- Biooxidation of bridged cycloketones using Baeyer-Villiger monooxygenases of various bacterial origin
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(Chemical Equation Presented) Bridged cycloketones were synthesized and utilized as substrates to study biooxidations mediated by Baeyer-Villiger monooxygenases (BVMO) of various bacterial origin. The required enzymes were heterologously produced by recombinant overexpression systems based on Escherichia coli to enable facile recycling of the required nicotinamide cofactors during the whole-cell biotransformations. Ketone precursors of various structural demands were chosen to evaluate steric limitations and flexibility of the active site of BVMOs. By desymmetrization of the prochiral substrates, four to six stereogenic centers were generated within a single biooxidation step. The enzyme library investigated in this study allowed access to antipodal lactone products with excellent enantioselectivity in several cases. Together with a distinct substrate acceptance profile, the recently proposed classification into two groups of cycloketone converting BVMOs was supported by the biotransformation results obtained within this study.
- Snajdrova, Radka,Braun, Ingbert,Bach, Thorsten,Mereiter, Kurt,Mihovilovic, Marko D.
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- Long-Range Photoinduced Through-Bond Electron Transfer and Radiative Recombination via Rigid Nonconjugated Bridges: Distance and Solvent Dependence
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A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthalene donor (D) and a 1,1-dicyanoethylene acceptor (A) interconnected by five different, rigid, nonconjugated bridges.The length of bridges varies with increments of two ?-bonds from four in 1(4) to 12 ?-bonds in 1(12), to provide donor-acceptor center-to-center separations (Rc) ranging from 7.0-14.9 Angstroem.In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer.The rate constant (ket) shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from >1011 s-1 for a four-bond separation to ca.4 x 108 s-1 for a 12-bond separation.In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10- and 12-bond separations, while it is not significantly decreased for the shorter homologues.Therefore the absence of electron transfer at 10- and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect.In solvents where electron transfer is thermodinamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridges were used or intermolecular electron transfer was studied.Through-bond interaction involving ?/? interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy).The observation of intramolecular charge-transfer absorption and emission for 1(4), 1(6), and 1(8) confirms the operation of such through-bond interaction.Rigid system like 1 can therefore not only provide more insight into the thermodynamics of electro transfer and its solvent dependence but especially also into the role of the nature of the coupling of donor and acceptor.The latter is of crucial importance for a better understanding of the factors governing the rates of electron transfer between redox centers in, e.g., biologically important redox proteins involved in photosynthesis and in the respiratory cycle.It is shown that the near-independence of the photoassisted charge separation dynamics on solvent polarity and the overall free-energy change (even for calculated variation of ca. 0.9 eV) is consistent with predictions of electron-transfer theory on the assumption that the solvent is a continuous dielectric.It is also shown how the parameters entering into the theoretical expressions (in particular, the intramolecular reorganization energy) may be correlated with those obtained from radiative transitions (e.g., charge recombination fluorescence).The dependence of the effective electron interaction element (J), which couples donor and acceptor, on the bridge length is discussed.
- Oevering, Henk,Paddon-Row, Michael N.,Heppener, Marc,Oliver, Anna M.,Cotsaris, E.,et al.
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p. 3258 - 3269
(2007/10/02)
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- Steric Effects on Reaction Rates - III. Application of Force-Field Calculations to Chromic Acid Oxidation of Alcohols
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The rates of oxidation with chromic acid of 15 bi- and polycyclic secondary alcohols have been measured and correlated with strain changes calculated by the MM1-program between the alcohols and the corresponding ketones.A correlation of the same quality is obtained upon representation of OH-strain by CH3-strain.The significance of the correlations with respect to the oxidation mechanism as well as the limitations of the applicability of force-field calculations to reactivity problems are discussed.
- Mueller, Paul,Blanc, Jacky,Lenoir, Dieter
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p. 1212 - 1220
(2007/10/02)
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