Creative Educational Work While Making Musical Electronic Presentation

При финансовой поддержке Российского гуманитарного научного фонда, проект № 08-06-14135

Electronic structure of americium sesquioxide probed by resonant inelastic x-ray scattering

The Am $5d$-$5f$ resonant inelastic x-ray scattering (RIXS) data of americium sesquioxide were measured at incident photon energies throughout the Am $O_{4,5}$ edges. The experiment was supported by calculations using several model approaches. While the experimental Am $O_{4,5}$ x-ray absorption spectrum of Am$_2$O$_3$ is compared with the spectra calculated in the framework of atomic multiplet and crystal-field multiplet theories and Anderson impurity model (AIM) for the Am(III) system, the recorded Am $5d$-$5f$ RIXS data are essentially reproduced by the crystal-field multiplet calculations. A combination of the experimental scattering geometry and theoretical analysis of the character of the electronic states probed during the RIXS process confirms that the ground state of Am$_2$O$_3$ is singlet $\Gamma_1$. An appearance of the low-intense charge-transfer satellite in the Am $5d$-$5f$ RIXS spectra at an energy loss of $\sim$5.5 eV, suggests weak Am $5f$-O $2p$ hybridization which is in agreement with AIM estimations of the $5f$ occupancy from spectroscopic data in Am$_2$O$_3$ as being 6.05 electrons

Theoretical study of resonant x-ray emission spectroscopy of Mn films on Ag

We report a theoretical study on resonant x-ray emission spectra (RXES) in the whole energy region of the Mn $L_{2,3}$ white lines for three prototypical Mn/Ag(001) systems: (i) a Mn impurity in Ag, (ii) an adsorbed Mn monolayer on Ag, and (iii) a thick Mn film. The calculated RXES spectra depend strongly on the excitation energy. At $L_3$ excitation, the spectra of all three systems are dominated by the elastic peak. For excitation energies around $L_2$, and between $L_3$ and $L_2$, however, most of the spectral weight comes from inelastic x-ray scattering. The line shape of these inelastic ``satellite'' structures changes considerably between the three considered Mn/Ag systems, a fact that may be attributed to changes in the bonding nature of the Mn-$d$ orbitals. The system-dependence of the RXES spectrum is thus found to be much stronger than that of the corresponding absorption spectrum. Our results suggest that RXES in the Mn $L_{2,3}$ region may be used as a sensitive probe of the local environment of Mn atoms.Comment: 9 pages, 11 figure

RNA recurrent motifs : identification and characterization

La détermination de la structure tertiaire du ribosome fut une étape importante dans la compréhension du mécanisme de la synthèse des protéines. Par contre, l’élucidation de la structure du ribosome comme tel ne permet pas une compréhension de sa fonction. Pour mieux comprendre la nature des relations entre la structure et la fonction du ribosome, sa structure doit être étudiée de manière systématique. Au cours des dernières années, nous avons entrepris une démarche systématique afin d’identifier et de caractériser de nouveaux motifs structuraux qui existent dans la structure du ribosome et d’autres molécules contenant de l’ARN. L’analyse de plusieurs exemples d’empaquetage de deux hélices d’ARN dans la structure du ribosome nous a permis d’identifier un nouveau motif structural, nommé « G-ribo ». Dans ce motif, l’interaction d’une guanosine dans une hélice avec le ribose d’un nucléotide d’une autre hélice donne naissance à un réseau d’interactions complexes entre les nucléotides voisins. Le motif G-ribo est retrouvé à 8 endroits dans la structure du ribosome. La structure du G-ribo possède certaines particularités qui lui permettent de favoriser la formation d’un certain type de pseudo-nœuds dans le ribosome. L’analyse systématique de la structure du ribosome et de la ARNase P a permis d’identifier un autre motif structural, nommé « DTJ » ou « Double-Twist Joint motif ». Ce motif est formé de trois courtes hélices qui s’empilent l’une sur l’autre. Dans la zone de contact entre chaque paire d’hélices, deux paires de bases consécutives sont surenroulées par rapport à deux paires de bases consécutives retrouvées dans l’ARN de forme A. Un nucléotide d’une paire de bases est toujours connecté directement à un nucléotide de la paire de bases surenroulée, tandis que les nucléotides opposés sont connectés par un ou plusieurs nucléotides non appariés. L’introduction d’un surenroulement entre deux paires de bases consécutives brise l’empilement entre les nucléotides et déstabilise l’hélice d’ARN. Dans le motif DTJ, les nucléotides non appariés qui lient les deux paires de bases surenroulées interagissent avec une des trois hélices qui forment le motif, offrant ainsi une stratégie élégante de stabilisation de l’arrangement. Pour déterminer les contraintes de séquences imposées sur la structure tertiaire d’un motif récurrent dans le ribosome, nous avons développé une nouvelle approche expérimentale. Nous avons introduit des librairies combinatoires de certains nucléotides retrouvés dans des motifs particuliers du ribosome. Suite à l’analyse des séquences alternatives sélectionnées in vivo pour différents représentants d’un motif, nous avons été en mesure d’identifier les contraintes responsables de l’intégrité d’un motif et celles responsables d’interactions avec les éléments qui forment le contexte structural du motif. Les résultats présentés dans cette thèse élargissent considérablement notre compréhension des principes de formation de la structure d’ARN et apportent une nouvelle façon d’identifier et de caractériser de nouveaux motifs structuraux d’ARN.Although determination of the ribosome tertiary structure has been an outstanding step towards elucidation of the mechanism of protein synthesis, the complexity of this structure does not provide an easy answer of how this large molecular complex works. In order to understand the nature of structure-function relationships in the ribosome, the ribosome structure itself should be subjected to thorough analysis. In the last years, we undertook systematic efforts toward identification and characterization of all recurrent structural motifs existing in the ribosomal RNA and in other RNA-containing molecules. The analysis of many instances of helix-helix packing in the ribosome structure allowed us to identify a new structural motif which we called “G-ribo”. In this motif, an interaction of the sugar edge of a guanosine in one helix with the ribose of a nucleotide from another helix was found to be at the origin of a complex network of concomitant inter-nucleotide interactions. In total, the G-ribo motif was found at eight locations within the ribosomal RNA. A surprising feature of this motif consists in its ability to favor the formation of pseudoknots of a particular type. In the ribosome structure, there are four pseudoknots whose formation is mediated by the G-ribo motif. Systematic analysis of the ribosome as well as the RNAseP crystal structures allowed for the identification of a new RNA motif, which we called “DTJ”, or Double-Twist Joint motif. This motif is made of three short RNA double helices, which stack one on top of another. In the contact zone of each pair of helices two consecutive base pairs are over-twisted compared to the regular helical twist of 32° of A-RNA. One nucleotide of the base pair is always directly connected to the one nucleotide of the over-twisted base pair, while the opposite nucleotides of these base pairs are connected with one or several unpaired nucleotides. Introduction of the helical over-twist between two consecutive base pairs breaks the inter-nucleotide stacking and destabilizes the RNA double helix. In the DTJ, the unpaired nucleotides that connect the two over-twisted base pairs interact with one of the three motif-forming helices, providing an elegant strategy for the stabilization of the whole arrangement. To determine the nucleotide sequence constraints imposed on the structure of recurrent RNA motifs in the functional ribosome we developed a new approach consisting in the selection of functional ribosomes from a combinatorial gene library in which certain nucleotides of the rRNA gene corresponding to a particular motif were randomized. Comparison of the constraints determined for different examples of the same motif allowed us to distinguish between constraints responsible for the integrity of the motif and for its interaction with surrounding elements, including ribosomal proteins. The work significantly improves our understanding of the principles of RNA structure formation and opens a new way to identify and characterize RNA motifs

The Rate of Homogenous Nucleation of Ice in Supercooled Water.

The homogeneous freezing of water is of fundamental importance to a number of fields, including that of cloud formation. However, there is considerable scatter in homogeneous nucleation rate coefficients reported in the literature. Using a cold stage droplet system designed to minimize uncertainties in temperature measurements, we examined the freezing of over fifteen hundred pure water droplets with diameters between 4 and 24 μm. Under the assumption that nucleation occurs within the bulk of the droplet, nucleation rate coefficients fall within the spread of literature data and are in good agreement with a subset of more recent measurements. To quantify the relative importance of surface and volume nucleation in our experiments, where droplets are supported by a hydrophobic surface and surrounded by oil, comparison of droplets with different surface area to volume ratios was performed. From our experiments it is shown that in droplets larger than 6 µm diameter (between 234.6 and 236.5 K), nucleation in the interior is more important than nucleation at the surface. At smaller sizes we cannot rule out a significant contribution of surface nucleation, and in order to further constrain surface nucleation experiments with smaller droplets are necessary. Nevertheless, in our experiments, it is dominantly volume nucleation controlling the observed nucleation rate

District newspapers as a chronicler of the Arkhangelsk North history

The article discusses the creation and operation of district periodicals in the Arkhangelsk Oblast. The author analyzed their role in solving problems in different periods and evaluated the place of the “district” in the relations of the state and local authorities. Based on a study of modern local periodicals, the author concluded that the press, freed from the CPSU dictates, became dependent on new power and was forced to cover mainly its activities and achievements. The way out of this situation is the creation of mu-nicipal newspapers

Electronic structure investigation of CoO by means of soft X-ray scattering

The electronic structure of CoO is studied by resonant inelastic soft X-ray scattering spectroscopy using photon energies across the Co 2p absorption edges. The different spectral contributions from the energy-loss structures are identified as Raman scattering due to d-d and charge-transfer excitations. For excitation energies close to the L3 resonance, the spectral features are dominated by quartet-quartet and quartet-doublet transitions of the 3d7 configuration. At excitation energies corresponding to the satellites in the Co 2p X-ray absorption spectrum of CoO, the emission features are instead dominated by charge-transfer transitions to the 3d8L-1 final state. The spectra are interpreted and discussed with the support of simulations within the single impurity Anderson model with full multiplet effects which are found to yield consistent spectral functions to the experimental data.Comment: 8 pages, 2 figures, 2 tables, http://link.aps.org/doi/10.1103/PhysRevB.65.20510