Femtosecond multidimensional imaging of a molecular dissociation

The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.     

A gaseous metal disk around a white dwarf

The destiny of planetary systems through the late evolution of their host stars is very uncertain. We report a metal-rich gas disk around a moderately hot and young white dwarf. A dynamical model of the double-peaked emission lines constrains the outer disk radius to just 1.2 solar radii. The likely origin of the disk is a tidally disrupted asteroid, which has been destabilized from its initial orbit at a distance of more than 1000 solar radii by the interaction with a relatively massive planetesimal object or a planet. The white dwarf mass of 0.77 solar mass implies that planetary systems may form around high-mass stars.     

不是欧氏环的主理想环

介绍了一个不是欧氏环的主理想环的例子.     

Tree ring indices: a circumpolar comparison

A graphic and statistical comparison of major trends in tree ring indices representative of interior Alaska, northern Urals, northern Scandinavia, and Labrador indicates a highly significant correlation for most 50-year intervals between 1650 and the present. This is suggestive of similarities in trends of summer temperature on a circumpolar scale.     

Echo-planar imaging: magnetic resonance imaging in a fraction of a second

Progress has recently been made in implementing magnetic resonance imaging (MRI) techniques that can be used to obtain images in a fraction of a second rather than in minutes. Echo-planar imaging (EPI) uses only one nuclear spin excitation per image and lends itself to a variety of critical medical and scientific applications. Among these are evaluation of cardiac function in real time, mapping of water diffusion and temperature in tissue, mapping of organ blood pool and perfusion, functional imaging of the central nervous system, depiction of blood and cerebrospinal fluid flow dynamics, and movie imaging of the mobile fetus in utero. Through shortened patient examination times, higher patient throughput, and lower cost per MRI examination, EPI may become a powerful tool for early diagnosis of some common and potentially treatable diseases such as ischemic heart disease, stroke, and cancer.     

Haptic illusion: apparent elongation of a disk rotated between the fingers

A disk (coin) turned end over end between thumb and forefinger feels longer to the turning hand. The illusion grows rapidly for 30 seconds but does not become asymptotic within 60 seconds. The illusion increases with coin size and turning rate, and is independent of holding pressure. It appears to involve illusory mechanisms in both hands.     

Myelin Membrane: a molecular abnormality

Myelin was isolated from cerebral white matter from a patient who had died of metachromatic leukodystrophy, and its lipid composition was analyzed. Although the lipid content was nearly normal, the myelin contained a three- to fourfold excess of cerebroside sulfate and a threefold deficiency of cerebroside compared to normal myelin. The deficiency of cerebroside and the excess of cerebroside sulfate may account for defective myelination in this disease.     

From stars to dust: looking into a circumstellar disk through chondritic meteorites

One of the most fundamental questions in planetary science is, How did the solar system form? In this special issue, astronomical observations and theories constraining circumstellar disks, their lifetimes, and the formation of planetary to subplanetary objects are reviewed. At present, it is difficult to observe what is happening within disks and to determine if another disk environment is comparable to the early solar system disk environment (called the protoplanetary disk). Fortunately, we have chondritic meteorites, which provide a record of the processes that operated and materials present within the protoplanetary disk.     

The molecular structure of a DNA-triostin A complex

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.     

Kinetics of delignification: a molecular approach

A treelike model has been proposed for the structure of lignin. Kinetics of delignification are formulated to account for the cleavage of linkages along the linear primary chains and at the cross-links. Experimental data obtained from isothermal delignification are used to verify this theory. Good agreement has been obtained in calculating the delignification curves. Activation energies are found to be 172 kilojoules per mole of cross-links and 132 kilojoules per mole of aryl ether bonds.     

水产动物目标探测与追踪技术及应用研究进展

水产动物目标探测与追踪研究对智慧渔业、精准养殖具有重要的意义,本文全面梳理了基于声学传感器、光学摄像机及声学与光学结合的水产动物数据采集技术,系统阐述了数据集构建、数据预处理、水产动物目标识别、水产动物目标跟踪等技术研究进展及其在海洋渔业领域的应用现状,分析了水产动物目标探测与追踪技术研究及应用中面临的问题和挑战,对未来的研究方向进行了展望,提出了加快水产动物水下目标探测与追踪数据集及开放共享服务平台构建、实现水产动物精准探测与实时追踪关键技术突破、推动基于水产动物精准探测与实时追踪的智能监控设备研发等建议,旨在为加速推动智慧渔业、精准养殖及提升海洋渔业现代化水平提供参考。     

Lunar surface chemistry: a new imaging technique

Detailed chemical maps of the lunar surface have been constructed by applying a new weighted-filter imaging technique to Apollo 15 and Apollo 16 x-ray fluorescence data. The data quality improvement is amply demonstrated by (i) modes in the frequency distribution, representing highland and mare soil suites, which are not evident before data filtering and (ii) numerous examples of chemical variations which are correlated with small-scale (about 15 kilometer) lunar topographic features.     

Hidden thermodynamics of mutant proteins: a molecular dynamics analysis

A molecular dynamics simulation method is used to determine the contributions of individual amino acid residues and solvent molecules to free energy changes in proteins. Its application to the hemoglobin interface mutant Asp G1(99) beta----Ala shows that some of the contributions to the difference in the free energy of cooperativity are as large as 60 kilocalories (kcal) per mole. Since the overall free energy change is only -5.5 kcal/mole (versus the experimental value of -3.4 kcal/mole), essential elements of the thermodynamics are hidden in the measured results. By exposing the individual contributions, the free energy simulation provides new insights into the origin of thermodynamic changes in mutant proteins and demonstrates the role of effects beyond those usually considered in structural analyses.     

X-ray standing waves: a molecular yardstick for biological membranes

Structural information on an atomic scale has been obtained for a Langmuir-Blodgett (LB) trilayer system by means of long-period x-ray standing waves. The LB trilayer of zinc and cadmium arachidate was deposited on a layered synthetic microstructure (LSM) consisting of 200 tungsten/silicon layer pairs with a 25 A period. A 30 A thermally induced inward collapse of the zinc atom layer that was initially located in the LB trilayer at 53 A above the LSM surface has been observed. The mean position and width of the zinc atom layer was determined with a precision of +/- 0.3 A.     

Nanoassembly of a fractal polymer: a molecular "Sierpinski hexagonal gasket"

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.     

Diacetylenic lipid tubules: experimental evidence for a chiral molecular architecture

Molecular self-assembly is of key importance for the rational design of advanced materials. To investigate the causal relation between molecular structure and the consequent self-assembled microstructure, self-assembled tubules of diacetylenic lipids were studied. Circular-dichroism studies give experimental evidence that the formation of tubules is driven by chiral molecular packing, in agreement with recent theories of tubules. On the basis of these results, a molecular mechanism for the formation of tubules is proposed.