RNA structure. Pulling on hair(pins)     

Fernandez JM  Chu S  Oberhauser AF 《Science (New York, N.Y.)》2001,292(5517):653-654

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现阶段宁夏农民合作经济组织的发展思考     

褚庆芳 《农业科学研究》2007,28(1):32-35

农民合作经济组织是在农村经济体制改革中兴起的,以其独特的组织形式、灵活的机制、广泛的适应性,在提高农业产业化经营水平及提高农民收入方面发挥了巨大的作用.为了更好地促进农民合作经济组织的发展,通过调查分析得出农民合作经济组织的运作经验为:扶持龙头企业,提高带动能力,围绕特色建协会,力争政策和资金支持,以利益联结、股份合作、提高科学技术服务于农民.提出了加快农村制度改革,科学定位,法律保障,提供公共支持,建立农工贸一体化生产经营体系的发展对策.  相似文献   

Removal of meteoric iron on polar mesospheric clouds     

Plane JM  Murray BJ  Chu X  Gardner CS 《Science (New York, N.Y.)》2004,304(5669):426-428

Polar mesospheric clouds are thin layers of nanometer-sized ice particles that occur at altitudes between 82 and 87 kilometers in the high-latitude summer mesosphere. These clouds overlap in altitude with the layer of iron (Fe) atoms that is produced by the ablation of meteoroids entering the atmosphere. Simultaneous observations of the Fe layer and the clouds, made by lidar during midsummer at the South Pole, demonstrate that essentially complete removal of Fe atoms can occur inside the clouds. Laboratory experiments and atmospheric modeling show that this phenomenon is explained by the efficient uptake of Fe on the ice particle surface.  相似文献   

Anomeric bond-character in the pyranose sugars     

H M Berman  S S Chu  G A Jeffrey 《Science (New York, N.Y.)》1967,157(796):1576-1577

Evidence from determinations of crystal structure relating to the shortening of carbon-oxygen bonds in the anomeric position in pyranose sugars is presented. There is a high degree of probability that the C(1)-O(1) bond is shortened by about 0.04 A relative to the other C-O single-bond lengths, except in the case of an axially oriented glycosidic group where there is evidence of a distinction between the two ring C-O bonds.  相似文献   

Boron isotope effect in superconducting zirconium dodecaboride     

Chu CW  Hill HH 《Science (New York, N.Y.)》1968,159(3820):1227-1228

The boron isotope effect in ZrB(12) has been determined to be T(c) infinity M(alpha), where alpha is equal to - 0.09 +/- 0.05, M is the boron mass, and T(c) is the supercondiucting transition temperature.  相似文献   

实验性慢性氟中毒家兔肝、肾动态超微病理学研究   总被引：1，自引：0，他引：1  

曲连东  滕国麟 《东北农业大学学报》1994,25(4):360-365

２４只２０～３０日龄体重０．７～１．２ｋｇ新西兰白兔随机分为实验组１５只（每日每只随饮水摄取ＮａＦ４５ｍｇ／ｋｇ体重）及健康对照组９只（饮用自来水）．两组在相同饲养管理条件下实验观察．分别在实验第３０，６０．９０ｄ时迫杀，对肝和肾进行超微病理学研究．结果表明，肝和肾实质细胞膜系统损伤明显，胶原原纤维再生增多，且有再生障碍及再生后的损伤性变化．  相似文献   

粗糙集神经网络在昆虫总科阶元分类学上的应用   总被引：2，自引：0，他引：2  

杜瑞卿  褚学英  王庆林  赵秋红  庞发虎 《中国农业大学学报》2007,12(1):33-38

为研究粗糙集和神经网络相结合方法的实践性,以及昆虫的数学形态特征在总科阶元上作为分类特征的可行性、可靠性和重要性。从总科角度对鳞翅目(Lepidoptera)和鞘翅目(Coleoptera)5个总科23种虫体图像中提取的昆虫面积、周长等11项数学形态特征进行粗糙集神经网络分析。结果表明:在总科阶元上,11项特征的可靠性大小为,面积、亮斑数>周长、横轴长、形状参数、圆形性、似圆度、偏心率>纵轴长、叶状性、球状性,与赵汗青等的统计分析结果中属性特征的重要性大多数一致;神经网络模式识别结果与传统分类结果完全一致。应用粗糙集理论进行昆虫数学形态特征分类结果准确;在昆虫总科阶元分类上粗糙集神经网络较统计学方法具有优势。  相似文献   

Crystal structure of inhibitor-bound human 5-lipoxygenase-activating protein     

Ferguson AD  McKeever BM  Xu S  Wisniewski D  Miller DK  Yamin TT  Spencer RH  Chu L  Ujjainwalla F  Cunningham BR  Evans JF  Becker JW 《Science (New York, N.Y.)》2007,317(5837):510-512

Leukotrienes are proinflammatory products of arachidonic acid oxidation by 5-lipoxygenase that have been shown to be involved in respiratory and cardiovascular diseases. The integral membrane protein FLAP is essential for leukotriene biosynthesis. We describe the x-ray crystal structures of human FLAP in complex with two leukotriene biosynthesis inhibitors at 4.0 and 4.2 angstrom resolution, respectively. The structures show that inhibitors bind in membrane-embedded pockets of FLAP, which suggests how these inhibitors prevent arachidonic acid from binding to FLAP and subsequently being transferred to 5-lipoxygenase, thereby preventing leukotriene biosynthesis. This structural information provides a platform for the development of therapeutics for respiratory and cardiovascular diseases.  相似文献   

Carotenogenesis up-regulation in Scenedesmus sp. using a targeted metabolomics approach by liquid chromatography-high-resolution mass spectrometry     

Chu FL  Pirastru L  Popovic R  Sleno L 《Journal of agricultural and food chemistry》2011,59(7):3004-3013

Carotenoids have potent antioxidant activity as well as therapeutic value, and their formation has been seen to be induced in algae by stress, including high-salt culture conditions. A differential profiling of carotenoids was conducted using a targeted metabolomics approach with accurate mass data generated by liquid chromatography-electrospray-time-of-flight (LC-ESI-TOF) mass spectrometry followed by postacquisition filtering based on isotope patterns and mass defects to detect carotenoids up-regulated in Scenedesmus sp. exposed to high-salt conditions. Algal cultures treated with high concentrations of sodium acetate or sodium chloride were found to cause an increase in various carotenoids. On the basis of differential analysis, astaxanthin and canthaxanthin increased upon salt treatment. Astaxanthin, in its free form and as fatty acid esters, was seen to increase in Scenedesmus sp. using accurate mass MS. A few other carotenoid compounds increased upon salt treatment, including echinenone and adonirubin, involved in the pathway of astaxanthin biosynthesis from β-carotene, as well as isomers of astaxanthin and canthaxanthin. A time course study of acetate treatment was done to observe the time-dependent up-regulation of carotenogenesis.  相似文献   

喉管直径对文丘里施肥器性能影响的数值模拟     

严海军  初晓一 《排灌机械》2011,29(4):359-363

基于雷诺时均Navier-Stokes方程和标准k-ε湍流模型,利用Fluent软件对文丘里施肥器的内部流动进行了数值模拟,并对数值计算方法的正确性进行了验证.模拟分析了喉管进口直径为4,5,6 mm,以及出口直径与进口直径之比λ为1.0,1.1,1.2,1.3和1.4时15种结构方案的吸肥性能.结果表明：喉管直径对喉管及扩散段内流动特性、吸肥性能和效率均有一定影响,但出口直径与进口直径之比λ对文丘里施肥器性能的影响要明显甚于喉管进口直径.根据效率最高原则,喉管直径比应为1.2～1.3.建立了λ=1.2或1.3时文丘里施肥器吸肥效率与流量比之间的回归模型,并应用回归模型预测了吸肥效率最大值.喉管进口直径4 mm的文丘里施肥器当直径比为1.3时吸肥效率可达到15.5%,与直径比为1.0相比,前者的扩散段内部流动平顺、无漩涡出现,且相同横截面的平均湍动能值要小.该研究结果为文丘里施肥器的优化设计提供了一定的理论参考.  相似文献