Interpretation of magnetic resonance data in the context of structural and chemical biology requires prior knowledge of the g-tensor directions for paramagnetic metallo-cofactors with respect to the protein structural frame. Access to this information is often limited by the strict requirement of suitable protein crystals for single-crystal electron paramagnetic resonance (EPR) measurements or the reliance on protons (with ambiguous locations in crystal structures) near the paramagnetic metal site. Here we develop a novel pulsed EPR approach with selective 13 C β -cysteine labeling of model [2Fe-2S] proteins to help bypass these problems. Analysis of the 13 C β -cysteine hyperfine tensors reproduces the g-tensor of the Pseudomonas putida ISC-like [2Fe-2S] ferredoxin (FdxB). Its application to the hyperthermophilic archaeal Rieske-type [2Fe-2S] ferredoxin (ARF) from Sulfolobus solfataricus, for which the single-crystal EPR approach was not feasible, supports the best-fit g x -, g z -, and g y -tensor directions of the reduced cluster as nearly along Fe-Fe, S-S, and the cluster plane normal, respectively. These approximate principal directions of the reduced ARF g-tensor, explored by 13 C pulsed EPR, are less skewed from the cluster molecular axes and are largely consistent with those previously determined by single-crystal EPR for the cytochrome bc 1 -associated, reduced Rieske [2Fe-2S] center. This suggests the approximate g-tensor directions are conserved across the phylogenetically and functionally divergent Rieske-type [2Fe-2S] proteins.
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