共查询到20条相似文献，搜索用时 375 毫秒
1.
S Subbiah 《Science (New York, N.Y.)》1991,252(5002):128-133
An ab initio approach to the phase problem in macromolecular x-ray crystallography is described. A random gas of hard-sphere point scatterers is allowed to condense under the constraint of the solvent fraction and the restraint of the observed Fourier amplitude data. Two applications to real macromolecular examples are discussed. This method produces an approximate outline of the bulk solvent regions and thus yields a low-resolution picture of the unit cell that can be extended to higher resolutions in special cases, such as through the use of molecular replacement or of noncrystallographic symmetry-based phase extension. 相似文献
2.
Molecular weight determinations of proteins by californium plasma desorption mass spectrometry 总被引：5，自引：0，他引：5
B Sundqvist P Roepstorff J Fohlman A Hedin P H?kansson I Kamensky M Lindberg M Salehpour G S?we 《Science (New York, N.Y.)》1984,226(4675):696-698
The plasma desorption mass spectrometry method is used to determine the molecular weights of larger molecules than before, to determine the molecular weights of proteins and peptides in mixtures, and to monitor protein modification reactions. Proteins up to molecular weight 25,000 can now be studied with a mass spectrometric technique. Protein-peptide mixtures that could not be resolved with conventional techniques were successfully analyzed by this technique. The precision of the method is good enough to permit one to follow the different steps in the conversion of porcine insulin to human insulin. 相似文献
3.
Craighead HG 《Science (New York, N.Y.)》2000,290(5496):1532-1536
Nanoelectromechanical systems are evolving, with new scientific studies and technical applications emerging. Mechanical devices are shrinking in thickness and width to reduce mass, increase resonant frequency, and lower the force constants of these systems. Advances in the field include improvements in fabrication processes and new methods for actuating and detecting motion at the nanoscale. Lithographic approaches are capable of creating freestanding objects in silicon and other materials, with thickness and lateral dimensions down to about 20 nanometers. Similar processes can make channels or pores of comparable dimensions, approaching the molecular scale. This allows access to a new experimental regime and suggests new applications in sensing and molecular interactions. 相似文献
4.
In outer space, high-energy irradiation of cryogenic ice mixtures of abundant water and carbon dioxide is expected to form solid carbonic acid. Experiments and thermodynamic analyses show that crystalline carbonic acid sublimates without decomposition. Free-energy considerations based on highly accurate molecular quantum mechanics, in combination with vapor pressures resulting from experimental sublimation rates, suggest that in the gas phase, a monomer and dimer of carbonic acid are in equilibrium, comparable to that of formic acid. Gaseous carbonic acid could be present in comets, on Mars and outer solar system bodies, in interstellar icy grains, and in Earth's upper atmosphere. 相似文献
5.
Kahlweit M 《Science (New York, N.Y.)》1988,240(4852):617-621
Water and oil can be made completely miscible by adding a sufficient amount of an amphiphilic compound, such as soap or a detergent. For historical reasons, such stable homogeneous solutions are called "microemulsions." In this article the term microemulsion is used in a more restrictive manner; at low concentrations of the amphiphile, mixtures of water, nonpolar solvents, and amphiphiles may separate into three coexisting liquid layers, namely, an aqueous phase, an amphiphile-rich phase, and an oil-rich phase. In the amphiphile-rich phase, which is the microemulsion in the narrower sense, one finds for thermodynamic reasons a maximum of the mutual solubility between water and oil, combined with a minimum of the interfacial tension between the aqueous and the oil-rich phase, properties that are of interest for both theory and application. The present state of art in this rapidly growing field of science is reviewed. 相似文献
6.
Genetic analysis of the human malaria parasite Plasmodium falciparum 总被引：48，自引：0，他引：48
D Walliker I A Quakyi T E Wellems T F McCutchan A Szarfman W T London L M Corcoran T R Burkot R Carter 《Science (New York, N.Y.)》1987,236(4809):1661-1666
Malaria parasites are haploid for most of their life cycle, with zygote formation and meiosis occurring during the mosquito phase of development. The parasites can be analyzed genetically by transmitting mixtures of cloned parasites through mosquitoes to permit cross-fertilization of gametes to occur. A cross was made between two clones of Plasmodium falciparum differing in enzymes, drug sensitivity, antigens, and chromosome patterns. Parasites showing recombination between the parent clone markers were detected at a high frequency. Novel forms of certain chromosomes, detected by pulsed-field gradient gel electrophoresis, were produced readily, showing that extensive rearrangements occur in the parasite genome after cross-fertilization. Since patients are frequently infected with mixtures of genetically distinct parasites, mosquito transmission is likely to provide the principal mechanisms for generating parasites with novel genotypes. 相似文献
7.
Schnur JM 《Science (New York, N.Y.)》1993,262(5140):1669-1676
The use of molecular self-assembly to fabricate microstructures suitable for advanced material development is described. Templating techniques that transform biomolecular self-assemblies into rugged and stable nano- and microstructures are described. By using a lipid-based microcylinder (tubule) as a paradigm, the path followed from research and development to emerging technological applications is detailed. This process includes modification of the lipid molecular structure, the formation and subsequent characterization of cylindrical microstructures, the use of these structures as templates for metallization, and the characterization and assessment of these hollow metal microcylinders for several potential applications. 相似文献
8.
The biological cell is equipped with a variety of molecular machines that perform complex mechanical tasks such as cell division or intracellular transport. One can envision employing these biological motors in artificial environments. We review the progress that has been made in using motor proteins for powering or manipulating nanoscale components. In particular, kinesin and myosin biomotors that move along linear biofilaments have been widely explored as active components. Currently realized applications are merely proof-of-principle demonstrations. Yet, the sheer availability of an entire ready-to-use toolbox of nanosized biological motors is a great opportunity that calls for exploration. 相似文献
9.
We report the discovery of an orientational transition of bent-core molecules in a background anisotropic medium, in this case a smectic liquid crystal made of rod-like molecules. The resulting director is apolar in nature, and the medium can be described as an orthogonal biaxial smectic. The detailed phase diagram of mixtures of the two types of compounds revealed the induction of two liquid crystalline phases that are specific to compounds with bent-core molecules. The chemical nature of the bounding surface had a marked influence on the observed textures. 相似文献
10.
An instrument providing a new, rapid, and accurate method of determining the number and critical radii of condensation nuclei with radii under 200 angstroms is described. Based on the principle of the cloud chamber, the instrument measures transient changes in the attenuation and scattering of a monochromatic light beam by the growing fog droplets. From data obtained the absolute number concentration and radii of condensation nuclei can be calculated. Preliminary studies of aerosol formation in beta-irradiated mixtures of air and sulfur dioxide showed that carbon monoxide and methane inhibit the formation of nuclei; relative rate constants can be deduced. Some applications of this instrument for environmental and basic research are pointed out. 相似文献
11.
Interfacial charge-transfer rate constants have been measured for n-type Si electrodes in contact with a series of viologen-based redox couples in methanol through analyses of the behavior of these junctions with respect to their current density versus potential and differential capacitance versus potential properties. The data allow evaluation of the maximum rate constant (and therefore the electronic coupling) for majority carriers in the solid as well as of the dependence of the rate constant on the driving force for transfer of delocalized electrons from the n-Si semiconducting electrode into the localized molecular redox species in the solution phase. The data are in good agreement with existing models of this interfacial electron transfer process and provide insight into the fundamental kinetic events underlying the use of semiconducting photoelectrodes in applications such as solar energy conversion. 相似文献
12.
Vapor profiles of pesticide mixtures over fallow agricultural fields reveal consistent rapid shifts in the relative proportions of the components in air. These relative concentration changes are correlated with molecular weight and size and reveal a significant molecular contribution to dispersion in turbulent air. Atmospheric studies that rely upon tracer gases, or use calculations that assume the equality of eddy dispersion coefficients, may be in serious error. 相似文献
13.
When two fluid phases are near a critical point, one of them will be excluded from contact with any third phase that happens to be present by a wetting film of the other critical phase. A simple and quite general strategy that may be used to induce a phase transition from complete wetting of the third phase to incomplete wetting is to add a new component to the fluid phases chosen to drive the two phases away from their critical point. This strategy is illustrated for methanol-cyclohexane mixtures. 相似文献
14.
Self-organized supramolecular organic nanostructures have potential applications that include molecular electronics, photonics, and precursors for nanoporous catalysts. Accordingly, understanding how self-assembly is controlled by molecular architecture will enable the design of increasingly complex structures. We report a liquid crystal (LC) phase with a tetragonal three-dimensional unit cell containing 30 globular supramolecular dendrimers, each of which is self-assembled from 12 dendron (tree-like) molecules, for the compounds described here. The present structure is one of the most complex LC phases yet discovered. A model explaining how spatial arrangement of self-assembled dendritic aggregates depends on molecular architecture and temperature is proposed. 相似文献
15.
以大庆外输含蜡原油为实验介质,利用自行研制的高压流变特性测试系统,测试了实验油样在氮气和天然气两种气质加压条件下,油气两相和油气水三相在不同剪切速率、含水率及压力下的粘温特性,探讨了氮气和天然气的溶解度对含蜡原油析蜡点和溶蜡点的影响。结果表明：在氮气和天然气两种气质加压条件下,油气两相和油气水三相混合物的视粘度均随剪切速率的增大而减小,且温度越低剪切稀释性越明显。在相同的压力条件下,原油含水率低于20%时,油样视粘度随着含水率的增大而增大。在低于泡点压力的条件下,当以天然气加压时,油气水三相和油气两相混合物的溶蜡点和析蜡点均随压力升高而降低,而以氮气加压时的变化规律与此相反,说明天然气溶解于原油有利于改善原油的流动性。 相似文献
16.
The self-assembly mechanism of alkanethiol monolayers on the (111) surface of gold was discovered with the use of an ultrahigh-vacuum scanning tunneling microscope. Monolayer formation follows a two-step process that begins with condensation of low-density crystalline islands, characterized by surface-aligned molecular axes, from a lower density lattice-gas phase. At saturation coverage of this phase, the monolayer undergoes a phase transition to a denser phase by realignment of the molecular axes with the surface normal. These studies reveal the important role of molecule-substrate and molecule-molecule interactions in the self-assembly of these technologically important material systems. 相似文献
17.
Circadian clocks are self-sustained biological oscillators that can be entrained by environmental cues. Although this phenomenon has been studied in many organisms, the molecular mechanisms of entrainment remain unclear. Three cyanobacterial proteins and adenosine triphosphate (ATP) are sufficient to generate oscillations in phosphorylation in vitro. We show that changes in illumination that induce a phase shift in cultured cyanobacteria also cause changes in the ratio of ATP to adenosine diphosphate (ADP). When these nucleotide changes are simulated in the in vitro oscillator, they cause phase shifts similar to those observed in vivo. Physiological concentrations of ADP inhibit kinase activity in the oscillator, and a mathematical model constrained by data shows that this effect is sufficient to quantitatively explain entrainment of the cyanobacterial circadian clock. 相似文献
18.
Micelle formation between 5-hydroxytryptamine and adenosine triphosphate in platelet storage organelles 总被引：8，自引：0，他引：8
As judged by analytical ultracentrifugation, 5-hydroxytryptamine and adenosine-5'-triphosphate form micelles in artificial mixtures and also in storage organelles containing 5-hydroxytryptamine of blood platelets of rabbits. Their average apparent molecular weights depend on the concentration and on the molar ratio of the two constituents. The 5-hydroxytryptamine and adenosine triphosphate of these 5-hydroxytryptamine organelles may be stored in vivo together as micelles with apparent molecular weights of several hundred thousands or more. 相似文献
19.
家蚕抗浓核病毒分子标记筛选及分析 总被引：3，自引：0，他引：3
在抗DNV的品种秋丰、高感品种华八35及其近等基因系BC6中,通过495个RAPD随机引物在各个品种的DNA混合物中进行PCR扩增,获得了一个与家蚕抗DNV基因相关的分子标记S366,对这个标记进行了克隆、测序,并且根据序列设计特异引物转换成SCAR标记,在多个敏感性和抗性品种中进行了验证,证明此分子标记真实可靠.通过生物信息学对测序片段进行分析,发现该片段序列与黄嘌呤脱氢酶基因有91%的同源性. 相似文献
20.
Polymer vesicles 总被引：1，自引：0，他引：1
Vesicles are microscopic sacs that enclose a volume with a molecularly thin membrane. The membranes are generally self-directed assemblies of amphiphilic molecules with a dual hydrophilic-hydrophobic character. Biological amphiphiles form vesicles central to cell function and are principally lipids of molecular weight less than 1 kilodalton. Block copolymers that mimic lipid amphiphilicity can also self-assemble into vesicles in dilute solution, but polymer molecular weights can be orders of magnitude greater than those of lipids. Structural features of vesicles, as well as properties including stability, fluidity, and intermembrane dynamics, are greatly influenced by characteristics of the polymers. Future applications of polymer vesicles will rely on exploiting unique property-performance relations, but results to date already underscore the fact that biologically derived vesicles are but a small subset of what is physically and chemically possible. 相似文献