Design and Evaluation of Artificial Hybrid Photoredox Biocatalysts

A pair of 9-mesityl-10-phenyl acridinium (Mes-Acr⁺ ) photoredox catalysts were synthesized with an iodoacetamide handle for cysteine bioconjugation. Covalently tethering of the synthetic Mes-Acr⁺ cofactors with a small panel of thermostable protein scaffolds resulted in 12 new artificial enzymes. The unique chemical and structural environment of the protein hosts had a measurable effect on the photophysical properties and photocatalytic activity of the cofactors. The constructed Mes-Acr⁺ hybrid enzymes were found to be active photoinduced electron-transfer catalysts, controllably oxidizing a variety of aryl sulfides when irradiated with visible light, and possessed activities that correlated with the photophysical characterization data. Their catalytic performance was found to depend on multiple factors including the Mes-Acr⁺ cofactor, the protein scaffold, the location of cofactor immobilization, and the substrate. This work provides a framework toward adapting synthetic photoredox catalysts into artificial cofactors and includes important considerations for future bioengineering efforts.