Solid-state NMR, Mossbauer, crystallographic, and density functional theory investigation of Fe-O-2 and Fe-O-2 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 follo wing Fe-O-2 analogue metalloporphyrins: Fe(5,10,15,20-tetraphenylporphyrina te) (nitrosobenzene)(1-methylimidazole); Fe(5,10,15,20-tetraphenylporphyrin ate) (nitrosobenzene)(pyridine); Fe(5,10,15,20-tetraphenylporphyrinate)(4 F e-O-2 analogue metalloporphyrins (2,3,7,8,12,13,17,18-octaethylporphyrinate ) (nitrosobenzene)(1-methylimidazole) and Co(2,3,7,8,12,13,17,18-octaethylp orphyrinate)(NO). Our results show that the porphyrin rings of the two tetr aphenylporphyrins containing pyridine are ruffled while the other three com pounds 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-O-2 bonding in meta lloporphyrins and metalloproteins. We find no evidence for two binding site s in oxypicket fence porphyrin, characterized by very different electric fi eld gradients. However, the experimental Mossbauer quadrupole splittings ca n be readily accounted for by fast axial rotation of the Fe-O-2 unit. Unlik e oxymyoglobin, the Mossbauer quadrupole splitting in PhNO . myoglobin does not change with temperature, due to the static nature of the Fe . PhNO sub unit, as verified by H-2 NMR of Mb .[H-2(5)]PhNO. Rotation of O-2 to a seco nd (minority) site in oxymyoglobin can reduce the experimental quadrupole s plittings, either by simple exchange averaging, or by an electronic mechani sm, without significant changes in the Fe-O-O bond geometry, or a change in sign of the quadrupole splitting. DFT calculations of the molecular electr ostatic potentials in CO, PhNO, and O-2-metalloporphyrin complexes show tha t the oxygen sites In the PhNO and O-2 complexes are more electronegative t han that in the CO system, which strongly supports the idea that hydrogen b onding to O-2 Will be a major contributor to O-2/CO discrimination in heme proteins.