Direct observation of the femtosecond excited-state cis-trans isomerization in bacteriorhodopsin

Femtosecond optical measurement techniques have been used to study the primary photoprocesses in the light-driven transmembrane proton pump bacteriorhodopsin. Light-adapted bacteriorhodopsin was excited with a 60-femtosecond pump pulse at 618 nanometers, and the transient absorption spectra from 560 to 710 nanometers were recorded from -50 to 1000 femtoseconds by means of 6-femtosecond probe pulses. By 60 femtoseconds, a broad transient hole appeared in the absorption spectrum whose amplitude remained constant for about 200 femtoseconds. Stimulated emission in the 660- to 710-nanometer region and excited-state absorption in the 560- to 580-nanometer region appeared promptly and then shifted and decayed from 0 to approximately 150 femtoseconds. These spectral features provide a direct observation of the 13-trans to 13-cis torsional isomerization of the retinal chromophore on the excited-state potential surface. Absorption due to the primary ground-state photoproduct J appears with a time constant of approximately 500 femtoseconds.     

Base sequence and higher-order structure induce the complex excited-state dynamics in DNA

The high photostability of DNA is commonly attributed to efficient radiationless electronic relaxation processes. We used femtosecond time-resolved fluorescence spectroscopy to reveal that the ensuing dynamics are strongly dependent on base sequence and are also affected by higher-order structure. Excited electronic state lifetimes in dG-doped d(A)20 single-stranded DNA and dG.dC-doped d(A)20.d(T)20 double-stranded DNA decrease sharply with the substitution of only a few bases. In duplexes containing d(AGA).d(TCT) or d(AG).d(TC) repeats, deactivation of the fluorescing states occurs on the subpicosecond time scale, but the excited-state lifetimes increase again in extended d(G) runs. The results point at more complex and molecule-specific photodynamics in native DNA than may be evident in simpler model systems.     

三体磨料磨损试验机设计研究

饲料环模制粒机在挤压加工苜蓿草颗粒成型过程中会对环模产生严重的磨损,为探究环模失效的磨损机理,新设计了一种三体磨料磨损试验机,模拟制粒工况.该试验机具有加热及相应的控制系统,温度偏差小于2%,可用于100℃左右温度条件下草粉等磨料与不同金属试样的磨损试验.     

Efficient deactivation of a model base pair via excited-state hydrogen transfer

We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 +/- 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1pipi* state and the electronic ground state with a 1pipi* charge-transfer state that is strongly stabilized by the transfer of a proton.     

The large-scale axisymmetric magnetic topology of a very-low-mass fully convective star

Understanding how cool stars produce magnetic fields within their interiors is crucial for predicting the impact of such fields, such as the activity cycle of the Sun. In this respect, studying fully convective stars enables us to investigate the role of convective zones in magnetic field generation. We produced a magnetic map of a rapidly rotating, very-low-mass, fully convective dwarf through tomographic imaging from time series of spectropolarimetric data. Our results, which demonstrate that fully convective stars are able to trigger axisymmetric large-scale poloidal fields without differential rotation, challenge existing theoretical models of field generation in cool stars.     

Control of electron localization in molecular dissociation

We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 --> D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.     

Specificity and stability in topology of protein networks

Molecular networks guide the biochemistry of a living cell on multiple levels: Its metabolic and signaling pathways are shaped by the network of interacting proteins, whose production, in turn, is controlled by the genetic regulatory network. To address topological properties of these two networks, we quantified correlations between connectivities of interacting nodes and compared them to a null model of a network, in which all links were randomly rewired. We found that for both interaction and regulatory networks, links between highly connected proteins are systematically suppressed, whereas those between a highly connected and low-connected pairs of proteins are favored. This effect decreases the likelihood of cross talk between different functional modules of the cell and increases the overall robustness of a network by localizing effects of deleterious perturbations.     

脱落酸(ABA)在气孔调节中的作用

ABA在气孔调节中具有非常重要的作用。本文介绍了ABA结合蛋白、保卫细胞中的ABA合成和ABA在气孔调节中不依赖Ca^2 的机制和依赖Ca^2 的机制。     

花坛造景在园林中的作用

从花坛的概念、花坛的分类、花坛的布置要求及植物的选择,花坛的施工与养护管理,花坛的作用,从5个大的方面对花坛造景在园林绿化中的作用作了较为系统的概括,同时从花坛的趋势及发展告诉人们：花坛艺术,既装扮着人们美好的生活空间,也将使人们的审美情趣提高到一个新的水平。     

钢琴伴奏在合唱中的作用

在声乐表演艺术中,能够将指挥、人声、乐器、复调融为一体的就是合唱艺术。而在合唱艺术中扮演着和声衬托、主旋律的陈述、前奏间奏尾奏的陈述、复调性副旋律的对比、并置性主题的对比和填补合唱中不足的角色就是钢琴的特色伴奏。钢琴特色伴奏在合唱艺术中越来越重要,并且作用也呈现这多样化的趋势发展。本文将合唱艺术中钢琴伴奏的地位及作用从多方面进行分析和介绍,展现出合唱的艺术魅力。     

钙在植物生理代谢中的作用

钙不仅是一种大量元素，而且也是一种转导许多生理过程的胞内信号分子。Ca^2 对植物细胞的结构和生理功能有重要作用，能维持细胞壁、细胞膜及膜结合蛋白的稳定性，参与胞内稳态和生长发育的调节过程，在细胞内起第二信使的作用。Ca^2 调节植物对环境胁迫的反应，在胁迫条件下，胞内Ca^2 常常显著增加，这种增加可以启动基因表达，激活一系列生化反应，使植物能够适应环境胁迫。     

绪论在“动物遗传学”教学中的作用

论述了绪论在动物遗传学教学中的作用;指出上好绪论课不仅能够激发学生的学习兴趣、掌握学习的主要内容,帮助学生打开动物遗传学学习的大门,而且能够通过对遗传学形成、发展过程中一些重大研究成果、杰出人物事迹的介绍培养学生的科学素养。     

猪附红细胞体解离方法的比较

为建立一种简便易行的猪附红细胞体的解离方法，对同一患有附红细胞体病猪的血液样本进行解离，比较了两种方法对附红细胞体的解离效果．结果表明，应用水浴法解离附红细胞体较化学试剂法解离效果好；再用超声波粉碎，经Sephadex G-200柱层析，即可取得纯度较高、提取量较大的附红细胞体抗原成分．为进一步研究附红细胞体的生物学特性及抗原分析奠定了基础．     

沼气在新农村建设中的作用浅析

长期以来．“三农”问题一直是决定我国全面建设小康社会进程和现代化进程的关键。农业丰．则基础牢;农村稳。则社会安;农民富,则国家昌。只有广大农村落后面貌明显改变,才能实现全面小康。为此．党中央审时度势,提出了建设社会主义新农村的重大战略决策．勾画出了“生产发展、生活富裕、乡风文明、村容整洁、管理民主”的新农村宏伟蓝图。     

ABA在植物与病原菌互作中的作用

脱落酸(ABA)是一种重要的植物激素,在植物受到生物或非生物压力时,其对植物抵御这些压力具有重要作用。脱落酸在生物胁迫下,通过干扰水杨酸(SA)、茉莉酸(JA)、乙烯(ET)防御信号途径,可负调控植物抗病性。然而,近年来研究发现,脱落酸可通过诱导胼胝质积累正调控抗病性,但目前关于此方面的报道国内较少。综述了近年来关于ABA对植物防御研究的一些进展,介绍ABA在植物防御中所起的作用及调节机制。     

High-pressure dissociation of carbonic and boric acids in seawater

The apparent dissociation constants of carbonic and boric acids were determined for pressures up to 654 atmospheres in seawater of 34.8 per mille salinity at 22 degrees C. Our values for the ratios of the apparent dissociation constants at 654 atmospheres to the constants at 1 atmosphere are 1.84 1.48, and 1.94 for K'(1), K'(2), and K'(B), respectively. At this pressure the commonly accepted values of Buch and Gripenberg for these ratios are 2.03 and 1.30 for K'(1) and K'(2), respectively.