Ferric heme compounds

Theoeretical models proposed by Griffith (36, 38) and Kotani (59, 60) are now generally employed for the interpretation of the spectral and magnetic properties of hemoproteins. In his recent review Weissbluth (106) gave a detailed account of the calculations involved in these models. The present discussion is limited to a brief outline of the treatment of low spin ferric heme compounds, and the presentation of some results which will be useful for the analysis of the NMR data. 

In paramagnetic molecules with anisotropic g-tensors electron-proton dipole-dipole coupling may contribute to the hyperfine shifts observed in the proton NMR spectra. From the data to be discussed in this section it would seem, however, that in low spin ferric heme compounds many of the qualitative spectral features are mainly determined by Fermitype contact shifts. 

As mentioned at the beginning of this section the size of the pseudocontact shifts in the NMR spectra could in principle be calculated for all the low spin ferric heme compounds if detailed data on the electronic g-tensors were available (Jesson (47)). Unfortunately the EPR data on the azides can not be used directly, because these complexes are not in a pure low spin state under the conditions of the NMR experiments (see section VI C). For the compounds in Figs. 10 through 20 no. successful single-crystal EPR studies were as yet reported. However only g-values determined in frozen solutions are presently available (Blumberg and Peisach (70) Salmeen and Palmer (95a)), e.g. for dicyanoferri-porphin at 1.4 °Kgi = 3.64, g 2.29, and gs 1.0 were found. 

A more detailed treatment, including the effects of spin delocalization on the pseudocontact shifts, might be warranted once single crystal EPR data will become available for several of the low spin ferric heme compounds. In the hemoproteins it would then be of special interest to investigate pseudocontact shifts for amino acid residues near the heme groups which could yield structural information in a similar way as the ring current shifts. 

Eq. (4), which relates the observed contact shifts of the proton resonances to their isotropic contact coupling constants, and hence to the spin densities on the ring carbon atoms, is valid only for systems with isotropic g-tensors. To obtain an estimate of the errors which might arise from its application to low spin ferric heme compounds, we shall briefly consider a more general form of the equation, which was given by Jesson (46) for tetragonal systems with more than one populated electronic state. 

In mitochondrial fragments and complex III from beef heart, the EPR signals were reported to be at g = 3.44 and g = 3.78 for cytochrome b and cytochrome br, respectively, and interpreted as low field resonance of low-spin ferric heme compounds (35,36). 

Figure 1. Typical X-band EPR spectra of high-spin (upper) and low-spin (lower) ferric heme compounds as examined in frozen solutions...

Compound I will accept a further electron from azurin and decays to a species known as compound II with a bimolecular rate constant of 6 X 10 M s (84). It is anticipated that compound II is a form of CCP containing two ferric hemes, but possibly not identical to the structurally defined fully oxidized enzyme as isolated. This is because during turnover at room temperature there is no obvious reason for the histidine ligand displaced from the peroxidatic heme iron to return. Consequently, it might be assumed that compound II is structurally related to the MV conformer rather than the resting enzyme. 

An important conclusion from this study of model compounds is the additional evidence obtained for the key role of the S = 3/2 spin state in the chemistry of ferric heme complexes. There are no complexes for which S = 5/2 and 1/2 spin states are close enough to interact without an even greater contribution of the S = 3/2 spin state. Thus, the widely used assumption ( ) of high-spin/low-spin thermal contributions to explain observable properties of heme complexes appears to be incorrect. Explanations involving high-spin/intermediate-spin interaction are much more plausible, since small energy separations between these states were found. 

A more promising approach could be the interaction of ferric heme-sulfur complexes with oxene donors like 3-chloroperacetic acid, cumene hydroperoxide or iodoso-benzene, since cytochrome P450 seems to react with these compounds by formation of the active oxygen species. Attempts in this direction have been made, but so far... 

Similar experiments with PpNiR together with time-resolved FTIR showed significant differences." Nitrite bound to heme d and was rapidly reduced to form an EPR-detectable ferrous NO-heme d complex with oxidized heme c. The rate constant for electron transfer was estimated to be 500s. In a slower phase FTIR showed development of a band at l,913cm formed with a rate constant of 38 s . The energy of this band is consistent with a heme-Fe -NO" species, and close to those of model compounds of ferric heme d with NO and A-methyl imidazole." " ... 

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