Catálogo de publicaciones - revistas

<jats:title>Black Hole Alignment</jats:title> <jats:p> In interpreting observations of black holes, it is important to assess the extent to which the accretion disks around black holes and the jets that they produce align with the spin of the black hole. Based on numerical simulations of magnetohydrodynamic fluid flows around rotating black holes, <jats:bold> McKinney <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="49" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1230811">49</jats:related-article> , published online 15 November) describe a previously unknown mechanism that acts to align the accretion disk and jet with the spin axis of black holes. Unlike previous mechanisms, this effect applies to thick disks, like the one thought to exist around the massive black hole at the center of our galaxy. </jats:p>

<jats:title>Negative Is Hotter</jats:title> <jats:p> A common-sense perception of temperature tells us that the lower the temperature, the colder it is. However, below absolute zero, there is a netherworld of negative temperatures, which are, counterintuitively, hotter than positive temperatures. Usually, such states are achieved in the laboratory and are characterized by a higher occupation of high-energy versus low-energy states. This is most easily done for systems that have a finite spectrum of energy states, bounded from above and below. <jats:bold> Braun <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="52" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1227831">52</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6115" page="42" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1232558">Carr</jats:related-article> </jats:bold> ) achieved negative temperature for a system in which its spectrum was only bounded on one side. Starting with a gas of <jats:sup>39</jats:sup> K bosonic atoms with repulsive interactions in a dipole trap and an optical lattice, a final state with negative temperature was reached where the atoms attract each other. </jats:p>

<jats:title>Atomic Spin-Transfer Torque</jats:title> <jats:p> Efficient electrical control of magnetism is a major goal of spin-based electronics. In many setups, spin-polarized current is used to switch the magnetization of a magnetic layer. This phenomenon, known as the spin-transfer torque (ST T), has mainly been studied on a larger scale. Working at the atomic scale, <jats:bold> Khajetoorians <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="55" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228519">55</jats:related-article> ) observed ST T in a structure of 5 to 7 magnetic atoms adsorbed on a metallic surface. The tip of a spinpolarized scanning tunneling microscope (STM) acted as the source of the spin-polarized current, and the reversal of the sign of the STM voltage resulted in the reversal of the preferred spin direction. By varying the temperature, the roles of different quantum processes were elucidated. These results will be of significance as spintronic components are further miniaturized. </jats:p>

<jats:title>Placing the O's in Ouabagenin</jats:title> <jats:p> Whereas enzymes are remarkably adept at selectively oxidizing saturated carbon centers, these reactions seriously challenge chemists. In a 19-step synthesis of ouabagenin, <jats:bold> Renata <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="59" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1230631">59</jats:related-article> ) showcase a range of creative indirect methods to install the six hydroxyl groups in the steroid's framework. These include a solid-state photochemical transformation, as well as dehydrogenation sequences that place olefins in proper position for oxygenation. The route also yields several intermediates poised for elaboration to distinct analogs for exploratory medicinal chemistry. </jats:p>

<jats:title>Faint Young Sun</jats:title> <jats:p> During the first ∼billion years after the formation of Earth, the Sun was significantly less luminous than today, delivering considerably less solar energy to our planet. Nevertheless, the geological record shows that Earth harbored liquid water and was not frozen—as would be expected on the basis of the reduced solar output. The presence of liquid water on Earth during this time is referred to as the “faint young Sun” paradox. <jats:bold>Wordsworth and Pierrehumbert</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="64" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1225759">64</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6115" page="44" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1232662">Kasting</jats:related-article> </jats:bold> ) suggest a new explanation for this mystery, invoking absorption of solar radiation owing to collisions between atmospheric H <jats:sub>2</jats:sub> and N <jats:sub>2</jats:sub> as a source of heating. This mechanism could have supplied enough extra warming to keep surface temperatures above 0°C. </jats:p>

<jats:title>ENSO Variability</jats:title> <jats:p> The El Niño–Southern Oscillation (ENSO) is the most energetic, quasiperiodic climate oscillation in the world—every few years warming large expanses of the surface equatorial Pacific Ocean surface and impacting temperatures and rainfall patterns across the globe. A pressing question, in the context of global warming, is whether ENSO might be affected by the rising atmospheric temperatures caused by anthropogenic greenhouse gas emissions. Climate models do not agree on the answer to this question, but one place to look for data about how global temperatures might influence ENSO is the record of past ENSO variability. <jats:bold> Cobb <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="67" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228246">67</jats:related-article> ) present a record of ENSO variability spanning the past 7000 years, in an attempt better to define its response to insolation forcing over this same period. The findings reveal high variability in ENSO behavior that has no clear dependence on insolation, which implies that a link to warming, if it exists, may be difficult to detect. </jats:p>

<jats:title>Building Cross Connectivity</jats:title> <jats:p> Chandelier cells innervate the initial segment of axons from pyramidal neurons and are thus placed to regulate pyramidal cell circuits in the brain. Chandelier cells of mice are marked by expression of the NKX2.1 transcription factor. <jats:bold> Taniguchi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="70" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1227622">70</jats:related-article> , published online 22 November) followed the development of these neurons and found that chandelier cells originate from the ventral germinal zone. The nascent cells migrate and integrate with cortical neurons following specific developmental pathways. </jats:p>

<jats:title>Next-Generation Biogeography</jats:title> <jats:p> In 1876, Alfred Russel Wallace mapped the zoogeographical regions of the world, based on the distributions and taxonomic relationships of broadly defined mammalian families. Wallace's classification of zoogeographical regions became a cornerstone of modern biogeography and a reference for a wide variety of biological disciplines, including global biodiversity and conservation sciences. <jats:bold> Holt <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="74" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228282">74</jats:related-article> , published online 20 December) present a next-generation map of wallacean zoogeographic regions, incorporating phylogenetic data on &gt;20,000 vertebrate species to discern and characterize their natural biogeographic patterns. </jats:p>

<jats:title>How the Crocodile Got Its Scales</jats:title> <jats:p> Mammalian hairs, avian feathers, and reptile scales differentiate and grow from genetically controlled units. Using three-dimensional (3D) computer graphics and computational biology to study scale generation, <jats:bold> Milinkovitch <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="78" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1226265">78</jats:related-article> , published online 29 November; see the cover) show that crocodilians' face and jaw scales do not follow this rule but emerge by physical cracking of the developing skin in a tension field. Thus, a crocodile's head scales are not genetically controlled developmental units but are random polygonal domains of keratinized skin that are generated from a self-organizing physical process. </jats:p>

<jats:title>Translating Polyproline</jats:title> <jats:p> Translation of messenger RNA into protein is carried out by the ribosome, together with a variety of accessory factors, which offer the potential for regulation of this critical step in gene expression (see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6115" page="38" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1233338"> <jats:bold>Buskirk and Green</jats:bold> </jats:related-article> ). <jats:bold> Ude <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="82" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1228985">82</jats:related-article> , published online 13 December), using bacterial genetics and an in vitro reconstituted translation system, and <jats:bold> Doerfel <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="85" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1229017">85</jats:related-article> , published online 13 December), using a model assay for peptide bond formation, find that the universally conserved bacterial elongation factor P (EF-P) (which is orthologous to the archaeal and eukaryotic initiation factor 5A) is required for the efficient translation of polyproline-containing polypeptides. Such short polyproline stretches (with runs of two, three, or more proline residues) would otherwise cause ribosomal stalling. </jats:p>