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 transformation, 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 (DM2+)(2), N,N′-trimethylene-2,2′- bipyridinium (DT2+)(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 CO2 to formate. The quantum yield for formate formation is in the range φ= 0.5–1.6%