Towards mediator design: Characterization of tris-(4,4′-substituted-2,2′-bipyridine) complexes of iron(II), ruthenium(II) and osmium(II) as mediators for glucose oxidase of Aspergillus niger and other redox proteins

Abstract

A range of novel tris-(4,4′-substituted-2,2′-bipyridine) complexes of the group VIII metals iron(II), ruthenium(II) and osmium(II) have been synthesized and characterized electrochemically with respect to their ability to act as electron-transfer mediators for redox enzymes, notably glucose oxidase (GOD, EC 1.1.3.4) of Aspergillus niger. The complexes present a wide range of redox potentials (−325 to +610 mV in phosphate-buffered saline solution relative to the standard calomel electrode) and high second-order rate constants k med for electron transfer from reduced glucose oxidase (to approx. 10 7 M −1 s −1) as determined by an electrochemical method. These rate constants were treated following Marcus theory to yield a value for the reorganization energy λ for the mediation reaction of 0.70 eV. Complexes bearing amino substituents gave particularly high rate constants with GOD, suggesting a specific interaction with the enzyme's active site. The tris-(4,4′-dimethoxy-2,2′-bipyridine) complex of osmium is a versatile mediator, presenting high rate constants k med for a variety of unrelated redox proteins. Enzyme electrodes, formed by coadsorption of a mediator with a redox enzyme on the surface of graphite rods, responded chronoamperometrically to the addition of substrate to the surrounding phosphate- buffered saline. The family of mediators may be useful for redox protein electron-transfer studies and for application in amperometric biosensors. By appropriate choice of the 4,4′ substituent, the physicochemical and electrochemical properties of a complex could be tuned to suit the intended redox protein.