Mechanics of collective unfolding

Mechanically induced unfolding of passive crosslinkers is a fundamental biological phenomenon encountered across the scales from individual macro-molecules to cytoskeletal actin networks. In this paper we study a conceptual model of athermal load-induced unfolding and use a minimalistic setting allowing one to emphasize the role of long-range interactions while maintaining full analytical transparency. Our model can be viewed as a description of a parallel bundle of N bistable units confined between two shared rigid backbones that are loaded through a series spring. We show that the ground states in this model correspond to synchronized, single phase configurations where all individual units are either folded or unfolded. We then study the fine structure of the wiggly energy landscape along the reaction coordinate linking the two coherent states and describing the optimal mechanism of cooperative unfolding. Quite remarkably, our study shows the fundamental difference in the size and structure of the folding-unfolding energy barriers in the hard (fixed displacements) and soft (fixed forces) loading devices which persists in the continuum limit. We argue that both, the synchronization and the non-equivalence of the mechanical responses in hard and soft devices, have their origin in the dominance of long-range interactions. We then apply our minimal model to skeletal muscles where the power-stroke in acto-myosin crossbridges can be interpreted as passive folding. A quantitative analysis of the muscle model shows that the relative rigidity of myosin backbone provides the long-range interaction mechanism allowing the system to effectively synchronize the power-stroke in individual crossbridges even in the presence of thermal fluctuations. In view of the prototypical nature of the proposed model, our general conclusions pertain to a variety of other biological systems where elastic interactions are mediated by effective backbones

Understanding Policy Change through Bricolage: The Case of Chile's Renewable Energy Policy

Chile is a country where path dependency made energy policy change extremely difficult by international standards. However, the country has recently become a renewable energy poster child thanks to a gradual process of policy change. How was this possible? This article contributes to discussions about policy change driven by ideas and to explaining the puzzling case of Chilean energy policy change. It does so by discussing the mechanism of bricolage—the recombination of old and new ideas by policy entrepreneurs—and its capacity to produce policy change in contexts of high path dependency. The article develops the political manifestations and consequences of bricolage and problematizes how actors continue to contest and change ideas' meaning after they have been institutionalized, a key question when analyzing processes of bricolage. The analysis is based on an array of data sources including interviews with key actors, newspaper notes, and legislative proceedings.1 Introduction 2 Literature review 3 Chile: A least-likely case of energy policy change 4 Methods, data, and analysis 5 Results: Explaining the change in Chilean energy policy 6 Conclusions and discussion Acknowledgements Appendix A Reference

The Origin of Enhanced Activity in the Suns of M67

We report the results of the analysis of high resolution photospheric line spectra obtained with the UVES instrument on the VLT for a sample of 15 solar-type stars selected from a recent survey of the distribution of H and K chromospheric line strengths in the solar-age open cluster M67. We find upper limits to the projected rotation velocities that are consistent with solar-like rotation (i.e., v sini ~< 2-3 km/s) for objects with Ca II chromospheric activity within the range of the contemporary solar cycle. Two solar-type stars in our sample exhibit chromospheric emission well in excess of even solar maximum values. In one case, Sanders 1452, we measure a minimum rotational velocity of vsini = 4 +/- 0.5 km/s, or over twice the solar equatorial rotational velocity. The other star with enhanced activity, Sanders 747, is a spectroscopic binary. We conclude that high activity in solar-type stars in M67 that exceeds solar levels is likely due to more rapid rotation rather than an excursion in solar-like activity cycles to unusually high levels. We estimate an upper limit of 0.2% for the range of brightness changes occurring as a result of chromospheric activity in solar-type stars and, by inference, in the Sun itself. We discuss possible implications for our understanding of angular momentum evolution in solar-type stars, and we tentatively attribute the rapid rotation in Sanders 1452 to a reduced braking efficiency.Comment: accepted by Ap

Relations entre contrôle de la source en mémoire épisodique et fonctionnement exécutif dans le vieillissement normal

Les travaux sur le vieillissement cognitif normal suggèrent l\u27existence de difficultés pour le contrôle de la source en mémoire épisodique, en lien avec une diminution du contrôle exécutif. Néanmoins, les approches utilisées dans la littérature ne permettent pas de savoir si toutes les dimensions du contrôle de la source se dégradent avec l\u27âge, ni de savoir si certains processus exécutifs sont plus impliqués que d\u27autres dans la diminution du contrôle de la source. Dans ce travail, nous avons étudié l\u27effet de l\u27âge sur le contrôle de la source à partir de tâches simples et originales évaluant le contrôle interne, le contrôle externe et le contrôle de la réalité. Plusieurs mécanismes exécutifs ont aussi été évalués. Les tâches ont été proposées à 24 participants jeunes et 22 participants âgés en bonne santé appariés par le sexe et le niveau de vocabulaire au Mill Hill. Les résultats ont montré une détérioration du contrôle de la source en mémoire épisodique, surtout pour le contrôle externe, chez les participants âgés. Ce déclin était très lié à la diminution des performances au test de Stroop. Il semblerait donc que le vieillissement normal s\u27accompagne d\u27une baisse assez globale du contrôle de la source en mémoire épisodique et que cette baisse s\u27expliquerait principalement par l\u27altération des compétences inhibitrices

Blind fluorescence structured illumination microscopy: A new reconstruction strategy

In this communication, a fast reconstruction algorithm is proposed for fluorescence \textit{blind} structured illumination microscopy (SIM) under the sample positivity constraint. This new algorithm is by far simpler and faster than existing solutions, paving the way to 3D and/or real-time 2D reconstruction.Comment: submitted to IEEE ICIP 201

Benzene formation in the inner regions of protostellar disks

Benzene (c-C6H6) formation in the inner 3 AU of a protostellar disk can be efficient, resulting in high abundances of benzene in the midplane region. The formation mechanism is different to that found in interstellar clouds and in protoplanetary nebulae, and proceeds mainly through the reaction between allene (C3H4) and its ion. This has implications for PAH formation, in that some fraction of PAHs seen in the solar system could be native rather than inherited from the interstellar medium.Comment: 9 pages, 2 colour figures, to be published in the Astrophysical Journal Letter

Toward a new interpretation of the mechanical behaviour of As-quenched low alloyed martensitic steels

International audienceThough as-quenched martensite exhibits a low uniform elongation in tension, it is highlighted that this phase has a very high strain-hardening which increases with carbon content and a large Bauschinger effect. Because usual dislocation storage can not explain reasonably this particular behaviour, an approach based on a continuum composite view of martensite (CCA) is developed suitable to capture all the experimental features

Internal Dust Correction Factors for Star Formation Rates Derived for Dusty \HII Regions and Starburst Galaxies

Star formation rates in galaxies are frequently estimated using the Balmer line fluxes. However, these can be systematically underestimated because dust competes for the absorption of Lyman continuum photons in the ionized gas. Here we present theoretical correction factors in a simple analytic form. T These factors scale as the product of the ionization parameter, ${\cal U}$, and the nebular O/H abundance ratio, both of which can now be derived from the observation of bright nebular line ratios. The correction factors are only somewhat dependent upon the photoelectron production by grains, but are very sensitive to the presence of complex PAH-like carbonaceous molecules in the ionized gas, providing that these can survive in such an environment.Comment: 13 pages, 1 figures, Accepted for publication in ApJ. (Feb 1, 2003

Dual Signaling System with an Extended-Tetrathiafulvalene–Phenanthroline Dyad Acting as an Electrooptical Cation Chemosensor

A tetrathiafulvalene donor has been annulated to 2,3-di(1H-2-pyrrolyl)quinoxaline affording a new chemosensor 1, which shows a unique optical selectivity and reactivity for the fluoride ion over other anions in CH2Cl2 leading to a colorimetric response. Electrochemical polymerization of 1 occurred in the presence of fluoride