Ferredoxin model

The procedure we follow is very similar to that of Harriman " . Again, as in the case of the 2-Fe ferredoxin model," these two methods, i.e. ROHF and... 

Studies of the catalytic or stoichiometric reactions of various 4-Fe-ferredoxin model complexes have been reported using the 3-/2— redox couple with a negative redox potential91 >. Relatively stable are the 2—/I — redox couple for [Fe(Z-cyst-Ile-Ala-OMe)J2 (Z = benzyloxycarbonyl) or [Fe4S4(tipbt)J2 (tripbt = 2,4,6-triiso-propylbenzene-thiolato) in A, A -dimethylformamide. Catalytic oxidation of benzoin by 1,4-benzoquinone in the presence of various cyst-containing peptide complexes or bulky thiolato complexes in DMF has been examined. The postulated mechanism of the catalytic oxidation of benzoin in the presence of [Fe4S2(SR)4]2 is illustrated in Fig. 7. 

Activation of an M-CO bond for nucleophilic substitution in anion radical metallo-complexes appears to be quite a general effect (Kaim 1987 Mao et al. 1989,1992 Shut et al. 1995 Klein et al. 1996). Such activation seems to be the basis of metal-cluster catalytic activity. The iron-sulfur cluster (Bu4N)2Fe4S4(SPh)4 deserves to be mentioned here. The cluster is considered as a ferredoxin model (Inoue Nagata 1986) it catalyzes an electron transfer from //-butyl lithium or phenyl lithium to 5-phenyl thiobenzoate or phenylbenzoate (Inoue Nagata 1986). 

Reductions of acetylene, ketone, isonitrile, or nitrile have been carried out using the —3/ —2 redox couple of many model complexes (58-61), but not using the peptide model complex. A proton transfer function has been observed in studies on the ferredoxin model complex, [Fe4S4(SPh)4]2, in toluene/water (62). 

The iron-sulphur centre is probably of the 2Fe-2S type [191,223]. It is a one-electron donor/acceptor with of approx. 280 mV in mitochondria (pH independent below pH 8 [224,225]). It exhibits an EPR spectrum in the reduced state that is somewhat anomalous for 2Fe-2S clusters (see Ref. 221). This, as well as the high midpoint redox potential, suggest that the iron ligands may be less electronegative than the four cysteine sulphurs of the plant ferredoxin model (see Ref. 226). The EPR spectrum of the FeS cluster is affected by the redox state of ubiquinone... 

The initial 6 A-resolution X-ray work showed one of the [4Fe 4S] clusters is 15 A and the other 22 A from the preceding iron-sulfur electron acceptor, FeS-X. Discussion of the iron-sulfur center FeS-X itself will be deferred to Chapter 31, where the interaction of the FeS-X domain with PsaC (FeS-A/FeS-B) and its involvement in electron transfer to FeS-A/B will also be addressed. More recent work by the Berlin group at 4 A resolution has refined the distance measurements to 15.4 and 22.2 A, respectively. Fig. 12 (B) shows a three-dimensional model of PsaC based on low-temperature EPR measurements by Kamlowski and coworkers on PS-I single crystals. Their interpretation was made by using the bacterial ferredoxin model as a PsaC substitute in the PS-I reaction center matrix, and adjusting its orientation for the best agreement with the EPR data. The g-tensor orientation of EeS-A and FeS-B in the PS-I single crystals determines the PsaC orientation relative to the two clusters. 

Siegel B. (1979). Preparation and redox properties of a cyclodextrin based ferredoxin model. Journal of Inorganic and Nuclear Chemistry 41 609-610. 

Noodleman, L., 8c Baerends, E. J. (1984). Electronic structure, magnetic properties, ESR, and optical spectra for 2-iron ferredoxin models by LCAO-Xa valence bond theory. Journal of the American Chemical Society, 106, 2316. 

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

In an earlier work, we have proposed a theoretical procedure for the spectroscopy of antiferromagnetically (AF) coupled transition-metal dimers and have successfully applied this approach to the electronic absorption spectrum of model 2-Fe ferredoxin. In this work we apply this same procedure to the [Fe2in - 82) P o - CeH48)2)2 complex in order to better understand the electronic structure of this compound. As in our previous work" we base our analysis on the Intermediate Neglect of the Differential Overlap model parameterized for spectroscopy (INDO/S), utilizing a procedure outlined in detail in Reference 4. 

Detailed MCD experiments carried out on the dithionite-reduced forms of the C56S and C60S mutants of C. pasteurianum ferredoxin have shown that a large fraction of the [2Fe-2S] centers have an iS = 9/2 ground state (148). These centers exhibit temperature-dependent features at ges = 10.1, 9.25, 6.00, and 3.00, corresponding to E/D = 0.16 with D = 1.1 cm (148). According to the model pre-... 

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