Electrocatalytic Volleyball: Rapid Nanoconfined Nicotinamide Cycling for Organic Synthesis in Electrode Pores

Abstract

In living cells, redox chains rely on nanoconfinement using tiny enclosures, such as the mitochondrial matrix or chloroplast stroma, to concentrate enzymes and limit distances that nicotinamide cofactors and other metabolites must diffuse. In a chemical analogue exploiting this principle, nicotinamide adenine dinucleotide phosphate (NADPH) and NADP + are cycled rapidly between ferredoxin–NADP + reductase and a second enzyme—the pairs being juxtaposed within the 5–100 nm scale pores of an indium tin oxide electrode. The resulting electrode material, denoted (FNR+E2)@ITO/support, can drive and exploit a potentially large number of enzyme‐catalysed reactions.

Electrified Nanoconfined Biocatalysis with Rapid Cofactor Recycling

“…The area under these peaks is proportional to the quantity of FNR undergoing direct electron exchange with the electrode, which is typically 100 −200 pmolcm ‐2 geometric area. An ideal planar monolayer formed by close‐packed spheres with a diameter equal to the globular diameter of FNR (6 nm) would correspond to a coverage of less than 5 pmolcm ‐2 ; therefore the much higher coverages observed show that FNR molecules must adsorb quite deeply into the porous layer . The peak currents increase directly with scan rate, as expected for a thin, non‐diffusing layer, and the narrow half‐height width of both peaks is consistent with each FNR undergoing a cooperative two‐electron exchange process via a one‐electron radical intermediate – the overall population of FNR molecules throughout the film experiencing a surprisingly uniform environment.…”

“…The initial electrode (FNR@ITO/graphite) has only FNR pre‐loaded, and the reaction is initiated by injecting ADH (=E2) into the cell solution, all other components being present. The current (rate) climbs from zero as the ADH enters the pores of the ITO layer to couple with the FNR that is already present . A complete buffer exchange performed once the current has reached a maximum value removes all enzyme from solution and restores the initial conditions of high reactant concentration and zero product: the slight increase being mainly due to a decrease in product inhibition.…”

“…This potent combination leads to a colossal amplification of rate. The ‘electrochemical leaf’, a hybrid bio‐nanomaterial technology, exploits this principle, mimicking nature to provide a novel and highly efficient way to investigate and perform biocatalysis with fine control and continuous live monitoring …”

“…The high occupancy of enzyme molecules with strong affinity for NADP(H) results in the concurrent nanoconfinement of nicotinamide cofactor molecules. Reactions continue for several minutes or more even after NADP(H) is removed from solution . In effect, the (FNR+E2)@ITO electrode behaves like a sponge, soaking up and retaining NADP(H) molecules which are then concentrated relative to the bulk solution .…”

“…The 'electrochemical leaf', a hybrid bio-nanomaterial technology, exploits this principle, mimicking nature to provide a novel and highly efficient way to investigate and perform biocatalysis with fine control and continuous live monitoring. [7][8][9] The central concept is that two or more different enzymes, one of which can feed electrons back and forth under the external control of an applied potential, work synergistically while confined within the nanoporous structure of an electronically conducting metal oxide material. The latter is presented as a 1-3 μm deep film [9] that is formed easily and cheaply by electrophoretic deposition of metal oxide nanoparticles (typically indium tin oxide -ITO) onto a conducting support such as titanium, ITO glass or graphite.…”