Catálogo de publicaciones - revistas

<jats:p> The three-dimensional structure of the complex between human H-Ras bound to guanosine diphosphate and the guanosine triphosphatase (GTPase)–activating domain of the human GTPase-activating protein p120 <jats:sup>GAP</jats:sup> (GAP-334) in the presence of aluminum fluoride was solved at a resolution of 2.5 angstroms. The structure shows the partly hydrophilic and partly hydrophobic nature of the communication between the two molecules, which explains the sensitivity of the interaction toward both salts and lipids. An arginine side chain (arginine-789) of GAP-334 is supplied into the active site of Ras to neutralize developing charges in the transition state. The switch II region of Ras is stabilized by GAP-334, thus allowing glutamine-61 of Ras, mutation of which activates the oncogenic potential, to participate in catalysis. The structural arrangement in the active site is consistent with a mostly associative mechanism of phosphoryl transfer and provides an explanation for the activation of Ras by glycine-12 and glutamine-61 mutations. Glycine-12 in the transition state mimic is within van der Waals distance of both arginine-789 of GAP-334 and glutamine-61 of Ras, and even its mutation to alanine would disturb the arrangements of residues in the transition state. </jats:p>

<jats:p>CD1 represents a third lineage of antigen-presenting molecules that are distantly related to major histocompatibility complex (MHC) molecules in the immune system. The crystal structure of mouse CD1d1, corresponding to human CD1d, at 2.8 â„« resolution shows that CD1 adopts an MHC fold that is more closely related to that of MHC class I than to that of MHC class II. The binding groove, although significantly narrower, is substantially larger because of increased depth and it has only two major pockets that are almost completely hydrophobic. The extreme hydrophobicity and shape of the binding site are consistent with observations that human CD1b and CD1c can present mycobacterial cell wall antigens, such as mycolic acid and lipoarabinomannans. However, mouse CD1d1 can present very hydrophobic peptides, but must do so in a very different way from MHC class Ia and class II molecules.</jats:p>

<jats:p>The infrared spectrum of hot water observed in a sunspot has been assigned. The high temperature of the sunspot (3200 K) gave rise to a highly congested pure rotational spectrum in the 10-micrometer region that involved energy levels at least halfway to dissociation. Traditional spectroscopy, based on perturbation theory, is inadequate for this problem. Instead, accurate variational solutions of the vibration-rotation Schrödinger equation were used to make assignments, revealing unexpected features, including rotational difference bands and fewer degeneracies than anticipated. These results indicate that a shift away from perturbation theory to first principles calculations is necessary in order to assign spectra of hot polyatomic molecules such as water.</jats:p>

<jats:p> Plasma outflows, escaping from Earth through the high-altitude polar caps into the tail of the magnetosphere, have been observed with a xenon plasma source instrument to reduce the floating potential of the POLAR spacecraft. The largest component of H <jats:sup>+</jats:sup> flow, along the local magnetic field (30 to 60 kilometers per second), is faster than predicted by theory. The flows contain more O <jats:sup>+</jats:sup> than predicted by theories of thermal polar wind but also have elevated ion temperatures. These plasma outflows contribute to the plasmas energized in the elongated nightside tail of the magnetosphere, creating auroras, substorms, and storms. They also constitute an appreciable loss of terrestrial water dissociation products into space. </jats:p>

<jats:p> Shock-induced melt veins in the Acfer 040 L5-6 (S6) chondrite contain a previously unknown set of high pressure phases consisting of amorphous grains similar in composition to majorite, MgSiO <jats:sub>3</jats:sub> -ilmenite, and ringwoodite. The amorphous grains have compositions that are similar to those of synthetic MgSiO <jats:sub>3</jats:sub> -perovskites from chemically complex systems and are inferred to be MgSiO <jats:sub>3</jats:sub> -perovskite that crystallized from the melt at high pressure and temperature and subsequently amorphized after pressure release. The ilmenite represents a natural occurrence of a potentially important mineral in Earth's mantle. The MgSiO <jats:sub>3</jats:sub> -perovskite–MgSiO <jats:sub>3</jats:sub> -ilmenite–ringwoodite assemblage is not predicted by phase equilibria studies, but appears to result from crystallization of a melt at pressures above 26 gigapascals. </jats:p>

<jats:p> The Galileo spacecraft performed six radio occultation observations of Jupiter's Galilean satellite Europa during its tour of the jovian system. In five of the six instances, these occultations revealed the presence of a tenuous ionosphere on Europa, with an average maximum electron density of nearly 10 <jats:sup>4</jats:sup> per cubic centimeter near the surface and a plasma scale height of about 240 ± 40 kilometers from the surface to 300 kilometers and of 440 ± 60 kilometers above 300 kilometers. Such an ionosphere could be produced by solar photoionization and jovian magnetospheric particle impact in an atmosphere having a surface density of about 10 <jats:sup>8</jats:sup> electrons per cubic centimeter. If this atmosphere is composed primarily of O <jats:sub>2</jats:sub> , then the principal ion is O <jats:sub>2</jats:sub> <jats:sup>+</jats:sup> and the neutral atmosphere temperature implied by the 240-kilometer scale height is about 600 kelvin. If it is composed of H <jats:sub>2</jats:sub> O, the principal ion is H <jats:sub>3</jats:sub> O <jats:sup>+</jats:sup> and the neutral temperature is about 340 kelvin. In either case, these temperatures are much higher than those observed on Europa's surface, and an external heating source from the jovian magnetosphere is required. </jats:p>

<jats:p> Sulfurization of meteoritic metal in H <jats:sub>2</jats:sub> S-H <jats:sub>2</jats:sub> gas produced three different sulfides: monosulfide solid solution [(Fe,Ni) <jats:sub>1−</jats:sub> <jats:sub> <jats:italic>x</jats:italic> </jats:sub> S], pentlandite [(Fe,Ni) <jats:sub>9−</jats:sub> <jats:sub> <jats:italic>x</jats:italic> </jats:sub> S <jats:sub>8</jats:sub> ], and a phosphorus-rich sulfide. The composition of the remnant metal was unchanged. These results are contrary to theoretical predictions that sulfide formation in the solar nebula produced troilite (FeS) and enriched the remaining metal in nickel. The experimental sulfides are chemically and morphologically similar to sulfide grains in the matrix of the Alais (class CI) carbonaceous chondrite, suggesting that these meteoritic sulfides may be condensates from the solar nebula. </jats:p>