Artificial Metalloproteins Containing Co₄O₄ Cubane Active Sites

Artificial metalloproteins (ArMs) containing Co₄O₄ cubane active sites were constructed via biotin-streptavidin technology. Stabilized by hydrogen bonds (H-bonds), terminal and cofacial Co^III-OH₂ moieties are observed crystallographically in a series of immobilized cubane sites. Solution electrochemistry provided correlations of oxidation potential and pH. For variants containing Ser and Phe adjacent to the metallocofactor, 1e^-/1H⁺ chemistry predominates until pH 8, above which the oxidation becomes pH-independent. Installation of Tyr proximal to the Co₄O₄ active site provided a single H-bond to one of a set of cofacial Co^III-OH₂ groups. With this variant, multi-e^-/multi-H⁺ chemistry is observed, along with a change in mechanism at pH 9.5 that is consistent with Tyr deprotonation. With structural similarities to both the oxygen-evolving complex of photosystem II (H-bonded Tyr) and to thin film water oxidation catalysts (Co₄O₄ core), these findings bridge synthetic and biological systems for water oxidation, highlighting the importance of secondary sphere interactions in mediating multi-e^-/multi-H⁺ reactivity.