Insight into a natural Diels-Alder reaction from the structure of macrophomate synthase

The Diels-Alder reaction, which forms a six-membered ring from an alkene (dienophile) and a 1,3-diene, is synthetically very useful for construction of cyclic products with high regio- and stereoselectivity under mild conditions. It has been applied to the synthesis of complex pharmaceutical and biologically active compounds. Although evidence on natural Diels-Alderases has been accumulated in the biosynthesis of secondary metabolites, there has been no report on the structural details of the natural Diels-Alderases. The function and catalytic mechanism of the natural Diels-Alderase are of great interest owing to the diversity of molecular skeletons in natural Diels-Alder adducts. Here we present the 1.70 A resolution crystal structure of the natural Diels-Alderase, fungal macrophomate synthase (MPS), in complex with pyruvate. The active site of the enzyme is large and hydrophobic, contributing amino acid residues that can hydrogen-bond to the substrate 2-pyrone. These data provide information on the catalytic mechanism of MPS, and suggest that the reaction proceeds via a large-scale structural reorganization of the product.     

Insight into the change in carbon structure and thermodynamics during anthracite transformation into graphite

The thermodynamic and kinetic mechanisms of Taixi anthracite during its graphitization process were explored. To understand the variation trends of carbon arrangement order, microcrystal size, and graphitization degree against temperature during the graphitization process, a series of experiments were performed using Raman spectroscopy and X-ray diffraction(XRD). Subsequently, the influencing factors of the dominant reaction at different temperatures were analyzed using thermodynamics and kinetics. The results showed that the graphitization process of Taixi anthracite can be divided into three stages from the perspective of reaction thermodynamics and kinetics. Temperature played a crucial role in the formation and growth of a graphitic structure. Meanwhile, multivariate mechanisms coexisted in the graphitization process. At ultrahigh temperatures, the defects of synthetic graphite could not be completely eliminated and perfect graphite crystals could not be produced. At low temperatures, the reaction is mainly controlled by dynamics, while at high temperatures, thermodynamics dominates the direction of the reaction.     

Insights into microtubule nucleation from the crystal structure of human gamma-tubulin

Microtubules are hollow polymers of alphabeta-tubulin that show GTP-dependent assembly dynamics and comprise a critical part of the eukaryotic cytoskeleton. Initiation of new microtubules in vivo requires gamma-tubulin, organized as an oligomer within the 2.2-MDa gamma-tubulin ring complex (gamma-TuRC) of higher eukaryotes. Structural insight is lacking regarding gamma-tubulin, its oligomerization and how it promotes microtubule assembly. Here we report the 2.7-A crystal structure of human gamma-tubulin bound to GTP-gammaS (a non-hydrolysable GTP analogue). We observe a 'curved' conformation for gamma-tubulin-GTPgammaS, similar to that seen for GDP-bound, unpolymerized alphabeta-tubulin. Tubulins are thought to represent a distinct class of GTP-binding proteins, and conformational switching in gamma-tubulin might differ from the nucleotide-dependent switching of signalling GTPases. A crystal packing interaction replicates the lateral contacts between alpha- and beta-tubulins in the microtubule, and this association probably forms the basis for gamma-tubulin oligomerization within the gamma-TuRC. Laterally associated gamma-tubulins in the gamma-TuRC might promote microtubule nucleation by providing a template that enhances the intrinsically weak lateral interaction between alphabeta-tubulin heterodimers. Because they are dimeric, alphabeta-tubulins cannot form microtubule-like lateral associations in the curved conformation. The lateral array of gamma-tubulins we observe in the crystal reveals a unique functional property of a monomeric tubulin.     

从文献数量的变化透视我国服装学科的发展

采用文献计量学方法,对1984年～2004年间我国服装学科研究论文的发表时间、文献分布情况进行了统计分析.通过曲线拟合,建立了文献数量随时间变化的回归方程并进行了有效性验证,从而证明了我国服装学科的文献数量随时间的变化近似符合指数曲线的变化趋势,其曲线与科学发展的初期阶段是相对应的,即我国服装学科正处于学科发展的初始阶段.总的发展趋势是从一般研究到多学科综合理论性研究,从物理性能研究深化到机理性研究,从主观评定转向量化的客观评定,并更强调其实用性、科学性和时代性.     

邓小平的世界眼光

邓小平观察、分析中国具有独到的世界眼光，这种世界眼光反映了邓小平深厚的马克思主义理论功底和科学的认识方法，主要体现在三个方面：一、邓小平根据对世界发展的趋势和国际格局的判断来确定中国共产党的基本路线、方针和政策，指导社会主义现代化建设；二、邓小平从世界发达国家和发展中国家现代化进程中总结现代化的一般规律、基本经验和教训，创造性地运用于当代中国社会主义现代化，研究中国社会主义现代化的特色。三、邓小平从世界社会主义运动的角度来分析、认识当代中国社会主义现代化的重要意义。     

从汉语言看女性的文化特征

本文从汉语词语本身和女性用语两个方面阐述了女性的文化特征。     

从继承性看基础理论化学的发展及"空洞"

基础理论化学的继承性围绕着提出经验问题和解决实际问题而展开.继承性发展既体现着科学理论的前因后果又表明科学发展的一脉性及与时俱进性.化学家总是在理论与现实的“空洞“间寻找继承发展的切入点,“空洞“的最终便是继承性发展,进而产生更完善的理论来指导科学实践活动.     

Insight into the 2004 Sumatra-Andaman earthquake from GPS measurements in southeast Asia

Data collected at approximately 60 Global Positioning System (GPS) sites in southeast Asia show the crustal deformation caused by the 26 December 2004 Sumatra-Andaman earthquake at an unprecedented large scale. Small but significant co-seismic jumps are clearly detected more than 3,000 km from the earthquake epicentre. The nearest sites, still more than 400 km away, show displacements of 10 cm or more. Here we show that the rupture plane for this earthquake must have been at least 1,000 km long and that non-homogeneous slip is required to fit the large displacement gradients revealed by the GPS measurements. Our kinematic analysis of the GPS recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre. It also provides evidence that the rupture propagated northward sufficiently fast for stations in northern Thailand to have reached their final positions less than 10 min after the earthquake, hence ruling out the hypothesis of a silent slow aseismic rupture.     

Insight into Flocculation Mechanism of Chitosan

Grain analyzer, turbidimeter, Zeta potentialinstrument and microscope with Panansonic CCD areused to analyse the distribution of the bentonitegrain, test the Zeta potential of bentonite andobserve the structure of flocs. Through thecomparison among chitosan, PAM, and aluminum     

试论二语听力的可教性

从认知语言学角度出发．二语听者在自下而上及自上而下信息处理方面存在着各种困难。教师应针对这些困难,采取培养听者自下而上及自上而下两种信息处理能力的教学方式。在课堂教学上应有意识地培养学生话语感知,单词识别及策略使用的能力以体现二语听力的可教性。二语听力“可教性”的观点认为听力课堂的功能不应仅体现在丰富真实的听力内容与活泼生动的教学形式上,也不能仅止于提供机会让学生练习听力。二语听力课堂的功能应更深层次地体现在培养学生对自己听力能力的责任意识及驾驭能力上。     

Insight into the effect of Ge modification on the oxidation of NbSi2 from first-principles calculations

The effect of Ge modification on the oxidation of Nb Si₂ was performed by using the first-principles method based on Density Functional Theory(DFT). The O₂ molecule absorption configurations on the ideal NbSi₂(001) surface with and without Ge substitution was constructed to study the initial oxidation behavior as well as oxygen diffusion in the bulk Nb Si₂ cell in the late oxidation stage. Ge substitution was found to decrease the absorption energy of ...     

Insight into tubulin regulation from a complex with colchicine and a stathmin-like domain

Microtubules are cytoskeletal polymers of tubulin involved in many cellular functions. Their dynamic instability is controlled by numerous compounds and proteins, including colchicine and stathmin family proteins. The way in which microtubule instability is regulated at the molecular level has remained elusive, mainly because of the lack of appropriate structural data. Here, we present the structure, at 3.5 A resolution, of tubulin in complex with colchicine and with the stathmin-like domain (SLD) of RB3. It shows the interaction of RB3-SLD with two tubulin heterodimers in a curved complex capped by the SLD amino-terminal domain, which prevents the incorporation of the complexed tubulin into microtubules. A comparison with the structure of tubulin in protofilaments shows changes in the subunits of tubulin as it switches from its straight conformation to a curved one. These changes correlate with the loss of lateral contacts and provide a rationale for the rapid microtubule depolymerization characteristic of dynamic instability. Moreover, the tubulin-colchicine complex sheds light on the mechanism of colchicine's activity: we show that colchicine binds at a location where it prevents curved tubulin from adopting a straight structure, which inhibits assembly.     

Cosmic gamma-ray background from structure formation in the intergalactic medium

The Universe is filled with a diffuse background of gamma-ray radiation, the origin of which remains one of the unsolved puzzles of cosmology. Less than one-quarter of the gamma-ray flux can be attributed to unresolved discrete sources, such as active galactic nuclei; the remainder appears to constitute a truly diffuse background. Here we show that the shock waves induced by gravity in the gas of the intergalactic medium, during the formation of large-scale structures like filaments and sheets of galaxies, produce a population of highly relativistic electrons. These electrons scatter a small fraction of the cosmic microwave background photons in the local Universe up to gamma-ray energies, thereby providing the gamma-ray background. The predicted diffuse flux agrees with the observed background across more than four orders of magnitude in photon energy, and the model predicts that the gamma-ray background, though generated locally, is isotropic to better than five per cent on angular scales larger than a degree. Moreover, the agreement between the predicted and observed background fluxes implies a mean cosmological density of baryons that is consistent with Big Bang nucleosynthesis.     

浅谈高职学生的思想政治教育

本文阐述了新时期高职生思想政治教育的紧迫性和重要性,分析了高职生思想政治现状,提出了加强和改进高职学生思想政治教育工作的四点对策。     

胸腺嘧啶二醇脱氧核苷单分子糖苷键断裂的理论研究

采用理论计算方法对c is-(5S,6R)胸腺嘧啶二醇脱氧核苷(dTg)单分子糖苷键断裂的热力学和动力学性质进行了研究.在气相中,所有稳定点的几何构型都采用B3LYP/6-31+G(d,p)方法进行全优化,并利用导电极化连续模型对气相优化构型进行单点计算确定水的溶剂效应,作者主要考虑了直接断裂和抽氢断裂两条路径.计算结果表明:两条反应机理均涉及较高的活化能,从而说明单分子分解并不能促进糖苷键的断裂,该研究为生物体内这一重要反应提供了更多有用的信息.     

探究英汉复句的省略

本文以Halliday提出的小句的信息功能的相关理论为理论支持,试图从语言功能的角度来解释英汉复句省略手法上的差异。具体分析是借助邱述德的语篇原则,通过探求说话人的交流意图和表达心理,细致地探究了英汉复句的省略手法的选择机制。     

局部气候调节探索屏障系统节能管理新思路

屏障系统动物设施节能设计[1]中采用较多的全新风单元式空调模式,现在多用于较大型综合性实验动物中心,例如江苏省医药、农药、兽药安全性评价与研究中心和浙江省实验动物中心等.它是通过合理划区,再为每一个区域配备一套全新风空气净化和空调系统,这种用时ON、不用时OFF的能源控制设施模式可以节省不必要的能耗,也便于维护保养.但是,多数情况下,因为每一个区域相对较大的空间,所以这种模式并不能真正实现控制区域的恒温恒湿要求.     

略论物质与宇宙基本问题的物理“终极探索”

当基本理论物理的研究正在指向所谓物质与宇宙本原的“终极问题”,是否意味着物理学已走到最后的认知“疆界”。本文拟从一个科学理论体系的不可能假定出发,阐述对这一基本命题的看法。     

盆地腹地隆起带内高角度变形带构造样式新解

在总结岩石变形理论和砂箱模拟实验研究进展基础上, 通过构造物理模拟实验和地震剖面的地质解释, 对塔里木盆地腹地巴楚隆起区的高角度变形带的构造属性进行了全新的分析和解释, 认为形成隆起的部分高角度构造为大型膝褶带。巴楚隆起内的膝褶带在地震剖面上表现为低反射、低信噪比, 且地震信息模糊带具有一定的宽度, 这是由于膝褶带内地层发生陡倾斜所致。根据最大有效力矩准则, 一定宽度的低反射带、近于平行的低反射带边界、低反射带与地层呈高角度及其钝角角分线对着挤压方向的几何学特征均表明塔里木盆地腹地巴楚隆起带内部存在膝褶带构造。     

新型胶原支架材料的结构特征与性能

为了克服猪脱细胞真皮基质作为组织工程支架材料渗透性差、降解速度过慢、免疫原性较强等缺点,采用多种化学、生物与物理综合方法处理猪皮制备了一种新型天然胶原支架材料,通过光学显微镜、扫描电镜观察以及体外降解时间、透水汽性、拉伸强度、孔隙率、收缩温度等的测定,对其性能进行了研究. 实验结果显示:支架中的成纤维细胞、脂肪细胞及组织纤维间质完全去除,胶原纤维得到了松散,并维持其原有的天然三维网络多孔结构;该材料透水汽性处于3 000 g·m-2·d-1左右,适合创面恢复;体外降解时间处于25～50 h之间,并可根据需要调整工艺条件控制降解时间;拉伸强度介于10.20～11.50 Mpa之间,具有良好的拉伸强度;收缩温度介于70～85 ℃之间. 上述结果表明该材料已解决了猪脱细胞真皮基质渗透性差、降解速度过慢的缺点,并且其透气性和拉伸强度高、降解性优良且可控,符合组织工程支架材料的要求.