Conformational properties of the guanine-binding site of ribonuclease T1 inferred from the X-ray structure and protein engineering

Recognition by ribonuclease T₁ of guanine bases via multidentatehydrogen bonding and stacking interactions appears to be mediatedmainly by a short peptide segment formed by one stretch of aheptapeptide, Tyr⁴²-Asn⁴³-Asn⁴⁴-Tyr⁴⁵-Gly⁴⁶-Gly⁴⁷-Phe⁴⁸. Thesegment displays a unique folding of the polypeptide chain—consistingof a reverse turn, Asn⁴⁴-Tyr⁴⁵-Glu⁴⁶-Gly⁴⁷, stabilized by ahydrogen-bond network involving the side chain of Asn⁴⁴, themain-chain atoms of Asn⁴⁴, Gly⁴⁷ and Phe⁴⁸ and one water molecule.The segment is connected to the C terminus of a ß-strandand expands into a loop region between Asn⁴³ and Ser⁵⁴. Lowvalues for the crystallographic thermal parameters of the segmentindicate that the structure has a rigidity comparable to thatof a ß-pleated sheet. Replacement of Asn⁴⁴ with alanineleads to a far lower enzymatic activity and demonstrates thatthe side chain of Asn⁴⁴ plays a key role in polypeptide foldingin addition to a role in maintaining the segment structure.Substitution of Asn⁴³ by alanine to remove a weak hydrogen bondto the guanine base destabilized the transition state of thecomplex by 6.3 kJ/mol at 37°C. In contrast, mutation ofGlu⁴⁶ to alanine to remove a strong hydrogen bond to the guaninebase caused a destabilization of the complex by 14.0 kJ/mol.A double-mutant enzyme with substitutions of Asn⁴³ by a histidineand Asn⁴⁴ by an aspartic acid, to reproduce the natural substitutionsfound in ribonuclease M_s, showed an activity and base specificitysimilar to that of the wild-type ribonuclease M_s. The segmenttherefore appears to be well conserved in several fungal ribonucleases.     

基于固定界面子结构模态的频响函数精确综合法

提出基于固定界面子结构模态的双协调结构频响函数精确综合方法。首先用固定界面子结构的中高阶模态与精确剩余约束模态表达子结构频域位移;再利用界面力与位移的协调关系进一步消去界面自由度,建立双协调的精确缩聚变换;然后根据里兹法,得到结构精确综合的频域运动方程与频响函数;最后通过数值结果验证该方法的综合能力与准确性。     

基于美国规范的桥梁下部结构计算

分析了美国公路桥梁设计规范(AASHTO),采用MIDAS软件,对某项目桥梁下部结构设计进行了介绍,并针对美国规范和中国规范进行了比较,为公路桥梁设计者了解美国规范提供参考。     

采用不同节点形式的三维钢框架-组合楼板子结构抗连续倒塌性能试验研究

为研究三维钢框架-组合楼板子结构的抗连续倒塌性能,对4个采用不同节点形式的1/3缩尺组合楼板试件在移除边柱工况下进行了拟静力试验。试件主梁与柱的连接形式为平齐式端板(FEP)节点、反向槽钢(RC)节点、狗骨式(RBS)节点与栓焊混合(WUFB)节点,次梁与柱及次梁与主梁的连接形式为腹板双角钢(DAC)节点或剪切板(FP)节点。通过特殊设计的6点加载系统实现位移控制下的均布荷载作用,获得了三维钢框架-组合楼板子结构加载全过程的荷载-位移曲线以及破坏模式,并分析了抗弯效应、悬链线效应及楼板的受拉薄膜效应对结构抗连续倒塌性能的影响。结果表明:对于主梁节点,采用栓焊混合连接的试件具有较高的初始刚度和极限承载力,而采用反向槽钢连接的试件则具有较好的延性,采用狗骨式连接的试件具有最好的能量吸收能力; 对于次梁节点,采用剪切板连接的试件呈现出明显的脆性破坏特征,而其他采用腹板双角钢连接的试件发生的是延性破坏; 在边柱失效情况下,抗弯效应在抵抗连续倒塌过程中一直起主导作用,楼板的受拉薄膜效应在结构的大变形阶段也发挥着一定的作用,而悬链线效应贡献较小,可忽略不计。     

筒体结构上部结构基础与桩筏共同作用的分析

通过对筒体结构上部结构、基础与桩筏共同作用的分析，详细介绍了子结构法，并建立了共同作用基本方程。指出共同作用分析方法特别适用于高层建筑结构。     

Static properties and stability of cylindrical ILTDBS reticulated mega-structure with double-layer grid substructures

For the cylindrical intersecting-latticed-three-dimensional-beams-system (ILTDBS) reticulated mega-structure with double-layer plane-plate grid substructures, the configuration, form parameters, the support styles of the main structure, and the support ways of the substructures on the main structure are analyzed firstly in this paper. Then the geometrically nonlinear mechanical model and corresponding stability analysis procedure are developed, and the structural stability including buckling modes, ultimate loads under different kinds of conditions and a series of parametric analyses are researched. Moreover, the general failure conditions of the structure are discussed. Finally the basic static properties of the structure are investigated, and a series of parametric analyses with respect to static properties of the structure are performed. All this work will provide theoretical guidance to the actual engineering application of this structure.     

Experience with Construction of Underground Sections of Buildings by the “From Top Down” Scheme

Experience gained with the method of installing the underground section of a building from the top down is described in an example of the construction of specific projects in Moscow, design, construction and monitoring problems are exposed, advantages and disadvantages of this method are indicated, and observational data are cited for the settlements of existing buildings located within the zone of influence of new construction.     

北京大兴国际机场航站楼中心区屋盖钢结构设计的关键问题

为满足北京大兴国际机场航站楼少柱、大空间的建筑需求,提出一种以可独立承载的大型子结构单元为基础、支承体系与屋盖结构一体化的巨型网格结构。并就结构设计中的关键问题,进行了竖向承载能力、水平抗侧能力、防连续倒塌、抗震性能研究及竖向地震作用分析,结果表明：以若干独立承载的大型子结构单元为基础构建的体系可大幅提升结构的承载能力和抗倒塌能力;两个支撑筒或支撑框架与C形柱组成三角形支承结构,承载力可提升300%以上;子结构之间的屋盖结构对巨型网格结构的传力机制影响较大,应考虑子结构独立模型和整体模型进行承载力包络设计;当支承构件承担的地震剪力小于其承担的重力荷载比例时,应对支承构件承担的地震剪力进行调整,以提高支承构件的抗侧能力;针对构件和支承系统层面的抗连续倒塌分析可有效评估不同构件和C形柱的重要性等级;采用静力法计算大跨屋盖的竖向地震响应偏小,应采用振型分解反应谱法或动力时程分析法进行复核;采用基础与层间结合的超大平面隔震技术可有效提高巨型网格结构的抗震能力。     

预制钢筋混凝土剪力墙结构拟动力子结构试验研究

在预制钢筋混凝土剪力墙结构拟静力试验研究的基础上,采用等效力控制方法对足尺试验模型进行了拟动力子结构试验,研究该结构在地震作用下的破坏过程和变形能力,获得了模型下降段试验数据,给出了结构各层的位移时程曲线、层间滞回曲线、骨架曲线、刚度退化曲线以及延性等,为该类结构在地震作用下的弹塑性分析提供了试验数据。试验结果表明：采用的水平连接技术是可靠的,能够很好地保证其在地震作用下连接性能;该结构模型属于延性结构,可以满足我国现行抗震设计规范7度设防区多道抗震设防的要求,符合“三水准”抗震设防目标,具有较好的整体抗震性能,经过合理设计可以在地震设防区使用。图15表5参15