Paramagnetic Properties of a Crystalline Iron–Sulfur Protein by Magic-Angle Spinning NMR Spectroscopy

We present the first solid-state NMR study of an iron–sulfur protein. The combined use of very fast (60 kHz) magic-angle spinning and tailored radiofrequency irradiation schemes allows the detection and the assignment of most of the ¹H and ¹³C resonances of the oxidized high-potential iron–sulfur protein I from Ectothiorhodospira halophila (EhHiPIP I), including those in residues coordinating the Fe₄S₄ cluster. For these residues, contact shifts as large as 100 and 400 ppm for ¹H and ¹³C resonances, respectively, were observed, which represent the most shifted solid-state NMR signals ever measured in metalloproteins. Interestingly, by targeting EhHiPIP I in a crystalline environment, we were able to capture distinct paramagnetic signatures from the two conformations present in the asymmetric unit. The magnetic properties of the system were verified by following the temperature dependence of the contact-shifted cysteine resonances.