| Abstract
| - The ENDOR response of 57Fe nuclei and protons of the high-potential iron−sulfur (HiPIP) proteiniso-II from Ectothiorhodospira halophila in frozen solutions, i.e., on nonoriented systems, has been exploitedto determine electronic and structural details of the oxidized [Fe4S4]3+ cluster and its protein environment.Two distinct 57Fe hyperfine couplings were resolved and assigned to pairs of highly symmetric ferric andmixed-valence iron ions in agreement with results of Mössbauer and ENDOR studies on related proteins andmodel compounds. From the analysis of dipolar contributions of the eight cysteine β-CH2− and five additionalprotons of residues close to the cluster, the spin population on the iron ions in the ferric and the mixed-valencepair was deduced. The symmetric spin vector coupling model yields coefficients, which suggest the existenceof a |7/2,3,1/2〉 state or an admixture of |9/2,4,1/2〉 and |7/2,4,1/2〉 as possible ground states of the cluster. Theidentification of the mixed-valence and ferric irons within the cluster was in agreement with NMR resultsbased on the sequence specific assignments of proton couplings. In addition, a unique orientation of the g-tensorwith respect to the molecular frame was found in the protein, the maximal g-tensor component being nearlyperpendicular to the cluster face containing the mixed-valence irons. The intermediate and minimal componentswere related to the vectors connecting the ferric and mixed-valence irons, respectively. The analysis of theisotropic parts of the cysteine β-CH2-proton interactions allowed establishment of a correlation with the NMRshifts of corresponding protons, obtained by applying different scaling factors for protons close to the ferricand mixed-valence pair, respectively. The empirical law used to describe the relationship between the geometricorientation of a CH bond and the observed isotropic interaction for these types of clusters could be verified.
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