Bonding of heme FeIII with dioxygen: observation and characterization of an incipient bond

Abstract

While ferrous heme (Fe^II) within hemoproteins binds dioxygen efficiently, it has not yet been possible to observe the analog complex with ferric heme (Fe^III). We present the first observation and characterization of the latter complex in a cooled ion trap. The bond formation enthalpy of ferric heme–O₂ has been derived from the Van't Hoff equation by means of temperature dependent measurements. The binding energy of the [heme Fe^III–O₂]⁺ ionic complex is rather strong as compared to that of [heme Fe^III–N₂]⁺, showing the formation of an incipient Fe–O bond, which is confirmed by the electronic absorption spectra of the two complexes. This first observation of the [heme Fe^III–O₂]⁺ complex lays the basis for the precise description of its electronic states.