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551.
Barros-Filho BA de Oliveira MC Mafezoli J Barbosa FG Rodrigues-Filho E 《Natural product communications》2012,7(6):771-773
The basidiomycete Lentinus strigellus was cultivated in three different culture media and the secondary metabolites produced under different culture conditions were isolated and identified. When cultivated in a liquid medium with peptone, L. strigellus afforded the benzopyrans, 2,2-dimethyl-6-methoxychroman-4-one, 4-hydroxy-2,2-dimethyl-6-methoxychromane and (3R,4S)-3,4-dihydroxy-2,2-dimethyl-6-methoxychromane. The indole alkaloid echinuline and the anthraquinone fiscione, both unprecedented for the genus Lentinus, were isolated from the mycelium of the fungus. When cultured in Czapek medium enriched with potato broth, the fungus afforded the same benzopyrans except (3S,4S)-3,4-dihydroxy-2,2-dimethyl-6-methoxychromane. Panepoxydone and isopanepoxydone were also isolated when the microorganism was grown in Czapek medium. 相似文献
552.
Fernando Benavente Silvia Medina‐Casanellas José Barbosa Victoria Sanz‐Nebot 《Journal of separation science》2010,33(9):1294-1304
In this study, we investigated the performance of several commercial sorbents (Sep‐pack® C18, tC18, C8 and tC2, Oasis® HLB, Isolute® ENV+, Strata?‐X and Oasis® MCX) for the determination of opioid peptides by solid‐phase extraction coupled on‐line to capillary electrophoresis (SPE‐CE). First, standard solutions were analyzed in order to achieve the lowest LOD and the best electrophoretic separations using UV detection. The best results were obtained using C18, C8 and tC2 sorbents, which were examined for the analysis of spiked human plasma samples. A double‐step sample clean‐up pretreatment, which consisted of precipitation with acetonitrile and filtration, was needed to prevent saturation of the on‐line SPE microcartridge. The filtration step was critical to obtain optimum analyte recovery and to clean up the sample matrix. A range of centrifugal filters and filtration conditions were tested and the recoveries of the sample pretreatment were evaluated by CE‐ESI‐MS. The LODs attained through SPE‐CE‐UV were approximately ten‐fold better with C18 than with C8 and tC2. The 0.1 μg/mL LODs achieved by C18‐SPE‐CE‐UV were further improved until we could detect 1 ng/mL concentrations of opioid peptides in plasma samples by C18‐SPE‐CE‐ESI‐MS, due to the outstanding selectivity of the MS detection. 相似文献
553.
554.
Abdelillah Karouit Essaid Sabir Fernando Ramirez-Mireles Luis Orozco Barbosa Abdelkrim Haqiq 《Journal of Mathematical Modelling and Algorithms》2013,12(3):291-309
In this paper, we present a game analysis of the Binary Exponential Backoff (BEB), a popular bandwidth allocation mechanism used by a large number of distributed wireless technologies. A Markov chain analysis is used to obtain equilibrium retransmission probabilities and throughput. Numerical results show that when the arrival probability increases, the behavior of mobile stations MSs become more and more aggressive resulting in a global deterioration of the system throughput. We then consider a non-cooperative game framework to study the operation and evaluate the performance of the BEB algorithm when a group of MSs competing with each other to gain access to the wireless channel. We focus our attention to the case when an MS acts selfishly by attempting to gain access to the channel using a higher retransmission probability as a means to increase its own throughput. As a means to improve the system performance, we further explore the use of two transmission mechanisms and policies. First, we introduce the use of multiple power levels (MPLs) for the data transmission. The use of multiple power levels results on a capture effect allowing the receiver to properly decode the message even in the presence of a collision. Under the proposed scheme, named MPL-BEB, the effect of the aggressive behavior, higher transmission probabilities, is diminished since the power level is chosen randomly and independently by each and every station. Second, we introduce a disutility policy for power consumption. The resulting mechanism, named MPL-BEB with costs, is of prime interest in wireless networks composed of battery-powered nodes. Under this scheme aggressive behavior is discouraged since each retransmission translates into the depletion of the energy stored in the battery. Via price of anarchy, our results identify a behavior similar to the well-know prisoner’s dilemma. A non-efficiency of Nash equilibrium is observed for all schemes (BEB, MPL-BEB, MPL-BEB with costs) under heavy traffic with a notable outperformance of MPL-BEB with costs over both MPL-BEB and BEB. 相似文献
555.
556.
In this work we have synthesized TiO2 nanoparticles, using either a sol–gel base catalysed process in the interior of CTAB reversed micelles (TiO2 CTAB sol), or the neutralization of a TiO2/H2SO4 solution in the interior of AOT reversed micelles. From the absorption and emission data of the TiO2 nanoparticles it is possible to conclude that in the sol–gel route there remains alkoxide groups in the structure, originating transitions lower than the energy gap of TiO2 semiconductor. These transitions disappear in the neutralization procedure, where the alkoxide groups are absent in the structure. We have assigned the observed indirect and direct optical transitions according to the anatase band structure. TiO2 Langmuir-Blodgett (LB) films were prepared either by direct deposition of titanium isopropoxide or by deposition of the TiO2 CTAB sol. These films showed photoluminescence, which was attributed to band-gap emission and to surface recombination of defect states. 相似文献
557.
Using molecular-dynamics simulations and integral equations (Rogers-Young, Percus-Yevick, and hypernetted chain closures) we investigate the thermodynamics of particles interacting with continuous core-softened intermolecular potential. Dynamic properties are also analyzed by the simulations. We show that, for a chosen shape of the potential, the density, at constant pressure, has a maximum for a certain temperature. The line of temperatures of maximum density (TMD) was determined in the pressure-temperature phase diagram. Similarly the diffusion constant at a constant temperature, D, has a maximum at a density rho(max) and a minimum at a density rho(min) < rho(max). In the pressure-temperature phase diagram the line of extrema in diffusivity is outside of the TMD line. Although this interparticle potential lacks directionality, this is the same behavior observed in simple point charge/extended water. 相似文献
558.
Slana GB de Azevedo MS Lopes RS Lopes CC Cardoso JN 《Beilstein journal of organic chemistry》2006,2(1):1
Using new variations of anionic aromatic chemistry, the total synthesis of oxygenated brazanquinones (1a-1c), derived from beta-brasan, a natural product isolated from Caesalpina echinata, via carbamates 2a-2c is described. 相似文献
559.
560.
Luís Gustavo Cofani dos Santos Elisabete A. De Nadai Fernandes Simone Silveira Nery da Silva Cofani dos Santos Márcio Arruda Bacchi Gabriel Adrián Sarriés Fernando Barbosa Júnior 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):207-211
Brazil started to export dairy products in the early 2000s and since then has slowly consolidated its position in the world dairy market. To ensure the position in the international market by improving the quality of milk produced in the country, the government created a national program and a network of laboratories for milk quality. The Normative Instruction 51 (Instrução Normativa 51—IN51) was introduced within the national program establishing parameters for milk quality and safety. In spite of not being included in the IN51, chemical elements are under thorough discussion as quality parameters for their nutritional and toxicological relevance. This work aimed at evaluating the quality of milk from one of the major dairy regions in Brazil, located in the state of Rio Grande do Sul, by the determination of chemical elements, comparing the results with those previously obtained for milk samples from the main dairy region of the country, in the state of Minas Gerais. Samples were collected from cooling tanks in nineteen dairy farms. After freeze drying, chemical elements were determined by instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). For ten out of sixteen chemical elements determined, there were significant differences between samples from both producing regions (p < 0.01). 相似文献