Crystal structure of the β2 adrenergic receptor-Gs protein complex

G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signalling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β(2) adrenergic receptor (β(2)AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signalling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β(2)AR and nucleotide-free Gs heterotrimer. The principal interactions between the β(2)AR and Gs involve the amino- and carboxy-terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β(2)AR include a 14 ? outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an α-helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the α-helical domain of Gαs relative to the Ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signalling by a GPCR.     

ATP-induced helicase slippage reveals highly coordinated subunits

Helicases are vital enzymes that carry out strand separation of duplex nucleic acids during replication, repair and recombination. Bacteriophage T7 gene product 4 is a model hexameric helicase that has been observed to use dTTP, but not ATP, to unwind double-stranded (ds)DNA as it translocates from 5' to 3' along single-stranded (ss)DNA. Whether and how different subunits of the helicase coordinate their chemo-mechanical activities and DNA binding during translocation is still under debate. Here we address this question using a single-molecule approach to monitor helicase unwinding. We found that T7 helicase does in fact unwind dsDNA in the presence of ATP and that the unwinding rate is even faster than that with dTTP. However, unwinding traces showed a remarkable sawtooth pattern where processive unwinding was repeatedly interrupted by sudden slippage events, ultimately preventing unwinding over a substantial distance. This behaviour was not observed with dTTP alone and was greatly reduced when ATP solution was supplemented with a small amount of dTTP. These findings presented an opportunity to use nucleotide mixtures to investigate helicase subunit coordination. We found that T7 helicase binds and hydrolyses ATP and dTTP by competitive kinetics such that the unwinding rate is dictated simply by their respective maximum rates V(max), Michaelis constants K(M) and concentrations. In contrast, processivity does not follow a simple competitive behaviour and shows a cooperative dependence on nucleotide concentrations. This does not agree with an uncoordinated mechanism where each subunit functions independently, but supports a model where nearly all subunits coordinate their chemo-mechanical activities and DNA binding. Our data indicate that only one subunit at a time can accept a nucleotide while other subunits are nucleotide-ligated and thus they interact with the DNA to ensure processivity. Such subunit coordination may be general to many ring-shaped helicases and reveals a potential mechanism for regulation of DNA unwinding during replication.     

Mixed aromatic-aliphatic organic nanoparticles as carriers of unidentified infrared emission features

Unidentified infrared emission bands at wavelengths of 3-20 micrometres are widely observed in a range of environments in our Galaxy and in others. Some features have been identified as the stretching and bending modes of aromatic compounds, and are commonly attributed to polycyclic aromatic hydrocarbon molecules. The central argument supporting this attribution is that single-photon excitation of the molecule can account for the unidentified infrared emission features observed in 'cirrus' clouds in the diffuse interstellar medium. Of the more than 160 molecules identified in the circumstellar and interstellar environments, however, not one is a polycyclic aromatic hydrocarbon molecule. The detections of discrete and broad aliphatic spectral features suggest that the carrier of the unidentified infrared emission features cannot be a pure aromatic compound. Here we report an analysis of archival spectroscopic observations and demonstrate that the data are most consistent with the carriers being amorphous organic solids with a mixed aromatic-aliphatic structure. This structure is similar to that of the organic materials found in meteorites, as would be expected if the Solar System had inherited these organic materials from interstellar sources.     

马铃薯交联-羧甲基淀粉的结构与性能

以马铃薯淀粉为原料,环氧氯丙烷为交联剂,制备交联-羧甲基淀粉(CCMS).对比了CCMS与马铃薯原淀粉、交联淀粉(CS)、羧甲基淀粉(CMS)和羧甲基纤维素(CMC)的性质,结果表明CCMS糊液具有良好的抗老化能力、热稳定性、酸碱稳定性及抗剪切稳定性.采用傅里叶变换红外光谱(FT-IR)、环境扫描电镜(ESEM)和X射...     

Correlations for the ignition delay times of hydrogen/air mixtures

Correlations for the ignition delay times of hydrogen/air mixtures were developed using the method of High Dimensional Model Representation (HDMR).The hydrogen/air ignition delay times for initial conditions over a wide range of temperatures from 800 to 1600 K,pressures from 0.1 to 100 atm,and equivalence ratios from 0.2 to 10 were first calculated utilizing the full chemical mechanism.Correlations were then developed based on these ignition delay times.Two forms of correlations were constructed:the first one is an overall general model covering the whole range of the initial conditions;while the second one is a piecewise correlation model valid for initial conditions within different sub-domains.The performance of these correlations was studied through comparison with results from the full chemical mechanism as well as experimental data.It was shown that these correlations work well over the whole range of initial conditions and that the accuracy can be significantly improved by using different piecewise correlations for different sub-domains.Therefore,piecewise correlations can be used as an effective replacement for the full mechanism when the prediction of chemical time scale is needed in certain combustion modeling.     

Skin collagen fiber-based radar absorbing materials

By using skin collagen fiber (CF) as raw material,Schiff base structure containing CF (Sa-CF) was synthesized through CF-salicylaldehyde reaction.Then a novel radar absorbing material (Fe-Sa-CF) was prepared by chelating reaction between Sa-CF and Fe 3+.The coaxial transmission and reflection method was used to analyze the complex permittivity and complex magnetic permeability of these CF-based materials,and the radar cross section (RCS) method was used to investigate their radar absorbing properties in the frequency range of 1.0-18.0 GHz.Experimental results indicated that the conductivity of CF increased from initial 1.08×10-11 to 2.86×10-6 S/cm after being transferred into Fe-Sa-CF,and its dielectric loss tangent (tanδ) in the frequency range of 1.0-17.0 GHz also increased.These facts suggest that the Fe-Sa-CF is electric-loss type radar absorbing material.In the frequency range of 3.0-18.0 GHz,Sa-CF (1.0 mm in thickness) exhibited somewhat radar absorbing property with maximum radar reflection loss (RL) of-4.73 dB.As for Fe-Sa-CF,the absorbing bandwidth was broadened,and the absorbing intensity significantly increased in the frequency range of 1.0-18.0 GHz where a maximum radar RL of-9.23 dB was observed.In addition,the radar absorbing intensity of Fe-Sa-CF can be further improved by increasing membrane thickness.When the thickness reached to 2.0 mm,the RL values of Fe-Sa-CF were-15.0-18.0 dB in the frequency range of 7.0-18.0 GHz.Consequently,a kind of novel radar absorbing material can be prepared by chemical modification of collagen fiber,which is characterized by thin thickness,low density,broad absorption bandwidth and high absorption intensity.     

Quantum key distribution using four-level particles

We present a quantum key distribution protocol based on four-level particle entanglement. Furthermore, a controlled quantum key distribution protocol is proposed using three four-level particles. We show that the two protocols are secure.     

Role of Tip60 tumor suppressor in DNA repair pathway

To prevent the damage caused by DNA strand breaks, eukaryotic cells have evolved a series of highly conserved DNA repair mechanisms. The ubiquitously expressed acetyltransferase, Tip60, plays a central role in ATM (ataxia-telangiectasia mutated) activation which is involved in DNA repair. Recent work uncovered a new mechanism of ATM activation mediated by Tip60 and demonstrated that histone methylation, specifically, trimethylation of histone H3, is a key factor in the process. Here, we review the current understanding of how Tip60 is activated and how it activates ATM in response to DNA damage.     

谈谈字母词

全球几千种语言,“很少是自给自足的”(萨丕尔),时刻都在进行着跨语言文化的词语代码转化,并生成民族化的外来词。在200个国家中有120个国家,在民族化的同时兼顾国际化：拉丁文语种以借形为主,非拉丁文语种(俄语、日语)以借音为主(兼顾借形),而汉语以借意为主(约占75%)。20世纪初,在新文化浪潮的冲击下,中国开始出现了借形的新奇现象。数学和文学里分别出现了x+y=a,阿Q。紧接着自然科学和社会科学纷纷出现了诸如此类的现象。到了改革开放新时期,借形已成了新潮。所谓的“字母词”,就是借形词的一种。对于汉语借形词,有人坚决反对。较早的时候,有人把x+y=a改为“天+地=甲”,上演一出国粹闹剧。可是怎么也演不下去,因为罗马和希腊字母作为数学符号,常见的至少有60来个,而我们的天干地支总共才22个,不可能对译,而且用起来很别扭。后来,有人看阿Q很刺眼,可是阿Q站起来了,而且堂而皇之地走进了汉语词汇家族。鲁迅在20世纪30年代,还用过ABC、OK等许多字母词。时至今日,字母词已成百上千地出现在书报上,迫使严肃的辞书也不得不逐渐承认它们。一向有开明意识的《辞海》,1979年版附录了63条,1989年版附录了97条,1999年版附录了126条。一贯以规范著称的《现代汉语词典》第三版附录了39条,第四版附录了142条,第五版附录了182条。2001年上海辞书出版社出版了资深语言学家刘涌泉的《字母词词典》,内收2000多条。这也只是一般书报上多见的,如果再稍微扩大到专业书报,再加上近几年产生的,编一本《字母词大词典》,估计收词至少有5000条。字母词不仅频率不断增加,而且分布不断扩大。从自然科学技术领域,渐渐扩大到社会科学及生活领域。如WTO(世界贸易组织)、GNP(国民生产总值)、DVD(数字影碟)、VCD(影碟)、MTV(音乐电视)、IC(集成电路)卡、IP电话、IP卡电话、B超等等。刘涌泉先生给字母词概括了四个优点：快,简,明,广。此外,似乎还可以增加一个优点：补——填补词汇空缺(lexical gap),如A股、B股、A型血、B型血、A调、C调、OK等等。集“快,简,明,广,补”于一身的,有两个更典型的词：GMDSS,这是国际通用的求救信号,是汉语所无,而且换任何一个符号都不如它“快,简,明,广”;DNA,是脱氧核糖核酸,即遗传基因成分,是国际通用的测量公众科学素养的三个概念之一(另两个概念是“分子molecule”,“计算机软件computer software”)。与此相连带的,还有利用字母形状,借形表意,填补表形空缺的,如：U型管、S钩、T型台、A字裙、V字领、T型人才(横杠表示知识广,竖杠表示专业深)、X型人才(X表示交叉,指掌握两门交叉专业的人)。此外,字母词还有多种语用价值。有价值的,就是优变;无价值的,就是畸变。优存畸亡,适者生存。这就是历史的选择。这种选择总是以价值为尺度的。在生物界,名人达尔文有一句名言：“The Nature Selects the Fittest Survives.”被名家严复巧译成名句：“物竞天择,适者生存。”这是英汉翻译史上令人拍案叫绝的“绝译”。不仅词句绝佳,而且道理绝真。即使应用于词汇现象,类推也是成立的：词竞众择,适者生存。所谓“词竞”,是词语以其词形、语音、结构(含理据或内部形式)、语义、语用(含频率)等价值条件,跟并行词语、对应词语或相关词语竞争。例如：email/e-mail/Email,其中e-mail使用频率高,是多数人的选择,《现代汉语词典》便以它为规范词形。所谓“众择”,是语言的广大使用者的选择。广大使用者包括一般使用者和专家。选择,就是根据词语的各种价值和交际需要确定词语的取舍。选择的过程是：个体→群体→社团→全民。在这个过程中就形成一个或一些词语的使用趋势和习惯。而语言中的最高权威,就是众人的习惯。专家不要过早地表态或下结论。专家的任务,就是观察并描写习惯,发现趋势(趋势代表着方向,预示着规律),总结并完善规范标准,选择约定俗成的流行形式,引导言语活动,优化交际效能。例如：对BP机、PP机、呼机、寻呼机、无线传呼机等,要允许它们有一个被选择的过程,不要过早地定于一尊。对书写形式,特别是字母大小写表示不同意义的,要强调规范形式。如pH表示氢离子浓度指数,ph表示光照度单位“辐射”(非法定的)。对字母加汉字的混合词,不要采取过激的态度。有人讽刺这类词是“马褂加领带”。其实,这类中西合璧从阿Q时代就开始了,在国外早已有之,通称“混合词”,英语叫hybrid word,法、德、俄语分别叫mots hybrids,hybrids word,гибрид(或意译为полукалька),都是指语源不同的成分构成的词,可见混合词是许多语言共有的现象。这一现象刚出现时,人们难免看不惯,说三道四。最后,混合词终于以其准确性、科学性冲破习惯势力的森然门禁。总之,字母词从阿Q到T型人才,在中国形成了队伍,占领的地盘越来越大,其势不可阻挡。但是,不能滥用,更不能乱用。必须考虑交际对象、环境、语体等等因素。使用的指导思想只能是“科学实用,便于推行和普及,促进汉语健康发展”。     

螺噁嗪类化合物光致变色过程抗疲劳性研究进展

螺噁嗪以相对较高的化学稳定性和抗疲劳性而倍受瞩目,在商业和军事领域有着潜在的应用价值。其抗疲劳性能尚达不到商品化要求是目前限制其工业化的主要障碍之一,本文综述了此方面的研究进展。