Towards mediator design: II. Optimization of mediator global charge for the mediation of glucose oxidase of Aspergillus niger

Abstract

The effect of mediator global charge on the ability of a range of tris (4,4′-substituted-2,2′-bipyridine) complexes of the group VIII metals iron(II), ruthenium(II) and osmium(II) to act in their oxidized form as electron transfer acceptors for glucose oxidase (GOD, EC 1.1.3.4) of Aspergillus niger has been studied. Variation in the mediator global charge was examined by the use of new (4,4′-substituted-2,2′-bipyridine) ligands bearing one or more permanent positive charges in each of their substituted groups. The syntheses of these ligands and complexes are described, as is the electrochemical characterization of the resulting complexes. The rate of electron transfer from reduced glucose oxidase to oxidized mediator was seen to depend on the global mediator charge, with a maximum second-order rate constant being observed for complexes with a 5+ charge. This is interpreted as the manifestation of the anionic nature of the enzyme's active site and a possible selective electrostatic interaction of the cationic mediator with anionic groups therein. In addition, the values of k med obtained from 5+ global charge complexes (usually 10 7 M −1 s −1) were several times greater than the values recorded previously by us (10 6 M −1 s −1) for complexes of similar redox potential and structure but lacking the extra positive charge. This enhanced reaction rate with oxidized mediator decreases the competition for reduced GOD by dissolved oxygen. The 5+ cationic complexes yielded enzyme electrodes with a response less sensitive to the oxygen tension of the surrounding aqueous medium, indicating retention of fast mediation in the adsorbed state; more efficient amperometric glucose biosensors could result from the use of these complexes.