A photoactive semisynthetic metalloenzyme exhibits complete selectivity for CO₂ reduction in water

A series of artificial metalloenzymes containing a ruthenium chromophore and [Ni^II(cyclam)]²⁺, both incorporated site-selectively, have been constructed within an azurin protein scaffold. These light-driven, semisynthetic enzymes do not evolve hydrogen, thus displaying complete selectivity for CO₂ reduction to CO. Electrostatic effects rather than direct excited-state electron transfer dominate the ruthenium photophysics, suggesting that intramolecular electron transfer from photogenerated Ru^I to [Ni^II(cyclam)]²⁺ represents the first step in catalysis. Stern-Volmer analyses rationalize the observation that ascorbate is the only sacrificial electron donor that supports turnover. Collectively, these results highlight the important interplay of elements that must be considered when developing and characterizing molecular catalysts.