A QM/MM study of the catalytic mechanism of aspartate ammonia lyase

Aspartate ammonia lyase (Asp) is one of three types of ammonia lyases specific for aspartate or its derivatives as substrates, which catalyzes the reversible reaction of l-aspartate to yield fumarate and ammonia. In this paper, the catalytic mechanism of Asp has been studied by using combined quantum-mechanical/molecular-mechanical (QM/MM) approach. The calculation results indicate that the overall reaction only contains two elementary steps. The first step is the abstraction of C_β proton of l-aspartate by Ser318, which is calculated to be rate limiting. The second step is the cleavage of C_αN bond of l-aspartate to form fumarate and ammonia. Ser318 functions as the catalytic base, whereas His188 is a dispensable residue, but its protonation state can influence the active site structure and the existing form of leaving amino group, thereby influences the activity of the enzyme, which can well explain the pH dependence of enzymatic activity. Mutation of His188 to Ala only changes the active site structure and slightly elongates the distance of C_β proton of substrate with Ser318, causing the enzyme to remain significant but reduced activity.     

Oracle数据库内存参数调优

Oracle数据库在使用过程中,会遇到性能下降的问题,对其进行调整的方法很多,这里主要介绍使oracle的内存参数如何设置以优化Oracle的性能,并在实际的工作中灵活应用,使oracle的内存性能达到最优配置,并进行合理的内存使用,提升应用程序反应速度。     

基于CC1110的井下无线传感器监测网络的设计与实现

介绍了一种基于CC1110的矿井下的无线传感器(WSNs)的构建方法,包括传感器节点的硬件构成和网络拓扑结构、协议的设计.该WSNs可实时地对井下的环境参数进行检测,实验证明:系统运行稳定,效果良好.     

互联网背景下图书馆管理信息化建设方法探究

现如今已经进入了互联网的时代,互联网贯穿在人们生活中的各个领域,人们的日常生活和工作离不开互联网的支持。因此,图书馆管理也要随着时代的发展进行改革创新,为了给读者提供优质的服务。目前我国为了促使图书馆的可持续发展推进了图书管理信息化建设的工作,但在此项目实施的过程中产生了诸多的问题。鉴于此,文章通过对图书管理信息化建设的现状及产生的问题进行分析,并针对问题提出一些有效的措施,希望能够为图书馆工作人员提供参考建议。     

大鼠胼胝体中突触结构的电镜观察

关于胼胝体中突触的在存在及其结构特点未见报道。选用２生后一周未睁眼Ｗｉｓｔａｒ大鼠４０只，随机分为摘除右侧眼球、左侧眼球、双侧眼球及正常对照四组，各组１０只。经灌流、固定、分别在胼胝体膝部、干部及压部取材后，电镜下观察了胼胝体中突触的结构。结果如下：１．胼胝 大多为化学性突触。２．各组及同组胼胝体各部中突触的美元数不同。３．胼胝体中突触数受视觉的影响。最后就胼胝体中突触的机能意义进行了讨论。     

青海扎日加地区晚三叠世侵入岩锆石U-Pb年龄、地球化学特征及其地质意义

青海扎日加地区分布有大量中酸性侵入岩,岩性组合主要由中细粒黑云母花岗闪长岩、斑状黑云母二长花岗岩、细粒正长花岗岩3类组成,岩石中SiO_2(67.38%~76.03%)含量逐渐增高,Al_2O_3(13.62%~15.48%)、CaO(0.6%~3.02%)、MgO(0.12%~1.69%)、K_2O(3.00%~4.51%)、Ti O_2(0.05%~0.66%),碱度率AR=2.01~3.59,大于1;里特曼指数δ在1.42~1.98,属过铝质—钙碱性或碱性花岗岩;稀土总量中等(∑REE=83.09×10-6~222.27×10-6),稀土元素标准化曲线为向右陡倾的富集型曲线,铕处"V"型较明显,同时各曲线近于平行;岩石中Rb、Th明显富积,Ba、Nb、Hf、Zr不同程度亏损,以上特征显示岩石具壳型花岗岩特征。在斑状黑云母二长花岗岩中获得LA-ICP-MS锆石U-Pb年龄为(203.34±0.72)Ma,表明巴颜喀拉山浊积盆地区的同碰撞花岗岩发生在晚三叠世并持续至早侏罗世,在构造阶段上属于从挤压构造体制向伸展构造体制转化的转折期。     

基于密度聚类与SVM的非法广播信号识别研究

针对非法广播信号的危害,以及传统人工检测效率低的问题,提出一种基于密度聚类与SVM的信号识别模型。首先,采用标准欧式距离对特征信号进行提取;其次,以聚类样本为基础,采用SVM分类器对信号分类;最后,以青海广播电视局中波台整点时刻前后300帧的数据为样本,以静音信号作为评价指标,对信号进行识别。结果表明,在正常信号中加入非法信号后,频谱中有少量的静音信号,且SVM训练时间和识别正确率都要优于传统算法。     

柴东欧南凹陷上石炭统克鲁克组构造背景、物质来源及沉积环境

柴达木盆地东部（简称柴东）欧南凹陷上石炭统克鲁克组获得油气发现,但该层段的构造背景、物质来源与沉积环境动态演化过程研究依然较薄弱。通过对欧南凹陷东北侧柏树沟露头上石炭统克鲁克组（C₂k）细粒沉积岩开展元素地球化学研究,结果表明,欧南凹陷在C₂k沉积期是阿尼玛卿洋壳向北俯冲控制的主动大陆边缘活动背景下弧后裂陷盆地的组成部分,南部的柴北缘加里东构造带向其提供酸性岩浆岩为主要母岩的陆源碎屑,北部的宗务隆海槽由于枕状玄武岩喷发和低温蚀变产生溶解硅,经海底洋流上升引发硅质浮游生物群落的繁盛而沉积生物硅。C₂k沉积期处于全球范围晚古生代冰期的第Ⅲ期事件,共识别出3期次间冰期（升温事件A、B和C）。古气候温度升高引发冰川型海侵,水体随之加深,缺氧程度增强,海水的回灌和升温蒸发作用使盐度升高而咸化;其中升温B期的温度升高强度和海平面上升幅度均强于升温A期和C期,对应于C₂k最大海泛面,古水体达到弱还原状态,宗务隆海槽方向以硅质生物为代表的海洋低等水生生物供给数量增多,其他时段主要受柴北缘构造带方向陆源有机质的影响。以上研究结果及认识为探索柴达木盆地石炭纪构造—沉积—古地理演化过程和有机质富集机制提供了依据。     

Microcomputer-assisted temperature programmed desorption studies

An IBM-PC assisted ultra-high vacuum system for temperature programmed desorption studies is described. With the use of a self-constructed interface board and a commercial IEEE interface, the microcomputer controls the heating of the adsorbent and stores the partial pressure-temperature-time data from a quadrupole mass spectrometer and thermocouple. Hardware and software details are presented and discussed. Applications of the system are illustrated by the results of carbon monoxide desorption from CdTe(110) surface.     

Supercritical CO2 oil extraction from Chinese star anise seed and simultaneous compositional analysis using HPLC by fluorescence detection and online atmospheric CI‐MS identification

BACKGROUND: Supercritical CO₂ was utilised to extract Chinese star anise seed oil (CSASO), and a three‐level Box–Behnken factorial design from response surface methodology was applied to optimise the extraction conditions, including pressure, temperature and amount of modifier (ethanol). The compositional analysis of fatty acids in CSASO was performed by HPLC with fluorescence detection using 2‐(11H‐benzo[a]carbazol‐11‐yl)‐ethyl‐4‐methylbenzenesulfonate (BCETS) as labelling reagent. Identification was carried out by online atmospheric chemical ionisation–mass spectrometry. RESULTS: The optimum extraction conditions were as follows: extraction pressure, 27.72 MPa, extraction temperature, 46.22 °C, and amount of modifier, 8.58 vol.%. The experimental result showed that the maximum extraction yield was 25.31 ± 0.22% (w/w) under the conditions proposed. The compositional analysis indicated that CSASO mainly contained C18:2, C18:1, C18:3, C20:4, C16, C18 and C20 fatty acids. CONCLUSION: In this study, a fast, simple and high‐efficiency supercritical technique for extracting oil from Chinese star anise seed was developed. Simultaneous determination of fatty acids in CSASO using BCETS as the labelling reagent with HPLC fluorescence detection and online mass spectroscopy identification has been successfully achieved. Copyright © 2010 Society of Chemical Industry