Iron III , low spin

Low spin iron(III) occurs with strong ligands and often with hexacoordination. A typical complex is Fe(CN) -. The short electron relaxation times have allowed the, 3C and, 5N investigation of the complexes [31]. Its NMRD in water indicates a Tj 10-13 s [32]. The electronic configuration of a generic low symmetry low spin iron(EI) complex is shown in Fig. 5.16. In distorted octahedral coordination, 

In D4h (tetragonal) symmetry there is only one unpaired electron in two degenerate orbitals of correct symmetry to give rise to n bonds with a n orbital of the porphyrin moiety [34]. The H NMR spectrum of Fe(protoporphyrin IX)-imidazole-cyanide is reported in Fig. 5.17 [35]. The free rotation of the imidazole ring about the metal-nitrogen bond, which is fast on the NMR timescale, simulates a tetragonal symmetry as far as the chemical shifts are concerned [36]. The four methyls are all downfield, though to a quite smaller value than in the case of 

The NMR spectrum of the heme group of oxidized horse heart cytochrome c is shown in Fig. 5.18 [41,42]. Cytochrome c is a heme protein where the iron atom is hexacoordinated, with a histidine and a methionine as axial ligands. As common for low spin Fe(III) systems, it presents relatively sharp signals and a 

Separation of contact and pseudocontact shifts in low-spin bis-imidazole iron(III) porphyrins [37,38] (labeling as in Fig. 5.7B) 

Position Hyperfine shift (ppm) Pseudocontact shift (ppm) Contact shift (ppm) 

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At the present time we have no certain knowledge of the state of the heme in these 450 nm species. We do not know if there are heme aggregates although they are unlikely. It is therefore reasonable to look at systems where the haem is aggregated as well as those where it is not in order to see how the absorption spectra can be mimic-ed. It seems reasonable to assume that the iron is low-spin in the carbon monoxide, isocyanide, and nitric oxide complexes as no high-spin iron complexes of this type are known. In the high-spin or low-spin state it may be that the thiol is weakly bound, if at all, for Fe(II) heme in models or in hemoglobin does not bind to thiols. In an attempt to understand these spectra we shall use a semi-empirical approach based on the theoretical discussion in the previous article (52) and elaborated in what follows immediately. Only Fe(II) complexes will be analysed as the Fe(III) proteins have been previously examined (52). 

In recent studies, involving the prediction of electronic and EPR spectra of hex-aamine complexes of chromium(III), low spin iron(II), cobalt(III), nickel(II) and copper(II)[90,166,197,3021 with a combination of molecular mechanics and AOM (angular overlap model) calculations, the two effects could be separated (Table 10.4 see also text below) because the structural factors (steric crowding molecular mechanics) and the electronic factors (inductive effects AOM) are parameterized separately. 

Figure 8.8 The catalytic cycle of the CYP enzymes. The substrate binds to a hydro-phobic site of the enzyme (I —> II). This leads to a shift in the valence electrons of iron, from low-spin to high-spin status, and an uptake of an electron from cytochrome P450 reductase (II —> III). Oxygen is then added (III —> IV) to the iron and another electron is added (IV V). Some of the intermediates may lose reactive oxygen species (IV —> II), which has harmful consequences for the cell. There are some electronic rearrangements (V —> VI —> VII). The product (RO) leaves the enzyme and its ground state is restored (VII —> I). RO is often more toxic than the substrate (R) and may be an alcohol, a phenol, or an epoxide that can be rendered harmless by other enzymes and made ready for excretion.

In ferricytochrome c, state III is the physiologically active one, again with histidine-18 and methionine-80 as the heme ligands. These two strong field ligands force the potentially unpaired iron electrons to pair and give the iron a low spin, and the intact methionine-80 bond produces the characteristic 695-nm absorption. Fluoride ion cannot displace methionine from the iron as a ligand, cyanide displaces methionine slowly, and azide and NO do so rapidly. 

The MM-AOM approach - that is, AOM calculations based on molecular mechanics refined structures - vas used to predict d-electron transitions of chromium(III), low-spin iron(III), low-spiniron(II), cobalt(III) and nickel(II) hexaamines and copper (II) tetraamines [132,190, 220, 383, 384, 387,391, 392]. AOM calculations allow one to compute d-electron energy levels based on the geometry of the chromophore and the bonding parameters (e<, and Ct ) for all ligand atoms [389,390]. Usually, the AOM is used to interpret electronic properties (UV-vis-NIR, EPR spectra, magnetic moments). However, for the prediction of spectroscopic data based on an established structure (experimental or molecular mechanics), a known and transferable set of electronic parameters must be used. This is not, a priori, a given property of the AOM approach, and therefore is problematic [132, 220, 277, 393]. The successful application of the M M-AOM approach with constant parameter sets for chromium(III), cobalt(III), low-spin iron(111), low-spin iron(II) and nickel(I I) hexaamines and copper(II) tetraamines [132,190, 220, 383, 384, 391, 392] does not imply that the electronic parameters are... 

In the electron transfer cytochromes the iron of heme is bound in a hexacoordinate low-spin state, with two protein ligands (typically histidine and/or methionine) above and below the heme plane. They serve as electron carrier proteins in mitochondria and endoplasmatic organelles as well as in bacterial redox chains. At least three classes of cytochromes, a, b and c, are known. They can alternate between an oxidized Fe(III) low-spin state with a single unpaired eleetron and a reduced Fe(II) low-spin form with no unpaired electrons. Since iron remains low spin, electron transfer is greatly facilitated. The best characterized family are the c cytochromes. 

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