Immobilization using Cu(II) and Zr(IV): Persistent and highly efficient activation of galactose oxidase by in-situ generation of hydroxyl radicals in concert with in-situ generation of O2

Abstract

5-Hydroxymethylfurfural (HMF) can be oxidized to 2,5-diformylfuran (DFF) by galactose oxidase (GO) with H2O2 as the byproduct. The active site of GO is composed of the ECu(II)/(Cys-Tyr)• complex. In this work, GO was encapsulated using non-enzymic Cu(II) (NECu(II)) and Zr(IV) in phosphate buffer saline, forming the GO-inorganic hybrid nanoflowers GO@NECu(II)&Zr(IV). Under the synergistic catalysis of coordinated NECu(II)) and Zr(IV), the byproduct H2O2 is decomposed to generate hydroxyl radicals •OH and dioxygen O2. On the basis of ample experimental evidence, it has been discovered that •OH activate the encapsulated GO by reforming ECu(II)/(Cys-Tyr)• (active). The mechanism of •OH activation was studied by quantum mechanical/molecular mechanical calculation. Dioxygen O2, as the co-substrate of encapsulated GO, is used for the O2 reduction half-reaction. The generation of •OH is highly associated with the O2 reduction half-reaction, which provides a source of H2O2. The O2-to-H2O2-to-(•OH + O2) conversion is sustained through iterative use of O2. Under the synergistic catalysis of NECu(II) and Zr(IV), the in-situ generation of •OH is in concert with the in-situ generation of O2. The sustained generations of •OH and O2 in a concerted manner contribute to that, activation of the encapsulated GO is persistent and highly efficient. As a result of the activation, for the conversion of 5-hydroxymethylfurfural (100 mM), the hybrid GO@NECu(II)&Zr(IV) (with 0.75 mg/mL GO encapsulated) achieved a 98.1% conversion of 5-hydroxymethylfurfural within 10 h. At the identical reaction conditions, free GO (0.75 mg/mL) achieved a 5-hydroxymethylfurfural conversion of 22.2%. During the catalysis of the hybrid, it is not necessary to add any exogenous additives. All the conversion reactions were carried out in pure water.