Thioethers axial

Some of the earlier work on simpler models can be discussed in more detail here. Much of this focuses on the role of Met-80 in controlling the redox potential of cytochrome c. Low-spin tetraphenylporphyrin complexes of Fe" and Fe " with thioether axial ligands have similar bond lengths (Fe—S = 2.34 and 2.33, 2.35 A respectively). Increase in Fe—S bond length on going... 

When n electron donors are involved, the XB is preferentially along the axis of the donated lone pair on D. For instance, XBs around ethers, thioethers, and amines feature a tetrahedral arrangement with preferential axial directions for the XBs around hexacyclic amines, and equatorial directions for hexacyclic thioethers [16,17,124,139-142] (Fig. 6). 

Following the synthetic route for the preparation of thioether appended por-phyrazines developed by Schramm and Hoffman (2), a series M[pz(S-Et)g] M = Mn, Co, Fe, were prepared by Ricciardi and co-workers (113-116). Their goal was to study the effect of the central metal and the immediate electronic environment, that is, oxidation state and axial ligand, on the physicochemical properties of the pz (Scheme 10). 

The distorted octahedral species [10] and [11] are the essential part of the cytochromes acting as redox catalysts. In these, a very specific porphyrin redox potential may have been adjusted by an appropriate choice of the axial ligands exerting a cis effect on the porphyrin system transmitted through the iron atom. In cytochrome c, these axial ligands are the imidazole of a histidine and the thioether function of methionine, as in [10], and in the cytochromes a or b5 they are presumably two imidazoles of histidine, as in [77] (7). 

Therefore, we investigated a molecular interaction of cardiolipin with cytochrome c. As shown in Figure 2, cytochrome c consists of 5 helices and inter-helical loops which harbor a heme c prosthehc group by covalent thioether-bonds through cysteine-14 and -17 residues. Ferric ion, centered in the pyrrole ring, is axially liganded by histidine-18 and methionine-80 residue. The lower half of the protein consishng of flexible random coils is a rather soft stmcture and has a space between the heme c plate and P-loops inside the smah basic protein. 

Fig. 5.18. 360 MHz H NMR spectra of oxidized horse heart cytochrome c. The labeled signals are assigned to a = 8-CH3, b = 3-CH3, c = 5-CH3, d = thioether bridge 2-CH3, e = axial methionine S-CH3 the resonances at 7.4 ppm (1-CH3) and 3.1 ppm (thioether bridge 4-CH3) are not shown. Chemical shifts are in ppm from DSS (adapted from [42]) (labeling as in Fig. 5.7B).

Because of their structural and spectroscopic analogies with the hemo-chromes Fe(P)L2, e.g. the protoporphyrin derivative 3 (L, L = Py or 1-Meim), the corresponding 4d and 5d homologs are named ruthenochromes or osmo-chromes . The hemochromes derive their name from the cytochromes, the widespread electron-carrying heme proteins. Cytochrome b (coordination type F, M = Fe) has two imidazole donors from histidine side chains at the central iron, cytochrome c (coordination type G, M = Fe) an imidazole and a methyl-thioether function from a methionine. F is an axially symmetrical, G an axially unsymmetrical system. 

It should finally be mentioned that asymmetric and metal-free sulfoxidation can also be achieved by use of flavinium (84) [131] or simpler chiral iminium cations (85) as catalysts [132] (Scheme 10.18). For the axially chiral flavinophane 84 enantiomeric excesses up to 65% were reported by Toda et al. (for methyl p-tolyl sulfide as substrate), at typical catalyst loadings of ca. 10 mol% (relative to the thioether). The same substrate gave the sulfoxide with 32% ee when the iminium cation 85 was used [132],... 

Figure 10. Calculated effects of axial thioether ligation upon copper d orbitals. The relative energies of the antibonding orbitals and their predominant copper d character are indicated for the two sites shown. The energies of the half-occupied level have been set to zero. Reproduced from Ref. 14.

The a-deprotonation of conformationally constrained thioethers can proceed with high diastereoselectivity (Scheme 2.16) [54]. That equatorial protons are removed much more rapidly than axial protons suggests that stabilization of carbanions Qt -S-Q by sulfur is mainly a result of hyperconjugation between the carbanion lone pair and the antibonding S-Ca- orbital [55]. 

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