Faculté des sciences

Création de métalloenzymes artificielles basées sur la technologie biotine-avidine pour la réduction asymétrique de dérivés carbonylés par transfert hydrogénant

Letondor, Christophe ; Ward, Thomas R. (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2006 ; 1866.

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    Summary
    Homogenous- and enzymatic catalysis are in many respects complementary for the synthesis of enantiopure compounds. As the subtle details that govern chiral discrimination are difficult to predict, improving the performance of such catalysts often relies on trial-and-error procedures. Homogenous catalysts are optimized by chemical modification of the first coordination sphere and enzymes can be improved by modification of gene encoding the protein. The supramolecular anchoring of a biotinylated organometallic catalyst into a host protein ((Strept)avidin) affords versatile artificial metalloenzymes for the reduction of ketones by transfer hydrogenation. A first chemogenetic optimisation allows us to identify highly selective hybrid catalysts for the asymmetric reduction of carbinols. The artificial metalloenzyme [6-(p-cymene)Ru(Biot-p-L)Cl]  P64G affords the 1-(4-metylphenyl)ethanol with up to 94 % ee. In order to get more insight into the catalytic site, a saturation mutagenesis is performed at the position 112 of streptavidin. This choice for the second genetic optimisation round is suggested by docking studies which reveal that this position lies closest to the biotinylated metal upon incorporation into streptavidin. This screening of nineteen mutants combined with twenty-one biotinylated complexes affords enantioenriched alcohols in up to 97 % ee (R) or 84 % (S). Based on these results, we suggest that the enantioselection is mostly dictated by CH- interactions between the substrate and the 6-bond arene. However, in many cases, this complex – protein interactions can be outweighed by substrate – protein interactions to afford the opposite enantiomers of products, especially in the presence of cationic residues. Combined with a chemical optimisation, the mutagenesis techniques allow to create a large library of hybrid catalysts. As a preliminary test, in order to get a maximum of benefits of this powerful combinatorial approach, the artificial metalloenzyme technology is associated with high throughput screening of enantioselective catalysts by immunoassay.