Sulfur-centered complexes

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

Complex III transfers electrons from reduced coenzyme Q (UQH2) to cytochrome c. Because it contains two b-type cytochromes, one cytochrome Cj (cyt Cj), and one iron-sulfur center, complex III is sometimes referred to as the cytochrome bcj complex. The cytochromes (Figure 10.6) are a series of electron transport proteins that contain a heme prosthetic group similar to those found in hemoglobin and myoglobin. Electrons are transferred one at a time as each oxidized iron atom (Fe3+) is reversibly reduced to Fe2+. The movement of electrons from UQH2 to cytochrome c is a complex, multistep process. Because UQ is lipid-soluble, it diffuses within the inner membrane between the electron donors in complex I or II and the electron acceptor in complex... 

Without question, the most significant advance in the use of sulfur-centered nucleophiles was made by Shibasaki, who discovered that 10 mol% of a novel gallium-lithium-bis(binaphthoxide) complex 5 could catalyze the addition of tert-butylthiol to various cyclic and acyclic meso-epoxides with excellent enantioselectiv-ities and in good yields (Scheme 7.11) [21], This work builds on Shibasaki s broader studies of heterobimetallic complexes, in which dual activation of both the electrophile and the nucleophile is invoked [22]. This method has been applied to an efficient asymmetric synthesis of the prostaglandin core through an oxidation/ elimination sequence (Scheme 7.12). 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

Fe-4S] + + clusters are certainly the most ubiquitous iron-sulfur centers in biological systems. They play the role of low potential redox centers in ferredoxins, membrane-bound complexes of the respiratory... 

Despite its weakness, the anisotropy of the g tensor of iron-sulfur centers can be used to determine the orientation of these centers or that of the accommodating polypeptide in relation to a more complex system such as a membrane-bound complex. For this purpose, the EPR study has to be carried out on either partially or fully oriented systems (oriented membranes or monocrystals, respectively). Lastly, the sensitivity of the EPR spectra of iron-sulfur centers to structural changes can be utilized to monitor the conformational changes induced in the protein by different factors, such as the pH and the ionic strength of the solvent or the binding of substrates and inhibitors. We return to the latter point in Section IV. 

Similar difficulties have been encountered in the case of complex enzymes such as fumarate reductase and nitrate reductase from E. coli, in which substituting certain Cys ligands led to the loss of several if not all the iron-sulfur centers (171, 172). However, in the case of nitrate reductase, which possesses one [3Fe-4S] and three [4Fe-4S] centers, it was possible to remove selectively one [4Fe-4S]... 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

Treatment of l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene with SC12 and S0C12 resulted in 87 and 88, respectively (67). Complex 87 can be converted into the corresponding fluoride (89) via reaction with AgF. Surprisingly, however, reduction of the sulfur center was ob-... 

Complexes of bismuth chloride with 2 equiv of the AT,AT-substituted dithioxamide ligands of the general formulas [BiCl3 RNHC(S)C(S) NHR 2] (R = Me, Et, Pr, nBu, CH2Ph) likely involve five-membered chelate interactions (123) as demonstrated by the structurally characterized example [BiCl3 EtNHC(S)C(S)NHEt 2] Me2CO, which involves a seven-coordinate pentagonal bipyramidal environment for bismuth (124), with chlorine atoms in axial positions [Bi-Cl 2.635(10) and 2.717(1) A] and one equatorial [Bi-Cl 2.651(12) A] with four sulfur centers [Bi-S 2.818(13)-3.042(12) A],... 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

More complex iron-sulfur centers in the MoFe-protein of Mo-nitrogenase are discussed next. 

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