Solid-State Oxygen-17 Nuclear Magnetic Resonance Spectroscopic Studies of [¹⁷O₂] Picket Fence Porphyrin, Myoglobin, and Hemoglobin

We have studied a model compound for oxyhemoglobin and oxymyoglobin, the iron—dioxygen complex of “picket fence porphyrin” (5,10,15,20-tetrakis(α,α,α,α-Opivalamidophenyl)porphyrinato)iron (II) ((1-Melm)O₂), as well as oxymyoglobin and oxyhemoglobin themselves, by using ¹⁷O solid-state nuclear magnetic resonance spectroscopy. For the model picket fence porphyrin, the principal components of the chemical shift tensors for both bridging and terminal oxygens in the Fe-O₂unit have been determined, and the isotropic chemical shifts occur at 1200–1600 and 2000 ppm, respectively, somewhat deshielded from the ~ 1750 and ~2500 ppm values found by Gerothanassis et al. in solution (J. Am. Chem. Soc. 1989, 111, 7006–7012). The anisotropics of the shift tensors are very large for both oxygens (Δδ = ~2200 ppm for the bridging oxygen and Δδ = 3350 ppm for the terminal oxygen, at 77 K). From partial averaging of the shift tensors at room temperature, due to fast axial rotation of the dioxygen ligand, an Fe—O—O bond angle of ~140° has been derived for the model system. Temperature dependence studies indicate essentially no change in the isotropic chemical shift of the terminal oxygen down to 4.2 K, while there is an apparent low-frequency shift of the bridging oxygen on cooling to 77 K, possibly due to the freezing in of one conformational substate. Spectra of oxymyoglobin and oxyhemoglobin, at 77 K, are very similar to those of the model compound at low temperature. Our results indicate that the ¹⁷O nuclear quadrupole coupling constants must be relatively small for both oxygens (≲5 MHz) in all systems, much smaller than the 8.5- and 20-MHz values found for ozone, suggesting extensive π-delocalization in the Fe—O—O fragment. Our results are also consistent with an overwhelmingly spin paired configuration, both in the model system, and in oxyhemoglobin and oxymyoglobin themselves.