Artificial, Photoinduced Activation of Nitrogenase Using Directed and Mediated Electron Transfer Processes

Light-driven, bias-free nitrogenase-based bioelectrochemical cell for ammonia generation

Nitrogen fixation is a key process that sustains life on Earth. Nitrogenase is the sole enzyme capable of fixing nitrogen under ambient conditions. Extensive research efforts have been dedicated to elucidating the enzyme mechanism and its artificial activation through high applied voltage, photochemistry, or strong reducing agents. Harnessing light irradiation to minimize the required external bias can lower the process's high energy investment. Herein, we present the development of photo-bioelectrochemical cells (PBECs) utilizing BiVO4/CoP or CdS/NiO photoanodes for nitrogenase activation toward N2 fixation. The constructed PBEC based on BiVO4/CoP photoanode requires minimal external bias (200 mV) and suppresses O2 generation that allows efficient activation of the nitrogenase enzyme, using glucose as an electron donor. In a second developed PBEC configuration, CdS/NiO photoanode was used, enabling bias-free activation of the nitrogenase-based cathode to produce 100 μM of ammonia at a faradaic efficiency (FE) of 12%. The ammonia production was determined by a commonly used fluorescence probe and further validated using 1H-NMR spectroscopy. The presented PBECs lay the foundation for biotic-abiotic systems to directly activate enzymes toward value-added chemicals by light-driven reactions.

Analysis of the Ammonia Production Rates by Nitrogenase

Ammonia (NH3) is produced industrially by the Haber–Bosch process from dinitrogen (N2) and dihydrogen (H2) using high temperature and pressure with an iron catalyst. In contrast to the extreme conditions used in the Haber–Bosch process, biology has evolved nitrogenase enzymes, which operate at ambient temperature and pressure. In biological settings, nitrogenase requires large amounts of energy in the form of ATP, using at least 13 GJ ton−1 of ammonia. In 2016, Brown et al. reported ATP-free ammonia production by nitrogenase. This result led to optimism that the energy demands of nitrogenase could be reduced. More recent reports confirmed the ATP-free production of ammonia; however, the rates of reaction are at least an order of magnitude lower. A more detailed understanding of the role of ATP in nitrogenase catalysis is required to develop ATP-free catalytic systems with higher ammonia production rates. Finally, we calculated the theoretical maximal ammonia production rate by nitrogenase and compared it to currently used Haber–Bosch catalysts. Somewhat surprisingly, nitrogenase has a similar theoretical maximum rate to the Haber–Bosch catalysts; however, strategies need to be developed to allow the enzyme to maintain operation at its optimal rate.

Tailoring Quantum Dot Sizes for Optimal Photoinduced Catalytic Activation of Nitrogenase

Many efforts have been directed towards elucidating the nitrogenase structure, its biocatalytic activity, and methods to artificially activate it by external stimuli. Here, we investigated how semiconductor nanoparticles (NPs) with sizes ranging between 2.3-3.5 nm form nano-biohybrids with the nitrogenase enzyme and enable its photoinduced biocatalytic activity. We examined two homogenously synthesized quantum dots (QDs), CdS, CdSe, and two nitrogenase variants, the wild-type and a cysteine-mutated. We show that the cysteine-mutated variant does not enhance the hydrogen generation amounts, as compared with the wild type. Nevertheless, we show that the 2.3 nm-sized CdSe NPs facilitate an eightfold increase compared with larger CdSe NPs. The obtained results were investigated using electrochemical techniques, transmission electron microscopy, and further confirmed by time-resolved spectroscopic measurements, which allow us to determine the electron tranfer rate constant (k_ET ) of the different configurations.