Spectrophotometric kinetic study and analytical implications of the glucose oxidase-catalyzed reduction of [MIII(LL)2Cl2]+ complexes by D-glucose (M = Os and Ru, LL = 2,2'-bipyridine and 1,10-phenanthroline type ligands).

Abstract

Glucose oxidase-catalyzed reduction of cis-[MIII(LL)2Cl2]+ (M = Os and Ru) complexes to cis-[MII(LL)2Cl2] (LL = 2,2'-bipyridine and 1,10-phenanthroline type ligands) by D-glucose is a first-order process in the complex and the enzyme in aqueous buffered solution. The reaction follows Michaelis-Menten kinetics in D-glucose and the rate is independent of D-glucose concentration above 0.03 M. The reactivity decreases in the series [Ru(bpy)2Cl2]+ > [Os(phen)2Cl2]+ > [Os(4,4'-Me2bpy)2Cl2]+ > [Os(4,7-Me2phen)2Cl2]+. The measured second-order rate constant for the oxidation of reduced glucose oxidase by [Os(phen)2Cl2]+ in air equals 1.2 x 10(5) M-1 s-1 at pH 6,7, [D-glucose] 0.05 M, and 25 degrees C, which is ca. 20% less than that when the reaction solutions are purged with argon. In the case of [Ru(bpy)2Cl2]+ the rate constant equals 1.8 x 10(5) M-1 s-1 under similar conditions in air, showing higher reactivity of Ru complexes compared with Os ones. The reduction is pH-dependent with a maximum around 7. Added for solubilization of poorly soluble metal complexes, surfactants decrease the rates of the enzymatic reaction. The retardation effect increases in the series: cetyltrimethylammonium bromide < Triton X-100 << sodium dodecyl sulfate, i.e. on going from positively charged to neutral and then to negatively charged surfactants. The behavior of the OsIII and RuIII complexes toward reduced glucose oxidase contrasts to that of recently studied ferricenium cations. As opposed to the latter, the former do not show kinetically meaningful binding with the enzyme, and the Michaelis kinetics typical of the ferricenium case is not realized for the OsIII, and RuIII species. The systems OsIII- or RuIII-glucose oxidase are convenient for routine "one pot" spectrophotometric monitoring of the D-glucose content in samples, since the metal reduction to MII is accompanied by a strong increase in absorbance in the visible spectral region.