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'Mechanism' in keywords Facet   Publication Year 1996  [X]
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1Author    C. Kerst, P. Potzinger, H. Gg, WagnerRequires cookie*
 Title    The Mercury-Sensitized Photolysis of Pentamethyldisilane  
 Abstract    Two primary processes were observed in the Hg-sensitized photolysis of Me 5 Si 2 H: (I) hydrogen abstraction from the Si-H bond with a quantum yield of 0(1) = 0.85, (V) Si-Si bond breaking with 0(V) = 0.04. The hydrogen atoms formed in (/) undergo an H atom abstraction reaction (k(3)), as well as substitution reactions at the Si centers resulting in the formation of dimethylsilane and trimethylsilyl radical (k(4)) or trimethylsilane and dimethylsilyl radical (k(5)). The following branching ratios have been determined: M = 0.87. M 0.096, M = 0.034. k(3) + k(4) + k(5) k(3) + k(4) + k(5) k(3) + k(4) + k(5) The ratio of disproportionation (A: (2)) to combination (£(1)) for the pentamethyldisilyl radical has been determined with MeOH as the scavenger for 1-methyl-l-trimethylsilylsilene, 0.046 < k(2)/ A: C1) < 0.071. A mechanism with pertinent rate constants has been proposed which accounts for the results. 
  Reference    Z. Naturforsch. 51a, 105—115 (1996); received October 28 1995 
  Published    1996 
  Keywords    Hg-sensitized photolysis, pentamethyldisilane, mechanism, substitution reaction, silyl rad-ical disproportionation 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0105.pdf 
 Identifier    ZNA-1996-51a-0105 
 Volume    51 
2Author    N. G. Faleev3, S. N. Spirina3, V. S. Ivoilov3, T. V. Dem, R. S. Phillips0Requires cookie*
 Title    The Catalytic Mechanism of Tyrosine Phenol-Lyase from Erwinia herbicola: The Effect of Substrate Structure on pH-Dependence of Kinetic Parameters in the Reactions with Ring-Substituted Tyrosines  
 Abstract    Apparently hom ogeneous tyrosine phenol-lyase (TPL) from Erwinia herbicola has been prepared by a new method. The pH -dependencies o f the main kinetic parameters for the reactions of Erwinia TPL with tyrosine, 2-fluorotyrosine, 3-fluorotyrosine, 2-chlorotyrosine, and 3.4-dihydroxyphenylalanine (D O P A) have been studied. The pattern of pH-dependence ° f V^max depends on the nature of the substituent in the aromatic ring. For the substrates bearing small substituents (H, 2-F, 3-F) Kmax values were found to be pH-independent. For 2-chlorotyrosine and DO PA Vmax decreased at lower pH, the effect being described by equa­ tion with one pKa. Generally two bases are reflected in the pH dependence of Vmax/K m. The first base, probably is responsible for the abstraction of a-proton, while the second one, interacts with the phenolic hydroxyl at the stage of binding. The reaction of TPL with DOPA differs from the reactions with other tyrosines by the requirement of an additional base which is reflected in the pH-profiles of both and VmaJ K m. For the reaction of TPL from Citrobacter intermedins with DO PA only Vma J K m values could be determined. The activity of Citrobacter enzyme towards DO PA is considerably less than that of E. herbicola enzyme, and its maximal value is attained at higher pH. 
  Reference    Z. Naturforsch. 51c, 363 (1996); received N ovem ber 22 1995/February 27 1996 
  Published    1996 
  Keywords    Tyrosine Phenol-Lyase, Mechanism, Kinetics, Substrate Structure, pH -Dependence 
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 TEI-XML for    default:Reihe_C/51/ZNC-1996-51c-0363.pdf 
 Identifier    ZNC-1996-51c-0363 
 Volume    51