Photochemical and Enzymatic Synthesis of Malic Acid from Pyruvic Acid and HCO3 with Combination System of Zinc Chlorin-e6 and Malic Enzyme in Aqueous Medium

CO2 fi xation is a potential technology for the realization of photocatalytic CO2 reduction and synthesis of organic compounds from CO2 as a starting material. Many studies of CO2 fi xation or utilization have investigated ultra-visible light induced photocatalysis on semiconductors such as titanium dioxide1), silicon carbide1) and strontium titanate2) in heterogenous aqueous media. Biological CO2 fi xation or utilization systems have also received much attention, because organic compounds can be synthesized in homogenous aqueous media by using water soluble enzymes with CO2 fi xation function. For example, CO2 or HCO3 can be reduced to formic acid with formate dehydrogenase (FDH) and NADH. Therefore, a CO2 fi xation or utilization system that combines the photoreduction of NAD+ with photosensitizer and ferredoxin_NADP+_reductase (FNR), and HCO3 reduction with FDH can be established. We previously reported visible-light induced enzymatic formic acid synthesis from CO2 or HCO3 with FDH using the photosensitization of water-soluble zinc porphyrin and chlorophyll3)6). CO2 fixation based on malic acid synthesis combined with the photoreduction of NAD+ by the photosensitization of ruthenium(II) polypyridyl complex or chemically modified chlorophyll and ferredoxin_NADP+_reductase (FNR), and malic acid synthesis from pyruvate and HCO3 with malic enzyme (ME) have also been reported7),8) . In this system, the carbon chain is extended from C3 (pyruvate) to C4 (malic acid) by photochemical and enzymatic methods in the aqueous medium. To further improve the reaction effi ciency using ruthenium(II) polypyridyl complexes, near-infrared absorption must be improved, which is weak because of the small extinction coeffi cient of such ruthenium(II) complexes above 600 nm. Chlorophylls, which act as the effective photosensitizer in the photosynthesis reaction9), and derivatives such as chlorin-e6 have absorption bands in the visible region. Thus, chlorophylls have high potential as visible light photosensitizers. Chlorophyll molecules are insoluble in aqueous media, but chlorin-e6 (Chl-e6) is formed by the hydrolysis of chlorophyll-a and contains three carboxylate groups in its molecule. Water-soluble zinc porphyrins also have absorption bands in the visible light region (380-600 nm), and are widely used as effective photosensitizers10). Zinc tetraphenylporphyrin tetrasulfonate (ZnTPPS) and zinc tetrakis(4-methylpyridyl)porphyrin (ZnTMPyP) are 272 Journal of the Japan Petroleum Institute, 50, (5), 272-277 (2007)