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Tribology and Lubrication Technology April 2017 : Page 15

In the first of two studies, Hu found that the key component in the nitrogenase of Azotobacter vine-landii that is responsible for con-verting carbon dioxide to carbon monoxide is the iron-protein asso-ciated, iron-sulfur (Fe 4 S 4 ) cluster. The capability of this iron protein to reduce carbon dioxide to carbon monoxide was first evaluated in an in vitro study. Hu says, “We used an in vitro strategy to find a chemical ap-proach for putting the iron-sulfur clusters into the specific redox state to convert carbon dioxide to carbon monoxide. This included using an oxidizing agent such as indigo disulfonate and a reducing agent such as dithionite to deter-mine the proper oxidation state for the iron-sulfur clusters.” Further analysis using electron paramagnetic resonance (EPR) was effective because the iron-sulfur clusters have free, unpaired elec-trons. Hu says, “EPR enabled us to determine the optimum oxidation state enabling the iron-sulfur clus-ters to donate electrons to facilitate the reduction of carbon dioxide.” The researchers also deter-mined that the reduction of carbon dioxide occurred more effectively in vivo than in vitro . Hu says, “We believe that the anaerobic environment of Azotobacter vinelandii provides more ideal reducing conditions. The presence of oxygen in vitro appears to hinder the formation of carbon monoxide.” The second study focused on evalu-ating the conversion of carbon mon-oxide to simple hydrocarbons by the vanadium nitrogenase of Azotobacter vinelandii . Growing the bacterium at ambient temperature was initially done in the presence of ammonia to suppress nitrogen fixation and encourage bio-mass formation. At this point, ammonia was re-moved and carbon monoxide added. The resulting nitrogenase converted carbon monoxide to ethylene, ethane and propane over an eight-hour period at ambient temperature. WWW .S TLE. OR G Figure 3 | The vanadium nitrogenase pres-ent in the bacterium Azotobacter vinelandii is an enzyme responsible for converting car-bon dioxide to simple hydrocarbons such as ethylene in a pathway where carbon monox-ide is an intermediate. (Figure courtesy of the University of California, Irvine.) oxide as a potential hazard.” Figure 3 shows a schematic model of the vanadium nitroge-nase, which is the enzyme that the researchers mainly worked with in these studies. Ribbe says, “We believe that the Azotobacter vinelandii process has the potential to generate hydrocar-bons in an analogous manner to the Fischer-Tropsch reaction. Using the nitrogenase is an advantage because the process is run at room tempera-ture and does not require the use of hydrogen. But our process is not as efficient and does not generate higher molecular weight hydrocar-bons that could be used in fuels or lubricant base stocks.” Future work will be focused on improving the efficiency and investigating if the enzyme can be modified to produce a single hydro-carbon instead of a mixture. Addi-tional information can be found in references to the two studies 2,3 or by contacting Hu at or Ribbe at REFERENCES 1. Canter, N. (2017), “Biofuel production using syngas fermen-tation,” TLT, 73 (1), pp. 14-15. 2. Rebelein, J., Stiebritz, M., Lee, C. and Hu, Y. (2017), “Activation and reduction of carbon dioxide by nitrogenase iron proteins,” Nature Chemical Biology , 13 , pp. 147-149. 3. Rebelein, J., Lee, C., Hu, Y. and Ribbe, M. (2016), “The in vivo hydrocarbon formation by vanadium nitrogenase follows a secondary metabolic pathway,” Nature Communications , 7 , Article # 13641. Ribbe says, “But after increasing the concentration of carbon monoxide above 15% of the gas phase or doing further incubation, hydrocarbon for-mation plateaued. We decided to stop the process by exposing the bacterium to air, which enabled the cells to ‘relax.’ After reintroducing carbon monoxide, hydrocarbon formation resumed at a comparable rate to when the process was initiated.” The researchers determined that hydrocarbon formation is a secondary metabolic pathway that is not required for cell growth. Ribbe says, “We pro-pose that this pathway was used in the past by microbes when the earth’s at-mosphere had a much higher concen-tration of carbon dioxide and is now used as a way to eliminate carbon mon-TRIBOL OG Y & L UBRIC A TION TE CHNOL OG Y Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at APRIL 2 017 • 15

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