NAVEL REDOX SURFACTANTS AND THEIR INTERACTIONS WITH GLUCOSE-OXIDASE OF ASPERGILLUS-NIGER

A number of novel redox surfactants (based on mixed bipyridine/dipyridylamine complexes of osmium (II) where the dipyridylamine ligand bears a saturated C-8, C-10, C-12, C-14, or C-16 alkyl chain) were synthesized and characterized electrochemically and biochemically as mediators for glucose oxidase (EC 1.1.3.4, GOD) of Aspergillus niger. These compounds exhibited critical micelle concentrations (CMCs) in phosphate-buffered saline solution (pH 7.4) in the range 10(-4) to 10(-3) M, the value decreasing with increasing chain length. Dependence of a number of properties (speed of mediation, redox potential, denaturing action on the enzyme, adsorption on an electrode surface) on the length of the mediator alkyl chain was observed. The presence of an alkyl chain decreased the rate of mediation relative to otherwise similar nonsurfactant mediators, and the longer the alkyl chain, the slower the rate of mediation, For each compound, mediation above the CMC was about tenfold slower than that observed below the CMC. However, for the case of mediator adsorbed on an electrode surface with GOD, longer chains gave increased physisorption of mixed micelles of enzyme and mediator. The compounds were incidentally found to inhibit the glucose oxidase activity of GOD in a complex manner; inhibition increased with increasing chain length and the deactivation, for any given compound, was more pronounced below the CMC than above. Glucose oxidase activity assays and study of the action of surfactants and mediators on the fluorescent properties of carboxy-fluorescein-labeled GOD led to the consideration of a model for redox surfactant-GOD interaction where three mechanisms may operate: first, a selective interaction of mediators with the GOD active site; second, a nondenaturing association of short-chain (< C-14) cationic surfactants; and third, a denaturing action of long-chain (greater than or equal to C-14) cationic surfactants. (C) 1994 John Wiley and Sons, Inc.