Carbon-13 Solid-State NMR Studies on Synthetic Model Compounds of [4Fe−4S] Clusters in the 2+ State

Abstract

Carbon-13 solid-state NMR is used to determine, via the temperature dependence of paramagnetic shifts, the spin ladder of magnetic levels and the related spin exchange parameters in [4Fe−4S]2+ clusters. This study has been carried out between 180 and 330 K in three model compounds of the active sites of 4Fe−4S proteins: [(C2D5)4N]2[Fe4S4(S13CD2C6D5)4], [(C2H5)4N]2[Fe4S4(SC(CH3)3)4], and [(C2H5)4N]2[Fe4Se4(SC(CH3)3)4]. The temperature dependencies of both the isotropic and anisotropic parts of the 13C paramagnetic shift tensors have been measured and analyzed. Antiferromagnetic exchange coupling constants among iron atoms of J ≈ 430, 380, and 370 cm-1 have been deduced from fits of their temperature dependencies. Additional contributions of the double-exchange term B favoring electronic delocalization and of a ΔJ term associated with the compression of the cubane geometry are also discussed. We also show that connections between these measurements and those of magnetic susceptibility are especially close in the case of the [4Fe−4S]2+ redox state.

Sensitivity enhancement, assignment, and distance measurement in 13C solid-state NMR spectroscopy for paramagnetic systems under fast magic angle spinning

“…13 C SSNMR is a potentially powerful method for characterizing paramagnetic organic systems without the requirement of isotope-labeled materials. In contrast, for paramagnetic organic complexes, applications of 13 C SSNMR and SSNMR of other dilute spin-1/2 nuclei have been notably limited, despite the success of previous studies [30][31][32][33][34][35][36][37][38]. 13 C paramagnetic shift dispersions can range from several hundreds to a few thousands of ppm, while those in 1 H SSNMR can vary from several tens to several hundreds of ppm [34,38,39].…”

“…In spite of the excellent result, the moderate spinning speed is useful only for systems having a small 1 H-1 H dipolar flip-flop rate due to a large 1 H shift dispersion, isotopic dilution, or molecular motions [34,38,40]. For this reason, 2 D-labeled paramagnetic compounds have been generally utilized in 13 C SSNMR to remove 1 H-13 C couplings without 1 H RF decoupling [33,34,37,41]. Furthermore, polarization transfer by CP, a vital technique in 13 C SSNMR , has been ineffective for many paramagnetic systems because of the large shift dispersions, except in a few successful cases for compounds having small shift dispersions [30,36,37].…”

“…However, recoupling techniques have not been applied to paramagnetic systems because of the limited sensitivity and the lack of effective decoupling methods. Only a few 2D SSNMR experiments have been performed for 13 C-labeled small paramagnetic systems [35,37]. Thus, even if 13 C SSNMR spectra can be obtained for unlabeled systems, reliable signal assignments have required selectively 13 C-and/or 2 D-labeled samples [34,39].…”

Paramagnetic shifts in solid-state NMR of proteins to elicit structural information

“…Small paramagnetic molecules have been studied through magic angle spinning (MAS) SSNMR for decades (27)(28)(29)(30)(31)(32)(33). Paramagnetism in the solid state causes problems connected with the large shift anisotropy, inhomogeneous broadening (34), and the difficulties in obtaining efficient proton decoupling (30,32).…”

Redox-Related Chemical Shift Perturbations on Backbone Nuclei of High-Potential Iron−Sulfur Proteins

“…[26][27][28][29][30][31] On the other hand, the reduced state contains an [Fe 4 S 4 ] 2+ cluster that has a diamagnetic ground state and only experiences a residual paramagnetism due to population of paramagnetic excited states. 32,33 The residual magnetic moment of the reduced state corresponds to less than half of that of the oxidized protein. 34 This state has virtually no magnetic anisotropy.…”