Metal-Organic Framework-functionalized Electrodes for NADH Cofactor Regeneration

Overview & Synopsis

Enzymatic electrocatalysis combines the advantage of regio- and stereoselective biocatalysts with the sustainability of electrocatalysis, making it a green and efficient catalytic technology with great potential in the synthesis of high-value chemicals. Enzymes, which are used in biological processes, can be classified into six main categories, oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Among these enzymes, oxidoreductases are widely used in pharmaceutical synthesis, bioenergy transformation, and the crafting of fine chemical products. Oxidoreductases are enzymes that specifically catalyze oxidation-reduction reactions within biological systems. In a typical oxidoreductase electrocatalysis reaction, NAD-dependent dehydrogenases will act as a catalyst to transfer hydride between NADH and substrates. Nevertheless, most NAD-dependent dehydrogenases will continuously consume NADH during reactions, limiting their widespread industrial application. Given the high cost of cofactor, its effective and selective regeneration are very important for the viability of enzymatic processes. To address this challenge, we immobilized [Rh(Cp*)(bpy)Cl]⁺ onto the Zr₆ nodes of NU-1000 that was in situ grown on the surface of graphite felt, using solvent-assisted ligand incorporation, and applied it in a flow bioreactor. Then, we employed a gas diffusion electrode to oxidize H₂, providing a clean proton source for the electrochemical regeneration of NADH. In enzymatic electrocatalytic synthesis using L-lactate dehydrogenase (LDH) as a model reaction, the system achieved total turnover numbers of 19600 and 1750 for [Rh(Cp*)(bpy)Cl]⁺ and NAD⁺, respectively, after 48 hours, with turnover frequencies reaching 2350 h⁻¹ and 210 h⁻¹, demonstrating excellent catalytic performance. Additionally, this study also explores the development of electrodes for the selective regeneration of NADH driven by H₂. Among the materials evaluated, Pt-NU-1000-GF electrode shows exceptional regeneration efficiency, being 49 times higher than that of the Pt-GF electrode, with selectivity exceeding 99%. Mechanistic studies revealed that the Pt nanoparticles lowered the surface potential, enabling the efficient transfer of electrons to Rh complexes and facilitating the production of fully active NADH. The electrode also maintained 72% of its activity after 5 runs, indicating impressive stability for practical applications. Furthermore, the research investigated the potential for simultaneous NAD⁺ regeneration and H₂ production using the Pt-Rh-NU-1000. Although NAD⁺ regeneration improved under acidic conditions, H₂ production was limited due to insufficient catalyst activity and possible side reactions. These findings highlight the challenges and opportunities in developing efficient systems for sustainable energy and chemical processes. Additionally, efforts also were made to increase the pore size of MOFs for the co-immobilization of cobaltocene and enzymes within the MOF, aiming to reduce mass transfer distances and further enhance the efficiency of NADH regeneration. In summary, this study explores the complex interactions within the developed catalytic systems and lays the groundwork for future research in bioconversion technologies and renewable energy applications, emphasizing the critical role of effective cofactor management in enzymatic processes and showcasing the sustainable prospects of hydrogen catalysis for industrial applications.