The [Fe-Fe]-hydrogenase maturation protein HydF from Thermotoga maritima is a GTPase with an iron-sulfur cluster.

International audienceThe active site of [Fe-Fe]-hydrogenases is composed of a di-iron complex, where the two metal atoms are bridged together by a putative di(thiomethyl)amine molecule and are also ligated by di-nuclear ligands, namely carbon monoxide and cyanide. Biosynthesis of this metal site is thought to require specific protein machinery coded by the hydE, hydF and hydG genes. The HydF protein has been cloned from the thermophilic organism Thermotoga maritima, purified and characterized. The enzyme possesses specific amino-acid signatures for GTP-binding and is able to hydrolyze GTP. The anaerobically reconstituted TmHydF protein binds a [4Fe-4S] cluster with peculiar EPR characteristics: an S=1/2 signal presenting a high field shifted g-value together with a S=3/2 signal, similar to those observed for [4Fe-4S] clusters ligated by only three cysteines. HYSCORE spectroscopy experiments were carried out in order to determine the nature of the cluster's fourth ligand and its exchangeability was demonstrated with the formation of a [4Fe-4S]-imidazole complex

Investigation of native defects in BaSi2 epitaxial films by electron paramagnetic resonance

We investigated photoresponse, photoluminescence (PL), and electron paramagnetic resonance (EPR) spectra of 0.5 μm thick BaSi2 films grown by molecular beam epitaxy using various Ba-to-Si deposition rate ratios (R Ba/R Si). BaSi2 films (R Ba/R Si = 2.2) showed the highest photoresponsivity at room temperature. In contrast, BaSi2 films with R Ba/R Si away from 2.2 showed low photoresponsivity but intense sub-bandgap PL at 9 K. An anisotropic EPR line was observed below 20 K for such BaSi2 films. The EPR line disappeared for BaSi2 films passivated with atomic hydrogen. Thereby, the PL and EPR signals are interpreted to originate from native defects in the BaSi2 films

Quantum simulations and experiments on Rabi oscillations of spin qubits: intrinsic {\sl vs} extrinsic damping

Electron Paramagnetic Resonance experiments show that the decay of Rabi oscillations of ensembles of spin qubits depends noticeably on the microwave power and more precisely on the Rabi frequency, an effect recently called "driven decoherence". By direct numerical solution of the time-dependent Schr\"odinger equation of the associated many-body system, we scrutinize the different mechanisms that may lead to this type of decoherence. Assuming the effects of dissipation to be negligible ($T_1=\infty$), it is shown that a system of dipolar-coupled spins with -- even weak-- random inhomogeneities is sufficient to explain the salient features of the experimental observations. Some experimental examples are given to illustrate the potential of the numerical simulation approach.Comment: Accepted for publication in Physical Review

Lesion-induced DNA weak structural changes detected by pulsed EPR spectroscopy combined with site-directed spin labelling

Double electron-electron resonance (DEER) was applied to determine nanometre spin–spin distances on DNA duplexes that contain selected structural alterations. The present approach to evaluate the structural features of DNA damages is thus related to the interspin distance changes, as well as to the flexibility of the overall structure deduced from the distance distribution. A set of site-directed nitroxide-labelled double-stranded DNA fragments containing defined lesions, namely an 8-oxoguanine, an abasic site or abasic site analogues, a nick, a gap and a bulge structure were prepared and then analysed by the DEER spectroscopic technique. New insights into the application of 4-pulse DEER sequence are also provided, in particular with respect to the spin probes’ positions and the rigidity of selected systems. The lesion-induced conformational changes observed, which were supported by molecular dynamics studies, confirm the results obtained by other, more conventional, spectroscopic techniques. Thus, the experimental approaches described herein provide an efficient method for probing lesion-induced structural changes of nucleic acids

Etude par RPE résolue en temps et en spin trapping détecté par spectrométrie de masse de mécanismes radicalaires dans la photochimie des pyrimidines et des nitroaromatiques

Cet ouvrage décrit l'étude expérimentale de radicaux libres d'intérêt biologique en solution et à température ambiante. Les mécanismes explorés concernent : d'une part la dégradation photochimique du matériel génétique, plus précisément la photochimie radicalaire de dérives des pyrimidines en UV-C en solutions aqueuse et organique. D'autre part le mode d'action de certains précurseurs photochimiques de substances bioactives, en particulier la photoréduction des composés nitroaromatiques. Les radicaux libres impliqués dans ces mécanismes ont une durée de vie très courte (de l'ordre de la microseconde). Pour les observer, nous avons mis au point un montage de RPE résolue en temps ou les radicaux sont produits in situ par un laser impulsionnel (5NS) et dont la résolution temporelle est de 100 NS. Les radicaux observés mettent en évidence la compétition de mécanismes d'échange d'électron et d'échange d'hydrogène dans la photoréduction des nitroaromatiques. L'interprétation des spectres a été confirmée par des calculs DFT des couplages hyperfins. Le même type de compétition a été observé lors de la photolyse des pyrimidines en solution dans l'éthylène glycol. En outre, les radicaux créés par ionisation biphotonique de dérivés de l'uracile en solution aqueuse ont été identifiés pour la première fois par RPE. L'étude de la photochimie des dérivés de l'uracile a été complétée par des expériences de spin trap détecté par HPLC couplée à la spectrométrie de masse en mode tandem. Un mécanisme radicalaire original a ainsi été détecté lors de l'irradiation UV-C d'uridine en solution aqueuse et a été également observé lors de l'irradiation de Brins d'Arn.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Iron–Sulfur Clusters in “Radical SAM” Enzymes: Spectroscopy and Coordination

International audienceThere is increasing evidence for the versatility of the coordination of iron–sulfur clusters in biology. In addition to cysteine residues as the most favored ligand and providing sulfur coordination, oxygenbased (aspartate, tyrosinate …) and nitrogen-based (histidine, arginine …) residues have also been observed as ligands to the clusters. Furthermore, low-molecular-weight substrates (citrate in the case of aconitase) and cofactors (S-adenosylmethionine, SAM, in the case of “Radical SAM” enzymes) have been shown to bind to one of the iron atoms of the [4Fe–4S] clusters where they are then activated. In this chapter we discuss the potential as well as the limitations of ENDOR and HYSCORE spectroscopy for characterizing metalloprotein coordination and, more specifically, the cluster–SAM complexes that are essential intermediates in pyruvate formate lyase-activating enzyme, lysine 2,3 aminomutase, and ribonucleotide reductase activating enzyme. These three systems are prototypes for the “Radical SAM” enzyme superfamily, whose chemistry seems to be extensively utilized in the metabolism of all living organisms

Study of Photoinduced N -Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

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A Rigorous Framework To Interpret Water Relaxivity. The Case Study of a Gd(III) Complex with an α-Cyclodextrin Derivative

International audienceWe present a general theoretical framework suitable for an economical, but rigorous, analysis of the relaxivity and EPR data of paramagnetic metal complexes. This framework is based on the so-called Grenoble method that properly accounts for the fluctuations of the “static” zero-field splitting Hamiltonian and avoids the misinterpretation of experimental data, which occurs with the Solomon, Bloembergen, and Morgan (SBM) formalism and may lead to erroneous conclusions. The applicability of the SBM approximation is discussed. Our approach is implemented in the case of a new Gd3+ chelate with a cyclodextrin derivative ligand hexakis(2-O-carboxymethyl-3,6-anhydro)-α-cyclodextrin (ACX), designed to obtain lanthanide complexes of enhanced stability in comparison to natural cyclodextrins. The introduction of carboxymethyl units on the six residual hydroxyl groups of an α-per-3,6-anhydro cyclodextrin leads to mono- and binuclear Ln3+ complexes with log β110 ≈ 7.5. The GdACX complex induces large water proton relaxivity in 0.1 M KCl aqueous solution. The molecular parameters governing the longitudinal (r1) and transverse (r2) relaxivities above 1 T are obtained through simple SBM-like theoretical expressions and complementary experimental techniques. The metal hydration state, the translational diffusion coefficient of the complex, and its rotational correlation time are derived from luminescence lifetime studies, pulse-field gradient NMR, and deuteron quadrupolar relaxation, respectively. The high relaxivity induced by the GdACX complex is attributed to its high hydration state in the presence of potassium ions and to a rotational correlation time lengthened by the hydrophilic character of the ACX scaffold

Activation of the anaerobic ribonucleotide reductase by S-adenosylmethionine.

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