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PREBIOTIC CHEMISTRY
The human appetite for understanding the origin of life has led scientists to speculate on how amino and nucleic acids could have formed in the primordial soup of chemicals thought to exist on Earth long ago. Somehow, researchers in this field agree, unreactive nitrogen in the early atmosphere was converted to the more reactive ammonia--before nitrogen-fixing bacteria were around to do the job. Now, chemistry professors Günter Kreisel and Wolfgang Weigand at the University of Jena, in Germany, and coworkers have developed a lab method to prepare NH3 from N2 under conditions that they believe could have existed during the prebiotic era [Angew. Chem. Int. Ed., 42, 1540 (2003)]. They use inorganic FeS as a substitute for the nitrogenase enzyme, made up of an FeS-based protein and an MoFeS-based protein, that bacteria use for the transformation. Several researchers have explored using aqueous and nonaqueous suspensions of iron and other transition-metal compounds to reduce N2, the researchers note. But the aqueous systems require temperatures above 100 °C and pressures above 50 atm. In the University of Jena method, ammonia is produced in water at 70–80 °C and atmospheric pressure. The researchers precipitate the black FeS by reacting FeSO4 with Na2S. The pH of the suspension is lowered by adding H2SO4, which leads to formation of H2S as a hydrogen source. Ultrapure N2 is then bubbled into the freshly prepared FeS, and over a period of weeks N2 is slowly reduced to NH3, driven by oxidation of the sulfide to disulfide, FeS2. The NH3 is recovered as (NH4)2SO4. The low yield of about 0.1% is what might be expected under prebiotic conditions, the researchers note. They speculate that N2 adsorbs on the surface of the FeS particles and is subsequently attacked by hydrogen ions. One of the key observations of the experiments is that freshly prepared FeS is required: When the research team tried commercial or previously prepared FeS, NH3 didn't form. Upon microscopic investigation, the researchers discovered that the freshly precipitated FeS has a very rugged surface compared with older material, and they believe it must contain Fe–S structures that allow N2 binding. "Considering the long reaction times, as well as the large amounts of iron sulfide and H2S available on the primordial Earth, a synthesis of ammonia similar to the one we describe could have contributed significantly to the ammonia budget on the early Earth," the researchers write.
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