Pyridoxal photoenzymes for asymmetric radical-radical cross-couplings.

Abstract

Non-native photoenzymes have enabled myriad asymmetric bond-forming events that are otherwise challenging or at present 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' in which no known photoenzymatic activity has been characterized¹. 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 using non-native benzylamine 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 benzylamines 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 effect³. The emergent photoexcited intermediates of PLP identified in this work greatly expand the potential avenues for valuable bond-forming events by PLP-dependent enzymes.