Abstract
A ^1^H 2D NMR investigation of the oxidized 2[4Fe‐4S] ferredoxins from Clostridium pasteurianum and C. acidurici was carried out. Sequential assignments through standard HOHAHA and NOESY procedures were obtained for the 2–7, 23–36 and 52–55 sequence segments of both proteins. Additional assignments of the 15–17 and 44–46 segments, linking the clusters, and of a few other residues were made by taking advantage of sequence differences between the two proteins. In summary, only the immediate vicinity of the cysteine ligands could not be identified by the procedure. In these small 2[4Fe‐4S] ferredoxins, a large proportion of the resonances is sensitive to the presence of the paramagnetic centres and does not easily display NOE correlations. Nevertheless, long‐range NOE peaks that could be observed shed light on the solution structure of these proteins. The close interaction between the N‐ and C‐termini, previously evidenced by x‐ray crystallography, was confirmed for both proteins in solution. Small differences between the ferredoxins from C. pasteurianum and C. acidurici were detected in the flexible and variable external 25–30 loop and also in the region separating the clusters. These differences may correlate with small dissimilarities previously observed between some properties of these ferredoxins.