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

Abstract

 The oxidized Fe₇S₈ ferredoxin from Bacillus schlegelii, containing both [Fe₃S₄]⁺ and [Fe₄S₄]²⁺ clusters, has been investigated by ¹H 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 β-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 ⁵⁷Fe. It is shown that the magnetic interactions are more asymmetric in Fe₇S₈ ferredoxins than in Fe₃S₄ ferredoxins. The NMR non-observability of the β-CH₂ protons of coordinated cysteines in the one-electron-reduced form has been discussed.