Studies of the electron-nuclear coupling between Fe(III) and 14N in cytochrome P-450 and in a series of low spin heme compounds.

We have observed the nuclear modulation pattern in the envelope of electron spin echoes for various low spin paramagnetic heme proteins including cytochrome c, myoglobin hydroxide, myoglobin mercaptoethanol, and cytochrome P-450, using the three-pulse-stimulated echo method. We have also carried out similar experiments with model compounds containing either [14N]- or [15N]imidazole. In many of the compounds studied, we have been able to identify the nuclear modulation effects arising from 14N of the porphyrin ring and have been able to characterize and interpret the modulation effects due to 14N of various nitrogenous axial ligands. We have found that the heme of low spin ferric cytochrome P-450 is coordinated to a nitrogenous ligand, probably imidazole. We have also demonstrated that the remote 14N of the imidazole ligand in a [14N]imidazole-heme-NO-model compound is coupled differently than in myoglobin nitroxide, demonstrating the direct effect of the protein of metal ligand bonding.