Kombinatorische und evolutionsgesteuerte Methoden zur Bildung enantioselektiver Katalysatoren

Learning from Directed Evolution: Theoretical Investigations into Cooperative Mutations in Lipase Enantioselectivity

Molecular modeling with classical force-fields has been used to study the reactant complex and the tetrahedral intermediate in lipase-catalyzed ester hydrolysis in 20 enzyme/substrate combinations. The R and S enantiomers of alpha-methyldecanoic acid ester served as substrates for the wild-type lipase from Pseudomonas aeruginosa and nine selected mutants. After suitable preparation of initial structures from an available wild-type crystal structure, each system was subjected to 1 ns CHARMM force-field molecular dynamics simulations. The resulting geometric and energetic changes allow interpretation of some experimentally observed effects of mutations, particularly with regard to the "hot spots" at residues 155 and 162. The replacement S155F enhances S enantiopreference through a steric relay involving Leu162. The double mutation S53P + L162G improves S enantioselectivity by creating a new binding pocket for the S enantiomer with an additional stabilizing hydrogen bond to His83. The simulations provide insight into remote and cooperative effects of mutations.

A low background high-throughput screening (HTS) fluorescence assay for lipases and esterases using acyloxymethylethers of umbelliferone

Esters and acyloxymethyl ethers of umbelliferone were evaluated as fluorogenic substrates for lipases and esterases with respect to stability and resistance to non-specific hydrolysis. Isobutyryloxymethyl ether 1c and pivaloxymethyl ether 1d were found to be optimal substrates for enzyme assays, particularly with respect to HTS applications.

Assembly of designed oligonucleotides as an efficient method for gene recombination: a new tool in directed evolution

A new and practical method for gene recombination with formation of libraries of mutant genes is presented. The method is based on the assembly of appropriately prepared oligonucleotides whose design is guided by sequence information. High recombination frequency with formation of full-length products is achieved by controlled overlapping of the designed oligomers. This process (ADO) minimizes self-hybridization of parental genes, which constitutes a significant advantage over conventional family shuffling as used in the directed evolution of functional enzymes. ADO was applied to the recombination of two lipase family genes from Bacillus subtilis (LipA and LipB). In a library of 3000 lipase variants created by this method, several were found that display increased enantioselectivity in a model reaction involving the hydrolysis of a meso-diacetate.

Second-generation MS-based high-throughput screening system for enantioselective catalysts and biocatalysts

A high-throughput method is described, where the enantioselectivity of approximately 10 000 catalysts or biocatalysts can be determined per day. The method is based on electrospray mass spectrometric techniques using an eight-channel multiplexed (MUX) sprayer system connected to a time-of-flight mass spectrometer. The inlet of the ion source is controlled by a stepping rotor that is continuously moving from one sprayer to the next with a recording time of 100 ms for each channel and a delay time of 50 ms, thus allowing a spectrum to be obtained from each channel every 1.2 s. One cycle, where eight samples are being sprayed in parallel, requires around 70 s, which allows a 96-well microtiter plate to be screened in 14 min. Integration of two pseudo-enantiomers (S)-glycidyl phenyl ether and (R)-D5-glycidyl phenyl ether is necessary to quantify the enantiomeric excess (ee-value), where one enantiomer is isotopically labeled to allow easy identification of the mass spectrometric signals. Errors of ~2% for the ee-values indicate that in addition to the significant improvement in sample throughput this is also a precise method for high-throughput screening. This second-generation assay is useful for combinatorial enantioselective transition-metal catalysis and in the directed evolution of enantioselective enzymes.

The aldol addition reaction: an old transformation at constant rebirth

The main recent conceptual advances in asymmetric aldol reactions are presented. Methods ranging from stoichiometric chiral auxiliary-mediated to direct, catalytic reactions are covered, including the Mukaiyama aldol reactions which use stoichiometric base and silylating reagents, but catalytic (substoichiometric) amounts of the chiral inductor. The salient features of each new development are noted, paying special attention to practical concerns and to the potential implementation for large scale production. After examination of pros and cons of each strategy, gaps and limitations that deserve further investigation are highlighted.

Catalyzed Asymmetric Arylation Reactions

Addition and substitution reactions with carbon nucleophiles are fundamental processes in organic synthesis, and the development of general catalytic asymmetric variants thereof is still a major challenge today. In contrast to enantioselective alkyl transfer reactions, the corresponding arylations have not yet reached a high level of maturity. The existing protocols are either of no general applicability or are limited in terms of selectivity. This article summarizes established routes for catalytic asymmetric aryl transfer together with the latest developments in this area. The scope and limitations of this reaction are discussed.

Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.