Iron picket fence model

Tabushi, I., M. Kodera, and M. Yokoyama (1985). Kinetics and mechanism of reductive dioxygen activation catalyzed by the P-450 model system—iron picket fence as a catalytic center. J. Am. Chem. Soc. 107, 4466 473. 

Table 3. Comparison of some geometry parameters of iron picket-fence prophyrin obtained by QM/MM modeling with the experimental X-ray data . The distances R are given in A, and the angles in degrees. R(Fe Nj) are the iron-nitrogen bond distances h is the out-of-plane displacement of the Fe atom R(N-N), R(O-O), and R(C-C) are the corresponding interligand interatomic distances between, respectively, the nitrogen, oxygen, and butyl-carbon atoms of the four "fence" ligands.

Illustrative examples of calculations with these methods are given. The ONIOM method yields reasonable results for energies. Model and real metallobiochemical systems treated by the QM/MM/CT method illustrate its feasibility with quite reasonable results in modeling iron picket-fence porphyrin, vitamin Bn, aquocobalamin, and coenzyme Bn. 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

In order to prepare and isolate solid-state, crystalline, oxygenated iron-heme model complexes, chemists learned to synthesize (by self-assembly methods) and oxygenate many types of hindered porphyrins. For instance, capped porphyrins were synthesized by direct condensation of a suitable tetraaldehyde with four pyrrole molecules.37 Picket-fence porphyrins such as I e(TPP)((V-MeIm) (where TPP = meso-tetraphenylporphyrin and /V-Melm = (V-methylimidazole)... 

Support for the vtew that the globin portion of the molecule produces a constraint upon the iron atom (which would otherwise move into the heme pocket) comes from the behavior of myoglobin and model compounds (such as the picket-fence corn-pounds with I- or 2-mcthylimidazole mimicking the porphyrin and histidine), which are easier to study than the more complex hemoglobin ... 

Chemical model systems, in particular the picket fence 33 (Figure 10.13), have been particularly useful in the studies on hemoglobin. The small chemical models may be crystallized and their structures determined with far higher precision than the structures of the actual proteins can be. Binding measurements may be made on the models without the complications that arise from the protein structure. The precise displacement of the iron atom from the heme and the geometry of iron-ligand bonds were first measured in the models.29,33,34 The Fe—02 bond is bent, whereas the Fe—CO bond is linear (structures 10.15). The Fe—02 bond is bent at 156° in hemoglobin.35... 

The chemical viability of the first proposal is confirmed by studies on model porphyrins, where the axial imidazole group is hindered to prevent movement towards the plane of the porphyrin. Thus, binding of dioxygen to iron(II) picket-fence porphyrins with N-methylimidazole as an... 

To limit the formation of 6-coordinate iron(II) porphyrins some kind of steric hinderance is deliberately introduced, such as in the picket fences approach77 (17-E-IX) or the strapped (17-E-X) and roofed models (17-E-XI). 

One of the representative models is the picket fence iron porphyrin reported by Coltman and his coworkers (Figure 4). The picket fence porphyrin as a structural model for myoglobin has four bulky pivaloyl groups at the peripheral positions to prevent two of these iron porphyrins from coming together to form a /u.-oxo dimer. The first significant result in their study was to demonstrate the 3D structure of the stable... 

Figure 7 Tailed iron porphyrins used as models of c)7ochrome P450. (a) model designed by Battersby and coworkers, attached at two points (b) TPP derivative with a pendant aryl thiol capable of bonding to the metal (c) a picket fence derivative with a pendant alkyl thiol (d) a chelated heme with a thiolate ligand (e) precursor of a model designed by Tatsimo et al., attached to the porphyrin at two points (f) another TPP derivative with a pendant aryl thiolate ...

---