Catálogo de publicaciones - revistas

<jats:p>Nearly a century has been spent collecting and preserving genetic diversity in plants. Germplasm banks—living seed collections that serve as repositories of genetic variation—have been established as a source of genes for improving agricultural crops. Genetic linkage maps have made it possible to study the chromosomal locations of genes for improving yield and other complex traits important to agriculture. The tools of genome research may finally unleash the genetic potential of our wild and cultivated germplasm resources for the benefit of society.</jats:p>

<jats:p> Neutron scattering experiments have revealed a fascinating interplay between the hole doping, the spin fluctuations, and the superconductivity of the cuprate superconductors. Recently, electrochemical techniques have been used to produce large single crystals of La <jats:sub>2</jats:sub> CuO <jats:sub>4+</jats:sub> <jats:sub> <jats:italic>y</jats:italic> </jats:sub> , which has mobile oxygen dopants. Staging behavior of the excess oxygen has been demonstrated, and the low-energy spin fluctuations in stage 6 La <jats:sub>2</jats:sub> CuO <jats:sub>4+</jats:sub> <jats:sub> <jats:italic>y</jats:italic> </jats:sub> have been measured. The spin fluctuations are incommensurate with the lattice and have spatial, energy, and temperature dependencies very much like those in La <jats:sub>2−</jats:sub> <jats:sub> <jats:italic>x</jats:italic> </jats:sub> Sr <jats:sub> <jats:italic>x</jats:italic> </jats:sub> CuO <jats:sub>4</jats:sub> , with similar high transition temperature. This establishes the universality of the incommensurate spin fluctuations among the La <jats:sub>2</jats:sub> CuO <jats:sub>4</jats:sub> -based superconductors. </jats:p>

<jats:p>Intramolecular carbon, hydrogen, and sulfur isotope ratios were measured on a homologous series of organic sulfonic acids discovered in the Murchison meteorite. Mass-independent sulfur isotope fractionations were observed along with high deuterium/hydrogen ratios. The deuterium enrichments indicate formation of the hydrocarbon portion of these compounds in a low-temperature environment that is consistent with that of interstellar clouds. Sulfur-33 enrichments observed in methanesulfonic acid could have resulted from gas-phase ultraviolet irradiation of a precursor, carbon disulfide. The source of the sulfonic acid precursors may have been the reactive interstellar molecule carbon monosulfide.</jats:p>

<jats:p> Single <jats:bold>-</jats:bold> molecule fluorescence spectroscopy of a multichromophoric conjugated polymer (molecular weight ∼20,000) revealed surprising single-step photobleaching kinetics and acute jumps in fluorescence intensity. These jumps were shown not to result from spectral diffusion and were attributed to fluctuations in the quantum yield of emission for the molecules. The data indicate efficient intramolecular electronic energy transfer along the polymer chain to a localized fluorescence-quenching polymer defect. The defects are created by reversible photochemistry of the polymer. These findings have implications for the use of conjugated polymers in light-emitting diode displays and sensors. </jats:p>

<jats:p>A highly selective, colorimetric polynucleotide detection method based on mercaptoalkyloligonucleotide-modified gold nanoparticle probes is reported. Introduction of a single-stranded target oligonucleotide (30 bases) into a solution containing the appropriate probes resulted in the formation of a polymeric network of nanoparticles with a concomitant red-to-pinkish/purple color change. Hybridization was facilitated by freezing and thawing of the solutions, and the denaturation of these hybrid materials showed transition temperatures over a narrow range that allowed differentiation of a variety of imperfect targets. Transfer of the hybridization mixture to a reverse-phase silica plate resulted in a blue color upon drying that could be detected visually. The unoptimized system can detect about 10 femtomoles of an oligonucleotide.</jats:p>

<jats:p> A new high-temperature superconducting compound system, Ba <jats:sub>2</jats:sub> Ca <jats:sub> <jats:italic>n</jats:italic> </jats:sub> <jats:sub>–</jats:sub> <jats:sub>1</jats:sub> Cu <jats:sub> <jats:italic>n</jats:italic> </jats:sub> O <jats:sub> <jats:italic>x</jats:italic> </jats:sub> [02( <jats:italic>n</jats:italic> –1) <jats:italic>n</jats:italic> –Ba] interstitially doped with calcium or (Ca,Cu) has been identified to exhibit a transition temperature up to 126 kelvin, the highest for a superconductor without a volatile toxic element. 02( <jats:italic>n</jats:italic> –1) <jats:italic>n</jats:italic> –Ba has body-centered tetragonal symmetry and an unusual charge-reservoir block. The compounds offer interesting opportunities for high-temperature superconductivity science and technology. </jats:p>

<jats:p> The minerals (Mg,Fe)SiO <jats:sub>3</jats:sub> -ilmenite and -perovskite were identified in the shock-induced veins in the Tenham chondritic meteorite. Both phases are inferred to have transformed from pyroxene at high pressures and temperatures by shock metamorphism. Columnar-shaped ilmenite grains, one of two types of morphologies, have a topotaxial relationship with neighboring pyroxene grains, indicating shear transformation. Granular-shaped perovskite grains showed a diffraction pattern consistent with orthorhombic perovskite, but these grains were not stable under the electron beam irradiation and became amorphous. The higher iron concentration in both phases compared with those experimentally reported may suggest their metastable transition from enstatite because of shock compression. </jats:p>

<jats:p>It has been suggested that European Middle Pleistocene humans, Neandertals, and prehistoric modern humans had a greater sexual dimorphism than modern humans. Analysis of body size variation and cranial capacity variation in the large sample from the Sima de los Huesos site in Spain showed instead that the sexual dimorphism is comparable in Middle Pleistocene and modern populations.</jats:p>