H-1 NMR studies of the Fe7S8 ferredoxin from Bacillus schlegelii: A further attempt to understand Fe3S4 clusters

The oxidized Fe7S8 ferredoxin from Bacillus schlegelii, containing both [Fe3S4](+) and [Fe4S4](2+) clusters, has been investigated by H-1 NMR spectroscopy. An extensive sequence-specific assignment of the hyperfine-shifted resonances has been obtained by making use of a computer-generated structural model. The pattern and the temperature dependence of the hyperfine shifts of the beta-CH2 protons of the cysteines coordinating the [Fe3S4](+) cluster are rationalized in terms of magnetic interactions between the iron ions. The same approach holds for the hyperfine coupling with Fe-57. It is shown that the magnetic interactions are more asymmetric in Fe7S8 ferredoxins than in Fe3S4 ferredoxins. The NMR non-observability of the beta-CH2 protons of coordinated cysteines in the one-electron-reduced form has been discussed.