共查询到20条相似文献，搜索用时 31 毫秒
1.
考虑核子结构中可能存在的五夸克分量qqqqq,在组分夸克模型框架下对核子的自旋结构进行了研究.结果表明,在核子波函数中考虑了qqqqq分量后,核子中奇异夸克对自旋的贡献和极化中子的自旋分布与实验结果符合较好. 相似文献
2.
Recent results with Tokamak experiments provide insights into the problem of magnetic confinement. They demonstrate how to avoid anomalous transport and thus solve the major problems of Tokamak reactors: size, the production of 14-megaelectron volt neutrons, and maintenance. An alternate confinement system, the field-reversed configuration, confines beams of protons and boron-11. For the proton-boron-11 fusion reaction, the fusion products are all charged particles for which direct conversion is feasible and neutron flux is negligible. 相似文献
3.
Rosner G 《Science (New York, N.Y.)》2000,290(5499):2083-2084
Nucleons (protons and neutrons) have been extensively studied over the past 30 or 40 years, but our knowledge of their internal structure remains rather limited, largely because the nucleon's constituents, the quarks, are so tightly bound that the nucleon cannot be taken apart. As Rosner discusses in his Perspective, scattering experiments are providing increasingly detailed information about the spatial distribution of charges, spins, and currents within the nucleon. He highlights the work by Hasty et al., which goes a long way toward determining the role of "strange" quarks, originally thought not to play a role in ordinary matter. 相似文献
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Nuclear collisions can compress nuclear matter to densities achieved within neutron stars and within core-collapse supernovae. These dense states of matter exist momentarily before expanding. We analyzed the flow of matter to extract pressures in excess of 10(34) pascals, the highest recorded under laboratory-controlled conditions. Using these analyses, we rule out strongly repulsive nuclear equations of state from relativistic mean field theory and weakly repulsive equations of state with phase transitions at densities less than three times that of stable nuclei, but not equations of state softened at higher densities because of a transformation to quark matter. 相似文献
6.
Olive KA 《Science (New York, N.Y.)》1991,251(4998):1194-1199
A transition from normal hadronic matter (such as protons and neutrons) to quark-gluon matter is expected at both high temperatures and densities. In physical situations, this transition may occur in heavy ion collisions, the early universe, and in the cores of neutron stars. Astrophysics and cosmology can be greatly affected by such a phase transition. With regard to the early universe, big bang nucleosynthesis, the theory describing the primordial origin of the light elements, can be affected by inhomogeneities produced during the transition. A transition to quark matter in the interior by neutron stars further enhances our uncertainties regarding the equation of state of dense nuclear matter and neutron star properties such as the maximum mass and rotation frequencies. 相似文献
7.
Jiang L Althoff EA Clemente FR Doyle L Röthlisberger D Zanghellini A Gallaher JL Betker JL Tanaka F Barbas CF Hilvert D Houk KN Stoddard BL Baker D 《Science (New York, N.Y.)》2008,319(5868):1387-1391
The creation of enzymes capable of catalyzing any desired chemical reaction is a grand challenge for computational protein design. Using new algorithms that rely on hashing techniques to construct active sites for multistep reactions, we designed retro-aldolases that use four different catalytic motifs to catalyze the breaking of a carbon-carbon bond in a nonnatural substrate. Of the 72 designs that were experimentally characterized, 32, spanning a range of protein folds, had detectable retro-aldolase activity. Designs that used an explicit water molecule to mediate proton shuffling were significantly more successful, with rate accelerations of up to four orders of magnitude and multiple turnovers, than those involving charged side-chain networks. The atomic accuracy of the design process was confirmed by the x-ray crystal structure of active designs embedded in two protein scaffolds, both of which were nearly superimposable on the design model. 相似文献
8.
Yamanouchi K 《Science (New York, N.Y.)》2002,295(5560):1659-1660
High-intensity lasers are opening up a new realm of light-matter interactions. In his Perspective, Yamanouchi reviews recent progress in this field, focusing on two intensity regimes: the Coulombic regime, which mostly deforms molecular structures and causes tunneling ionization, and the relativistic regime, where high-intensity lasers produce x-rays and high-energy particles and may cause nuclear fusion reactions. Efforts are under way to increase laser intensity further for accessing the next frontier. 相似文献
9.
Hasty R Hawthorne-Allen AM Averett T Barkhuff D Beck DH Beise EJ Blake A Breuer H Carr R Covrig S Danagoulian A Dodson G Dow K Farkhondeh M Filippone BW Gao J Herda MC Ito TM Jones CE Korsch W Kramer K Kowalski S Lee P McKeown RD Mueller B Pitt M 《Science (New York, N.Y.)》2000,290(5499):2117-2119
The violation of mirror symmetry in the weak force provides a powerful tool to study the internal structure of the proton. Experimental results have been obtained that address the role of strange quarks in generating nuclear magnetism. The measurement reported here provides an unambiguous constraint on strange quark contributions to the proton's magnetic moment through the electron-proton weak interaction. We also report evidence for the existence of a parity-violating electromagnetic effect known as the anapole moment of the proton. The proton's anapole moment is not yet well understood theoretically, but it could have important implications for precision weak interaction studies in atomic systems such as cesium. 相似文献
10.
Gamma-ray spectral lines have recently been reported coming from the celestial object SS 433, which is known to emit high-speed jets in opposite directions. The proposed identification of the lines as coming from fusion reactions on nitrogen nuclei as part of the carbon-nitrogen-oxygen cycle operating in the jets has now received observational support. Predictions of strengths and widths of additional lines which, if seen, would provide valuable new information about conditions giving rise to the jets are presented. 相似文献
11.
Chimeras of two isoprenoid synthases catalyze all four coupling reactions in isoprenoid biosynthesis
The carbon skeletons of over 55,000 naturally occurring isoprenoid compounds are constructed from four fundamental coupling reactions: chain elongation, cyclopropanation, branching, and cyclobutanation. Enzymes that catalyze chain elongation and cyclopropanation are well studied, whereas those that catalyze branching and cyclobutanation are unknown. We have catalyzed the four reactions with chimeric proteins generated by replacing segments of a chain-elongation enzyme with corresponding sequences from a cyclopropanation enzyme. Stereochemical and mechanistic considerations suggest that the four coupling enzymes could have evolved from a common ancestor through relatively small changes in the catalytic site. 相似文献
12.
H Nam NE Lewis JA Lerman DH Lee RL Chang D Kim BO Palsson 《Science (New York, N.Y.)》2012,337(6098):1101-1104
Enzymes are thought to have evolved highly specific catalytic activities from promiscuous ancestral proteins. By analyzing a genome-scale model of Escherichia coli metabolism, we found that 37% of its enzymes act on a variety of substrates and catalyze 65% of the known metabolic reactions. However, it is not apparent why these generalist enzymes remain. Here, we show that there are marked differences between generalist enzymes and specialist enzymes, known to catalyze a single chemical reaction on one particular substrate in vivo. Specialist enzymes (i) are frequently essential, (ii) maintain higher metabolic flux, and (iii) require more regulation of enzyme activity to control metabolic flux in dynamic environments than do generalist enzymes. Furthermore, these properties are conserved in Archaea and Eukarya. Thus, the metabolic network context and environmental conditions influence enzyme evolution toward high specificity. 相似文献
13.
Antibodies to Asp-Asp-Glu-Asp can inhibit transport of nuclear proteins into the nucleus 总被引：18，自引：0，他引：18
Y Yoneda N Imamoto-Sonobe Y Matsuoka R Iwamoto Y Kiho T Uchida 《Science (New York, N.Y.)》1988,242(4876):275-278
The signal sequence of simian virus 40 (SV40) large T-antigen for translocation into the nucleus is composed of positively charged amino acids Lys-Lys-Lys-Arg-Lys. Rabbit antibodies to a synthetic peptide containing the negatively charged amino acid sequence Asp-Asp-Asp-Glu-Asp were obtained. Indirect immunofluorescence of the antigens recognized by the antibody was punctate at the nuclear rim or the nuclear surface, depending on the plane of focus. The antibody blocked transport of nuclear proteins into the nucleus. The antigens recognized by the antibody were predominantly localized to the nuclear pores. 相似文献
14.
Karol PJ 《Science (New York, N.Y.)》1984,226(4681):1425-1427
Interaction mean free paths are calculated for projectile fragments from relativistic heavy-ion interactions in emulsions and plastic. There are important discrepancies between emulsion results and those from other techniques. "Anomalous" secondaries (anomalons) may prove to be interpretable by conventional nuclear physics: short mean free paths may be due to convoys of collinear light fragments from peripheral interactions (for example, with neutrons) or from low-excitation particle-unbound fragments that decay in flight. 相似文献
15.
Brasefield CJ 《Science (New York, N.Y.)》1959,129(3363):1610
Exhaust gases are electrically charged, usually positively but sometimes negatively. This charge does not depend on the electric potential of the vehicle. Whether the charge is positive or negative seems to depend on whether water droplets or carbon particles predominate in the exhaust. 相似文献
16.
RNA as an RNA polymerase: net elongation of an RNA primer catalyzed by the Tetrahymena ribozyme 总被引：7，自引：0，他引：7
A catalytic RNA (ribozyme) derived from an intervening sequence (IVS) RNA of Tetrahymena thermophila will catalyze an RNA polymerization reaction in which pentacytidylic acid (C5) is extended by the successive addition of mononucleotides derived from a guanylyl-(3',5')-nucleotide (GpN). Cytidines or uridines are added to C5 to generate chain lengths of 10 to 11 nucleotides, with longer products being generated at greatly reduced efficiency. The reaction is analogous to that catalyzed by a replicase with C5 acting as the primer, GpNs as the nucleoside triphosphates, and a sequence in the ribozyme providing a template. The demonstration that an RNA enzyme can catalyze net elongation of an RNA primer supports theories of prebiotic RNA self-replication. 相似文献
17.
Mechanism of the selective catalytic reduction of nitric oxide by ammonia elucidated by in situ on-line fourier transform infrared spectroscopy 总被引：1，自引：0，他引：1
Topsøe NY 《Science (New York, N.Y.)》1994,265(5176):1217-1219
The selective catalytic reduction reaction of nitric oxide bv ammonia over vanadia-titania catalysts is one of the methods of removing NOx pollution. In the present study, it has been possible to identify the reaction mechanism and the nature of the active sites in these catalysts by combining transient or steady-state in situ (Fourier transform infrared spectroscopy) experiments directly with on-line activity studies. The results suggest a catalytic cycle that consists of both acid and redox reactions and involves both surface V-OH (Br?nsted acid sites) and V=O species. A fundamental microkinetic model is proposed, which accounts for the observed industrial kinetics performance. 相似文献
18.
Poxleitner MK Carpenter ML Mancuso JJ Wang CJ Dawson SC Cande WZ 《Science (New York, N.Y.)》2008,319(5869):1530-1533
The diplomonad parasite Giardia intestinalis contains two functionally equivalent nuclei that are inherited independently during mitosis. Although presumed to be asexual, Giardia has low levels of allelic heterozygosity, indicating that the two nuclear genomes may exchange genetic material. Fluorescence in situ hybridization performed with probes to an episomal plasmid suggests that plasmids are transferred between nuclei in the cyst, and transmission electron micrographs demonstrate fusion between cyst nuclei. Green fluorescent protein fusions of giardial homologs of meiosis-specific genes localized to the nuclei of cysts, but not the vegetative trophozoite. These data suggest that the fusion of nuclei, or karyogamy, and subsequently somatic homologous recombination facilitated by the meiosis gene homologs, occur in the giardial cyst. 相似文献
19.
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. 相似文献
20.
Spheroid chromatin units (v bodies) 总被引：118，自引：0，他引：118
Linear arrays of spherical chromatin particles (nu bodies) about 70 angstroms in diameter have been observed in preparations of isolated eukaryotic nuclei swollen in water, centrifuged onto carbon films, and positively or negatively stained. These bodies have been found in isolated rat thymus, rat liver, and chicken erythrocyte nuclei. Favorable views also reveal connecting strands about 15 angstroms wide between adjacent particles. 相似文献