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 (O-2) in heme compounds, a study of the UV/Vis action spectra and binding energies of heme-O-2 molecules has been undertaken in the gas phase. The binding reaction of protonated ferrous heme [Fe-II-hemeH](+) with O-2 has been studied in the gas phase by determining the equilibrium of complexed [Fe-II-hemeH(O-2)](+) with uncomplexed protonated heme in an ion trap at controlled temperatures. The binding energy of O-2 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 kJmol(-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(O-2)] complex, as deduced from its absorption spectrum. It also differs from that of ferric heme [Fe-III-heme(O-2)](+), as evidenced by the absorption spectra. Protonated heme creates a specific bond that cannot accommodate strong sigma donation.