an iron L-edge X-ray absorption study of the active centre

Iron L-edge X-ray absorption spectra of the active centre of myoglobin in the met-form, in the reduced form and upon ligation to O2, CO, NO and CN are presented. The strength of ligation with the iron centre is finger-printed through the variation of the L3 : L2 intensity ratio. Charge Transfer Multiplet calculations are performed and give qualitative information about oxidation states as well as charge transfer

Couplage AFM/Raman et spectroscopie Raman exaltée par effet de pointe de nanostructures

Pour mieux comprendre leurs propriétés, diverses nanostructures individuelles ont été étudiées à l aide d une technique couplant microscopie à force atomique et spectroscopie Raman confocale. Sous excitation lumineuse polarisée, la composition chimique, la structure et la présence de défauts a pu être précisée dans des nanobâtonnets d oxydes métalliques (ZnO et a-Fe2O3). Sous irradiation laser résonnante, les spectres de nanotubes de carbone monoparoi enrobés de polymères ont révélé notamment l absence de transfert de charge polymère-nanotube et un effet de désolvatation. Finalement, des feuillets de graphène oxydé et des ADNs double-brin peignés ont pu être préparés et caractérisés par spectroscopie Raman exaltée par effet de pointe en atteignant une résolution spatiale latérale voisine du rayon de courbure de l apex de la pointe utilisée ( 12 nm), bien plus faible que la limite de diffraction, ce qui ouvre la voie à de nouveaux travaux spectroscopiques à l échelle nanométrique.To better understand their properties, various nanostructures have been studied using a technique combining atomic force microscopy and confocal Raman spectroscopy. Under polarized light excitation, the chemical composition, the structure and the presence of defects has been described in metal oxides nanorods (ZnO et a-Fe2O3). Under resonant laser excitation, Raman spectra of polymer-wrapped single-walled carbon nanotubes have revealed the absence of polymer-nanotube charge transfer and an effect due to desolvation. Finally, graphene oxide sheets and combed double-stranded DNAs have been prepared and characterized using tip-enhanced Raman spectroscopy with a lateral spatial resolution down to the curvature radius of the apex of the used tip ( 12 nm), well below the diffraction limit, which opens new opportunities for spectroscopic works at the nanometer scale.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

On the enzymatic activity of catalase: an iron L-edge X-ray absorption study of the active centre

Catalase and methaemoglobin have very similar haem groups, which are both ferric, yet catalase decomposes hydrogen peroxide to water and oxygen very efficiently, while methaemoglobin does not. Structural studies have attributed this behaviour to their different distal environments. Here we present Fe L-2,L-3-edge X-ray absorption spectra of these proteins in physiological solutions, which reveal clear differences in their electronic structures, in that pi back-donation of the Fe atom occurs in catalase, which confers on it a partial ferryl (Fe4+) character, while this is not the case in methaemoglobin. The origin of the Fe4+ character stems from the proximal tyrosine residue. We also find that both systems are in a high spin state. Temperature effects influence the spectra of catalase only weakly, in agreement with previous studies of its chemical activity. We conclude that the high activity of catalase is not only determined by its distal environment but also by its partial ferryl character

Special Issue on “Raman Spectroscopy for Chemical and Structural Characterization in Biology”

Raman spectroscopy is a popular non-invasive spectroscopic technique for molecular characterization and imaging with a high spatial resolution [...

Unified dynamical description of pulsed magnetic field and pressure effects on the spin crossover phenomenon

International audienceThe effects of pulsed magnetic fields and pressure on the spin crossover phenomenon between high-spin and low-spin electronic states in ferrous compounds are investigated by introducing a model based on a macroscopic master equation written in the mean-field approximation. A bidirectional transition is predicted on the spontaneous thermal hysteresis and the light-induced thermal hysteresis (LITH) loops. The model also witnesses that a pressure pulse displays a mirror effect compared to a pulse of magnetic field and that different behaviors can be obtained depending on the intensity and shape of these pulses. In particular, we found that a rather long (∼1s) external perturbation must be used to reach a maximal spin conversion. In addition, the pulse intensity necessary to trigger a transition on the LITH loop is at least 30 times higher than for a spontaneous thermal hysteresis loop. As a final theoretical point, the correlation between the cooperativity and the response to a pulsed excitation is presented

The Physical Origin of the Venus Low Atmosphere Chemical Gradient

International audienceVenus shares many similarities with the Earth, but concomitantly, some of its features are extremely original. This is especially true for its atmosphere, where high pressures and temperatures are found at the ground level. In these conditions, carbon dioxide, the main component of Venus' atmosphere, is a supercritical fluid. The analysis of VeGa-2 probe data has revealed the high instability of the region located in the last few kilometers above the ground level. Recent works have suggested an explanation based on the existence of a vertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Our goal was then to identify which physical processes could lead to the establishment of this intriguing nitrogen gradient, in the deep atmosphere of Venus. Using an appropriate equation of state for the binary mixture CO 2-N 2 under supercritical conditions, and also molecular dynamics simulations, we have investigated the separation processes of N 2 and CO 2 in the Venusian context. Our results show that molecular diffusion is strongly inefficient, and potential phase separation is an unlikely mechanism. We have compared the quantity of CO 2 required to form the proposed gradient with what could be released by a diffuse degassing from a low volcanic activity. The needed fluxes of CO 2 are not so different from what can be measured over some terrestrial volcanic systems, suggesting a similar effect at work on Venus

Photoswitching of the spin crossover polymeric material [Fe(Htrz)₂(trz)](BF₄) under continuous laser irradiation in a Raman scattering experiment

Photoswitching of the [Fe(Htrz)2(trz)](BF4) spin crossover polymeric material has been investigated by means of confocal Raman spectroscopy upon continuous laser irradiation outside and inside its thermal hysteresis loop. In both cases, the evolution of HS and LS Raman marker bands show that light excitation can trigger the LS to HS transition, but the long-lived HS state can be populated only within the hysteresis loop. Local sample heating can explain this light-induced effect leading to a narrowing of the thermal hysteresis loop for laser intensities higher than 0.02 mW/μm2, in strong accordance with previous time-resolved pump-probe experiments on similar materials