共查询到20条相似文献，搜索用时 15 毫秒
1.
Despite long study, a molecular picture of the mechanism of water reorientation is still lacking. Using numerical simulations, we find support for a pathway in which the rotating water molecule breaks a hydrogen bond (H-bond) with an overcoordinated first-shell neighbor to form an H-bond with an undercoordinated second-shell neighbor. The H-bond cleavage and the molecular reorientation occur concertedly and not successively as usually considered. This water reorientation mechanism involves large-amplitude angular jumps, rather than the commonly accepted sequence of small diffusive steps, and therefore calls for reinterpretation of many experimental data wherein water rotational relaxation is assumed to be diffusive. 相似文献
2.
We report the incrementally staged design, synthesis, characterization, and operation of a molecular machine that behaves like a nanoscale elevator. The operation of this device, which is made of a platformlike component interlocked with a trifurcated riglike component and is only 3.5 nanometers by 2.5 nanometers in size, relies on the integration of several structural and functional molecular subunits. This molecular elevator is considerably more complex and better organized than previously reported artificial molecular machines. It exhibits a clear-cut on-off reversible behavior, and it could develop forces up to around 200 piconewtons. 相似文献
3.
Newkome GR Wang P Moorefield CN Cho TJ Mohapatra PP Li S Hwang SH Lukoyanova O Echegoyen L Palagallo JA Iancu V Hla SW 《Science (New York, N.Y.)》2006,312(5781):1782-1785
Mathematics and art converge in the fractal forms that also abound in nature. We used molecular self-assembly to create a synthetic, nanometer-scale, Sierpinski hexagonal gasket. This nondendritic, perfectly self-similar fractal macromolecule is composed of bis-terpyridine building blocks that are bound together by coordination to 36 Ru and 6 Fe ions to form a nearly planar array of increasingly larger hexagons around a hollow center. 相似文献
4.
Ramirez AP Haddon RC Zhou O Fleming RM Zhang J McClure SM Smalley RE 《Science (New York, N.Y.)》1994,265(5168):84-86
Elemental carbon can be synthesized in a variety of geometrical forms, from three-dimensional extended structures (diamond) to finite molecules (C(60) fullerite). Results are presented here on the magnetic susceptibility of the least well-understood members of this family, nanotubes and C(60) fullerite. (i) Nanotubes represent the cylindrical form of carbon, intermediate between graphite and fullerite. They are found to have significantly larger orientation-averaged susceptibility, on a per carbon basis, than any other form of elemental carbon. This susceptibility implies an average band structure among nanotubes similar to that of graphite. (ii) High-resolution magnetic susceptibility data on C(60) fullerite near the molecular orientational-ordering transition at 259 K show a sharp jump corresponding to 2.5 centimeter-gram-second parts per million per mole of C(60). This jump directly demonstrates the effect of an intermolecular cooperative transition on an intramolecular electronic property, where the susceptibility jump may be ascribed to a change in the shape of the molecule due to lattice forces. 相似文献
5.
DEUTSCH HF 《Science (New York, N.Y.)》1963,141(3579):435-436
A basic protein with a molecular weight near 17,000, related serologically to the normal serum gamma(2)-globulins, has been isolated in crystalline form from the urine of a patient with multiple myeloma. Proteins of this size along with the Bence-Jones molecules whose inolecular weight is about 35,000 can provide subunits for studies of the structure of the serum gamma-globulins. 相似文献
6.
Gagnon E Ranitovic P Tong XM Cocke CL Murnane MM Kapteyn HC Sandhu AS 《Science (New York, N.Y.)》2007,317(5843):1374-1378
The direct observation of molecular dynamics initiated by x-rays has been hindered to date by the lack of bright femtosecond sources of short-wavelength light. We used soft x-ray beams generated by high-harmonic upconversion of a femtosecond laser to photoionize a nitrogen molecule, creating highly excited molecular cations. A strong infrared pulse was then used to probe the ultrafast electronic and nuclear dynamics as the molecule exploded. We found that substantial fragmentation occurs through an electron-shakeup process, in which a second electron is simultaneously excited during the soft x-ray photoionization process. During fragmentation, the molecular potential seen by the electron changes rapidly from nearly spherically symmetric to a two-center molecular potential. Our approach can capture in real time and with angstrom resolution the influence of ionizing radiation on a range of molecular systems, probing dynamics that are inaccessible with the use of other techniques. 相似文献
7.
Computer-aided molecular design 总被引：6，自引：0，他引：6
J A McCammon 《Science (New York, N.Y.)》1987,238(4826):486-491
Theoretical chemistry, as implemented on fast computers, is beginning to yield accurate predictions of the thermodynamic and kinetic properties of large molecular assemblies. In addition to providing detailed insights into the origins of molecular activity, theoretical calculations can be used to design new molecules with specific properties. This article describes two types of calculations that show special promise as design tools, the thermodynamic cycle-perturbation method and the Brownian reactive dynamics method. These methods can be applied to calculate equilibrium and rate constants that describe many aspects of molecular recognition, stability, and reactivity. 相似文献
8.
The molecular basis of neuronal excitability 总被引：25，自引：0，他引：25
W A Catterall 《Science (New York, N.Y.)》1984,223(4637):653-661
Neurons process and transmit information in the form of electrical signals. Their electrical excitability is due to the presence of voltage-sensitive ion channels in the neuronal plasma membrane. In recent years, the voltage-sensitive sodium channel of mammalian brain has become the first of these important neuronal components to be studied at the molecular level. This article describes the distribution of sodium channels among the functional compartments of the neuron and reviews work leading to the identification, purification, and characterization of this membrane glycoprotein. 相似文献
9.
植物的每个细胞都包含着该物种的全部遗传信息,具备发育成完整植株的遗传能力,这被称为植物细胞的全能性。体细胞胚胎(体胚)发生是指在没有受精的情况下,由体细胞或营养细胞发育成胚胎,是诱导植物细胞全能性的一种形式。体胚发生在种质资源保存、种苗生产、分子育种和植物基础研究等方面都有着广泛的应用,已成为重要的植物生物技术工具和研究平台。多年来的分子遗传学研究表明:体胚发生受到由众多转录因子、激素信号途径及表观遗传修饰等构成的复杂网络的调控。本研究概述了植物体胚发生的途径,并重点综述了体胚发生关键基因的功能与调控机制、体胚发生的表观遗传修饰以及体胚发生关键基因在基因工程中的应用。随着研究的深入和新技术的出现,体胚发生过程中涉及的代谢组分动态变化、转录调控、激素信号转导与表观遗传调控等复杂生物学过程有望得到更深入地阐释,将更进一步地解析植物体胚发生的分子调控机制。此外,利用体胚发生关键基因的功能与调控机制,开发更高效的体胚诱导和遗传转化方法,有望为更多植物的基因功能研究和遗传改良提供新的思路和技术。参81 相似文献
10.
The molecular structure of a DNA-triostin A complex 总被引：31，自引：0，他引：31
A H Wang G Ughetto G J Quigley T Hakoshima G A van der Marel J H van Boom A Rich 《Science (New York, N.Y.)》1984,225(4667):1115-1121
The molecular structure of triostin A, a cyclic octadepsipeptide antibiotic, has been solved complexed to a DNA double helical fragment with the sequence CGTACG (C, cytosine; G, guanine; T, thymine; A, adenine). The two planar quinoxaline rings of triostin A bis intercalate on the minor groove of the DNA double helix surrounding the CG base pairs at either end. The alanine residues form hydrogen bonds to the guanines. Base stacking in the DNA is perturbed, and the major binding interaction involves a large number of van der Waals contacts between the peptides and the nucleic acid. The adenine residues in the center are in the syn conformation and are paired to thymine through Hoogsteen base pairing. 相似文献
11.
Chen LX Jäger WJ Jennings G Gosztola DJ Munkholm A Hessler JP 《Science (New York, N.Y.)》2001,292(5515):262-264
The determination of the structure of transient molecules, such as photoexcited states, in disordered media (such as in solution) usually requires methods with high temporal resolution. The transient molecular structure of a reaction intermediate produced by photoexcitation of NiTPP-L2 (NiTPP, nickeltetraphenylporphyrin; L, piperidine) in solution was determined by x-ray absorption fine structure (XAFS) data obtained on a 14-nanosecond time scale from a third-generation synchrotron source. The XAFS measurements confirm that photoexcitation leads to the rapid removal of both axial ligands to produce a transient square-planar intermediate, NiTPP, with a lifetime of 28 nanoseconds. The transient structure of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same structure as the ground state in a noncoordinating solvent before it recombines with two ligands to form the more stable octahedrally coordinated NiTPP-L2. 相似文献
12.
Kong J Franklin NR Zhou C Chapline MG Peng S Cho K Dai H 《Science (New York, N.Y.)》2000,287(5453):622-625
Chemical sensors based on individual single-walled carbon nanotubes (SWNTs) are demonstrated. Upon exposure to gaseous molecules such as NO(2) or NH(3), the electrical resistance of a semiconducting SWNT is found to dramatically increase or decrease. This serves as the basis for nanotube molecular sensors. The nanotube sensors exhibit a fast response and a substantially higher sensitivity than that of existing solid-state sensors at room temperature. Sensor reversibility is achieved by slow recovery under ambient conditions or by heating to high temperatures. The interactions between molecular species and SWNTs and the mechanisms of molecular sensing with nanotube molecular wires are investigated. 相似文献
13.
Model studies in molecular recognition 总被引：5，自引：0，他引：5
J Rebek 《Science (New York, N.Y.)》1987,235(4795):1478-1484
Recognition at the molecular level is a fundamental characteristic of biochemical systems. Recent models developed in bioorganic chemistry have revealed the importance of complementarity in size, shape, and functional groups in molecular recognition. Structures that feature a cleft are particularly effective in regard to complementarity since functional groups attached to the interior of the cleft converge on substrates held inside. The molecular clefts offer the advantage of efficient construction; their surfaces can be tailored for specific applications. This article describes their use for recognition of acids, bases, amino acids, metal ions, and neutral substrates. Their ability to provide microenvironments complementary to asymmetric molecules and their future promise are discussed. 相似文献
14.
15.
de Jong JJ Lucas LN Kellogg RM van Esch JH Feringa BL 《Science (New York, N.Y.)》2004,304(5668):278-281
In nature, key molecular processes such as communication, replication, and enzyme catalysis all rely on a delicate balance between molecular and supramolecular chirality. Here we report the design, synthesis, and operation of a reversible, photoresponsive, self-assembling molecular system in which molecular and supramolecular chirality communicate. It shows exceptional stereoselectivity upon aggregation of the molecules during gel formation with the solvent. This chirality is locked by photochemical switching, a process that is subsequently used to induce an inverted chiral supramolecular assembly as revealed by circular dichroism spectroscopy. The optical switching between different chiral aggregated states and the interplay of molecular and supramolecular chirality offer attractive new prospects for the development of molecular memory systems and smart functional materials. 相似文献
16.
Gust D Moore TA Moore AL Lee SJ Bittersmann E Luttrull DK Rehms AA Degraziano JM Ma XC Gao F Belford RE Trier TT 《Science (New York, N.Y.)》1990,248(4952):199-201
A synthetic five-part molecular device has been prepared that uses a multistep electron transfer strategy similar to that of photosynthetic organisms to capture light energy and convert it to chemical potential in the form of long-lived charge separation. It consists of two covalently linked porphyrin moieties, one containing a zinc ion (P(Zn)) and the other present as the free base (P). The metailated porphyrin bears a carotenoid polyene (C) and the other a diquinone species (Q(A)-Q(B)). Excitation of the free-base porphyrin in a chloroform solution of the pentad yields an initial charge-separated state, C-P(Zn)-P(.+).-Q(A)(-)-Q(B), with a quantum yield of 0.85. Subsequent electron transfer steps lead to a final charge-separated state, C(.+)-P(Zn)-P-Q(A)-Q(B)(.-), which is formed with an overall quantum yield of 0.83 and has a lifetime of 55 microseconds. Irradiation of the free-base form of the pentad, C-P-P-Q(A)-Q(B), gives a similar charge-separated state with a lower quantum yield (0.15 in dichloromethane), although the lifetime is increased to approximately 340 microseconds. The artificial photosynthetic system preserves a significant fraction ( approximately 1.0 electron volt) of the initial excitation energy (1.9 electron volts) in the long-lived, charge-separated state. 相似文献
17.
三聚氰胺改性脲醛树脂,是业界普遍采用的脲醛树脂改性方法之一。三聚氰胺的加入可以有效改善脲醛树脂的耐水、耐候性,降低游离甲醛含量等。然而,三聚氰胺能否有效参与到脲醛树脂反应体系当中,形成有效的共缩聚成分,对树脂的应用性能更为关键。基于此,文中设置了在终反应摩尔比以及第1阶段摩尔比保持不变的条件下,通过改变反应原料用量,即三聚氰胺与尿素的用量,合成了一系列三聚氰胺-尿素-甲醛(MUF)共缩聚树脂,利用电喷雾电离质谱仪(ESI-MS)对合成的不同MUF共缩聚树脂的分子量分布情况进行了表征。比较不同样品所测图谱,结果表明,在合成反应过程中,通过改变三聚氰胺(M)与尿素(U)的用量对MUF共缩聚树脂分子量的形成、分布及共缩聚成分有较大影响,当M用量占M与U总质量比例增加至40%时,MUF共缩聚树脂分子量的分布发生了质的改变,同时,共缩聚成分的比例也有明显增加。因此,可以认为,在该试验条件下,增加M的用量可以有效促进共缩聚反应的进程,同时,真正意义上的共缩聚树脂的形成,要求M的加入量至少应控制在40%。 相似文献
18.
叶序（Phyllotaxis）是指植物叶片在茎或枝条上坐落有序的排列方式。早期对叶序发育的研究工作主要集中在叶序形态学观察上,随着分子生物学的不断发展,科学家们开始揭示了这一过程背后的分子机制。研究表明,叶序发育受到多个基因的精细调控,同时,生长素和细胞分裂素等激素在叶序发育中起着重要的作用。大量的分子组分、物理因素以及复杂的调控网络共同决定了植物叶序的模式。对近年来国内外有关叶序模式调控的分子机制研究进展进行综述。 相似文献
19.
Carilli CL Lewis GF Djorgovski SG Mahabal A Cox P Bertoldi F Omont A 《Science (New York, N.Y.)》2003,300(5620):773-775
Images of the molecular CO 2-1 line emission and the radio continuum emission from the redshift 4.12 gravitationally lensed quasi-stellar object (QSO) PSS J2322+1944 reveal an Einstein ring with a diameter of 1.5". These observations are modeled as a star-forming disk surrounding the QSO nucleus with a radius of 2 kiloparsecs. The implied massive star formation rate is 900 solar masses per year. At this rate, a substantial fraction of the stars in a large elliptical galaxy could form on a dynamical time scale of 108 years. The observation of active star formation in the host galaxy of a high-redshift QSO supports the hypothesis of coeval formation of supermassive black holes and stars in spheroidal galaxies. 相似文献
20.
竹子生物技术育种研究进展 总被引：13，自引：4，他引：13
竹子组培是竹子育种的一个重要途径。研究人员对20余属70多个竹种开展了组织培养研究，阐明了外植体不同基因型、激素种类和浓度以及不同培养基成分对竹子组培中愈伤组织诱导形成和植株再生以及诱导生根效率的影响，并建立成熟的组织培养技术体系，为竹子育种工作奠定了坚实的基础。利用分子标记技术对竹子遗传分化开展研究是竹子育种研究的一个热点，近年来进展迅速。通过研究初步阐明了竹子的遗传变异，为竹种的育种和分类提供了理论依据。参44 相似文献