Ferredoxin peptide model complexes

B. [2Fe 2S] Plant-Type Ferredoxin Peptide Model Complexes... 

D. [4Fe-4S] Ferredoxin Peptide Model Complexes and Model Reactions... 

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). 

Complete sequences of ca. 50 different plant-type ferredoxins(Fd) are known. The invariant sequences nearest to the 2-Fe core are confirmed to be Pro-Tyr-Ser-Cys-Arg-Ala-Gly-Ala-Cys-Ser-Thr-Cys-Ala-Gly and Leu-Thr-Cys-Val. 2Fe-2S complexes of oligopeptides with the Cys-X-Y-Cys sequence have been synthesized by ligand exchange reactions (7,23). We have examined the redox potentials of these model complexes, and the results are shown in Table I. The reversibility improved remarkably and the potential approached the value of the native proteins as the sequence more closely simulated that of the proteins. It is conjectured that hydrogen bonds from the peptide N-H s to thiolate and/or sulfide groups increase the stability of the reduced cluster. It is likely that peptide sequences like those found in the proteins favor the formation of such hydrogen bonds. 

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. 

The synthetic [2Fe-2S] model complex of the 20-peptide complex exhibits two LMCT absorption maxima at 423 and 461 nm in DMF, maxima which are near to those of the native plant-type ferredoxin (423 and 466 nm) (69). Two redox couples for — 3/—2 were observed at — 0.64 V versus SCE and at —0.96 V versus SCE in DMF. One of them is very close to the value (—0.64 V versus SCE) of native ferredoxin. The 20-peptide complex containing invariant sequences Cys-A-B-C-D-Cys-X-Y-Cys and Leu-Thr-Cys-Val possesses all essential factors for a model of the active site except for the peptide conformation. The positive-shifted redox potential of the 20-peptide complex in DMF is undoubtedly due to the interactions between the Fe2S22+ core and adjacent amino-acid residues, giving rise to NH--S hydrogen bonding. 

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