H NMR Study of High-Spin Ferric Natural Porphyrin Derivatives as Models of Methemoproteins

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High-field 1H NMR spectra have been recorded and analyzed for a series of five-and six-coordinate, high-spin ferric complexes of natural porphyrin derivatives. Protoporphyrins with α-and β-deuterated vinyl groups have been synthesized that reveal the location of the elusive vinyl Ha resonances in the models. Deuterium labeling of individual methyl groups reveals that the increasing spread of the methyl isotropic shifts as the porphyrin 2,4 substituents are made more electron withdrawing is similar to that observed in low-spin ferric complexes. This supports a direct influence of the substituents on the asymmetry of the ligand molecular orbital and argues against a role of the raising of the orbital degeneracy in the low-spin species. Strongly electron-withdrawing 2,4 substituents induce a methyl spread similar to that observed in metaquomyoglobins, suggesting that the in-plane asymmetry in proteins arises primarily from peripheral heme-apoprotein interactions. Comparison of pyrrole substituent shift patterns in the five-and six-coordinate models suggests that the pyrrole proton/methyl shift ratio may serve as a useful indicator of the state of occupation of the sixth site in high-spin hemoproteins. © 1979, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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