Dioxygen Binding to Protonated Heme in the Gas Phase, an Intermediate Between Ferric and Ferrous Heme

With a view to characterizing the influence of the electronic structure of the Fe atom on the nature of its bond with dioxygen (O₂ ) in heme compounds, a study of the UV/Vis action spectra and binding energies of heme-O₂ molecules has been undertaken in the gas phase. The binding reaction of protonated ferrous heme [Fe^II -hemeH]⁺ with O₂ has been studied in the gas phase by determining the equilibrium of complexed [Fe^II -hemeH(O₂ )]⁺ with uncomplexed protonated heme in an ion trap at controlled temperatures. The binding energy of O₂ to the Fe atom of protonated ferrous heme was obtained from a van't Hoff plot. Surprisingly, this energy (1540±170 cm^-1 , 18.4±2 kJ mol^-1 ) is intermediate between those of ferric heme and ferrous heme. This result is interpreted in terms of a delocalization of the positive charge over the porphyrin cycle, such that the Fe atom bears a fractional positive charge. The resulting electron distribution on the Fe atom differs notably from that of a purely low-spin ferrous heme [Fe^II -heme(O₂ )] complex, as deduced from its absorption spectrum. It also differs from that of ferric heme [Fe^III -heme(O₂ )]⁺ , as evidenced by the absorption spectra. Protonated heme creates a specific bond that cannot accommodate strong σ donation.