999 resultados para 41-368


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A comprehensive study was conducted on mesoporous MCM-41. Spectroscopic examinations demonstrated that three types of silanol groups, i.e., single, (SiO)3Si-OH, hydrogen-bonded, (SiO)3Si-OH-OH-Si(SiO)3, and geminal, (SiO)2Si(OH)2, can be observed. The number of silanol groups/nm2, ?OH, as determined by NMR, varies between 2.5 and 3.0 depending on the template-removal methods. All these silanol groups were found to be the active sites for adsorption of pyridine with desorption energies of 91.4 and 52.2 kJ mol-1, respectively. However, only free silanol groups (involving single and geminal silanols) are highly accessible to the silylating agent, chlorotrimethylsilane. Silylation can modify both the physical and chemical properties of MCM-41.

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The discovery of mesoporous molecular sieves, MCM-41, which possesses a regular hexagonal array of uniform pore openings, aroused a worldwide resurgence in this field. This is not only because it has brought about a series of novel mesoporous materials with various compositions which may find applications in catalysis, adsorption, and guest-host chemistry, but also it has opened a new avenue for creating zeotype materials. This paper presents a comprehensive overview of recent advances in the field of MCM-41. Beginning with the chemistry of surfactant/silicate solutions, progresses made in design and synthesis, characterization, and physicochemical property evaluation of MCM-41 are enumerated. Proposed formation mechanisms are presented, discussed, and identified. Potential applications are reviewed and projected. More than 100 references are cited.

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Several mechanisms have been proposed to explain the action of enzymes at the atomic level. Among them, the recent proposals involving short hydrogen bonds as a step in catalysis by Gerlt and Gassman [1] and proton transfer through low barrier hydrogen bonds (LBHBs) [2, 3] have attracted attention. There are several limitations to experimentally testing such hypotheses, Recent developments in computational methods facilitate the study of active site-ligand complexes to high levels of accuracy, Our previous studies, which involved the docking of the dinucleotide substrate UpA to the active site of RNase A [4, 5], enabled us to obtain a realistic model of the ligand-bound active site of RNase A. From these studies, based on empirical potential functions, we were able to obtain the molecular dynamics averaged coordinates of RNase A, bound to the ligand UpA. A quantum mechanical study is required to investigate the catalytic process which involves the cleavage and formation of covalent bonds. In the present study, we have investigated the strengths of some of the hydrogen bonds between the active site residues of RNase A and UpA at the ab initio quantum chemical level using the molecular dynamics averaged coordinates as the starting point. The 49 atom system and other model systems were optimized at the 3-21G level and the energies of the optimized systems were obtained at the 6-31G* level. The results clearly indicate the strengthening of hydrogen bonds between neutral residues due to the presence of charged species at appropriate positions. Such a strengthening manifests itself in the form of short hydrogen bonds and a low barrier for proton transfer. In the present study, the proton transfer between the 2'-OH of ribose (from the substrate) and the imidazole group from the H12 of RNase A is influenced by K41, which plays a crucial role in strengthening the neutral hydrogen bond, reducing the barrier for proton transfer.

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Two geometrid moths Chiasmia inconspicua and Chiasmia assimilis, identified as potential biological control agents for prickly acacia Acacia nilotica subsp. indica, were collected in Kenya and imported into quarantine facilities in Australia where laboratory cultures were established. Aspects of the biologies of both insects were studied and CLIMEX® models indicating the climatically favourable areas of Australia were developed. Host range tests were conducted using an approved test list of 74 plant species and no-choice tests of neonate larvae placed on both cut foliage and potted plants. C. inconspicua developed through to adult on prickly acacia and, in small numbers, Acacia pulchella. C. assimilis developed through to adult on prickly acacia and also in very small numbers on A. pulchella, A. deanei, A. decurrens, and A. mearnsii. In all experiments, the response on prickly acacia could be clearly differentiated from the responses on the non-target species. Both insects were approved for release in Australia. Over a three-year period releases were made at multiple sites in north Queensland, almost all in inland areas. There was no evidence of either insect's establishment and both colonies were terminated. A new colony of C. assimilis was subsequently established from insects collected in South Africa and releases of C. assimilis from this new colony were made into coastal and inland infestations of prickly acacia. Establishment was rapid at one coastal site and the insect quickly spread to other infestations. Establishment at one inland area was also confirmed in early 2006. The establishment in coastal areas supported a CLIMEX model that indicated that the climate of coastal areas was more suitable than inland areas.

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Table beet production in the Lockyer Valley of south-eastern Queensland is known to be adversely affected by soilborne root disease from infection by Pythium spp. However, little is known regarding the species or genotypes that are the causal agents of both pre- and post-emergence damping off. Based on RFLP analysis with HhaI, HinfI and MboI of the PCR amplified ITS region DNA from soil and diseased plant samples, the majority of 130 Pythium isolates could be grouped into three genotypes, designated LVP A, LVP B and LVP C. These groups comprised 43, 41 and 7% of all isolates, respectively. Deoxyribonucleic acid sequence analysis of the ITS region indicated that LVP A was a strain of Pythium aphanidermatum, with greater than 99% similarity to the corresponding P. aphanidermatum sequences from the publicly accessible databases. The DNA sequences from LVP B and LVP C were most closely related to P. ultimum and P. dissotocum, respectively. Lower frequencies of other distinct isolates with unique RFLP patterns were also obtained with high levels of similarity (>97%) to P. heterothallicum, P. periplocum and genotypes of P. ultimum other than LVP B. Inoculation trials of 1- and 4-week-old beet seedlings indicated that compared with isolates of the LVP B genotype, a higher frequency of LVP A isolates caused disease. Isolates with the LVP A, LVP B and LVP C genotypes were highly sensitive to the fungicide Ridomil MZ, which suppressed radial growth on V8 agar between approximately four and thirty fold at 5 μg/mL metalaxyl and 40 μg/mL mancozeb, a concentration far lower than the recommended field application rate.

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The feasibility of state-wide eradication of 41 invasive plant taxa currently listed as ‘Class 1 declared pests’ under the Queensland Land Protection (Pest and Stock Route Management) Act 2002 was assessed using the predictive model ‘WeedSearch’. Results indicated that all but one species (Alternanthera philoxeroides) could be eradicated, provided sufficient funding and labour were available. Slightly less than one quarter (24.4%) (n = 10) of Class 1 weed taxa could be eradicated for less than $100 000 per taxon. An additional 43.9% (n = 18) could be eradicated for between $100 000 and $1M per taxon. Hence, 68.3% of Class 1 weed taxa (n = 28) could be eradicated for less than $1M per taxon. Eradication of 29.3% (n = 12) is predicted to cost more than $1M per taxon. Comparison of these WeedSearch outputs with either empirical analysis or results from a previous application of the model suggests that these costs may, in fact, be underestimates. Considering the likelihood that each weed will cost the state many millions of dollars in long-term losses (e.g. losses to primary production, environmental impacts and control costs), eradication seems a wise investment. Even where predicted costs are over $1M, eradication can still offer highly favourable benefit:cost ratios. The total (cumulative) cost of eradication of all 41 weed taxa is substantial; for all taxa, the estimated cost of eradication in the first year alone is $8 618 000. This study provides important information for policy makers, who must decide where to invest public funding.

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Handwritten on verso: zu obest mein Atelier (My studio on the top floor)

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Aluminium iodide reduces sulphonyl chlorides to disulphides and sulphoxides to sulphides under mild conditions in acetonitrile.

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Handwritten on verso: zu obest mein Atelier (My studio on the top floor)

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Mesoporous MCM-41 type silicas containing molybdenum and cobalt have been prepared with pore sizes in the range 30-38 Angstrom and 54-59 Angstrom. Catalytic properties of these materials have been examined with respect to the oxidation of cyclooctene and aniline.

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The short-lived radionuclide Ca-41 plays an important role in constraining the immediate astrophysical environment and the formation timescale of the nascent solar system due to its extremely short half-life (0.1 Myr). Nearly 20 years ago, the initial ratio of Ca-41/Ca-40 in the solar system was determined to be (1.41 +/- 0.14) x 10(-8), primarily based on two Ca-Al-rich Inclusions (CAIs) from the CV chondrite Efremovka. With an advanced analytical technique for isotopic measurements, we reanalyzed the potassium isotopic compositions of the two Efremovka CAIs and inferred the initial ratios of Ca-41/Ca-40 to be (2.6 +/- 0.9) x 10(-9) and (1.4 +/- 0.6) x 10(-9) (2 sigma), a factor of 7-10 lower than the previously inferred value. Considering possible thermal processing that led to lower Al-26/Al-27 ratios in the two CAIs, we propose that the true solar system initial value of Ca-41/Ca-40 should have been similar to 4.2 x 10(-9). Synchronicity could have existed between Al-26 and Ca-41, indicating a uniform distribution of the two radionuclides at the time of CAI formation. The new initial Ca-41 abundance is 4-16 times lower than the calculated value for steady-state galactic nucleosynthesis. Therefore, Ca-41 could have originated as part of molecular cloud materials with a free decay time of 0.2-0.4 Myr. Alternative possibilities, such as a last-minute input from a stellar source and early solar system irradiation, could not be definitively ruled out. This underscores the need for more data from diverse CAIs to determine the true astrophysical origin of Ca-41.

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Three refractory coarse grained CAIs from the Efremovka CV3 chondrite, one (E65) previously shown to have formed with live Ca-41, were studied by ion microprobe for their Al-26-Mg-26 and Be-10-B-10 systematic in order to better understand the origin of Be-10. The high precision Al-Mg data and the inferred Al-26/Al-27 values attest that the precursors of the three CAIs evolved in the solar nebula over a period of few hundred thousand years before last melting-crystallization events. The initial Be-10/Be-9 ratios and delta B-10 values defined by the Be-10 isochrons for the three Efremovka CAIs are similar within errors. The CAI Be-10 abundance in published data underscores the large range for initial Be-10/Be-9 ratios. This is contrary to the relatively small range of Al-26/Al-27 variations in CAIs around the canonical ratio. Two models that could explain the origin of this large Be-10/Be-9 range are assessed from the collateral variations predicted for the initial delta B-10 values: (i) closed system decay of Be-10 from a ``canonical'' Be-10/Be-9 ratio and (ii) formation of CAIs from a mixture of solid precursors and nebula gas irradiated during up to a few hundred thousand years. The second scenario is shown to be the most consistent with the data. This shows that the major fraction of Be-10 in CAIs was produced by irradiation of refractory grains, while contributions of galactic cosmic rays trapping and early solar wind irradiation are less dominant. The case for Be-10 production by solar cosmic rays irradiation of solid refractory precursors poses a conundrum for Ca-41 because the latter is easily produced by irradiation and should be more abundant than what is observed in CAIs. Be-10 production by irradiation from solar energetic particles requires high Ca-41 abundance in early solar system, however, this is not observed in CAIs. (C) 2013 Elsevier B.V. All rights reserved.

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Se llevó a cabo un estudio durante el período lluvioso del año 2005, ‘Rancho Agropecológico’ “EBENEZER” Comarca ‘Hoja Chigüe’ Niquinohomo, Masaya, Nicaragua. El objetivo fue determinar la producción de materia seca y composición química de la biomasa a diferentes frecuencias de corte en Morera (Morus sp).Se utilizó un diseño de bloques completos al azar (BCA) con tres repeticiones. Las frecuencias de corte fueron; 30, 45 y 60 días de rebrote. Las variables de estudio fueron longitud de rebrotes (cm), rendimiento de materia seca (kg de MS/ha/corte), porcentajes de materia seca, proteína cruda, fibra cruda, calcio y fósforo. Se realizaron análisis de varianza (ANDEVA) y separaciones de medias, usando Duncan (P<0.05). Las variables codificadas en porcentajes se transforma ron, según, transformaciones arco seno de la raiz cuadrada de la proporción, con el fin de ajustar los datos porc entuales a una distribución normal, posteriormente se realizaron comparaciones de medias usando Duncan. Los resultados indican diferencias significativas entre tratamientos para las variables; longitud de rebrote (67 y 36 cm para frecuencias de 60 y 30 días);rendimiento de materia seca (1,368 y 480 kg/ha/corte, para las mismas frecuencias). Se encontraron diferencias significativas para la materia seca (27.50, 17.17 % para 60 y 30 días), fibra cruda (19.41 y,16.30 % para las mismas frecuencias), y Fósforo. Esta ultima variable presentó un comportamiento diferente; los mayores porcentajes a los 30 días (0.75) y los menores porcentajes a los 45 días, no presentándose diferencias entre 45 y 60 días (0.42). Referente a la proteína cruda y calcio, no presentaron diferencias significativas. El estudio permitió demostrar diferencias marcadas en la producción y composición química del forraje de Morera (Morus sp), observándose que, a diferencias de otros forrajes, los parámetros de calidad no presentan una disminución drástica a medida que se aumenta la edad de rebrote , con las frecuencias estudiadas.