Oxyhemoglobin

The 0-0 stretch that is observed by difference infrared techniques at around 1105 cm - I for oxyhemoglobin and oxymyoglobin ]49 clearly categorizes the dioxygen moiety as a superoxo species; that is, the order of the 0-0 bond is about 1.5. Considerable ink has been spilled about the nature of the Fe-02 fragment since Pauling's original suggestion 150 in 1948 that dioxygen binds to iron in an end-on bent fashion:

He subsequently reaffirmed this geometry, and proposed that hydrogen bonding between the coordinated dioxygen and the distal imidazole H- N group was important in stabilizing the Fe-02 species. 137 In an alternative model Weiss proposed that a low-spin Fe III center (5 = !) was very strongly antiferromagnetically coupled to a superoxide anion radical (5 = !).151 A triangular peroxo mode has also been advanced. 152,153 The problem has been how to resolve the observed diamagnetism of oxyhemoglobin 92,98 with UV-visible, x-ray absorption, and resonance Raman spectroscopic characteristics 154 that are distinctly different from those of Fell systems (such as carbonmonoxyhemoglobin and lowspin six-coordinated hemochromes, such as Fe(Porph)(Pyh) and from unambiguously Fe III systems (such as chloromethemoglobin or cyanomethemoglobin). Any adequate theoretical treatment must also explain how iron-porphyrin systems can bind not only O2, but also CO, NO, alkyl isocyanides, and alkylnitroso moieties. A simple qualitative model presented by Wayland and coworkers 129,155 conveniently summarizes ligand-binding geometries of cobalt and iron porphyrins. Although a reasonable quantitative theoretical consensus exists for 1: 1 cobalt-dioxygen species, the same cannot be said yet for irondioxygen systems.