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排序方式: 共有710条查询结果,搜索用时 15 毫秒
701.
Let M be a compact manifold of dimension n, P=P(h) a semiclassical pseudodifferential operator on M, and u=u(h) an L
2 normalized family of functions such that P(h)u(h) is O(h) in L
2(M) as h↓0. Let H⊂M be a compact submanifold of M. In a previous article, the second-named author proved estimates on the L
p
norms, p≥2, of u restricted to H, under the assumption that the u are semiclassically localized and under some natural structural assumptions about the principal symbol of P. These estimates are of the form Ch
−δ(n,k,p) where k=dim H (except for a logarithmic divergence in the case k=n−2, p=2). When H is a hypersurface, i.e., k=n−1, we have δ(n,n−1, 2)=1/4, which is sharp when M is the round n-sphere and H is an equator. 相似文献
702.
A comparative investigation of the presence of psychological conditions in high achieving eighth graders from TIMSS 2007 Mathematics 总被引:1,自引:0,他引:1
This study is a second analysis of Trends in International Mathematics and Science Study (TIMSS) 2007 background questionnaires to investigate high achieving eighth-grade students’ possession of three elements of Krutetskii’s (The Psychology of Mathematical Abilities in Schoolchildren, The University of Chicago, 1976) psychological conditions. Analyses were made between high achieving eighth graders and their lower achieving counterparts, and between Korean high achievers and their high achieving peers across ten high performing TIMSS participating countries. After reviewing 44,643 students across selected countries, we conclude that a larger percentage of students with mathematical talent demonstrate positive attitudes toward mathematics, value mathematics, and have self-confidence in their ability to learn mathematics than their peers without high achievement in mathematics. However, a larger portion of high achieving Korean students displayed low self-confidence and valued mathematics less than their peers from other high performing countries. Findings from this study will provide insight into some educational issues in science, technology, engineering and mathematics education. 相似文献
703.
A new technique for the rapid quantification of orientation-dependent etch rates, which uses micromachined test patterns and optical microscopy, has been developed. The etching of silicon in KOH etchants with and without isopropanol was studied. Etch rates measured with this technique are in good agreement with conventionally measured rates. In most cases, the etch rate anisotropies are well described by a simple model that is based on step-flow etching. Kinetic Monte Carlo simulations of etching were used to test the simple model and to generate approximate morphologies of the etched surfaces. Vicinal Si(110) surfaces display unusual, orientation-dependent etch rates in some etchants; the functional form of the etch rate anisotropy suggests that a morphological transition occurs on these highly reactive faces. In moderately concentrated KOH solutions where isopropanol is readily soluble, the measured etch rate anisotropies suggest that isopropanol stabilizes step-flow etching. 相似文献
704.
Nayely Leyva-Lpez Melissa Peraza-Arias Anaguiven Avalos-Soriano Crisantema Hernndez Cynthia E. Lizrraga-Velzquez J. Basilio Heredia 《Molecules (Basel, Switzerland)》2021,26(20)
Oxidative stress in aquatic organisms might suppress the immune system and propagate infectious diseases. This study aimed to investigate the protective effect of polyphenolic extracts from spent coffee grounds (SCG) against oxidative stress, induced by H2O2, in C. viridis brain cells, through an in vitro model. Hydrophilic extracts from SCG are rich in quinic, ferulic and caffeic acids and showed antioxidant capacity in DPPH, ORAC and FRAP assays. Furthermore, pretreatment of C. viridis brain cells with the polyphenolic extracts from SCG (230 and 460 µg/mL) for 24 h prior to 100 µM H2O2 exposure (1 h) significantly increased antioxidant enzymes activity (superoxide dismutase and catalase) and reduced lipid peroxidation (measured by MDA levels). These results suggest that polyphenols found in SCG extracts exert an antioxidative protective effect against oxidative stress in C. viridis brain cells by stimulating the activity of SOD and CAT. 相似文献
705.
Bruna F. Mazzeu Tatiana M. Souza-Moreira Andrew A. Oliveira Melissa Remlinger Lidiane G. Felippe Sandro R. Valentini Rafael V. C. Guido Cleslei F. Zanelli Maysa Furlan 《Molecules (Basel, Switzerland)》2021,26(22)
Friedelin, a pentacyclic triterpene found in the leaves of the Celastraceae species, demonstrates numerous biological activities and is a precursor of quinonemethide triterpenes, which are promising antitumoral agents. Friedelin is biosynthesized from the cyclization of 2,3-oxidosqualene, involving a series of rearrangements to form a ketone by deprotonation of the hydroxylated intermediate, without the aid of an oxidoreductase enzyme. Mutagenesis studies among oxidosqualene cyclases (OSCs) have demonstrated the influence of amino acid residues on rearrangements during substrate cyclization: loss of catalytic activity, stabilization, rearrangement control or specificity changing. In the present study, friedelin synthase from Maytenus ilicifolia (Celastraceae) was expressed heterologously in Saccharomyces cerevisiae. Site-directed mutagenesis studies were performed by replacing phenylalanine with tryptophan at position 473 (Phe473Trp), methionine with serine at position 549 (Met549Ser) and leucine with phenylalanine at position 552 (Leu552Phe). Mutation Phe473Trp led to a total loss of function; mutants Met549Ser and Leu552Phe interfered with the enzyme specificity leading to enhanced friedelin production, in addition to α-amyrin and β-amyrin. Hence, these data showed that methionine 549 and leucine 552 are important residues for the function of this synthase. 相似文献
706.
Radhika Vaid Erol Yildirim Melissa A. Pasquinelli Martin W. King 《Molecules (Basel, Switzerland)》2021,26(24)
Polylactic acid (PLA) is a widely used bioresorbable polymer in medical devices owing to its biocompatibility, bioresorbability, and biodegradability. It is also considered a sustainable solution for a wide variety of other applications, including packaging. Because of its widespread use, there have been many studies evaluating this polymer. However, gaps still exist in our understanding of the hydrolytic degradation in extreme pH environments and its impact on physical and mechanical properties, especially in fibrous materials. The goal of this work is to explore the hydrolytic degradation of PLA fibers as a function of a wide range of pH values and exposure times. To complement the experimental measurements, molecular-level details were obtained using both molecular dynamics (MD) simulations with ReaxFF and density functional theory (DFT) calculations. The hydrolytic degradation of PLA fibers from both experiments and simulations was observed to have a faster rate of degradation in alkaline conditions, with 40% of strength loss of the fibers in just 25 days together with an increase in the percent crystallinity of the degraded samples. Additionally, surface erosion was observed in these PLA fibers, especially in extreme alkaline environments, in contrast to bulk erosion observed in molded PLA grafts and other materials, which is attributed to the increased crystallinity induced during the fiber spinning process. These results indicate that spun PLA fibers function in a predictable manner as a bioresorbable medical device when totally degraded at end-of-life in more alkaline conditions. 相似文献
707.
Julia G. Knapp Dr. Xijun Wang Dr. Andrew S. Rosen Xingjie Wang Dr. Xinyi Gong Dr. Matthew Schneider Dr. Tatyana Elkin Dr. Kent O. Kirlikovali Dr. Melissa Fairley Dr. Matthew D. Krzyaniak Prof. Michael R. Wasielewski Prof. Nathan C. Gianneschi Prof. Randall Q. Snurr Prof. Omar K. Farha 《Angewandte Chemie (International ed. in English)》2023,62(29):e202305526
The interactions between uranium and non-innocent organic species are an essential component of fundamental uranium redox chemistry. However, they have seldom been explored in the context of multidimensional, porous materials. Uranium-based metal–organic frameworks (MOFs) offer a new angle to study these interactions, as these self-assembled species stabilize uranium species through immobilization by organic linkers within a crystalline framework, while potentially providing a method for adjusting metal oxidation state through coordination of non-innocent linkers. We report the synthesis of the MOF NU-1700 , assembled from U4+-paddlewheel nodes and catecholate-based linkers. We propose this highly unusual structure, which contains two U4+ ions in a paddlewheel built from four linkers—a first among uranium materials—as a result of extensive characterization via powder X-ray diffraction (PXRD), sorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), in addition to density functional theory (DFT) calculations. 相似文献
708.
Yuyuan Xie Gary H. Wikfors Mark S. Dixon Lisa Guy Melissa Krisak Yaqin Li 《Photochemistry and photobiology》2023,99(3):1010-1019
Nonphotochemical quenching (NPQ) is known to depress in vivo fluorescence (IVF) of chlorophyll a (Chla) in aquatic environments, which makes it difficult to interpret the hour-to-hour variations in Chla measured by in situ fluorometers. We hypothesized that ratios between quenched and unquenched IVF are a function of both NPQ and photochemical quenching. In this study, two diatom model species Thalassiosira pseudonana (CCMP1335) and Thalassiosira weissflogii (CCMP1047) incubated under a sinusoidal light:dark cycle were studied; IVF was recorded continuously, and Chla and photo-physiological variables were measured seven times a day. The maximal decline in Chla-specific IVF (IVFB) attributable to quenching was 50% under the experimental settings. An NPQ and photochemical quenching-based modeling equation exhibited a better match to the measured IVFB than equations representing the sole NPQ effect. Photochemical quenching induced by measuring light beam varied substantially during the day, and the part of the model for this process is excitation intensity-dependent (which is differed between models of in situ fluorometers, implying no straightforward method to correct Chla for all instrument models, instrument-specific parameterization is required). The forms of the IVFB-light relationship are discussed as well. The findings foster a holistic understanding of NPQ effects on in vivo Chla fluorometry. 相似文献
709.
Melissa Manuszak Guerrini Bernadette Charleux Jean‐Pierre Vairon 《Macromolecular rapid communications》2000,21(10):669-674
The application of atom transfer radical polymerization (ATRP) to the homopolymerization of 2‐hydroxyethyl acrylate and 2‐(methacryloyloxy) ethyl trimethylammonium chloride at the surface of a crosslinked polystyrene latex functionalized with alkyl bromide groups is reported. Polymerization was carried out using the surface groups of the dialyzed latex as initiators. The resulting hydrophobic core, hydrophilic shell latexes, were analyzed by FTIR, 13C‐NMR spectroscopy, and dynamic light scattering. 相似文献
710.
Fernando Galembeck Carlos Alberto Paula Leite Maria do Carmo V.M. da Silva Amauri José Keslarek Carlos Alberto Rodrigues Costa Érico Teixeira-Neto Márcia Maria Rippel Melissa Braga 《Macromolecular Symposia》2002,189(1):15-26
This work reviews new probe and electron microscopy approaches for the detection of charged domains in insulating polymers, as well as for the identification of the charge-bearing species: scanning electric potential microscopy (SEPM), electric force microscopy (EFM) and energy-loss spectroscopy imaging in the transmission electron microscope (ESI-TEM). The SEPM and EFM micrographs show patterned domains bearing excess electric charges and extending for tens of nanometers, in polymer latex particles and films. The charged species are identified by ESI-TEM as emulsion polymerization initiator and surfactant residues, as well as the associated counter-ions. Charged domains are also observed in common thermoplastic polymers, producing unexpectedly large electric potential gradients. 相似文献