Solid-state NMR, Mossbauer, crystallographic, and density functional theory investigation of Fe-O₂ and Fe-O₂ analogue metalloporphyrins and metalloproteins

We have synthesized and studied via solid-state NMR, Mossbauer spectroscopy, single-crystal X-ray diffraction, and density functional theory the following Fe-O₂ analogue metalloporphyrins: Fe(5,10,15,20- tetraphenylporphyrinate) (nitrosobenzene)(1-methylimidazole); Fe(5,10,15,20- tetraphenylporphyrinate) (nitrosobenzene)(pyridine); Fe(5,10,15,20- tetraphenylporphyrinate)(4-nitroso-N,N-dimethylaniline)(pyridine); Fe(2,3,7,8,12,13,17,18-octaethylporphyrinate) (nitrosobenzene)(1- methylimidazole) and Co(2,3,7,8,12,13,17,18-octaethylporphyrinate)(NO). Our results show that the porphyrin rings of the two tetraphenylporphyrins containing pyridine are ruffled while the other three compounds are planar: reasons for this are discussed. The solid-state NMR and Mossbauer spectroscopic results are well reproduced by the DFT calculations, which then enable the testing of various models of Fe-O₂ bonding in metalloporphyrins and metalloproteins. We find no evidence for two binding sites in oxypicket fence porphyrin, characterized by very different electric field gradients. However, the experimental Mossbauer quadrupole splittings can be readily accounted for by fast axial rotation of the Fe-O₂ unit. Unlike oxymyoglobin, the Mossbauer quadrupole splitting in PhNO · myoglobin does not change with temperature, due to the static nature of the Fe · PhNO subunit, as verified by ²H NMR of Mb · [²H₅]PhNO. Rotation of O₂ to a second (minority) site in oxymyoglobin can reduce the experimental quadrupole splittings, either by simple exchange averaging, or by an electronic mechanism, without significant changes in the Fe-O-O bond geometry, or a change in sign of the quadrupole splitting. DFT calculations of the molecular electrostatic potentials in CO, PhNO, and O₂-metalloporphyrin complexes show that the oxygen sites in the PhNO and O₂ complexes are more electronegative than that in the CO system, which strongly supports the idea that hydrogen bonding to O₂ will be a major contributor to O₂/CO discrimination in heme proteins.