共查询到20条相似文献，搜索用时 31 毫秒
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The scanning tunneling microscope (STM) can be used to measure current-voltage characteristics on an atomic scale. The attachment of copper phthalocyanine molecules, in contrast to a variety of other molecules, to graphite changes the electrical characteristics of the STM from relatively symmetric to highly asymmetric or rectifying. Evidence is presented to show that the asymmetry arises because of the electronic energy levels of the copper phthalocyanine. The organic molecules were bonded to the graphite by an acid-base reaction that may have wide applicability. 相似文献
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J Bailey A Chrysostomou JH Hough TM Gledhill A McCall S Clark F Menard M Tamura 《Science (New York, N.Y.)》1998,281(5377):672-674
Strong infrared circular polarization resulting from dust scattering in reflection nebulae in the Orion OMC-1 star-formation region has been observed. Circular polarization at shorter wavelengths might have been important in inducing chiral asymmetry in interstellar organic molecules that could be subsequently delivered to the early Earth by comets, interplanetary dust particles, or meteors. This could account for the excess of L-amino acids found in the Murchison meteorite and could explain the origin of the homochirality of biological molecules. 相似文献
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Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels. 相似文献
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Conformational variability of NAD+ in the free and bound states: a nicotinamide sandwich in NAD+ crystals 总被引：1，自引：0，他引：1
X-ray analysis of the free-acid crystal form of the coenzyme nicotinamide adenine dinucleotide (NAD+) revealed a conformational difference between the free NAD+ molecule and one bound in enzymes or complexed to Li+ ions. The pyrophosphate group showed asymmetry in the phosphate-oxygen bonds of the phosphate-oxygen-phosphate link; this bond at the nicotinamide side of the link is longer than that at the adenosine side by 0.04 angstrom. The crystal structure showed a novel intermolecular stacking of adenine and water molecules on opposite sides of nicotinamide that gives rise to a nicotinamide sandwich. 相似文献
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Zeolites are crystalline nanoporous aluminosilicates widely used in industry. In order for zeolites to find applications as innovative materials, they need to be organized into large two- and three-dimensional (2D and 3D) arrays. We report that uniformly aligned polyurethane films can serve as templates for the synthesis of uniformly aligned 2D and possibly 3D arrays of silicalite-1 crystals, in which the orientations of the crystals are controlled by the nature of the polymers. We propose that the supramolecularly organized organic-inorganic composites that consist of the hydrolyzed organic products and the seed crystals are responsible for this phenomenon. 相似文献
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A known host-guest assembly, organized only by means of relatively weak dispersive forces, exhibits hitherto unappreciated thermal stability. The hexagonal close-packed arrangement of calix[4]arene contains lattice voids that can occlude small, highly volatile molecules. This host-guest system can be exploited to retain a range of freons, as well as methane, not only well above their normal boiling points, but also at relatively high temperatures and low pressures. The usually overlooked van der Waals interactions in organic crystals can indeed be used in a highly stable supramolecular system for gas storage. 相似文献
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在本研究中,首先在二十个籼粳亚种四倍体水稻F1代中筛选出高结实杂交组合:Dur/Sg9912 (命名为PSR120),然后在田间随机取样的200多株PSR120F2代植株,并考察其结实率。用统计软件分析结实率,结果表明,多倍体水稻F2结实率变异大,分布不对称。 相似文献
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Evolutionary time has a characteristic direction as demonstrated by the asymmetry of clade diversity diagrams in large statistical samples. Evolutionary groups generally concentrate diversity during their early histories, producing a preponderance of bottom-heavy clades among those that arise early in the history of a larger group. This pattern holds across taxonomic levels and across differences in anatomy and ecology (marine invertebrates, terrestrial mammals). The quantitative study of directionality in life's history (replacing vague, untestable, and culturally laden notions of "progress") should receive more attention from paleobiologists. 相似文献
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Electronically excited molecules, being better electron donors and acceptors than their ground states, form charge-transfer complexes (exciplexes) which can lead to radical ions. Exciplex emission is widely used to probe polymers and organized media such as membranes and micelles. Exciplexes are also intermediates in photoreactions that lead to unique products. Photochemical electron-transfer processes, which are the basis of silver halide photography and electrophotography, are involved in many reactions of wide scope. Recent studies have led to the discovery of several electron-transfer photooxygenations with a diversity that will probably rival that of singlet oxygen. Both exciplex emission and photochemical electron transfer play important roles in organic photochemistry. 相似文献
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Collagen-coated cellulose sponge: three dimensional matrix for tissue culture of Walker tumor 256 总被引：6，自引：0，他引：6
Three-dimensional growth of large populations of cells in vitro has been observed in the interstices of a matrix consisting of collagen-coated cellu lose sponge. The growth of Walker tumor 256 in this composite matrix is com pared with that found in a matrix composed of either cellulose sponge alone or collagen sponge alone. The composite matrix is superior to either one. Collagen coated cellulose sponge may provide a simple tool for the study of social interaction of cells in the formation of organized elementary tissue structures. 相似文献
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本文首先深入分析了农产品跨境电商中信息不对称的表现及成因,其后阐述了信息不
对称对农产品跨境电商活动的影响,最后提出了一系列农产品跨境电商中的信息不对称问题的
解决策略和措施。 相似文献
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农民科技培训的问题探讨 总被引：2，自引：0，他引：2
我国是一个农业大国,农民科技文化素质的高低对促进现代农业发展和社会主义新农村建设有着重要的作用。在农业的发展过程中,农民科技素质的提升,决定着农业经济发展的整体效益和速度,有组织、有力度地加强农民的科技培训工作,是农业技术推广部门的基本任务。伴随着全面建设小康社会步伐的日益加快,加强农民科技教育培训工作显得尤为重要。针对当前农民在科技培训方面面临的问题提出相应的对策。 相似文献
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A pair of coronary arteries supplies the interventricular septum of the heart of Mus musculus. The members of this pair tend to be of unequal size, which permits distiniction between specimens displaying left asymmetry and those showing righit asymmetry. Three inbred strains (C57BL/1O, DBA/1, and Bulb/c) differ with regard to this asymmetry. However, variation exists within strains, which suggests that nongenetic factors also influence the development of the asymmetry. Right arterial asymmetry is dominant over left asymmetry. 相似文献
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N. A. Saveljeva E. E. Belova D. A. Korotchenkov O. S. Mishina A. A. Kolontsov 《Russian Agricultural Sciences》2014,40(4):242-243
The treatment of buckwheat with biostimulants Floravit E and Atonic Plus in dilutions 10?2–10?3 and 3 × 10?2, accordingly, had no effect on the stability and development of plants, as one can judge by the fluctuating asymmetry of metrical and meristic traits. Treatment with Atonic Plus led to a reduction of fluctuating asymmetry when control plants had a high value of asymmetry. Floravit E in the dilution 10?1 caused an increase of fluctuating asymmetry of half leaf width. The same effect was observed when growing buck-wheat in a lead-contaminated soil. 相似文献
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Schierbaum KD Weiss T van Veizen EU Engbersen JF Reinhoudt DN Göpel W 《Science (New York, N.Y.)》1994,265(5177):1413-1415
It is shown by angle-resolved x-ray photoelectron spectroscopy that cavitands derived from resorcin[4]arenes provided with four dialkylsulfide chains form stable monolayers on gold surfaces that are well organized by self-assembly. The cavitand headgroups at the surface of the resorcin[4]arene monolayer act as molecular recognition sites for small organic molecules with remarkable selectivity for perchloroethylene (C(2)Cl(4)). Comparative thermal desorption experiments indicate binding sites with high interaction energies of C(2)Cl(4) at the surface of the resorcin[4]arene monolayers. Fast and reversible "host-guest" interactions were found by the monitoring of extremely small mass changes (in the nanogram range) with a quartz microbalance oscillator provided with gold electrodes coated by resorcin[4]arene monolayers. 相似文献
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In earthquake hazard assessment studies, the focus is usually on horizontal ground motion. However, records from the 14 June 2008 Iwate-Miyagi earthquake in Japan, a crustal event with a moment magnitude of 6.9, revealed an unprecedented vertical surface acceleration of nearly four times gravity, more than twice its horizontal counterpart. The vertical acceleration was distinctly asymmetric; the waveform envelope was about 1.6 times as large in the upward direction as in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bouncing on a trampoline to account for this asymmetry and the large vertical amplitude. The finding of a hitherto-unknown mode of strong ground motion may prompt major progress in near-source shaking assessments. 相似文献