Redox fragment

MoCl2(Tp )(NO)] reacts with an equimolar quantity of the zinc121 and copper122 Schiff base complexes yielding heterobinu-clear complexes in which the redox fragment [MoClTp (NO)]+ is attached to the functionalized Schiff base derived by... 

We are now exploring a different approach to the synthesis of multicentre redox systems, in which the redox fragments are held together not by covalent bonds, but through coordinative interactions. Assembled systems in which more subunits are linked together through non-covalent interactions have been defined as supramolecular systems [9]. In this sense, coordina-tively linked multicentre systems belong to the realm of supramolecular chemistry and, in particular, they shovdd be defined as supramolecular coordination compounds. 

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

Various methods have been used to determine the redox potentials (Table XI). Very commonly, EPR-monitored chemical redox titration is performed, which can be used to measure the redox potential not only in isolated complexes but also in membrane preparations. In general, there is good agreement between redox potentials determined in membranes, isolated complexes, or isolated Rieske proteins or fragments the only exception is the water-soluble Rieske fragment from spinach bef complex where differences of more than 50 mV have been observed by the same group but using different methods (31). 

Fig. 18. pH dependence of the oxidized Rieske fragment from bovine heart mitochondria (ISF). (a) Redox potential determined by cyclic voltammetry. The line was fitted to the data points, giving = 7.6 and pi a, x2 = 9-2. (b) CD intensity of the oxidized... 

FIGURE 6.34 Regeneration of a-tocopherol by base-catalyzed fragmentation of 5a-toco-pheryl ascorbate (47) followed by a redox process. 

In a similar vein, various electron acceptors yielding anion radicals that undergo rapid unimolecular decomposition also facilitate the efficacy of Scheme 1 by effectively obviating the back-electron transfer. For example, the nitration of enol silyl ether with tetranitromethane (TNM) occurs rapidly (despite an unfavorable redox equilibrium)78 owing to the fast mesolytic fragmentation of the TNM anion radical79 (Scheme 15). 

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