Photochemical fixation of carbon dioxide: Enzymic photosynthesis of malic, aspartic, isocitric, and formic acids in artificial media

Photosensitized regeneration of 1,4-dihydronicotinamide adenine dinucleotide phosphate (NADPH) with an artificial photosystem allows the enzymic fixation of CO₂ through carboxylation of α-oxo acids using sacrificial electron donors. Pyruvic acid is carboxylated to malic acid and α-oxoglutaric acid is carboxylated to isocitric acid with the malic enzyme and isocitrate dehydrogenase (ICDH) as biocatalysts, φ = 1.9%. Malic acid formed through the photosensitized process is used as a synthetic building block for subsequent sesquestered enzymic transformations, and its conversion into aspartic acid is accomplished with fumarase and aspartase as biocatalysts. Photoreduction of CO₂ to formate is accomplished in the presence of formate dehydrogenase (FDH) as catalyst. Photosensitized reduction of different bipyridinium relay systems, i.e. N,N′-dimethyl-4,4′-bipyridinium (MV²⁺) (1), N,N′-dimethyl-2,2′-bipyridinium (DM ²⁺) (2), N,N′-trimethylene-2,2′-bipyridinium (DT ²⁺) (3), and N,N′-tetramethylene-2,2′-bipyridinium (DQ²⁺) (4), to the corresponding radical cations yields reduced relays that act as cofactors for FDH, which mediates the reduction of CO ₂ to formate. The quantum yield for formate formation is in the range φ = 0.5-1.6%.