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2001 (1)
1977 (1)
1Author    JohnG. Palmer, PaulA. Doemeny, GerhardN. SchrauzerRequires cookie*
 Title    The Chemical Evolution of a Nitrogenase Model, XXIII. The Nature of the Active Site and the Role of Homocitric Acid in MoFe-Nitrogenase  
 Abstract    The iron-molybdenum cofactor (FeMo-co) of bacterial nitrogenase is a heterometallic cluster of composition MoFe7S9 that is attached to the apoprotein by a coordinative Mo-N bond to the imidazole group of hisa442, and by a Fe-S bond to cysa.215. The molybdenum atom of FeMo-co in the enzyme in addition is coordinated to one molecule of homocitrate (he), which is required for maximal N2 reducing activity. The molybdenum atom in the enzyme-bound FeMo-co thus is hexacoordinated and cannot react with substrates unless free coordination sites are made available. It is proposed that the reactions of the substrates o f nitrogenase occur at a molybdenum active site consisting of a mononuclear molybdenum homocitrate complex attached to hisaAA2 of the apoprotein that in the functional enzyme is generated from FeMo-co by a reversible, redox-linked dissociation of the Fe7S9-c;y5' cluster. Studies with catalytic model systems consisting of complexes of molybdenum with imidazole and hydroxo-carboxylate ligands support this proposal and provide a rationale for the specific activating effect of homocitrate in nitrogenase. 
  Reference    Z. Naturforsch. 56b, 386—393 (2001); received January 17 2001 
  Published    2001 
  Keywords    Nitrogen, Acetylene, Homocitric Acid 
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 TEI-XML for    default:Reihe_B/56/ZNB-2001-56b-0386.pdf 
 Identifier    ZNB-2001-56b-0386 
 Volume    56 
2Author    S.Requires cookie*
 Title    Strahlenchemische Cycloadditionen mit Äthylen und Acetylen (Produktbildung und Lösungsmittelwahl)  
 Abstract    R a d ia tio n Chemical C ycloaddition Reactions w ith Ethylene and Acetylene M a n f r e d J . M i r e a c h 1, M a r l i s F. M i r b a c h 1 u n d A l f o n s S a u The (2 + 2) cycloadditions of maleic acid anhydride (MSA) and 1,3-cyclohexadiene (CHD) to ethylene and acetylene were initiated with gamma-rays in different solvents. The four-membered ring products were formed with high selectivity and surprisingly high energy yields in solvents with triplett energies smaller than ethylene and acetylene but higher than MSA and CH D resp. In benzonitrile as a solvent MSA reacts with ethylene to give cyclobutane-l,2-dicarboxylic acid anhydride (1) (G = 0.04) and with acetylene to give cyclobut-3-ene-l,2-dicarboxylic acid anhydride (2) (G = 3.7). The cyclobutene derivative (2) is formed in >80% yield and the bicyclopropyl derivative (3), which is the main product in the photochemical reaction, with a yield of <20%. 1,3-Cyclohexadiene was also added to ethylene to give bicyclo(4,2,0)oct-2-ene (G = 0.4) and bicyclo(2,2,2)oct-2-ene (G = 0.1) under gamma irradiation in methylnaphthalene as a solvent. Under similar conditions addition to acetylene gave bicyclo(4,2,0)octa-2,7-diene (G = 0.04) and bicyclo(2,2,2)octa-2,5-diene (G = 0.3), respectively. 
  Reference    (Z. Naturforsch. 32b, 281—284 [1977]; eingegangen am 6. Oktober 1976) 
  Published    1977 
  Keywords    Cycloaddition, Ethylene, Acetylene, Maleic Acid Anhydride, 1, 3-Cyclohexadiene 
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 TEI-XML for    default:Reihe_B/32/ZNB-1977-32b-0281.pdf 
 Identifier    ZNB-1977-32b-0281 
 Volume    32