Mechanism of electric-pulse-induced resistance switching in manganites

We investigate the electric-pulse-induced resistance switching in manganite systems. We find a "complementarity" effect where the contact resistance of electrodes at opposite ends show variations of opposite sign and is reversible. The temperature dependence of the magnitude of the effect reveals a dramatic enhancement at a temperature T*, below the metal-insulator transition. We qualitatively capture these features with a theoretical model, providing evidence for the physical mechanism of the resistance switching. We argue that doping control of the electronic state of the oxide at the interfaces is the mechanism driving the effect.     

Solvent-free synthesis of functionalized pyridine derivatives using Wells-Dawson heteropolyacid as catalyst

Wells-Dawson heteropolyacids (H₆P₂W₁₈O₆₂·24H₂O) were used as catalysts in the Hantzsch-like multicomponent condensation reaction with 3-formylchromones as aldehyde component, a β-ketoester and ammonium acetate, under solvent-free conditions at 80 °C. Although the desired products were obtained, functionalized pyridines were the main reaction product and became the alternative route to dihydropyridine ring formation. Based on the proposed mechanisms for the formation of each of the obtained products, the multicomponent reaction was modified to afford only the functionalized pyridines (60-99%). Our procedure represents a clean alternative for the synthesis of several highly functionalized pyridines.     

Development and validation of a rapid method for microcystins in fish and comparing LC-MS/MS results with ELISA

Microcystins (MCs) are the most common cyanotoxins found worldwide in freshwater, brackish, and marine environments. The rapid and accurate analysis of MCs and nodularin (Nod-R) in fish tissue is important for determining occurrence, following trends, and monitoring exposure for risk assessment and other purposes. The aim of this study was to develop a streamlined and reliable sample preparation method for eight MCs (MC-RR, MC-YR, MC-LR, MC-WR, MC-LA, MC-LY, MC-LW, and MC-LF) and Nod-R in fish, and conduct a validation of the new method using liquid chromatography–tandem mass spectrometry (LC-MS/MS) for analysis and compare the results with a commercial enzyme-linked immunosorbent assay (ELISA) kit. Different sample preparation methods were compared, and a simple extraction protocol with acidified acetonitrile/water (3:1) followed by hexane partitioning cleanup was found to be most effective. Thorough validation of the final method was conducted, and 90–115% recoveries were achieved for all analytes except for MC-RR, which gave 130% average recovery (isotopically labeled internal standards were unavailable to correct for possible biases). The use of electrospray ionization in the negative mode gave few interferences and minimal matrix effects in the LC-MS/MS analysis overall. Precision was typically 10–20% RSD among multiple days in experiments, detection limits were <10 ng/g in the fish tissue (catfish, basa, and swai filets), and no false-positives or false-negatives occurred in blind analyses of many spiked samples. The ELISA was unable to distinguish between MCs but was found to correctly assess the presence or absence of MCs and Nod-R in the blind-fortified fish tissues. The capability of these approaches to measure covalently bound MCs was not assessed.     

Trace analysis of pesticides in paddy field water by direct injection using liquid chromatography-quadrupole-linear ion trap-mass spectrometry

A multiresidue method was developed for the quantification and confirmation of 70 pesticides in paddy field water. After its filtration, water was injected directly in a liquid chromatograph coupled to a hybrid triple quadrupole-linear ion trap-mass spectrometer (QqLIT). The list of target analytes included organophosphates, phenylureas, sulfonylureas, carbamates, conazoles, imidazolinones and others compounds widely used in different countries where rice is cropped. Detection and quantification limits achieved were in the range from 0.4 to 80 ng L(-1) and from 2 to 150 ng L(-1), respectively. Correlation coefficients for the calibration curves in the range 0.1-50 μg L(-1) were higher than 0.99 except for diazinon (0.1-25 μg L(-1)). Only 9 pesticides presented more than 20% of signal suppression/enhancement, no matrix effect was observed in the studied conditions for the rest of the target pesticides. The method developed was used to investigate the occurrence of pesticides in 59 water samples collected in paddy fields located in Spain and Uruguay. The study shows the presence of bensulfuron methyl, tricyclazole, carbendazim, imidacloprid, tebuconazole and quinclorac in a concentration range from 0.08 to 7.20 μg L(-1).     

Anomalies in supercooled NaCl aqueous solutions: a microscopic perspective

In this work we studied the effect of NaCl on the thermodynamic and dynamic properties of supercooled water, for salt concentrations between 0.19 and 1.33 mol?kg(-1), using molecular dynamic simulations for TIP5P∕E water model and ion parameters specially designed to be used in combination with this potential. We studied the isobaric heat capacity (C(p)) temperature dependence and observed a maximum in C(p), occurring at T(m), that moves to lower temperature values with increasing salt concentration. Many characteristic changes were observed at scaled temperature T∕T(m) ～ 0.96, namely a minimum in the density of the system, a reduction of the slope of the number of hydrogen bonds vs. temperature, and a crossover from Vogel-Tamman-Fulcher to Arrhenius dynamics. Finally, at low temperatures we observed that water dynamics become heterogeneous with an apparently common relationship between the fraction of immobile molecules and T/T(m) for all studied systems.     

Supramolecular assembly of graphene with functionalized poly(fluorene-alt-phenylene): the role of the anthraquinone pendant groups

Hybrid materials composed of graphene and conjugated poly(fluorene-alt-phenylene) endowed with redox-active anthraquinone moieties are prepared. Remarkable changes in the electronic properties of the polymer are observed. Effective interactions among the pendant anthraquinone and graphene, likely due to π-π stacking, are confirmed.     

A non-adiabatic quantum-classical dynamics study of the intramolecular excited state hydrogen transfer in ortho-nitrobenzaldehyde

Ab initio surface-hopping dynamics calculations have been performed to simulate the intramolecular excited state hydrogen transfer dynamics of ortho-nitrobenzaldehyde (o-NBA) in the gas phase from the electronic S(1) excited state. Upon UV excitation, the hydrogen is transferred from the aldehyde substituent to the nitro group, generating o-nitrosobenzoic acid through a ketene intermediate. The semiclassical propagations show that the deactivation from the S(1) is ultrafast, in agreement with the experimental measurements, which detect the ketene in less than 400 fs. The trajectories show that the deactivation mechanism involves two different conical intersections. The first one, a planar configuration with the hydrogen partially transferred, is responsible for the branching between the formation of a biradical intermediate and the regeneration of the starting material. The conversion of the biradical to the ketene corresponds to the passage through a second intersection region in which the ketene group is formed.     

Antibacterial and cytotoxic activities of the sesquiterpene lactones cnicin and onopordopicrin

The antimicrobial and cytotoxic activities of chloroform extracts from the weeds Centaurea tweediei and C. diffusa, and the main sesquiterpene lactones isolated from these species, onopordopicrin and cnicin, respectively, were assayed. Results show that the chloroform extracts from both Centaurea species possess antibacterial activities against a panel of Gram-positive and Gram-negative bacteria. Remarkable antibacterial activity against methicillin-resistant Staphylococcus aureus was also measured. Both the extracts and the purified sesquiterpene lactones show high cytotoxicity against human-derived macrophages. Despite this cytotoxicity, C. diffusa chloroform extract and cnicin are attractive candidates for evaluation as antibiotics in topical preparations against skin-associated pathogens.     

A facile and efficient four-step enantioselective synthesis of (+)-vernolepin from (+)-minimolide, the major germacranolide of Mikania minima

Enantiomerically pure (+)-vemolepin was semi-synthesized for the first time using the synthon (6S,7R,8S)-8,14-diacetoxy-15-hydroxygermacra-1(10),4,11(13)-trien-6, 12-olide [(+)-minimolide], the major sesquiterpene lactone of the Argentinean vine Mikania minima. After performing four consecutive reactions (Cope rearrangement, two oxidations, and selective hydrolysis of the acetate groups) on the synthon (+)-minimolide, a (+)-vernolepin yield of ca. 40% was achieved, proving to be a suitable semi-synthetic strategy for the production of quantities between 0.5-1.0 g of (+)-vernolepin. The transformations described here mimetize the biogenetic pathway for the production of (+)-vernolepin in the genus Vernonia. The synthesized (+)-vernolepin, but not its precursors, shows antifungal activity similar to amphotericin B. The semi-synthesis reported here combines affordable and easily available chemical reagents with classical organic methodologies.     

Electron microscopy analysis of the nucleolus of Trypanosoma cruzi.

The nucleolus is the main site for synthesis and processing of ribosomal RNA in eukaryotes. In mammals, plants, and yeast the nucleolus has been extensively characterized by electron microscopy, but in the majority of the unicellular eukaryotes no such studies have been performed. Here we used ultrastructural cytochemical and immunocytochemical techniques as well as three-dimensional reconstruction to analyze the nucleolus of Trypanosoma cruzi, which is an early divergent eukaryote of medical importance. In T. cruzi epimastigotes the nucleolus is a spherical intranuclear ribonucleoprotein organelle localized in a relatively central position within the nucleus. Dense fibrillar and granular components but not fibrillar centers were observed. In addition, nuclear bodies resembling Cajal bodies were observed associated to the nucleolus in the surrounding nucleoplasm. Our results provide additional morphological data to better understand the synthesis and processing of the ribosomal RNA in kinetoplastids.