Setting an Upper Limit on the Myoglobin Iron(IV)Hydroxide pK a: Insight into Axial Ligand Tuning in Heme Protein Catalysis

To provide insight into the iron­(IV)­hydroxide pK a of histidine ligated heme proteins, we have probed the active site of myoglobin compound II over the pH range of 3.9–9.5, using EXAFS, Mössbauer, and resonance Raman spectroscopies. We find no indication of ferryl protonation over this pH range, allowing us to set an upper limit of 2.7 on the iron­(IV)­hydroxide pK a in myoglobin. Together with the recent determination of an iron­(IV)­hydroxide pK a ∼ 12 in the thiolate-ligated heme enzyme cytochrome P450, this result provides insight into Nature’s ability to tune catalytic function through its choice of axial ligand.