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  • 1
    Electronic Resource
    Electronic Resource
    The Important Role of Active Site Water in the Catalytic Mechanism of Human Carbonic Anhydrase II – A Semiempirical MO Approach to the Hydration of CO2 † (1998)
    Springer
    Journal of molecular modeling 4 (1998), S. 355-365 
    Details
    ISSN: 0948-5023
    Keywords: Keywords Human Carbonic Anhydrase II ; Semiempirical MO Theory ; AM1 ; Enzyme Catalysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The approach of CO2 to a series of active site model complexes of human carbonic anhydrase   II (HCAII) and its catalytic hydration to bicarbonate anion have been investigated using semiempirical MO theory (AM1). The results show that direct nucleophilic attack of zinc-bound hydroxide to the substrate carbon occurs in each model system. Further rearrangement of the bicarbonate complex thus formed via a rotation-like movement of the bicarbonate ligand can only be found in active site model systems that include at least one additional water molecule. Further refinement of the model complex by adding a methanol molecule to mimic Thr-199 makes this process almost activationless. The formation of the final bicarbonate complex by an internal (intramolecular) proton transfer is only possible in the simplest of all model systems, namely {[Im3Zn(OH)]+·CO2}. The energy of activation for this process, however, is 36.8 kcal·mol−1 and thus too high for enzymatic catalysis. Therefore, we conclude that within the limitations of the model systems presented and the level of theory employed, the overall mechanism for the formation of the bicarbonate complex comprises an initial direct nucleophilic attack of zinc-bound hydroxide to carbon dioxide followed by a rotation-like rearrangement of the bicarbonate ligand via a penta-coordinate Zn2+ transition state structure, including the participation of an extra active site water molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s008940050094
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  • 2
    Electronic Resource
    Electronic Resource
    Regio- and Stereoselective Cyclization Reactions of Unsaturated Silyl Enol Ethers by Photoinduced Electron Transfer - Mechanistic Aspects and Synthetic Approach (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1583-1596 
    Details
    ISSN: 1434-193X
    Keywords: Cyclizations ; Electron transfer ; High-pressure effects ; Radical cations ; Silyl enol ethers ; Solvent effects ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidative photoinduced electron transfer (PET) reactions have been performed with various silyl enol ethers and silyloxy-2H-chromones bearing an olefinic or silylacetylenic side chain. The reactions result in regioselective ring closure with the formation of bi- to tetracyclic ring systems with a well-defined ring juncture, e.g. perhydrophenanthrenones 13 or benzo-annellated xanthenones 24. Our investigations have focussed on the optimization of this cyclization method with regard to irradiation time and product yield. The irradiation times could be reduced by using the cosensitized PET method. Modifying the substrate at the silyl group led to enhanced yields. In addition, we found that solvent and pressure dependences are important tools, allowing control of the regiochemistry. Both the synthesis of 6-endo products by radical cationic reaction pathways, as well as 5-exo ring closure by radical intermediates was achieved. Mechanistic details, including findings from deuterium labelling experiments, are discussed.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Kinetic and thermodynamic study of the mechanism of reversible intermolecular electron-transfer reactions between horse heart cytochrome c and a series of cobalt imine complexes (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 39-46 
    Details
    ISSN: 0947-6539
    Keywords: cobalt ; cytochrome c ; electron transfer ; kinetics ; volume profile ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Kinetics and Thermodynamics of the Reversible Outer-Sphere Electron-Transfer Reactions between Horse Heart Cytochrome cII/III and [Co(phen)3]3+/2+ and [Co(bpy)3]3+/2+ were studied in detail, in particular as a function of temperature and pressure. It was possible to construct a volume profile for both reactions from the pressure data. The transition state was found to be halfway between the reactant and product states on a volume basis in all studied systems. This is in agreement with the λ≠ parameter estimated from the Marcus theory. For all the systems investigated, the differences in the activation volumes are in good agreement with the reaction volumes determined from spectrophotometric and electrochemical measurements at elevated pressure, and from the difference in the partial molar volumes of the metal complexes. The activation and reaction volumes of the bipyridine system are significantly smaller than those of the corresponding phenanthroline and terpyridine systems. A detailed mechanistic analysis is presented. The results show that the different kinetic and thermodynamic techniques employed complement one another.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970030108
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  • 4
    Electronic Resource
    Electronic Resource
    REACTIVITY OF NUCLEOPHILES TOWARDS X-3-(p-TOLYLSULFONYL)-1,2,3-BENZOXATHIAZOLE 2,2-DIOXIDES: KINETICS, ACTIVATION VOLUMES AND MECHANISM (1997)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 175-181 
    Details
    ISSN: 0894-3230
    Keywords: cyclic sulfamates ; nucleophiles ; reactivity ; kinetics ; mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The kinetics of the reaction of four X-3-(p-tolylsulfonyl)-1,2,3-benzoxathiazole 2,2-dioxides (X=5-Cl, 5-Br, 5-NO2 and 6-NO2) with hydroxide ion and imidazole in aqueous acetonitrile and aqueous ethanol solutions were investigated at various pressures. The volumes of activation were all found to be negative and consistent with a bimolecular reaction involving considerable solvent electrostriction. No significant dependence on the solvent composition was found. © 1997 by John Wiley & Sons, Ltd.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Electronically Tuned trans Labilization Controls the Substitution Behavior of RhodoximesThe authors acknowledge financial support from the Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, MURST, and the Max Buchner Forschungsstiftung. (1995)
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 34 (1995), S. 2245-2247 
    Details
    ISSN: 0570-0833
    Keywords: kinetics ; reaction mechanisms ; rhodium compounds ; substitutions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/anie.199522451
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