Non-Native Bioelectrocatalytic C-C and C-N Bond Formation via Cobalamin Dependent Enzymes

Abstract

Bioelectrocatalysis involves the use of biomaterials to catalyze redox reactions at an electrode. Biocatalysis offers improved selectivity and typically involves milder reaction conditions, while electrocatalysis can involve the use of renewable energy as an electron source. Cobalamin (B 12 ) dependent enzymes catalyze C-C and C-N bond forming reactions via radical mechanisms through the formation of a Co(III)-alkyl intermediate. Homolysis of the Co(III)-C bond leads to radical coupling with various substrates to form the product. However, these alkyl radicals can react through a variety of mechanistic pathways, leading to a mixture of products. Controlling the alkyl radical intermediate would lead to improved reaction selectivity.
Here, we report on CarH*, an engineered variant of a B 12 dependent photoreceptor protein, which catalyzes styrene C-H alkylation by trapping an alkyl radical intermediate in its protein scaffold. The reversible Co-alkyl bond homolysis protects the radical, allowing for selective olefin addition. However, to initiate this reactivity, the cobalt center of CarH* must be reduced through stoichiometric addition of the reducing agent titanium (III) citrate. Thus, we are developing a mediated electrochemical system to achieve this transformation without the use of a stoichiometric reductant and to investigate new reactivity in CarH* and similar variants. A diffusive redox mediator was found to deliver electrons to CarH* and drive alkyl radical formation. This methodology offers a more sustainable and general approach to electrochemical non-native biocatalysis and can be applied to discover new reactivity in B 12 dependent enzymes.