Aerosol-OT and Igepal Water-in-Oil Microemulsions: Compartmentation and Properties of Methemoglobin

Abstract

We report static fluorescence quenching experiments in the ionic AOT and the nonionic Igepal water-in-oil microemulsions and show that compartmentation, which is well known in the AOT system, is also effective in Igepal/cyclohexane, but not in water-rich Igepal/n-hexane microemulsions. We investigate the reactions of metal-ion binding to murexide, using the pressure jump relaxation method. Strong interactions with the ionic surfactant decrease the ligand binding strength in the AOT microemulsion and droplet collision determines the kinetics when the reaction is fast. No interactions and no rate limiting steps are observed in the Igepal microemulsions. We further investigate the reactions of the solubilized ferriheme proteins methemoglobin and metmyoglobin with the ionic ligands azide and fluoride. In Igepal microemulsions the proteins behave similarly to the aqueous solution. In the AOT system they do not have their native form and show optical absorption spectra dependent on the size of the water pool. This may be interpreted as an AOT induced shift of the monomer-dimer equilibrium of hemin adsorbed at the denatured protein.