Catálogo de publicaciones - revistas

<jats:p>Angiogenesis is thought to depend on a precise balance of positive and negative regulation. Angiopoietin-1 (Ang1) is an angiogenic factor that signals through the endothelial cell–specific Tie2 receptor tyrosine kinase. Like vascular endothelial growth factor, Ang1 is essential for normal vascular development in the mouse. An Ang1 relative, termed angiopoietin-2 (Ang2), was identified by homology screening and shown to be a naturally occurring antagonist for Ang1 and Tie2. Transgenic overexpression of Ang2 disrupts blood vessel formation in the mouse embryo. In adult mice and humans, Ang2 is expressed only at sites of vascular remodeling. Natural antagonists for vertebrate receptor tyrosine kinases are atypical; thus, the discovery of a negative regulator acting on Tie2 emphasizes the need for exquisite regulation of this angiogenic receptor system.</jats:p>

<jats:p> On the basis of x-ray diffraction data to a resolution of 2.9 angstroms, atomic models of most protein components of the bovine cytochrome bc <jats:sub>1</jats:sub> complex were built, including core 1, core 2, cytochrome b, subunit 6, subunit 7, a carboxyl-terminal fragment of cytochrome c <jats:sub>1</jats:sub> , and an amino-terminal fragment of the iron-sulfur protein. The positions of the four iron centers within the bc <jats:sub>1</jats:sub> complex and the binding sites of the two specific respiratory inhibitors antimycin A and myxothiazol were identified. The membrane-spanning region of each bc <jats:sub>1</jats:sub> complex monomer consists of 13 transmembrane helices, eight of which belong to cytochrome b. Closely interacting monomers are arranged as symmetric dimers and form cavities through which the inhibitor binding pockets can be accessed. The proteins core 1 and core 2 are structurally similar to each other and consist of two domains of roughly equal size and identical folding topology. </jats:p>

<jats:p> The isolated, young, sunlike star TW Hya and four other young stars in its vicinity are strong x-ray sources. Their similar x-ray and optical properties indicate that the stars make up a physical association that is on the order of 20 million years old and that lies between about 40 and 60 parsecs (between about 130 and 200 light years) from Earth. TW Hya itself displays circumstellar CO, HCN, CN, and HCO <jats:sup>+</jats:sup> emission. These molecules probably orbit the star in a solar-system–sized disk viewed more or less face-on, whereas the star is likely viewed pole-on. Being at least three times closer to Earth than any well-studied region of star formation, the TW Hya Association serves as a test-bed for the study of x-ray emission from young stars and the formation of planetary systems around sunlike stars. </jats:p>

<jats:p> Marine cores from the continental shelf edge of British Columbia (Canada) demonstrate that sea level at the shelf edge was 153 meters below present 14,000 calendar years ago and more than 30 meters lower than the maximum eustatic low of −120 meters. Dated artifacts, including stone tools, indicate that humans occupied this region by at least 10,200 calendar years before present (B.P.). Local sea level rose rapidly (5 centimeters per year) during the period of early human occupation as a result of eustatic sea-level rise and glacio-isostatic forebulge movement. This shelf edge site was first elevated and then subsided. The exposed shelf edge was available for human occupation and may have served as a migration route during times of lowered sea levels between 13,500 and 9500 <jats:sup>14</jats:sup> C years B.P. </jats:p>

<jats:p>The provenance of ice-rafted debris (IRD) in four Arctic sediment cores implies that icebergs from the northwestern Laurentide ice sheets drifted across the western Arctic Ocean along the 180°–0° meridian toward Fram Strait during mid- to late Pleistocene deglaciations within the last 700,000 years. This iceberg drift was different from the present-day Beaufort Gyre circulation and resembled a dislocated transpolar drift (TPD). Sea ice mainly followed the iceberg trajectories but also frequently drifted from the Russian shelves eastward into the Amerasian Basin.</jats:p>

<jats:p>Treatment of an overcrowded triselenatribismane, 2,4,6-tris(bis(trimethylsilyl)methyl)phenyl-1,3,5-triselena-2,4,6-tribismane, with hexamethylphosphorous triamide in toluene at 100°C resulted in the quantitative formation of a stable dibismuthene [TbtBi=BiTbt, where Tbt is 2,4,6-tris(bis(trimethylsilyl)methyl)phenyl], a compound containing a double bond formed between two bismuth atoms. The compound formed as deep purple crystals upon cooling. Ultraviolet-visible and Raman spectra, x-ray crystallographic structural analysis, and theoretical calculations provided evidence for the double bond character of the Bi–Bi bond.</jats:p>

<jats:p> Measurements of the influence of reactant alignment on the rates of chemical reactions provide direct information concerning the atomic motions necessary for chemical transformation. Data presented here show that at low collision energy, the dissociative adsorption of deuterium (D <jats:sub>2</jats:sub> ) on the (111) surface of copper has a much higher probability for broadside than for end-on collisions. Furthermore, this steric preference is sensitive to the kinetic energy of the incident molecule, almost disappearing as the energy increases to 0.8 electron volt. This study shows that the dynamic conditions of a surface chemical reaction can profoundly influence the associated steric requirements. </jats:p>

<jats:p> Quasi-particles (QPs) are excitations of the superconducting state. The behavior of QPs in Bi <jats:sub>2</jats:sub> Sr <jats:sub>2</jats:sub> CaCu <jats:sub>2</jats:sub> O <jats:sub>8</jats:sub> in a magnetic field was probed by measurement of the thermal conductivity κ. An anomaly in κ was observed at low temperatures. At a transition field H <jats:sub>k</jats:sub> , κ displayed a sharp break in slope, followed by a plateau region in which it ceased to change with increasing field. The nonanalytic nature of the break at H <jats:sub>k</jats:sub> suggests a phase transition of the condensate to a state in which the QP current is zero (the system remains superconducting). Detailed measurements of the new regime are presented, and implications for the QPs and the superconducting state are discussed. </jats:p>

<jats:p>Resonant Raman and nuclear magnetic resonance spectroscopies from single gallium arsenide quantum dots are demonstrated. The nuclei were probed through changes in the optical spectra of the quantum dot exciton arising from exciton-nuclear interactions. This approach allowed the application of optical spectroscopy with its extremely high sensitivity and selectivity. The experiments had a lateral spatial resolution of about 10 nanometers and probe a volume that was five orders of magnitude smaller than that of previous semiconductor nuclear spectroscopic studies.</jats:p>