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Transient Structures (transient + structure)
Selected AbstractsSeasonal evolution of Titan's dark polar hood: midsummer disappearance observed by the Hubble Space TelescopeMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Ralph D. Lorenz ABSTRACT Titan, Saturn's largest moon, has a dense organic-laden atmosphere that displays dramatic seasonal variations in composition and appearance. Here we document the evolution of the dark polar hood, first seen in 1980 by Voyager 1 around the north pole, and report quantitative measurements of the hood's disappearance from the south pole in 2002,2003 using previously unpublished observations with the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). These data support a model of the hood as a transient structure associated with downwelling during polar winter. [source] Sensitivity of secondary structure propensities to sequence differences between ,- and ,-synuclein: Implications for fibrillationPROTEIN SCIENCE, Issue 12 2006Joseph A. Marsh Abstract The synucleins are a family of intrinsically disordered proteins involved in various human diseases. ,-Synuclein has been extensively characterized due to its role in Parkinson's disease where it forms intracellular aggregates, while ,-synuclein is overexpressed in a majority of late-stage breast cancers. Despite fairly strong sequence similarity between the amyloid-forming regions of ,- and ,-synuclein, ,-synuclein has only a weak propensity to form amyloid fibrils. We hypothesize that the different fibrillation tendencies of ,- and ,-synuclein may be related to differences in structural propensities. Here we have measured chemical shifts for ,-synuclein and compared them to previously published shifts for ,-synuclein. In order to facilitate direct comparison, we have implemented a simple new technique for re-referencing chemical shifts that we have found to be highly effective for both disordered and folded proteins. In addition, we have developed a new method that combines different chemical shifts into a single residue-specific secondary structure propensity (SSP) score. We observe significant differences between ,- and ,-synuclein secondary structure propensities. Most interestingly, ,-synuclein has an increased ,-helical propensity in the amyloid-forming region that is critical for ,-synuclein fibrillation, suggesting that increased structural stability in this region may protect against ,-synuclein aggregation. This comparison of residue-specific secondary structure propensities between intrinsically disordered homologs highlights the sensitivity of transient structure to sequence changes, which we suggest may have been exploited as an evolutionary mechanism for fast modulation of protein structure and, hence, function. [source] Reticulocyte binding protein homologues are key adhesins during erythrocyte invasion by Plasmodium falciparumCELLULAR MICROBIOLOGY, Issue 11 2009Tony Triglia Summary The Apicomplexan parasite responsible for the most virulent form of malaria, Plasmodium falciparum, invades human erythrocytes through multiple ligand,receptor interactions. The P. falciparum reticulocyte-binding protein homologue (PfRh or PfRBL) family have been implicated in the invasion process but their exact role is unknown. PfRh1 and PfRh4, members of this protein family, bind to red blood cells and function in merozoite invasion during which they undergo a series of proteolytic cleavage events before and during entry into the host cell. The ectodomain of PfRh1 and PfRh4 are processed to produce fragments consistent with cleavage in the transmembrane domain and released into the supernatant, at about the time of invasion, in a manner consistent with rhomboid protease cleavage. Processing of both PfRh1 and PfRh4, and by extrapolation all membrane-bound members of this protein family, is important for function and release of these proteins on the merozoite surface and they along with EBA-175 are important components of the tight junction, the transient structure that links the erythrocyte via receptor,ligand interactions to the actin,myosin motor in the invading merozoite. [source] Why is limb regeneration possible in amphibians but not in reptiles, birds, and mammals?EVOLUTION AND DEVELOPMENT, Issue 2 2003Frietson Galis SUMMARY The capacity to regenerate limbs is very high in amphibians and practically absent in other tetrapods despite the similarities in developmental pathways and ultimate morphology of tetrapod limbs. We propose that limb regeneration is only possible when the limb develops as a semiautonomous module and is not involved in interactions with transient structures. This hypothesis is based on the following two assumptions: To an important extent, limb development uses the same developmental mechanisms as normal limb development and developmental mechanisms that require interactions with transient structures cannot be recapitulated later. In amniotes limb development is early, shortly after neurulation, and requires inductive interactions with transient structures such as somites. In amphibians limb development is delayed relative to amniotes and has become decoupled from interactions with somites and other transient structures that are no longer present at this stage. The limb develops as a semi-independent module. A comparison of the autonomy and timing of limb development in different vertebrate taxa supports our hypothesis and its assumptions. The data suggest a good correlation between self-organizing and regenerative capacity. Furthermore, they suggest that whatever barriers amphibians overcame in the evolution of metamorphosis, they are the same barriers that need to be overcome to make limb regeneration possible in other taxa. [source] The external gills of anuran amphibians: Comparative morphology and ultrastructureJOURNAL OF MORPHOLOGY, Issue 10 2008M. Nokhbatolfoghahai Abstract The external gills of anuran amphibians are transient structures, covered by the development of the operculum and regressing soon afterwards. Their functional role has been regarded as equivocal. However, detailed morphological analysis has been limited. Analysis of 21 species from six families using scanning and transmission electron microscopy revealed diversity at the anatomical and cellular levels in extent and length of gill filaments, numbers of surface ciliated cells, width of water-blood barrier distance, and evidence of gill motility. The most highly developed external gills were found in species with delayed hatching, such as Phyllomedusa trinitatis, or in species in which hatchlings hang from the surface film of temporary ponds, such as Phrynohyas venulosa in which gills added 26,38% to body surface area. In one family, the bufonids, all four species examined had poorly developed gills, but in other families where we examined several species, the hylids and leptodactylids, there was considerable diversity of external gills, suggesting flexible adaptation to incubation and hatching environment. J. Morphol., 2008. © 2008 Wiley-Liss, Inc. [source] The impact of mergers on relaxed X-ray clusters , I. Dynamical evolution and emergent transient structuresMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006Gregory B. Poole ABSTRACT We report on the analysis of a suite of smoothed particle hydrodynamics simulations (incorporating cooling and star formation) of mergers involving idealized X-ray clusters whose initial conditions resemble relaxed clusters with cool compact cores observed by Chandra and XMM. The simulations sample the most-interesting, theoretically plausible, range of impact parameters and progenitor mass ratios. We find that all mergers evolve via a common progression. We illustrate this progression in the projected gas density, X-ray surface brightness, Sunyaev,Zel'dovich, temperature, and gas-entropy maps. Several different classes of transient ,cold front' like features can arise over the course of a merger. Each class is distinguished by a distinct morphological signature and physical cause. We find that all these classes are present in Chandra and XMM observations of merging systems and propose a naming scheme for these features: ,comet-like' tails, bridges, plumes, streams and edges. In none of the cases considered do the initial cool compact cores of the primary and the secondary get destroyed during the course of the mergers. Instead, the two remnant cores eventually combine to form a new core that, depending on the final mass of the remnant, can have a greater cooling efficiency than either of its progenitors. We quantify the evolving morphology of our mergers using centroid variance, power ratios and offset between the X-ray and the projected mass maps. We find that the centroid variance best captures the dynamical state of the cluster. It also provides an excellent indicator of how far the system is from virial and hydrostatic equilibrium. Placing the system at z= 0.1, we find that all easily identified observable traces of the secondary disappear from a simulated 50-ks Chandra image following the second pericentric passage. The system, however, takes an additional ,2 Gyr to relax and virialize. Observationally, the only reliable indicator of a system in this state is the smoothness of its X-ray surface brightness isophotes, not temperature fluctuations. Temperature fluctuations at the level of ,T/T, 20 per cent, can persist in the final systems well past the point of virialization, suggesting that the existence of temperature fluctuations, in and of themselves, does not necessarily indicate a disturbed or unrelaxed system. [source] Excited-state molecular structures captured by X-ray transient absorption spectroscopy: a decade and beyondACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2010Lin X. Chen Transient molecular structures along chemical reaction pathways are important for predicting molecular reactivity, understanding reaction mechanisms, as well as controlling reaction pathways. During the past decade, X-ray transient absorption spectroscopy (XTA, or LITR-XAS, laser-initiated X-ray absorption spectroscopy), analogous to the commonly used optical transient absorption spectroscopy, has been developed. XTA uses a laser pulse to trigger a fundamental chemical process, and an X-ray pulse(s) to probe transient structures as a function of the time delay between the pump and probe pulses. Using X-ray pulses with high photon flux from synchrotron sources, transient electronic and molecular structures of metal complexes have been studied in disordered media from homogeneous solutions to heterogeneous solution,solid interfaces. Several examples from the studies at the Advanced Photon Source in Argonne National Laboratory are summarized, including excited-state metalloporphyrins, metal-to-ligand charge transfer (MLCT) states of transition metal complexes, and charge transfer states of metal complexes at the interface with semiconductor nanoparticles. Recent developments of the method are briefly described followed by a future prospective of XTA. It is envisioned that concurrent developments in X-ray free-electron lasers and synchrotron X-ray facilities as well as other table-top laser-driven femtosecond X-ray sources will make many breakthroughs and realise dreams of visualizing molecular movies and snapshots, which ultimately enable chemical reaction pathways to be controlled. [source] Motif Reconstruction in Clusters and Layers: Benchmarks for the Kawska,Zahn Approach to Model Crystal FormationCHEMPHYSCHEM, Issue 4 2010Theodor Milek Abstract A recently developed atomistic simulation scheme for investigating ion aggregation from solution is transferred to the morphogenesis of metal clusters grown from the vapor and layers deposited on a substrate surface. Both systems are chosen as benchmark models for intense motif reorganization during aggregate/layer growth. The applied simulation method does not necessarily involve global energy minimization after each growth event, but instead describes crystal growth as a series of structurally related configurations which may also include local energy minima. Apart from the particularly favorable high-symmetry configurations known from experiments and global energy minimization, we also demonstrate the investigation of transient structures. In the spirit of Ostwald's step rule, a continuous evolution of the aggregate/layer structure during crystal growth is observed. [source] | |||||||||||||