Pyridoxal photoenzymes for asymmetric radical–radical cross-couplings

Non-native photoenzymes have enabled a myriad of asymmetric bond-forming events that are otherwise challenging or currently impossible with small molecule catalysis.1,2 These reactions require enzymes with cofactors that are strong absorbers in the visible region with reasonably long-lived excited states, such as flavin and nicotinamide. However, there exists a substantial chromophoric cofactor “dark space” where no known photoenzymatic activity has been characterized.1 Increased knowledge of the photophysics of the cofactors in the “dark space” would increase the types of bonds that photoenzymes can form by accessing new excited state intermediates in enzyme classes with divergent reactivities and selectivities. Here, we establish pyridoxal 5’-phosphate (PLP) as a photoenzymatic cofactor by leveraging the excited state quinonoid intermediate as a potent single-electron reductant. We overcome the poor photophysical properties of the native quinonoid intermediate by employing non-native benzyl amine substrates and exploiting Förster resonance energy transfer mechanism from an exogenous photosensitizer to access the quinonoid excited state. This redox neutral approach enables an asymmetric radical-radical cross-coupling between benzyl amines and reductive radical precursors through concomitant generation and localization of a radical pair in an enzyme active site—overcoming the typical challenges associated with this reaction by removing the necessity for radical sorting and the persistent radical effect. 33 The emergent photoexcited intermediates of PLP identified in this work greatly expands the potential avenues for valuable bond forming events by PLP-dependent enzymes.