Abstract
An electrochemically driven nicotinamide recycling system, referred to as the 'electrochemical leaf' has unique attributes that may suit it to the small-scale industrial synthesis of high-value chemicals. A complete enzyme cascade can be immobilized within the channels of a nanoporous electrode, allowing complex reactions to be energized, controlled and monitored continuously in real time. The electrode is easily prepared by depositing commercially available indium tin oxide (ITO) nanoparticles on a Ti support, resulting in a network of nanopores into which enzymes enter and bind. One of the enzymes is the photosynthetic flavoenzyme, ferredoxin NADP+ reductase (FNR), which catalyzes the quasi-reversible electrochemical recycling of NADP(H) and serves as the transducer. The second enzyme is any NADP(H)-dependent dehydrogenase of choice, and further enzymes can be added to build elaborate cascades that are driven in either oxidation or reduction directions through the rapid recycling of NADP(H) within the pores. In this Article, we describe the measurement of key enzyme/cofactor parameters and an essentially linear scale-up from an analytical scale 4 mL reactor with a 14 cm2 electrode to a 500 mL reactor with a 500 cm2 electrode. We discuss the advantages (energization, continuous monitoring that can be linked to a computer, natural enzyme immobilization, low costs of electrodes and low cofactor requirements) and challenges to be addressed (optimizing minimal use of enzyme applied to the electrode).