1,721,984 research outputs found
Main Belt Comets: A new class of small bodies in the solar system
No full textComets and asteroids have traditionally been considered two distinct separate populations of small bodies in the solar system, according to their different dynamical, observational, and compositional characteristics. The discovery of a new class of objects, the so-called, Main Belt Comets (MBCs), exhibiting a clear cometary activity but having at the same time orbits indistinguishable from the ones of asteroids in the Main Belt provided further evidence that asteroids and comets, rather than two distinct separate classes, represent the end-members of a continuum of small bodies, with compositions from the very rocky to the very icy. Their study is nowadays deepening our knowledge of the formation mechanisms of the solar system and of the distribution of volatile materials in the protoplanetary disk. In this paper the present knowledge of MBCs is reviewed in terms of physical properties derived from observations, dynamical studies about the origin and formation, thermal modeling of the nuclei, investigations about the activation mechanisms, and the eventual contribution to the presence of water on our planet. An overview of the large-scale surveys dedicated to their discovery and of the detection techniques used so far is also given. Moreover, open question and indications for future observations and modeling are outlined. © 2011 Elsevier Ltd. All rights reserved
Integration of XAS and NMR techniques for the structure determination of metalloproteins. Examples from the study of copper transport proteins
No full textNuclear magnetic resonance (NMR) is a powerful technique for protein structure determination in solution. However, when dealing with metalloproteins, NMR methods are unable to directly determine the structure of the metal site and its coordination geometry. The capability of X-ray absorption spectroscopy (XAS) to provide the structure of a metal ion bound to a protein is then perfectly suited to complement the process of the structure determination. This aspect is particularly relevant in structural genomic projects where high throughput of structural results is the main goal. The synergism of the two techniques has been exploited in the structure determination of bacterial copper transport proteins. © 2005 International Union of Crystallography Printed in Great Britain - all rights reserved
Asteroids Close-Up: What We Have Learned from Twenty Years of Space ExplorationAsteroids
No full textAsteroids are objects of fundamental scientific importance for several reasons. They are the remnant debris from the formation of the inner solar system. They offer therefore the unique opportunity to study the original materials and the mechanisms which formed the terrestrial planets. Moreover, they can be the objects of a strong interaction with Earth's biosphere. Asteroids may have played a role in bringing water and organic substances on the Earth, influencing the formation of life. With catastrophic impacts they may have changed in the past the evolutionary path of life forms and may still constitute a serious threat to the human presence on the planet. Finally, asteroids represent an extraordinary source of minerals which can be exploited for the increasing necessities of our civilization and for the future exploration and colonization of the solar system
Light scattering properties of cometary dust
No full textAims:The nature of cometary dust is still elusive. In order to constrain the physical properties of dust in cometary coma, we modeled the observed scattering parameters, investigating different morphological and compositional effects.
Methods: We used fractal aggregates as dust particles. The scattering behavior of extremely elongated dust particles, similar to those formed in micro-gravity aggregation experiments and more compact aggregates, have been studied with the discrete-dipole approximation theory.
Results: Results show that particles comparable in size to the incident wavelength provide the best qualitative fits. A silicate composition gives a good fit to the polarization and the phase function, but is characterized by large albedo. Organic and mixed compositions provide low albedos but do not fit the polarization and phase function curves well. Compact particles seem to reproduce the observed parameters better, indicating a possible morphological metamorphism of dust since the time of formation. The observed different polarization classes may be explained in terms of different monomer sizes, if a cluster of spheres models are used
Human SOD1 before Harboring the Catalytic Metal: solution structure of copper-depleted, disulfide-reduced form
No full textSOD1 has to undergo several post-translational modifications before reaching its mature form. The protein requires insertion of zinc and copper atoms, followed by the formation of a conserved S-S bond between Cys-57 and Cys-146 (human numbering), which makes the protein fully active. In this report an NMR structural investigation of the reduced SH-SH form of thermostable E, Zn-as-SOD1 (E is empty; as is C6A, C111S) is reported, characterizing the protein just before the last step leading to the mature form. The structure is compared with that of the oxidized S-S form as well as with that of the yeast SOD1 complexed with its copper chaperone, CCS. Local conformational rearrangements upon disulfide bridge reduction are localized in the region near Cys-57 that is completely exposed to the solvent in the present structure, at variance with the oxidized forms. There is a local disorder around Cys-57 that may serve for protein-protein recognition and may possibly be involved in intermolecular S-S bonds in familial amyotrophic lateral sclerosis-related SOD1 mutants. The structure allows us to further discuss the copper loading mechanism in SOD1
Carbonic-anhydrase - An Example of How the Cavity Governs the Reactivity At the Zinc Ion
No full textThe recent availability of X-ray structures and new biophysical measurements have shed further light on the detailed mechanism of carbonic anhydrase and its inhibition. It is noted that in some instances the structural information obtained through X-ray analysis of single crystals and NMR measurements in solution disagree. We take these conflicting results as possible conformations close in energies, both of which can be used to design the enzymatic pathway
The origin of water on Earth: stars or diamonds?: A conversation among astronomers and geologists
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High resolution iron derivative of bullfrog M ferritin: structural insight into ferroxidase process
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