with rate constants for the equilibrium attainment of the complex. Evidence has been deduced for the two-step addition of imidazole to hemin. Micellar effects on this reaction are discussed in terms of selective substrate concentration, electrostatic interactions, and alterations of the effective dissociation constants and of the microenvironments of the reactants. The reactions of imidazole and cyanide ions on hemin micellar solutions are compared.