Ligand Energy Controls the Heme-Fe Valence in Aqueous Myoglobins

We use resonant X-ray emission spectroscopy and model calculations to quantify the ligand: heme-Fe energy structure of aqueous myoglobins. For reduced (Fe²⁺) and oxidized (Fe³⁺) states, the removal or addition of an electron primarily involves charge changes on the ligand-site, and not the Fe-site. The results indicate a finite positive/negative charge-transfer energy Δ between the heme-Fe 3d and ligand valence electronic states for Fe²⁺/Fe³⁺. Thus, the energy difference between the ligand and Fe 3d states (+Δ or -Δ) determines the charge properties of myoglobins. The study provides a reliable method for characterizing ligand-metal binding of biological systems in solution.