Development of artificial metalloenzymes for highly efficient catalytic processes.

Waste-free chemical processes are pivotal for a sustainable society. In nature, large and complex protein structures have evolved to achieve extremely selective and efficient enzyme catalyzed conversions. Unfortunately, nature does not provide enzymes for the synthesis of the many pharmaceuticals, agrochemicals and fine-chemicals that are needed. Using our recently developed method (Angew. Chem. 2010, 49, 5315), we want to establish a paradigm shift in catalysis research by developing 'artificial enzymes' containing transition metals that nature has not acquired for the production of important products. In addition to the 'traditional' toolbox of ligand backbone constraints and steric and electronic ligand modification, the use of molecular recognition and secondary interactions by proteins will be exploited. Incorporation of well-designed transition metal phosphine and nitrogen complexes into proteins will lead to a new world of artificial metalloenzymes. Therefore, the aim of the proposed research is the development of artificial late transition metal based metalloenzymes for important catalytic transformations for which no enzymes are known, aspiring to selectivities and activities that rival enzymatic catalysis. The resulting artificial transition-metalloenzymes are anticipated to exhibit activities and selectivities comparable to natural enzymes, but the presence of the synthetic catalytic moiety will extend their reaction repertoire beyond the limits of natural enzymes. The catalytic performance of these hybrid catalysts can easily be optimized by using orthogonal structural-diversity generating procedures: molecular biology for the optimization of the protein structure and synthetic chemistry to tune the structure of the ligand. This so-called chemogenetic optimization strategy has already been shown to be a powerful approach in optimizing the catalytic properties of hybrid catalysts. To maximize the possibilities for functional interactions of the proteins with substrates and the phosphine-ligands, we will modify proteins naturally containing suitable binding pockets, like fatty acid binding proteins. Recently, the crystal structure of Human Peroxisomal Multifunctional Enzyme Type 2 (MFE-2) has been solved. This crystal structure revealed a large apolar molecular tunnel, containing a Triton X-100 molecule. This apolar tunnel of MFE-2 can be exploited for selective uptake of apolar substrates and correct positioning of the metal center can result in selective exposure of the center of reactivity of the substrate to the catalytic site, like in natural enzymes. Encapsulating substrates by these 'synthetic proteins' will ultimately enable clean conversion of apolar unfunctionalised alkenes as for instance in selective rhodium catalyzed hydroformylation of alkenes, a reaction for which no natural enzymes are known. We have recently developed a robust and reliable method to couple a wide variety of phosphines to virtually any protein via a single reactive cysteine residue. Preliminary results showed that a phosphine modified Human Peroxisomal Multifunctional Enzyme Type 2 (MFE-2) gave two orders of magnitude rate acceleration in the the aqueous rhodium catalyzed hydroformylation of long chain alkenes. We now want to explore the full potential of these hybrid homogeneous and biocatalysts in biotechnological applications. We propose to use MFE-2 and other proteins as templates for shape-selective rhodium catalyzed hydroformylation of long chained terminal and internal alkenes, a demanding transformation using existing technologies. Functionalization of their binding sites at different positions with various phosphine ligands is anticipated to result in improved selectivity for the terminal aldehydes, which are extremely desirable feedstocks for the chemical industry. The concept will be extended to other important catalytic reactions like C-C couplings, C1-selective oxidation of alkanes and C-H activation.