| Abstract
| - We have synthesized and studied via solid-state NMR, Mössbauer spectroscopy, single-crystal X-raydiffraction, 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 tetraphenylporphyrinscontaining pyridine are ruffled while the other three compounds are planar: reasons for this are discussed.The solid-state NMR and Mössbauer spectroscopic results are well reproduced by the DFT calculations, whichthen enable the testing of various models of Fe−O2 bonding in metalloporphyrins and metalloproteins. Wefind no evidence for two binding sites in oxypicket fence porphyrin, characterized by very different electricfield gradients. However, the experimental Mössbauer quadrupole splittings can be readily accounted for byfast axial rotation of the Fe−O2 unit. Unlike oxymyoglobin, the Mössbauer quadrupole splitting inPhNO•myoglobin does not change with temperature, due to the static nature of the Fe•PhNO subunit, as verifiedby 2H NMR of Mb•[2H5]PhNO. Rotation of O2 to a second (minority) site in oxymyoglobin can reduce theexperimental quadrupole splittings, either by simple exchange averaging, or by an electronic mechanism, withoutsignificant changes in the Fe−O−O bond geometry, or a change in sign of the quadrupole splitting. DFTcalculations of the molecular electrostatic potentials in CO, PhNO, and O2-metalloporphyrin complexes showthat 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 discriminationin heme proteins.
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