Abstract
The ENDOR response of 57Fe nuclei and protons of the high-potential iron−sulfur (HiPIP) protein iso-II from Ectothiorhodospira halophila in frozen solutions, i.e., on nonoriented systems, has been exploited to 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 and mixed-valence iron ions in agreement with results of Mossbauer and ENDOR studies on related proteins and model compounds. From the analysis of dipolar contributions of the eight cysteine β-CH2− and five additional protons of residues close to the cluster, the spin population on the iron ions in the ferric and the mixed-valence pair was deduced. The symmetric spin vector coupling model yields coefficients, which suggest the existence of 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. The identification of the mixed-valence and ferri...
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