Evolutionary and Combinatorial Methods in Asymmetric Catalysis

Professor Manfred Reetz described work on two areas – mixed chiral monodentate phosphorus ligands and the directed evolution of enantioselective enzymes.  Chiral monodentate ligands such as monophos are thought to involve a reactive intermediate that contains the metal and two ligand molecules.  But what happens if two different monodentate ligands are used?  The answer in many cases is improved ee’s.  So for the enamide reduction shown below better results are obtained with a mixed ligand system than either of the pure ligands (see Table 1).

Molecular models of the possible catalyst in the mixed ligand system suggest that the methyl group of the Binap-P-ch3 ligand lies close to one of the t-butyl methyl groups suggesting that  bidentate ligand shown below should be a good catalyst.  However it is still not as good a catalyst as the mixed ligand system.

Directed evolution enantioselective enzymes involves finding a micro-organism that shows some activity for catalysing a particular reaction and then optimising the enantioselectivity by mutagenesis suing one of a variety of techniques (site directed mutagenesis, error-prone polymerase chain reaction, saturation mutagenesis, cassette mutagenesis, DNA shuffling or staggered extension process).  Often several iterations are required to produce a suitable enzyme.  In one example of a mutant lipase, 5 mutations were required, but perhaps surprisingly all the new amino acids in the enzyme were remote from the active site.