Paramagnetic NMR to study iron sulfur proteins: 13C detected experiments illuminate the vicinity of the metal center

The robustness of NMR coherence transfer in proximity of a paramagnetic center depends on the relaxation properties of the nuclei involved. In the case of Iron-Sulfur Proteins, different pulse schemes or different parameter sets often provide complementary results. Tailored versions of HCACO and CACO experiments significantly increase the number of observed C^α/C’ connectivities in highly paramagnetic systems, by recovering many resonances that were lost due to paramagnetic relaxation. Optimized ¹³C direct detected experiments can significantly extend the available assignments, improving the overall knowledge of these systems. The different relaxation properties of C^α and C’ nuclei are exploited in CACO vs COCA experiments and the complementarity of the two experiments is used to obtain structural information. The two [Fe₂S₂]⁺ clusters containing NEET protein CISD3 and the one [Fe₄S₄]²⁺ cluster containing HiPIP protein PioC have been taken as model systems. We show that tailored experiments contribute to decrease the blind sphere around the cluster, to extend resonance assignment of cluster bound cysteine residues and to retrieve details on the topology of the iron-bound ligand residues.