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Low-temperature oxidation of [4Fe-4S] analogues : generation of a Fe/S cluster spectroscopically similar to the 3-Fe clusters in the 3-Fe ferredoxins

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  • Biology
  • Chemistry


324 Inorg. Chem. 1987, 26, 324-329 magnetic susceptibilities indicative of spin states significantly higher than 1/219 (Table 111). The room temperature magnetic susceptibility and the chemical shifts of the protons of the (4Fe-4Se)' clusters of reduced Se-substituted Cp Fd are larger than those of native reduced Cp Fd (Figure 2, Table 111). These observations may be correlated with the occurrence of high- multiplicity spin states a t low temperature. At cryogenic temperatures, the reduced Se-substituted clostridial ferredoxins display three spin states, namely S = I /* , S = 3/2 and S = 7/2335, and one may wonder whether a similar spin-state heterogeneity occurs a t functionally relevant temperatures. The IH N M R spectra of the reduced Fd from Bacillus polymyxa14 and Bacillus s tear~thermophi lus ,~~ which contain a single (4Fe-4s)' cluster, display eight to ten manetically shifted proton resonances, out of the 0-10 ppm range. The spectra of the reduced clostridial ferredoxins involve ca. twice as many (17-20) shifted proton resonances as those of the ferredoxins containing a single cluster8 (Table 11). It may therefrom be inferred that the two clusters in clostridial Fd differ from each other with respect to the chemical shifts of their neighboring protons. The reduced Se-substituted Cp Fd displays no more shifted proton resonances than its native counterpart (Table 11), which suggests the presence of only two different types of (4Fe-4Se)' clusters in the former protein; the occurrence of three sets of spin states would imply that ca. eight to ten proton resonances have escaped observation. This seems unlikely, since no additional proton resonances have been observed in the 170-330 ppm range. Proton resonances occurring at lower field would probably be difficult to detect, due to line broadening. They would however be associated with clusters having high magnetic susceptibility, and therefore the presence of very low field resonances would result in

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