Charge Separation Related to Photocatalytic H₂ Production from a Ru–Apoflavodoxin–Ni Biohybrid

The direct creation of a fuel from sunlight and water via photochemical energy conversion provides a sustainable method for producing a clean source of energy. Here we report the preparation of a solar fuel biohybrid that embeds a nickel diphosphine hydrogen evolution catalyst into the cofactor binding pocket of the electron shuttle protein, flavodoxin (Fld). The system is made photocatalytic by linking a cysteine residue in Fld to a ruthenium photosensitizer. Importantly, the protein environment enables the otherwise insoluble Ni catalyst to perform photocatalysis in aqueous solution over a pH range of 3.5–12.0, with optimal turnover frequency 410 ± 30 h –1 and turnover number 620 ± 80 mol H 2 /mol hybrid observed at pH 6.2. For the first time, a reversible light-induced charge-separated state involving a Ni(I) intermediate was directly monitored by electron paramagnetic resonance spectroscopy. Transient optical measurements reflect two conformational states, with a Ni(I) state formed in ∼1.6 or ∼185 μs that persists for several milliseconds as a long-lived charge-separated state facilitated by the protein matrix.