1H NMR of high-potential iron-sulfur protein from the purple non-sulfurbacterium Rhodoferax fermentans

Variation of average g values and effective exchange coupling constants among [2Fe-2S] clusters: a density functional theory study of the impact of localization (trapping forces) versus delocalization (double-exchange) as competing factors

A phenomenological model aimed at rationalizing variations in both average g-tensor values (gav identical with 1/3Sigmaigi ) and effective exchange coupling constants Jeff (defined as two-thirds of the energy difference between the S = 3/2 and S = 1/2 spin states) has been derived in order to describe the great variety of magnetic properties exhibited by reduced [2Fe-2S] clusters in proteins. The key quantity in the present analysis is the ratio Delta E/B computed from two competing terms. Delta Ecomprises various effects that result in trapping-site asymmetries: vibronic coupling and the chemical nature (S/N/O) and conformations of the ligands on the one hand and solvation terms, the hydrogen bonding network, etc., on the other. All of these additive terms (in a "bottom-up" approach) favor valence localization of the reducing electron onto one of the two iron sites. In contrast, the B term is the double-exchange term, which favors electronic delocalization. Both gav and Jeff can be expressed as functions of Delta E/ B. We have also shown that electronic localization generally favors small gav and large Jeff values (while the opposite is true for electronic delocalization) in a comparative study of the spectroscopic features of plant-type ferredoxins (Fd's) and Rieske centers (and related mutants). Two other types of problems were particularly challenging. The first of these involved deprotonated Rieske centers and the xanthine oxidase clusters II, which are characterized by very small Jeff values (40-45 cm (-1) with a J S A. S B model) correlated with unusually large gav values (in the range 1.97-2.01) as a result of an antisymmetric exchange coupling mechanism. The second concerned the analogous Fd's from Clostridium pasteurianum (Cp) and Aquifex aeolicus (Aa). Detailed Mössbauer studies of the C56S mutant of the Cp system revealed a mixture of clusters with valence-localized S = 1/2 and valence-delocalized S = 9/2 ground states. We relied on crystallographic structures of wild-type and mutant Aa Fd's in order to explain such a distribution of spin states.

1H and (13)C NMR Studies of an Oxidized HiPIP

1H-(13)C HETCOR NMR spectra have been recorded for the oxidized HiPIP I from Ectothiorhodospira halophila for which an extended (1)H assignment was available. The hyperfine shifts of the alpha and beta carbons of the coordinated cysteines, as well as those of their attached protons, have been discussed in terms of the current magnetic coupling models and of the mechanisms of spin density delocalization. Through HSQC spectra preceded by a proton 180 degrees pulse, the nonselective T(1) values of the protons have been accurately obtained. It is shown how the nuclear T(1) values can be used as constraints, together with NOEs, for solution structure determination even when the present magnetic coupling scheme occurs. The oxidized cluster is shown to have an effective relaxation time much shorter than that in the reduced state.

The primary structure of Rhodoferax fermentans high-potential iron-sulfur protein, an electron donor to the photosynthetic reaction center

The complete amino acid sequence of Rhodoferax fermentans high-potential iron-sulfur protein (Hipip), which is known to be an efficient electron donor to the photosynthetic reaction center, has been determined using both N-terminal and C-terminal analyses. The sequence contains 75 residues, with 11 positive charges, 10 negative charges, and one histidine residue. The molecular mass of apo-Hipip, determined by electrospray ionization mass spectrometry, is 7849.64 Da. Multiple sequence alignment, based both on primary and tertiary structure information, reveals conservation of Tyr19 and Gly75 (Chromatium vinosum numbering) in addition to the four [Fe4S4]-bound cysteines. The Hipip from Rf. fermentans is most similar (57% similarity) to the Hipip from Rubrivivax gelatinosus, a photosynthetic bacterium belonging to the beta-1 subgroup of the proteobacteria.

Use of 1H longitudinal relaxation times in the solution structure of paramagnetic proteins. Application to [4Fe-4S] proteins

The accuracy of the solution structures determined by NMR is often poor around paramagnetic centers because the properties of the near protons are strongly perturbed by the electronic spin. The structural information contained in the relaxation rates of these protons has been extracted here by measuring the longitudinal relaxation times with the inversion-recovery total correlation spectroscopy (IR-TOCSY) sequence based on the recovery of cross peaks. In addition to measurements with nonselective inversion-recovery for nonoverlapping signals, reliable data have been obtained for a majority of main-chain protons from Chromatium vinosum high-potential ferredoxin. When a small and constant contribution from diamagnetism as well as the electronic spin distribution over the [4Fe-4S] cluster are taken into account, the shortest longitudinal relaxation times depend directly on the distance separating the protons from the paramagnetic center. This indicates that electron-nuclei dipolar interactions are the most efficient relaxation mechanism for these protons. However, the expected dependence of the relaxation rates as the sixth power of the distance has to be corrected because of induced relaxation among fast relaxing protons. This approach reveals that the solution structure of the protein is significantly different from the crystal structure around Phe-48. In addition, it provides an independent confirmation of the actual electronic structure of the [4Fe-4S]3+ cluster in the protein. The method devised in this work, which does not rely on specific enrichment, should be useful to improve the determination of NMR-derived solution structures of paramagnetic macromolecules.