Probing Aspergillus niger glucose oxidase with pentacyanoferrate(III) aza- and thia-complexes

Complexes of pentacyanoferrate(III) and biologically relevant ligands, such as pyridine, pyrazole, imidazole, histidine, and other aza- and thia-heterocycles, were synthesized. Their spectral, electrochemical properties, electron exchange constants, electronic structure parameters, and reactivity with glucose oxidase from Aspergillus niger were determined. The formation of the complexes following ammonia replacement by the ligands was associated with the appearance of a new band of absorbance in the visible spectrum. The constants of the complexes formation calculated at a ligand-pentacyanoferrate(III) concentrations ratio of 10:1, were 7.5 × 10^-5, 7.7 × 10^-5, and 1.8 × 10^-3 s^-1 for benzotriazole, benzimidazole, and aminothiazole ligands, respectively. The complexes showed quasi-reversible redox conversion at a glassy carbon electrode. The redox potential of the complexes spanned the potential range from 70 to 240 mV vs. saturated calomel electrode (SCE) at pH 7.2. For most of the complexes self-exchange constants (k₁₁) were similar to or larger than that of hexacyanoferrate(III) (ferricyanide). The complexes containing pyridine derivatives and thia-heterocyclic ligands held a lower value of k₁₁ than that of ferricyanide. All complexes reacted with reduced glucose oxidase at pH 7.2. The reactivity of the complex containing pyrazole was the largest in comparison to the rest of the complexes. Correlations between the complexes' reactivity and both the free energy of reaction and k₁₁ shows that the reactivity of pentacyanoferrates obeys the principles of Marcus's electron transfer theory. The obtained data suggest that large negative charges of the complexes decrease their reactivity.