Pyridoxal Photoenzymes Enable Asymmetric Radical–Radical Cross-Coupling Reactions

Non-native photoenzymes are expanding the toolbox for forging asymmetric bonds—often in ways small-molecule catalysis struggles to replicate. A key limitation, however, is that many photoenzymatic reactions depend on cofactors whose excited states are both strongly absorbing in the visible range and long-lived. Within this landscape lies a largely unexplored “dark space” of chromophoric cofactor states: regions of photophysical behavior for which no photoenzymatic activity has been clearly characterized. Filling that gap could unlock entirely new reaction manifolds by enabling access to previously inaccessible excited-state intermediates.

In a new study published in Nature, Sorensen, Wang, Ouyang and co-workers establish pyridoxal 5’-phosphate (PLP) as a photoenzymatic cofactor. The central idea is to exploit PLP’s excited-state chemistry—specifically the formation of a quinonoid intermediate—as an unusually potent single-electron reductant. In principle, such reductive capacity could enable radical-based bond formation inside enzyme active sites.

A major obstacle is that the native quinonoid intermediate suffers from poor photophysical performance, limiting productive excited-state lifetimes and reactivity. To overcome this, the researchers combine substrate engineering with photophysical coupling. They use non-native benzyl amine substrates designed to better support the generation and functional engagement of the quinonoid species.

But substrate redesign alone is not the full solution. The team further leverages Förster resonance energy transfer (FRET), using an exogenous photosensitizer to transfer excitation energy efficiently into the PLP-bound quinonoid manifold. This strategy bypasses the limitations of directly photoexciting the quinonoid intermediate, instead “feeding” the relevant excited state through energy transfer.

With these advances, the authors demonstrate a redox-neutral route to asymmetric radical–radical cross-coupling between benzyl amines and reductive radical precursors. Rather than relying on external radical sorting or handling persistently reactive radical populations, the method generates and localizes a radical pair within the enzyme active site.

This localization is crucial: it enables controlled coupling while mitigating common challenges in radical chemistry, including off-pathway reactions and loss of stereocontrol. The resulting selectivity arises from the spatial and temporal constraints imposed by the enzyme’s microenvironment.

Together, the work maps emergent PLP photochemical intermediates into the enzyme toolkit and suggests that PLP-dependent photoenzymes can reach beyond flavin and nicotinamide-based designs. By expanding the accessible excited-state “options” in the cofactor dark space, the platform offers a new route to valuable bond-forming reactions with asymmetric control.