35 research outputs found

    Involvement of Mechanical Cues in the Migration of Cajal-Retzius Cells in the Marginal Zone During Neocortical Development

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    Emerging evidence points to coordinated action of chemical and mechanical cues during brain development. At early stages of neocortical development, angiogenic factors and chemokines such as CXCL12, ephrins, and semaphorins assume crucial roles in orchestrating neuronal migration and axon elongation of postmitotic neurons. Here we explore the intrinsic mechanical properties of the developing marginal zone of the pallium in the migratory pathways and brain distribution of the pioneer Cajal-Retzius cells. These neurons are generated in several proliferative regions in the developing brain (e.g., the cortical hem and the pallial subpallial boundary) and migrate tangentially in the preplate/marginal zone covering the upper portion of the developing cortex. These cells play crucial roles in correct neocortical layer formation by secreting several molecules such as Reelin. Our results indicate that the motogenic properties of Cajal-Retzius cells and their perinatal distribution in the marginal zone are modulated by both chemical and mechanical factors, by the specific mechanical properties of Cajal-Retzius cells, and by the differential stiffness of the migratory routes. Indeed, cells originating in the cortical hem display higher migratory capacities than those generated in the pallial subpallial boundary which may be involved in the differential distribution of these cells in the dorsal-lateral axis in the developing marginal zone

    QuĂ­mics catalans al mĂłn: Xavier Fradera

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    Densitats bioelectroniques simplificades. Desenvolupament computacional i aplicacio en l'analisi d'interaccions electro-electrto en molecules

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    Centro de Informacion y Documentacion Cientifica (CINDOC). C/Joaquin Costa, 22. 28002 Madrid. SPAIN / CINDOC - Centro de Informaciòn y Documentaciòn CientìficaSIGLEESSpai

    New Insights in Chemical Reactivity by Means of Electron Pairing Analysis

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    According to the Lewis model, valence electrons in closed-shell atoms and molecules can be arranged into pairs of electrons shared between bonded atoms and lone pairs that belong to a single atom. Within this scheme, ionic bonding arises from the transfer of electrons between atoms, while covalent bonding is related to the sharing of electrons between atoms. Over the years, this simple model has proven to be extremely useful for the description of the bonding patterns in many molecules, and to describe the electronic rearrangements taking place during chemical reactions. However, a physically accurate description of the electron pairing in atoms and molecules has to be based on the electron-pair density. Within the theory of atoms in molecules, one can define atomic localization and delocalization indices which describe the intraand interatomic distribution of the electron pairs in a molecule. Therefore, these indices can be considered as a physically sound and numerically accurate extension of the Lewis model. In this paper, we use localization and delocalization indices to study the electron-pair reorganization taking place in five different reactions: two intramolecular rearrangements, a nucleophilic substitution, an electrophilic addition, and a Diels-Alder cycloaddition. For each reaction, we perform a comparative analysis of the electron-pairing patterns in reactants, transition states, and products. The evolution of electron-pairing along the reaction path is also studied. In all cases, the use of localization and delocalization indices provides useful insights on the electronic rearrangements taking place during the reactions

    Effects of Solvation on the Pairing of Electrons in a Series of Simple Molecules and in the Menshutkin Reaction

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    The effects of solvation on the pairing of electrons in molecules have been analyzed in a series of molecules and in the Menshutkin reaction between ammonia and methyl chloride. Solvation in water and chloroform has been modeled by means of the polarizable continuum model. A comparative analysis of the electron-pair structure of all the molecules studied has been carried out in the framework of the atoms in molecules theory. In particular, atomic populations and localization and delocalization indices have been used for describing the electron-pair characteristics of all the molecules. In general, this analysis shows that solute-solvent interactions modify the electron-pair distribution of the solute increasing the polarization of the molecular bonds. The electron-pair characteristics of the different stationary points found in the Menshutkin reaction have been also analyzed in detail. The evolution of electron pairing along the reaction path has also been followed in vacuo and in water. Comparison of the results obtained reveal that the main difference between the reaction in gas phase and in water is the structural and electronic advance of the transition state towards the reactant side. Finally, the Menshutkin reaction with explicit representation of the solvent molecules has been studied to discuss the relevance of charge-transfer and specific interactions between the solvent and the solute

    Sobre els mapes de semblança quàntica molecular

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    Partint de les definicions usuals de Mesures de Semblança Quàntica (MSQ), es considera la dependència d'aquestes mesures respecte de la superposició molecular. Pel cas particular en qnè els sistemes comparats siguin una molècula i un Àtom i que les mesures es calculin amb l'aproximació EASA, les MSQ esdevenen funcions de les tres coordenades de l'espai. Mantenint fixa una de les tres coordenades, es pot representar fàcilment la variació del valor de semblança en un pla determinat, i obtenir els anomenats mapes de semblança. En aquest article, es comparen els mapes de semblança obtinguts amb diferents MSQ per a sistemes senzillsQuantum Similarity Measures (QSM) allow to compare quantitatively any pair of molecules or, in general, quantum systems. Severa1 definitions of QSM exist, but, in all cases, these measures are dependent on the mutual superposition of the molecules or systems compared. In this paper, QSM between a molecule and a single atom are considered. Within the EASA approximation, which considers atoms as having spherical simmetry, these measures depend only on three spatial coordinates, and can be thus considered as spatial functions. By fixing one of these coordinates, the so called QSM maps can be easily obtained, representing the variation of the QSM value throughout a region of a plane. Some of these maps were obtained for the benzene 1 nitrogen and chlorobenzene / nitrogen systems, using different QSM and computational schemes. In all cases, the topology of the QSM maps matches a pattern defined by the heavy atoms of the considered molecule, while hydrogen atoms are nearly ignored in the map. This follows the well-known fact that maximal density values in molecules are located over the atomic coordinates, and suggests a close connection between QSM maps and the molecular density function. Comparisons of QSM maps of the same system helps to show the behaviour of different operator QSM. QSM maps have also been useful in order to test some of the computational schemes used in the EASA QSM computatio
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