Iron ligands

In the same manner, the S-monoxidized iron complex can be oxidized with a second equivalent of 3-chloroperoxybenzoic acid and subsequent irradiation to give the stable 1-benzothiepin 1,1-dioxide in 76% yield.23 (For removal of the iron ligand, see also Section 2.2.1.). 

The greater lability of 1-benzothiepin 1-oxides, compared to the parent compounds, may lead to differences in chemical behavior. Thus, treatment of the tricarbonyliron complex of 1-benzo-thiepin 1-oxide (8, X = SO) with ammonium cerium(IV) nitrate in acetone at — 30 °C leads, with the loss of sulfur monoxide, to naphthalene. In contrast, the iron ligand can be removed selectively from the corresponding 1-benzothiepin by ammonium cerium(IV) nitrate.23 92 For the synthesis of 1-benzothiepin 1-oxide, see Section 2.1.4.1,... 

The preferential conversion of the aluminum enolatc 2 c to diastereomer 3 is consistent with the boat-like transition state A where the enolate has adopted the usual antiperiplanar E geome-try42-50. The aldehyde substituent occupies the lower energy equatorial position of a boat-like transition state which places the bulky dialkylaluminum moiety away from the iron ligands. Possible transition states for the other observed diastereomeric products are also illustrated. 

In addition to the variation in electronic configuration, the geometric details of the coordination sphere and the properties of iron-ligand bonds (different a- or 71-donor strength) also influence the isomer shift as observed for a series of compounds ... 

Iron-Ligand Vibrations in Spin-Crossover Complexes... 

The driving force for the temperature-dependent spin crossover (SCO) is the entropy difference between the HS and the LS isomers which arises mainly from a shift of the vibrational frequencies when passing from the HS to the LS state [97-99]. This frequency shift has been studied by IR- and Raman-spectroscopy and recently also by NIS [23, 39, 87]. The NIS method is isotope ( Fe) selective and, therefore, its focus is on iron-ligand bond-stretching vibrations which exhibit the most prominent contribution to the frequency shift upon SCO [87]. 

Table 9.4 Assignment of vibrational iron-ligand modes based on a DFT calculation on the model with 21 atoms...

In tracing the evolutionary development of iron ligands it is of interest to examine the machinery employed by organisms which carry out reactions on those substances believed to have been present on the primitive Earth. Specific substrates acted on by this group include, besides ferrous iron itself, hydrogen sulfide, hydrogen gas, methane and reduced nitrogen compounds. Species which perform photosynthesis may be presumed to have the capacity to synthesize protoporphyrin IX since this substance is an intermediate in chlorophyll biosynthesis (43). 

There are two types of electron transport those involving flavoproteins and iron-sulfur proteins, and those requiring only flavoproteins. The X-ray crystal structure of the soluble cytochrome P450 from Pseudomonas putida grown on camphor (P-450-CAM) has been determined (Poulos et ah, 1985), as have several others. The haem group is deeply embedded in the hydrophobic interior of the protein, and the identity of the proximal haem iron ligand, based on earlier spectroscopic studies (Mason et ah, 1965) is confirmed as a specific cysteine residue. 

Figure 10.2. Amino acid seqnence alignment of D. vulgaris HUdenborongh DcrH residues 824-959 (top) with Phascolopsis gouldii Hr (bottom). Letters indicate residues that are common to both sequences, which are also conserved in all known Hr sequences boldface indicates residues furnishing iron ligands in Hr. Reprinted with permission from Xiong et al. (2000), copyright 2000 American Chemical Society.

Cytochromes are electron-transfer proteins having one or several haem groups. Cytochrome c binds to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochrome c has been proved to be a useful model system for studying the relationship between protein structure and thermostability due to the availability of its three-dimensional structure from a wide variety of organisms, both mesophiles and thermophiles. 

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