Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes--40Mg and 42Al--that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.     

South-polar features on Venus similar to those near the north pole

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.     

Structural basis for cofactor-independent dioxygenation in vancomycin biosynthesis

Enzyme-catalysed oxidations are some of the most common transformations in primary and secondary metabolism. The vancomycin biosynthetic enzyme DpgC belongs to a small class of oxygenation enzymes that are not dependent on an accessory cofactor or metal ion. The detailed mechanism of cofactor-independent oxygenases has not been established. Here we report the first structure of an enzyme of this oxygenase class in complex with a bound substrate mimic. The use of a designed, synthetic substrate analogue allows unique insights into the chemistry of oxygen activation. The structure confirms the absence of cofactors, and electron density consistent with molecular oxygen is present adjacent to the site of oxidation on the substrate. Molecular oxygen is bound in a small hydrophobic pocket and the substrate provides the reducing power to activate oxygen for downstream chemical steps. Our results resolve the unique and complex chemistry of DpgC, a key enzyme in the biosynthetic pathway of an important class of antibiotics. Furthermore, mechanistic parallels exist between DpgC and cofactor-dependent flavoenzymes, providing information regarding the general mechanism of enzymatic oxygen activation.     

Magnetoelectrics: is CdCr2S4 a multiferroic relaxor?

Materials showing simultaneous ferroelectric and magnetic ordering are attracting a great deal of interest because of their unusual physics and potential applications. Hemberger et al. have reported relaxor-like dielectric properties and colossal magnetocapacitance (in excess of 500%) for the cubic spinel compound CdCr2S4 and related isomorphs, concluding that CdCr2S4 is a multiferroic relaxor. We argue here, however, that their results might also be explained by a conductive artefact.     

No extreme bipolar glaciation during the main Eocene calcite compensation shift

Major ice sheets were permanently established on Antarctica approximately 34 million years ago, close to the Eocene/Oligocene boundary, at the same time as a permanent deepening of the calcite compensation depth in the world's oceans. Until recently, it was thought that Northern Hemisphere glaciation began much later, between 11 and 5 million years ago. This view has been challenged, however, by records of ice rafting at high northern latitudes during the Eocene epoch and by estimates of global ice volume that exceed the storage capacity of Antarctica at the same time as a temporary deepening of the calcite compensation depth approximately 41.6 million years ago. Here we test the hypothesis that large ice sheets were present in both hemispheres approximately 41.6 million years ago using marine sediment records of oxygen and carbon isotope values and of calcium carbonate content from the equatorial Atlantic Ocean. These records allow, at most, an ice budget that can easily be accommodated on Antarctica, indicating that large ice sheets were not present in the Northern Hemisphere. The records also reveal a brief interval shortly before the temporary deepening of the calcite compensation depth during which the calcite compensation depth shoaled, ocean temperatures increased and carbon isotope values decreased in the equatorial Atlantic. The nature of these changes around 41.6 million years ago implies common links, in terms of carbon cycling, with events at the Eocene/Oligocene boundary and with the 'hyperthermals' of the Early Eocene climate optimum. Our findings help to resolve the apparent discrepancy between the geological records of Northern Hemisphere glaciation and model results that indicate that the threshold for continental glaciation was crossed earlier in the Southern Hemisphere than in the Northern Hemisphere.     

Female mate-choice drives the evolution of male-biased dispersal in a social mammal

Dispersal has a significant impact on lifetime reproductive success, and is often more prevalent in one sex than the other. In group-living mammals, dispersal is normally male-biased and in theory this sexual bias could be a response by males to female mate preferences, competition for access to females or resources, or the result of males avoiding inbreeding. There is a lack of studies on social mammals that simultaneously assess these factors and measure the fitness consequences of male dispersal decisions. Here we show that male-biased dispersal in the spotted hyaena (Crocuta crocuta) most probably results from an adaptive response by males to simple female mate-choice rules that have evolved to avoid inbreeding. Microsatellite profiling revealed that females preferred sires that were born into or immigrated into the female's group after the female was born. Furthermore, young females preferred short-tenured sires and older females preferred longer-tenured sires. Males responded to these female mate preferences by initiating their reproductive careers in groups containing the highest number of young females. As a consequence, 11% of males started their reproductive career in their natal group and 89% of males dispersed. Males that started reproduction in groups containing the highest number of young females had a higher long-term reproductive success than males that did not. The female mate-choice rules ensured that females effectively avoided inbreeding without the need to discriminate directly against close kin or males born in their own group, or to favour immigrant males. The extent of male dispersal as a response to such female mate preferences depends on the demographic structure of breeding groups, rather than the genetic relatedness between females and males.