<strong>Enzyme Inspired Artificial Photosynthesis</strong>

Abstract

In natural photosynthesis, green plants and algae have evolved highly delicate, complex structures to convert light to chemical energy. Inside the photosynthetic factories termed chloroplasts, a membrane-bound thylakoid is the site of the light-dependent reactions of photosynthesis. Following light harvesting by chlorophyll molecules, photoinduced electrons are passed through electron transport chain to the enzyme ferredoxin-NADP reductase, leading to the reduction of nicotinamide adenine dinucleotide phosphate (NAD(P)) to NAD(P)H, completing the storage of light energy in one kind of “energy currency” of the cells. Inspired by the photosynthesis, g-C3N4 was employed for NADH regeneration in the presence of [Cp*Rh(bpy)Cl]Cl as an electron and proton mediator. Inspired by the photosynthetic thylakoid membrane of chloroplasts, the rational design of mesoporous structured colloids made up of graphitic carbon nitride nanosheets were synthesized and were applied in photocatalytic NADH regeneration for sustainable enzymatic synthesis. The in situ NADH regeneration rate is high enough to reverse the biological pathway of the three consecutive dehydrogenase enzymes (formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase, respectively), which allows for instance the successive conversion of formaldehyde to methanol and the reduction of carbon dioxide into methanol and other enzymatic reactions. The synthesis sequence developed here thereby might open further possibilities by combining with microfluidics to engineer high efficiency artificial photosynthesis systems in the future. Nitrogen is an essential nutrient for sustaining life and molecular dinitrogen (N2) must be fixed to bio-available ammonia (NH3) or nitrate (NO3-) for metabolism. Nitrogen is “fixed” in nature by nitrogenase enzymes under ambient temperature and pressure. Currently, over half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the widespread use of the industrial Haber-Bosch process, which operates under high temperature (400-500 °C) and high pressure (200-250 bar) in the presence of metallic iron catalyst. Photochemical conversion provides a promising approach to convert nitrogen into ammonia by using solar energy. Previous attempts used titania, Fe2Ti2O7, Ru/Au/Nb-SrTiO3, diamond, BiOBr, but these produced only impractically low efficiencies or abological. Inspired by the structure and function of MoFe protein in binding and reducing N2 also based on our previous works, we are motivated to prepare a MoFe protein mimicked chalcogel that incorporated both FeMoco-like Mo2Fe6S8 and P-cluster-like Fe4S4 clusters linked together with Na4Sn2S6. The resultant chalcogels were performed in photocatalytic N2 fixation and fairly higher N2 reduction. The initial binding of N2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15N2 isotope experiments confirm that the generated NH3 derives from N2. Density functional theory (DFT) electronic structure calculations suggest that the N2 binding is thermodynamically favorable only with the highly reduced active clusters.  The work should shed some light on the ongoing efforts of achieving nitrogen fixation under mild, ambient, and environmentally friendly conditions.