Catálogo de publicaciones - revistas

<jats:title>Explaining Anomalous Early Isotopes</jats:title> <jats:p> Meteorites contain some of the oldest materials formed in the solar system, including silicate minerals similar in composition to those on Earth. These meteorites, however, often differ in their oxygen isotopic composition, implying that their formation involved a different chemical mechanism than younger solar-system materials. <jats:bold> Chakraborty <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="463" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1242237">463</jats:related-article> ) performed a series of experiments and found that mixing of SiO gas and OH leads to a mass-independent isotope fractionation in SiO <jats:sub>2</jats:sub> solid products similar to those observed in the oldest meteorites. This oxidation reaction may thus have served as the first source for silicates with anomalous oxygen isotopic compositions in the solar nebula. </jats:p>

<jats:title>On the Brink of Melting</jats:title> <jats:p> The considerable pressures and temperatures of Earth's iron-rich inner core make it a challenge to compare measurements made in experimental systems with observed seismic data. Computational simulations of core materials may reconcile any apparent differences. <jats:bold> Martorell <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="466" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1243651">466</jats:related-article> , published online 10 October) used ab initio simulations to predict the elastic properties of iron at core pressures. As temperatures approached the melting point of pure iron, the material was predicted to weaken to the point that seismic waves would be slowed considerably. An inner core with a small percentage of light elements like oxygen and silicon near its melting temperature would correspond well with measured seismic velocities. </jats:p>

<jats:title>Making an Impact</jats:title> <jats:p> How big a role do unconventional combinations of existing knowledge play in the impact of a scientific paper? To examine this question, <jats:bold> Uzzi <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="468" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1240474">468</jats:related-article> ) studied 17.9 million research articles across five decades of the Web of Science, the largest repository of scientific research. Scientific work typically appeared to draw on highly conventional, familiar mixtures of knowledge. The highest-impact papers were not the ones that had the greatest novelty, but had a combination of novelty and otherwise conventional combinations of prior work. </jats:p>

<jats:title>Piecing Together Hydrogenase</jats:title> <jats:p> Microbial hydrogenase enzymes generally use iron to catalyze the reversible formation of hydrogen from protons and electrons. Key to their efficiency is a set of iron-coordinating ligands, including CO and cyanide. <jats:bold> Kuchenreuther <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="472" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1241859">472</jats:related-article> ) examined how the HydG maturase enzyme breaks down the amino acid tyrosine to derive these diatomic ligands for assembly of the diiron class of hydrogenases. The first step involves abstraction of an H atom from the phenolic OH substituent of the side chain. Electron paramagnetic resonance spectroscopy revealed a radical intermediate that subsequently results from heterolysis of the bond tethering the side chain to the α-carbon. With the side chain thus jettisoned, the residual dehydroglycine could be transformed into CO and CN <jats:sup>−</jats:sup> . </jats:p>

<jats:title>Exploiting Redundancy</jats:title> <jats:p> The genetic code is redundant—multiple codons can code for the same amino acid. So-called synonymous codon changes within genes can nonetheless have substantial affects on protein expression, which have been attributed to changes in the structure of 5′ messenger RNAs, among other factors. <jats:bold> Goodman <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="475" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1241934">475</jats:related-article> , published online 26 September) built and measured the expression of a synthetic library of 14,000 variant N-terminal sequences of 137 <jats:italic>Escherichia coli</jats:italic> genes to show that, unexpectedly, rare codons had a bigger effect on increasing protein expression than more common codons. Increased RNA structure downstream of translation initiation appeared to represent the major determinant of expression differences owing to codon usage. </jats:p>

<jats:title>Farming or Fishing</jats:title> <jats:p> Evidence has been mounting that most modern European populations originated from the immigration of farmers who displaced the hunter-gatherers of the Mesolithic. <jats:bold> Bollongino <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="479" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1245049">479</jats:related-article> , published online 10 October) present analyses of palaeogenetic and isotopic data from Neolithic human skeletons from the Blätterhöhle burial site in Germany. The analyses identify a Neolithic freshwater fish–eating hunter-gatherer group, living contemporaneously and in close proximity to a Neolithic farming group. While there is some evidence that hunter-gatherer women may have admixed into the farming population, it appears likely that marriage or cultural boundaries between the groups persisted for over two millennia. Thus, the transition from the Mesolithic involved a more complex pattern of coexistence among humans of different genetic origins and cultures in the Neolithic, rather than a more abrupt transition. </jats:p>

<jats:title>Conform to the Norm</jats:title> <jats:p> Human societies have always enforced compliance with norms of acceptable behavior among their members by threatening punishment. It has been proposed that the human brain may have developed neural processes that support norm enforcement behavior and generate appropriate behavioral responses to social punishment threats. However, evidence for the neural circuitry underlying sanction-induced norm compliance in humans is limited. Using noninvasive brain stimulation, <jats:bold> Ruff <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6157" page="482" related-article-type="in-this-issue" vol="342" xlink:href="10.1126/science.1241399">482</jats:related-article> , published online 10 October) observed that alteration of the activity and excitability of the right lateral prefrontal cortex affected norm compliance, without affecting awareness of the content of the respective norms, or the expected sanctions. These alterations were much larger in a social, as compared to a nonsocial, context. </jats:p>

<jats:p>A weekly roundup of information on newly offered instrumentation, apparatus, and laboratory materials of potential interest to researchers.</jats:p>

<jats:p>Listen to stories on shaping liquids with nanoparticles, eating scorpions with ease, the first Americans, and more.</jats:p>

<jats:p> This issue of <jats:italic>Science</jats:italic> includes the program of the 2014 AAAS Annual Meeting. The theme of the AAAS Annual Meeting in Chicago, 13 to 17 February 2014, is "Meeting Global Challenges: Discovery and Innovation." </jats:p> <jats:p> A <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.sciencemag.org/cgi/reprint/342/6157/486.pdf">PDF</jats:ext-link> of the program as it appears in this issue is available here; for more information on the meeting (including registration forms and information on accommodations), please visit <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="www.aaas.org/meetings/">www.aaas.org/meetings/</jats:ext-link> . </jats:p>