Réaction en chaîne (nucléaire)

Résumé

vignette|redresse=1.3|Schéma d'une réaction en chaîne de fission nucléaire1. Un atome d' absorbe un neutron et se divise en deux nouveaux atomes (produits de fission), relâchant trois nouveaux neutrons et de l'énergie de liaison.2. L'un des neutrons est absorbé par un atome d' et ne continue pas la réaction, un autre neutron est simplement perdu. Cependant, un neutron entre en collision avec un atome d', qui se divise et relâche deux neutrons et de l'énergie de liaison.3. Ces deux neutrons entrent en collision avec des atomes d', qui se divisent et relâchent de un à trois neutrons, qui peuvent encore entretenir la réaction. En physique nucléaire, une réaction en chaîne se produit lorsqu'un neutron cause la fission d'un noyau atomique fissile produisant un plus grand nombre de neutrons, qui à leur tour causent d'autres fissions, permettant de poursuivre cette réaction. Une réaction en chaîne non contrôlée se produisant avec une quantité suffisamment importante de com

Source officielle

https://fr.wikipedia.org/wiki/R%C3%A9action_en_cha%C3%AEne_(nucl%C3%A9aire)

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Impact of MOF topology upon solvent organization, dynamics, and solution phase stability

Wendy Lee Queen, Xiaoning Yang

One large gap in research related to hybrid org./inorg. frameworks pertains to the inability to rationally design materials with targeted structural features and hence properties, a problem that stems from a lack of knowledge of their crystn. processes. Little is known, for example, about how specific reaction conditions relate to framework crystn. or whether frameworks are derived from the addn. of single metal-ions or larger aggregates of metal-ions and ligands that first form in soln. Thus, materials synthesis cannot keep pace with computational efforts aimed at structure and property prediction. In recent work we have demonstrated that binary solvothermal conditions used in synthesis may result in partitioning of the solvent's around growing MOF crystallites, based upon the relative hydrophobicity of the crystallite and of the two solvents (Figure 1). The current work, which is a combined exptl. and theor. study, is examg. how solvent organization and solvation free energy changes for two MOF's made of the same building blocks, but with different topologies. For instance, MOF-235 and MIL-101 contain the same triangular Fe3O core interlinked by 1, 4 benzene dicarboxylate ligands. While MOF-235 is a cationic framework with charge balancing anions in the channels that render it effectively nonporous, MIL-101 is neutral mesoporous framework, 3.4 nm, with surface areas over 3000 m2/g. Of specific interest is whether the solvation favorability of the two frameworks influences the final product outcome, or whether a specific reaction pathway during crystn. is essential for control of which MOF is produced.

American Chemical Society2015

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The Transcription Factor Nfix Requires RhoA-ROCK1 Dependent Phagocytosis to Mediate Macrophage Skewing during Skeletal Muscle Regeneration

Giuliana Rossi

Macrophages (MPs) are immune cells which are crucial for tissue repair. In skeletal muscle regeneration, pro-inflammatory cells first infiltrate to promote myogenic cell proliferation, then they switch into an anti-inflammatory phenotype to sustain myogenic cells differentiation and myofiber formation. This phenotypical switch is induced by dead cell phagocytosis. We previously demonstrated that the transcription factor Nfix, a member of the nuclear factor I (Nfi) family, plays a pivotal role during muscle development, regeneration and in the progression of muscular dystrophies. Here, we show that Nfix is mainly expressed by anti-inflammatory macrophages. Upon acute injury, mice deleted for Nfix in myeloid line displayed a significant defect in the process of muscle regeneration. Indeed, Nfix is involved in the macrophage phenotypical switch and macrophages lacking Nfix failed to adopt an anti-inflammatory phenotype and interact with myogenic cells. Moreover, we demonstrated that phagocytosis induced by the inhibition of the RhoA-ROCK1 pathway leads to Nfix expression and, consequently, to acquisition of the anti-inflammatory phenotype. Our study identified Nfix as a link between RhoA-ROCK1-dependent phagocytosis and the MP phenotypical switch, thus establishing a new role for Nfix in macrophage biology for the resolution of inflammation and tissue repair.

2020

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Metabolic "dark matter" describes currently unknown metabolic processes, which form a blind spot in our general understanding of metabolism and slow down the development of biosynthetic cell factories and naturally derived pharmaceuticals. Mapping the dark matter of metabolism remains an open challenge that can be addressed globally and systematically by existing computational solutions. In this work, we use 489 generalized enzymatic reaction rules to map both known and unknown metabolic processes around a biochemical database of 1.5 million biological compounds. We predict over 5 million reactions and integrate nearly 2 million naturally and synthetically-derived compounds into the global network of biochemical knowledge, named ATLASx. ATLASx is available to researchers as a powerful online platform that supports the prediction and analysis of biochemical pathways and evaluates the bio-chemical vicinity of molecule classes (https://lcsb-databases.epfl.ch/Atlas2).

NATURE PORTFOLIO2022

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