Molecule cascades

Carbon monoxide molecules were arranged in atomically precise configurations, which we call "molecule cascades," where the motion of one molecule causes the subsequent motion of another, and so on in a cascade of motion similar to a row of toppling dominoes. Isotopically pure cascades were assembled on a copper (111) surface with a low-temperature scanning tunneling microscope. The hopping rate of carbon monoxide molecules in cascades was found to be independent of temperature below 6 kelvin and to exhibit a pronounced isotope effect, hallmarks of a quantum tunneling process. At higher temperatures, we observed a thermally activated hopping rate with an anomalously low Arrhenius prefactor that we interpret as tunneling from excited vibrational states. We present a cascade-based computation scheme that has all of the devices and interconnects required for the one-time computation of an arbitrary logic function. Logic gates and other devices were implemented by engineered arrangements of molecules at the intersections of cascades. We demonstrate a three-input sorter that uses several AND gates and OR gates, as well as the crossover and fan-out units needed to connect them.     

Dynamics of cluster-surface collisions

The structure, energetics, and dynamics of shock conditions generated in a nano-cluster upon impact on a crystalline surface are investigated with molecular-dynamics simulations for a 561-atom argon cluster incident with a velocity of 3 kilometers per second onto a sodium chloride surface. The "piling-up" shock phenomenon occurring upon impact, coupled with cascades of energy and momentum transfer processes and inertial confinement of material in the interior of the cluster, creates a transient medium lasting for about a picosecond and characterized by extreme local density, pressure, and kinetic temperature. The nano-shock conditions and impulsive nature of interactions in the newly formed compressed nonequilibrium environment open avenues for studying chemical reactivity and dynamics catalysed via cluster impact.     

Surface charge heterogeneity in amphibole cleavage fragments and asbestos fibers

Aspect ratio and electrophoretic mobility data for amphibole particles reveal that short fibers and blocky cleavage fragments have a smaller net charge than highly elongated particles. Asbestos fibers and cleavage fragments of the same dimensions exhibit the same net negative surface charge but positively charged ends and negatively charged lateral surfaces.     

Quantification of primary versus secondary C-h bond cleavage in alkane activation: propane on pt

The trapping-mediated dissociative chemisorption of three isotopes of propane (C(3)H(8), CH(3),CD(2)CH(3), and C(3)D(8)) has been investigated on the Pt(110)-(1 x 2) surface, and both the apparent activation energies and the preexponential factors of the surface reaction rate coefficients have been measured. In addition, the probabilities of primary and secondary C-H bond cleavage for alkane activation on a surface were evaluated. The activation energy for primary C-H bond cleavage was 425 calories per mole greater than that of secondary C-H bond cleavage, and the two true activation energies that embody the single measured activation energy were determined for each of the three isotopes. Secondary C-H bond cleavage is also preferred on entropic grounds, and the magnitude of the effect was quantified.     

1株抗肿瘤新城疫病毒的毒力分析

[目的]分析1株抗肿瘤新城疫病毒(NDV)的毒力。[方法]通过传统毒力的测定方法,测定此株NDV的最小致死量的平均死亡时间(MDT)、脑内致病指数(ICPI)及静脉致病指数(IVPI)。通过RT-PCR扩增此株病毒基因组的多个片段,相邻扩增片段之间有部分核酸序列重叠且覆盖完整的融合蛋白(F)基因和血凝素神经氨酸酶(HN)基因的区域,并进行测序。[结果]此株NDV的MDT为105.6h,ICPI约为0.26,IVPI为0。测序结果表明,此株病毒是1株新的NDV,其F蛋白剪切位点为112RRQRRF117。[结论]此株NDV为弱毒株,其F蛋白剪切位点与强毒株一致。除F蛋白剪切位点外,NDV的毒力必然与其他因素相关。     

Controlling chemical turbulence by global delayed feedback: pattern formation in catalytic CO oxidation on Pt(110)

Control of spatiotemporal chaos is one of the central problems of nonlinear dynamics. We report on suppression of chemical turbulence by global delayed feedback using, as an example, catalytic carbon monoxide oxidation on a platinum (110) single-crystal surface and carbon monoxide partial pressure as the controlled feedback variable. When feedback intensity was increased, spiral-wave turbulence was transformed into new intermittent chaotic regimes with cascades of reproducing and annihilating local structures on the background of uniform oscillations. The global feedback further led to the development of cluster patterns and standing waves and to the stabilization of uniform oscillations. These findings are reproduced by theoretical simulations.     

Association of crossover points with topoisomerase I cleavage sites: a model for nonhomologous recombination

Nonhomologous DNA recombination is frequently observed in somatic cells upon the introduction of DNA into cells or in chromosomal events involving sequences already stably carried by the genome. In this report, the DNA sequences at the crossover points for excision of SV40 from chromosomes were shown to be associated with eukaryotic topoisomerase I cleavage sites in vitro. The precise location of the cleavage sites relative to the crossover points has suggested a general model for nonhomologous recombination mediated by topoisomerase I.     

Designing superoleophobic surfaces

Understanding the complementary roles of surface energy and roughness on natural nonwetting surfaces has led to the development of a number of biomimetic superhydrophobic surfaces, which exhibit apparent contact angles with water greater than 150 degrees and low contact angle hysteresis. However, superoleophobic surfaces-those that display contact angles greater than 150 degrees with organic liquids having appreciably lower surface tensions than that of water-are extremely rare. Calculations suggest that creating such a surface would require a surface energy lower than that of any known material. We show how a third factor, re-entrant surface curvature, in conjunction with chemical composition and roughened texture, can be used to design surfaces that display extreme resistance to wetting from a number of liquids with low surface tension, including alkanes such as decane and octane.     

Controlled Deposition of Size-Selected Silver Nanoclusters

Variable-temperature scanning tunneling microscopy was used to study the effect of kinetic cluster energy and rare-gas buffer layers on the deposition process of size-selected silver nanoclusters on a platinum(111) surface. Clusters with impact energies of 相似文献   

Segregation and mapping analysis of polymorphic HLA class I restriction fragments: detection of a novel fragment

An HLA-B7 complementary DNA clone was used as a hybridization probe to analyze the segregation pattern of polymorphic class I restriction fragments in several families whose HLA types had been determined by serological techniques. In one family in which a crossover in the HLA region had occurred, a specific genomic fragment was mapped with respect to the crossover. In another family, a novel genomic fragment present in one child and absent in all other family members was observed. With the exception of this novel fragment, all polymorphic class I fragments observed in this study segregated with a serologically defined parental haplotype, a result consistent with HLA linkage.     

Structure of the hinge region of the mu heavy chain of human IgM immunoglobulins

The amino acid sequence around the central disulfide bridge linking the mu heavy chains of the human immunoglobulin M monomer is unlike that in immunoglobulin G. This hinge area contains one of the five oligosaccharides of the mu chain, is low in proline, and is the site of tryptic cleavage to yield Fabmicro and Fcmicro fragments.     

Site-specific cleavage of human chromosome 4 mediated by triple-helix formation.

Direct physical isolation of specific DNA segments from the human genome is a necessary goal in human genetics. For testing whether triple-helix mediated enzymatic cleavage can liberate a specific segment of a human chromosome, the tip of human chromosome 4, which contains the entire candidate region for the Huntington's disease gene, was chosen as a target. A 16-base pyrimidine oligodeoxyribonucleotide was able to locate a 16-base pair purine target site within more than 10 gigabase pairs of genomic DNA and mediate the exact enzymatic cleavage at that site in more than 80 percent yield. The recognition motif is sufficiently generalizable that most cosmids should contain a sequence targetable by triple-helix formation. This method may facilitate the orchestrated dissection of human chromosomes from normal and affected individuals into megabase sized fragments and facilitate the isolation of candidate gene loci.     

Structure of the uncleaved human H1 hemagglutinin from the extinct 1918 influenza virus

The 1918 "Spanish" influenza pandemic represents the largest recorded outbreak of any infectious disease. The crystal structure of the uncleaved precursor of the major surface antigen of the extinct 1918 virus was determined at 3.0 angstrom resolution after reassembly of the hemagglutinin gene from viral RNA fragments preserved in 1918 formalin-fixed lung tissues. A narrow avian-like receptor-binding site, two previously unobserved histidine patches, and a less exposed surface loop at the cleavage site that activates viral membrane fusion reveal structural features primarily found in avian viruses, which may have contributed to the extraordinarily high infectivity and mortality rates observed during 1918.     

Discovery of young, isolated planetary mass objects in the final sigma Orionis star cluster

We present the discovery by optical and near-infrared imaging of an extremely red, low-luminosity population of isolated objects in the young, nearby stellar cluster around the multiple, massive star final sigma Orionis. The proximity (352 parsecs), youth (1 million to 5 million years), and low internal extinction make this cluster an ideal site to explore the substellar domain from the hydrogen mass limit down to a few Jupiter masses. Optical and near-infrared low-resolution spectroscopy of three of these objects confirms the very cool spectral energy distribution (atmospheric effective temperatures of 1700 to 2200 kelvin) expected for cluster members with masses in the range 5 to 15 times that of Jupiter. Like the planets of the solar system, these objects are unable to sustain stable nuclear burning in their interiors, but in contrast they are not bound to stars. This new kind of isolated giant planet, which apparently forms on time scales of less than a few million years, offers a challenge to our understanding of the formation processes of planetary mass objects.     

蔗糖/氮羟甲基树脂改性速生林木材的力学性能

以蔗糖为改性主剂、二羟甲基二乙烯脲(DMDHEU)为交联剂对人工速生林木材——杨树和辐射松进行改性处理,系统评价了蔗糖/DMDHEU水溶液真空加压浸渍和高温干燥固着处理对木材的表面能、胶合强度、抗弯强度、抗拉强度、握钉力、抗劈力等力学性能的影响。结果表明:与素材相比较,改性处理并不改变木材的表面自由能,木材的胶合性能未受影响。由于密度增加,改性杨木和辐射松的抗弯弹性模量提高了51%和84%,握钉力增加幅度达32%和115%。然而,改性处理导致木材的抗拉强度、抗剪切强度、抗劈力都有不同程度的下降。综上所述,该处理工艺获得的杨木和松木具有改善的粘、钉组装集成能力,但要避免用于承受高载荷的部件。     

Controlled deposition, soft landing, and glass formation in nanocluster-surface collisions

Molecular dynamics simulations have been used to investigate the dynamics and redistribution of energy during the impact of a nanocrystal with adsorbed liquid films. Although impact of a 32-molecule NaCl cluster on a solid surface at 3 kilometers per second leads to melting, disordering, fragmentation, and rebounding, the same size cluster colliding with a liquid neon film transfers its energy efficiently to the liquid for a controlled soft landing. Impact on a higher density film (argon) leads to rapid attenuation of the cluster velocity, accompanied by fast heating. Subsequent disordering, melting, and fast cooling by evaporation of argon quench the cluster to a glassy state. These results suggest a method for the controlled growth of nanophase materials.     

能量对蛋鸡性成熟过程中卵巢卵泡的影响

通过研究日粮能量水平对蛋鸡性成熟过程中卵巢卵泡的影响,结果表明:22周龄前,卵巢表面卵泡数随着周龄和能量水平的提高而增多。到22周龄,除低能Ⅱ组外,其它各组优势卵泡(10mm以上)明显增多,与低能Ⅱ组达差异显著水平(P<0.05),说明过低的能量饲料对性成熟过程中生殖器官的发育有显著的影响。14和18周龄,低能组初级卵泡的闭锁率明显高于对照和高能组,说明在生殖器官发育的早期,能量过低对初级卵泡的影响比对次级卵泡影响的程度要大。22周龄,高能Ⅰ组卵泡的闭锁率高于其它各组。     

利用RFLP标记水稻亲本遗传差异及其杂种优势中的利用

本研究以17个杂交水稻亲本、3个新株型株系和24个光壳稻、爪哇稻品种为DNA样品来源，通过RFLP（限制性片段长度多态性，Restriction Fragment Length Polymorphism,简称RFLP）标记技术，研究光壳稻和爪哇稻及其与温带粳稻之间的关系，探索RFLP标记水稻亲本遗传差异在杂交稻育种中利用的可能性。研究结果表明：42个探针共产生69个不同的限制性片段，其中20个（占47.6%）探针显示47个（占68.1%）多态性片段，至少在2个基因型间存在差异。每个具有多态性片段分别以1和0记录存在与否。由RFLP数据计算的Nei‘s遗传距离，创建聚类树状图。聚类分析结果表明，灿稻和粳稻容易被分开，温带粳稻又容易与光壳稻、爪哇稻分开，但光壳稻和爪哇稻混合聚在一起，光壳稻与温带粳稻之间的遗传距离要比爪哇稻与温带粳稻之间的遗传距离大。根据聚类图发现温带粳稻亚群内杂种优势较弱，亚群间即生态群间的杂种优势较强，群间即灿、粳亚种间杂种优势更强。利用光壳稻、爪哇稻选育不同生态群方向的恢复系和不育系，已配组育成了强优势的杂交稻组合。     

利用RFLP标记水稻亲本遗传差异及其在杂种优势中的利用

本研究以 17个杂交水稻亲本、3个新株型株系和 2 4个光壳稻、爪哇稻品种为DNA样品来源 ,通过RFLP(限制性片段长度多态性 ,RestrictionFragmentLengthPolymorphism ,简称RFLP)标记技术 ,研究光壳稻和爪哇稻及其与温带粳稻之间的关系 ,探索RFLP标记水稻亲本遗传差异在杂交稻育种中利用的可能性。研究结果表明 :42个探针共产生 6 9个不同的限制性片段 ,其中 2 0个 (占 47.6 % )探针显示 47个 (占 6 8.1% )多态性片段 ,至少在 2个基因型间存在差异。每个具有多态性片段分别以 1和0记录存在与否。由RFLP数据计算的Nei’s遗传距离 ,创建聚类树状图。聚类分析结果表明 ,籼稻和粳稻容易被分开 ,温带粳稻又容易与光壳稻、爪哇稻分开 ,但光壳稻和爪哇稻混合聚在一起 ,光壳稻与温带粳稻之间的遗传距离要比爪哇稻与温带粳稻之间的遗传距离大。根据聚类图发现温带粳稻亚群内杂种优势较弱 ,亚群间即生态群间的杂种优势较强 ,群间即籼、粳亚种间杂种优势更强。利用光壳稻、爪哇稻选育不同生态群方向的恢复系和不育系 ,已配组育成了强优势的杂交稻组合     

Regulated cleavage of a contact-mediated axon repellent

Contact-mediated axon repulsion by ephrins raises an unresolved question: these cell surface ligands form a high-affinity multivalent complex with their receptors present on axons, yet rather than being bound, axons can be rapidly repelled. We show here that ephrin-A2 forms a stable complex with the metalloprotease Kuzbanian, involving interactions outside the cleavage region and the protease domain. Eph receptor binding triggered ephrin-A2 cleavage in a localized reaction specific to the cognate ligand. A cleavage-inhibiting mutation in ephrin-A2 delayed axon withdrawal. These studies reveal mechanisms for protease recognition and control of cell surface proteins, and, for ephrin-A2, they may provide a means for efficient axon detachment and termination of signaling.