Thiophenol-Based Designer Enzymes for Asymmetric Hydrogen/Deuterium Atom Transfer

The enantioselective control of short-lived radical species in aqueous media represents a major challenge in modern catalysis. Herein, we report on the design and evolution of a designer enzyme with a genetically encoded thiophenol moiety that enables enantioselective hydrogen atom transfer (HAT) biocatalysis. By incorporating 4-mercaptophenylalanine (pSHF), a thiophenol-based non-canonical amino acid, into a hydrophobic pocket of the transcriptional regulator LmrR through stop codon suppression, we demonstrated that pSHF functions as a HAT catalytic center for the enantioselective control of prochiral radical species generated through photoinduced single electron transfer within a photocatalytic cycle by a Ru photocatalyst, resulting in a highly enantioselective photoinduced imine reduction reaction. A directed evolution campaign yielded an optimized variant that catalyzes the asymmetric reduction of cyclic imines with quantitative yields and excellent enantioselectivities (up to >99% yield and >99% ee). Deuterium atom transfer (DAT) was also achieved in a D2O-based buffer solution to install a single deuterium atom to chiral amine products (up to 98% D incorporation). Mechanistic investigations as well as computational calculations suggested that the thiophenol moiety of pSHF promotes the HAT/DAT to the radical species incorporated within the hydrophobic scaffold of the designer enzyme. This work demonstrates the potential for expanding new-to-nature enzymatic functions and broadening the scope of asymmetric HAT in radical chemistry.