The pH‐Induced Selectivity Between Cysteine or Histidine Coordinated Heme in an Artificial α‐Helical Metalloprotein

De Novo metalloprotein design assesses the relationship between metal active site architecture and catalytic reactivity. Herein, we use an α‐helical scaffold to control the iron coordination geometry when a heme cofactor is allowed to bind to either histidine or cysteine ligands, within a single artificial protein. Consequently, we uncovered a reversible pH‐induced switch of the heme axial ligation within this simplified scaffold. Characterization of the specific heme coordination modes was done by using UV/Vis and Electron Paramagnetic Resonance spectroscopies. The penta‐ or hexa‐coordinate thiolate heme (9≤pH≤11) and the penta‐coordinate imidazole heme (6≤pH≤8.5) reproduces well the heme ligation in chloroperoxidases or cyt P450 monooxygenases and peroxidases, respectively. The stability of heme coordination upon ferric/ferrous redox cycling is a crucial property of the construct. At basic pHs, the thiolate mini‐heme protein can catalyze O 2 reduction when adsorbed onto a pyrolytic graphite electrode.