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Metal nitroxyl interactions 46. epr spectra of low spin iron iii complexes of spin labeled tetraphenylporphyrins and their implications for the interpretation of epr spectra of spin labeled cytochrome p 450



Metal nitroxyl interactions 46. epr spectra of low spin iron iii complexes of spin labeled tetraphenylporphyrins and their implications for the interpretation of epr spectra of spin labeled cytochrome p 450



Journal of the American Chemical Society 108(4): 618-625



Low-spin iron(III) complexes of seven spin-labeled tetraphenylporphyrins have been prepared. The spin labels were attached by amide or amide and ether linkages (three .sbd. eight bonds) to the ortho position of one phenyl ring. The axial ligands were imidazole and 1-methylimidazole. Computer simulations of the frozen solution EPR spectra were done with perturbation calculations and fourth-order frequency shift perturbation calculations. In frozen solution the complexes with amide linkages between the phenyl and nitroxyl rings adopted two conformations. The populations of the conformations were solvent-dependent. In one conformation the values of the electron-electron spin-spin coupling constants, J, were between .+-. 0.22 and .+-. 0.28 cm-1. Addition of a CH2 group between the amide and the nitroxyl caused J to decrease by a factor of 3-7. The spin-spin interaction in the second conformation was much weaker than in the first conformation. Resolved spin-spin splitting was not observed for this conformation although the line widths for the nitroxyl signal in frozen solution were increased due to interaction with the iron. Broadening of the nitroxyl signal in frozen solution was also observed for complexes with longer ether linkages between the phenyl ring and the nitroxyl. In fluid solution the integrated intensities of the nitroxyl signals indicated that the observed spectra were due only to the conformations with weak spin-spin interaction. The EPR spectra, reported by other authors, of two spin labels coordinated to ferric cytochrome P450 were anlayzed with the computer programs developed for the iron porphyrin model systems. The values of J for the two complexes were .+-. 0.03 and .+-. > 0.4 cm-1. These spectra indicate that electron-electron exchange interaction as well as dipolar interaction must be considered in analyzing the spectra of spin-labeled biomolecules.

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Accession: 005886523

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DOI: 10.1021/ja00264a010


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