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981.
Fluorine is the thirteenth most abundant element in the earth's crust, but fluoride concentrations in surface water are low and fluorinated metabolites are extremely rare. The fluoride ion is a potent nucleophile in its desolvated state, but is tightly hydrated in water and effectively inert. Low availability and a lack of chemical reactivity have largely excluded fluoride from biochemistry: in particular, fluorine's high redox potential precludes the haloperoxidase-type mechanism used in the metabolic incorporation of chloride and bromide ions. But fluorinated chemicals are growing in industrial importance, with applications in pharmaceuticals, agrochemicals and materials products. Reactive fluorination reagents requiring specialist process technologies are needed in industry and, although biological catalysts for these processes are highly sought after, only one enzyme that can convert fluoride to organic fluorine has been described. Streptomyces cattleya can form carbon-fluorine bonds and must therefore have evolved an enzyme able to overcome the chemical challenges of using aqueous fluoride. Here we report the sequence and three-dimensional structure of the first native fluorination enzyme, 5'-fluoro-5'-deoxyadenosine synthase, from this organism. Both substrate and products have been observed bound to the enzyme, enabling us to propose a nucleophilic substitution mechanism for this biological fluorination reaction. 相似文献
982.
983.
微型燃气轮机向心透平导向器的流场分析与设计研究 总被引:5,自引:0,他引:5
对微型燃气轮机向心透平导向器叶片型线的设计原则进行了探讨,设计出了5种尾缘厚度的导向器叶片,并分别对其通道流场进行了全三维黏性数值模拟.研究结果表明:以该型线设计原则设计出的高亚音速向心透平导向器叶片具有较高的流动效率,并且可以有效减小向心透平的径向尺寸;尾缘相对厚度对导向器叶片流动特性及效率影响较大,尾缘相对厚度每增加1%.叶轮进口气流角将减小约0.03°,速度系数将降低约0.1%.考虑到目前高温合金精密铸造的工艺水平,导向器叶片尾缘厚度取0.6~1.0 mm、尾缘相对厚度小于15%可以获得较高的气动性能. 相似文献
984.
985.
Enhancement of cell growth in suspension cultures is urgently needed in plant cell culture engineering. This study investigates the relationship between morphological transformation and cell growth in callus and suspension cultures of saffron cells belonging to the cell line C96 induced from Crocus safivus L In the suspension culture, an unbalanced osmotic pressure between the intracell and extracell regions induced a large morphological transformation which affected normal division of the saffron cells. An increase in osmotic pressure caused by the addition of sucrose inhibits the vacuolation and shrinkage of cytoplasm in the cells. As the sucrose concentration increases, the total amount of accumulated biomass also increases. Besides the sucrose concentration, increased ionic strength and inoculation ratio also help restrain to a large extent the vacuolation and shrinkage of the cytoplasm in the suspended cells, which results in increased biomass. The conditions for optimal biomass are: Murashige and Skoog's (MS) medium with 40 g/L sucrose and 60% (v/v) inoculation ratio. 相似文献
986.
高校"两课"互动式课堂教学的实践探索 总被引:5,自引:0,他引:5
本文通过互动式教学规律来探究“两课”教学,在教学中运用问题讨论式、小课题作业、合作辩论法、学生讲课式等教学法,充分发挥教师和学生之间的互动。通过互动教学提高了教师专业水平,促进了学生的全面发展。 相似文献
987.
LIU Zijing YIN Hongyan DENG Gang YUAN Yuhua WANG Jinzhong GENG Yunqi & CHEN Qimin College of Life Sciences Nankai University Tianjin China Correspondence should be addressed to Chen Qimin 《科学通报(英文版)》2003,48(12):1247-1250
As a bovine lentivirus,Jembrana disease virus (JDV) is genetically similar to the bovine immunodeficiency virus (BIV). Unlike other lentiviruses which always have a long incubation period, e.g. 8—10 years for HIV, JDV infection causes an acute illness with shorter incubationpreriod and higher mortality. In the experimentally inocu-lated cattle, the incubation period varied from 8 to 12 d before the onset of clinical symptoms, which is very simi-lar to bovine plague with a high titer of J… 相似文献
988.
本文对超快扫速循环伏安法的研究进展做了一个简要的综述,着重讨论了关于电子仪器,电极与电化学池电位降补偿,背景扣除,数据处理等。所涉及的部分理论和技术问题,引述了27篇文献。 相似文献
989.
利用等幂和与判别素数的充要条件,获得了Stirling数的一些新的性质,并揭示了等幂和与Stirling的奇妙关系。 相似文献
990.
本文分析讨论了两种在码间干扰和噪声干扰下误码率的估计方法。第一种是基于泰勒级数展开的分析方法,第二种是基于傅立叶级数展开的分析法,该方法不用码间干扰的矩函数,可用于各种噪声干扰,计算代价低,适用于相干振幅或相位调制技术。 相似文献