Carbon dioxide, reduction fixation

Table 1 summarizes several redox transformations that can be accomplished in artificial photosynthetic assemblies including the photolysis of water, carbon dioxide reduction, and nitrogen fixation processes. The endoergicities of these transformations, and the number of electrons involved in the reduction processes, are also indicated in the table. It is evident that the energy per electron to drive the various transformations are met by visible light quanta. 

Carbon dioxide reduction to formate in the presence of ForDH represents a photochemical C02-fixation process. The enzyme recognizes in addition to MV " other 2,2-bipyridinium radical cations [82]. Reduction of nitrate to NO2 and reduction of nitrite to ammonia in the presence of NitraR and NitriR, respectively, allows the sequential 8e reduction of nitrate to ammonia. 

In particular, the photo(electro)chemical reduction of carbon dioxide is connected to one of the most important environmental problems necessitating an urgent solution, that is, the fixation of CO2 in the atmosphere. The feasibility of CO2... 

The photoassisted reduction of aqueous carbon dioxide in the presence of inorganic minerals has been examined as a model of prebiological photosynthesis,120 a potential precursor to the photosynthetic fixation of C02 by plants. 

Ozone causes both quantitative and qualitative changes in carbon dioxide fixation patterns. Wilkinson and Bames, using carbon dioxide-found a reduction in radioactivity in soluble sugars and increases in free amino acids and sugar phosphates in white pine after a 10-min exposure to ozone at 0.10 ppm. Miller observed a decrease in carbon dioxide-fixation in ponderosa pines that correlated with loss of chlorophyll, after exposure to ozone at 0.30-0.35 ppm. The Hill reaction rates of chloroplasts isolated from healthy and ozone-injured ponderosa pine indicated that both light and dark reactions of the chloroplasts from ozone-injured plants were depressed. Barnes found depressed photosynthesis and stimulated respiration in seedlings of four pine species of the southeastern United States after exposure to ozone at 0.15 ppm. 

Carbohydrates are formed in green plants by photosynthesis, which is the chemical combination, or fixation, of carbon dioxide and water by utilization of energy from the absorption of visible light. The overall result is the reduction of carbon dioxide to carbohydrate and the formation of oxygen ... 

The insertion of carbon dioxide into a transition metal-hydrogen bond may be seen as the first and crucial step in the reduction or fixation of C02. This insertion could proceed in either of two ways to produce a formate complex, either mono- or bi-dentate [(31) or (32), respectively], or to form a metallocarboxylic acid, (33). 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

Fig. 15.17. Current in milliamperes cm-2 for CO production from C02 using C02+ tetraphenyl porphyrins modified with various pyridyl derivatives. (Reprinted with permission from T. Atoguchi, A. Aramata, and M. Engo, C02 Reduction by Macrocyclic Transition Metal Complex-Modified Electrodes," in Proc. International Symposium on Chemical Fixation of Carbon Dioxide, p. 338, Fig. 5,1991.)...

---