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921.
Corrosion destroys more than three per cent of the world's GDP. Recently, the electrochemical decomposition of metal alloys has been more productively harnessed to produce porous materials with diverse technological potential. High-resolution insight into structure formation during electrocorrosion is a prerequisite for an atomistic understanding and control of such electrochemical surface processes. Here we report atomic-scale observations of the initial stages of corrosion of a Cu3Au111 single crystal alloy within a sulphuric acid solution. We monitor, by in situ X-ray diffraction with picometre-scale resolution, the structure and chemical composition of the electrolyte/alloy interface as the material decomposes. We reveal the microscopic structural changes associated with a general passivation phenomenon of which the origin has been hitherto unclear. We observe the formation of a gold-enriched single-crystal layer that is two to three monolayers thick, and has an unexpected inverted (CBA-) stacking sequence. At higher potentials, we find that this protective passivation layer dewets and pure gold islands are formed; such structures form the templates for the growth of nanoporous metals. Our experiments are carried out on a model single-crystal system. However, the insights should equally apply within a crystalline grain of an associated polycrystalline electrode fabricated from many other alloys exhibiting a large difference in the standard potential of their constituents, such as stainless steel (see ref. 5 for example) or alloys used for marine applications, such as CuZn or CuAl. 相似文献
922.
Emerging connections between DNA methylation and histone acetylation 总被引:18,自引:0,他引:18
Modifications of both DNA and chromatin can affect gene expression and lead to gene silencing. Evidence of links between
DNA methylation and histone hypoacetylation is accumulating. Several proteins that specifically bind to methylated DNA are
associated with complexes that include histone deacetylases (HDACs). In addition, DNA methyltransferases of mammals appear
to interact with HDACs. Experiments with animal cells have shown that HDACs are responsible for part of the repressive effect
of DNA methylation. Evidence was found in Neurospora that protein acetylation can in some cases affect DNA methylation. The available data suggest that the roles of DNA methylation
and histone hypoacetylation, and their relationship with each other, can vary, even within an organism. Some open questions
in this emerging field that should be answered in the near future are discussed. 相似文献
923.
DNA methylation is involved in epigenetic processes such as X-chromosome inactivation, imprinting and silencing of transposons. We have demonstrated previously that dim-2 encodes a DNA methyltransferase that is responsible for all known cytosine methylation in Neurospora crassa. Here we report that another Neurospora gene, dim-5, is required for DNA methylation, as well as for normal growth and full fertility. We mapped dim-5 and identified it by transformation with a candidate gene. The mutant has a nonsense mutation in a SET domain of a gene related to histone methyltransferases that are involved in heterochromatin formation in other organisms. Transformation of a wild-type strain with a segment of dim-5 reactivated a silenced hph gene, apparently by 'quelling' of dim-5. We demonstrate that recombinant DIM-5 protein specifically methylates histone H3 and that replacement of lysine 9 in histone H3 with either a leucine or an arginine phenocopies the dim-5 mutation. We conclude that DNA methylation depends on histone methylation. 相似文献
924.
Central nervous system stem cells in the embryo and adult 总被引:19,自引:0,他引:19
J. Frisén C. B. Johansson C. Lothian U. Lendahl 《Cellular and molecular life sciences : CMLS》1998,54(9):935-945
The central nervous system is generated from neural stem cells during embryonic development. These cells are multipotent
and generate neurons, astrocytes and oligodendrocytes. The last few years it has been found that there are populations of
stem cells also in the adult mammalian brain and spinal cord. In this paper, we review the recent development in the field
of embryonic and adult neural stem cells.
Received 26 March 1998; received after revision 27 April 1998; accepted 27 April 1998 相似文献
925.
通过对整经和上浆数学模型的研究.提出了织轴生产工艺参数控制的新方法.并利用这种新方法对有关的设备做了改进,产生了显著的经济效益。 相似文献
926.
无线通信系统以移动通信形式出现以来.已经快速席卷整个世界。这种系统正在给我们生活带来巨大变化,甚至可以说,无线通信正在改变着我们的生活方式。移动通信不仅仅是一种通信方式,它更是一种商业契机。在过去短短20年当中,手机已经快速从第一代发展到第三代。在未来的20年中,手持电话将会快速取代固定电话,数量将会达到大约7.5亿部。正如评论家所言,移动通信技术是自从计算机普及以来在远程通信当中最为重要的一项技术。 相似文献
927.
简要介绍水平排列硅光二极管色传感器的制作工艺,该器件波长在500nm到650nm范围内有一个单调的近于线性的光谱响应。入射光波长为600nm时,入射光强度化五个数量级,测出的波长基本不变,器件波长分辨率、最低入射光强、长时间测量的波长漂移以及波长温漂系数等特性的测量结果表明:该类色传感器已达高性能实用要求。 相似文献
928.
929.
930.
主要介绍了一种直线永磁振动电机的结构及其工作原理,并用有限元软件对其在不同位置时的磁场进行了分析,同时搭建实验台做了实际的测试,给出了误差及出现误差的原因。该电机结构简单,可靠性高,寿命长,易维护,并且控制电路简单,运行平稳,适用于小功率电磁间的开关系统中。 相似文献