| | 1 | Author
| C. Kerst, P. Potzinger, H. Gg, Wagner | Requires 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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| default:Reihe_A/51/ZNA-1996-51a-0105.pdf | | | | Identifier
| ZNA-1996-51a-0105 | | | | Volume
| 51 | |
| 2 | Author
| N. G. Faleev3, S. N. Spirina3, V. S. Ivoilov3, T. V. Dem, R. S. Phillips0 | Requires 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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| default:Reihe_C/51/ZNC-1996-51c-0363.pdf | | | | Identifier
| ZNC-1996-51c-0363 | | | | Volume
| 51 | |
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