Frontiers in surface scattering simulations

Theorists have recently made substantial progress in simulating reactive molecule-metal surface scattering but still face major challenges. The grand challenge is to develop an approach that enables accurate predictive calculations of reactions involving electronically excited states with potential curve crossings. This challenge is all the more daunting because collisions involving molecules heavier than H2 may be accompanied by substantial energy exchange with the surface vibrations (phonons), and because an electronic structure approach that allows molecule-surface interaction energies to be computed with chemical accuracy (1 kilocalorie per mole) is not yet available even for the electronic ground state of molecule-metal surface systems.     

Cell adhesion molecules

It has been proposed that cell-cell recognition occurs by means of local cell surface modulation of a small number of proteins rather than by expression of large numbers of different cell surface markers. Several different cell adhesion molecules (CAM's) have now been found in a number of vertebrate species in different tissues such as liver and striated muscle and even in a single complex structure such as the brain, where different molecules specific for neurons and glia have been identified. The neuron-specific molecule is involved in early embryonic events but also mediates neurite fasciculation, neuromuscular interaction, and orderly layering of neural tissue. It undergoes local surface modulation with loss of sialic acid during development. A failure of this process is closely correlated with connectional disorders in the staggerer mutant of the mouse. The accumulated data on this and other CAM's favor modulation theories rather than strict chemoaffinity theories of cell-cell recognition.     

Laser probing of chemical reaction dynamics

Lasers are used in increasingly sophisticated ways to carry out reactions between molecules in selected vibrational, rotational, and electronic states and to probe the product states of chemical reactions. Such investigations are providing unprecedented insights into chemical reaction dynamics, the study of the detailed motions that molecules undergo in simple chemical reactions. In many cases it is possible to describe the influence that specific types of molecular excitation have on reactive events. Experiments are also being carried out to leam about chemical reactivity as a function of the alignment of reagents. There is increasing excitement concerning the potential of laser methods to interrogate the transition states of molecular reactions.     

Viral RNA polymerases: electron microscopy of reovirus reaction cores

The Kleinschmidt technique has been used to observe reovirus cores that have synthesized messenger RNA. Some individual viral cores probably synthesize all ten messenger RNA molecules in vitro. Each messenger RNA molecule appears to be attached to a different site on the core surface, implying that there are probably a number of different enzymic sites in each core.     

Creating favorable geometries for directing organic photoreactions in alkanethiolate monolayers

The products of photoreactions of conjugated organic molecules may be allowed by selection rules but not observed in solution reactions because of unfavorable reaction geometries. We have used defect sites in self-assembled alkanethiolate monolayers on gold surfaces to direct geometrically unfavorable photochemical reactions between individual organic molecules. High conductivity and stochastic switching of anthracene-terminated phenylethynylthiolates within alkanethiolate monolayers, as well as in situ photochemical transformations, have been observed and distinguished with the scanning tunneling microscope (STM). Ultraviolet light absorbed during imaging increases the apparent heights of excited molecules in STM images, a direct manifestation of probing electronically excited states.     

Mount pinatubo aerosols, chlorofluorocarbons, and ozone depletion

The injection into the stratosphere of large quantities of sulfur during the June 1991 eruption of Mount Pinatubo (Philippines) and the subsequent formation of sulfate aerosol particles have generated a number of perturbations in the atmosphere with potential effects on the Earth's climate. Changes in the solar and infrared radiation budget caused by the eruption should produce a cooling of the troposphere and a warming of the lower stratosphere. These changes could affect atmospheric circulation. In addition, heterogeneous chemical reactions on the surface of sulfate aerosol particles render the ozone molecules more vulnerable to atmospheric chlorine and hence to man-made chlorofluorocarbons.     

Imaging and manipulating molecules on a zeolite surface with an atomic force microscope

The adsorption of neutral molecules and ions on the surfaces of zeolites was observed in real time with an atomic force microscope (AFM). Direct imaging of the surface of the zeolite clinoptilolite was possible by using a diluted tert-butyl ammonium chloride solution as a medium. Images of the crystal in different liquids revealed that molecules could be bound to the surface in different ways; neutral molecules of tert-butanol formed an ordered array, whereas tert-butyl ammonium ions formed clusters. These absorbed molecules were not rearranged by the AFM tip when used in an imaging mode. However, when a sufficiently large force was applied, the tip of the AFM could rearrange the tert-butyl ammonium ions on the zeolite surface. This demonstration of molecular manipulation suggests new applications, including biosensors and lithography.     

Real-time observation of molecular motion on a surface

The laser-induced movement of CO molecules over a platinum surface was followed in real time by means of ultrafast vibrational spectroscopy. Because the CO molecules bound on different surface sites exhibit different C-O stretch vibrational frequencies, the site-to-site hopping, triggered by excitation with a laser pulse, can be determined from subpicosecond changes in the vibrational spectra. The unexpectedly fast motion--characterized by a 500-femtosecond time constant--reveals that a rotational motion of the CO molecules, rather than pure translation, is required for this diffusion process. This conclusion is corroborated by density functional theory calculations.     

Gas-phase inorganic chemistry: monovalent derivatives of calcium and strontium

The chemistry and spectroscopy of monovalent derivatives of calcium and strontium are described. Laser-driven chemical reactions of calcium and strontium vapors with a variety of small molecules have provided many new free radicals. In general, these species are analogous to the stable molecules obtained in traditional inorganic or organometallic chemistry, but some new families of molecules have been discovered.     

Enzymes without borders: mobilizing substrates, delivering products

Many cellular reactions involve both hydrophobic and hydrophilic molecules that reside within the chemically distinct environments defined by the phospholipid-based membranes and the aqueous lumens of cytoplasm and organelles. Enzymes performing this type of reaction are required to access a lipophilic substrate located in the membranes and to catalyze its reaction with a polar, water-soluble compound. Here, we explore the different binding strategies and chemical tricks that enzymes have developed to overcome this problem. These reactions can be catalyzed by integral membrane proteins that channel a hydrophilic molecule into their active site, as well as by water-soluble enzymes that are able to capture a lipophilic substrate from the phospholipid bilayer. Many chemical and biological aspects of this type of enzymology remain to be investigated and will require the integration of protein chemistry with membrane biology.     

Studies of unusual simple molecules by neutralization-reionization mass spectrometry

Reactive or unstable molecules are key intermediates in many important reactions, but can be difficult to prepare for experimental studies. Species with missing (:CH-OH) or extra (H3) substituents can often be formed conveniently in the gas phase by neutralizing a beam of a more stable ionic counterpart (CH = O+H, H3+). Reionization of the neutral after approximately 10(-6) seconds tests its stability, whereas its unimolecular chemistry can be probed by preparing it with different amounts of internal energy. The resulting neutral products are reionized and mass analyzed. Isomers are then characterized by ion dissociation and a third mass-analysis step. Many unusual molecules have been characterized with this technique, which can also be used to probe complex unimolecular chemistry, such as that of cyclobutadiene and ethylene oxide.     

The process of tholin formation in Titan's upper atmosphere

Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH4 and N2). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (approximately 1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (approximately 8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.     

Oligomerization of intervening sequence RNA molecules in the absence of proteins

The intervening sequence RNA excised from the ribosomal RNA precursor of Tetrahymena forms linear and circular oligomers when exposed to a heating-cooling treatment in vitro. The reactions require no protein or external energy source. Oligomerization is different from other self-catalyzed reactions of the intervening sequence RNA in that it involves intermolecular rather than intramolecular recombination, producing RNA molecules that are substantially larger than the original. The observation that RNA molecules can catalyze their own oligomerization has possible implications for the evolution of chromosomes and for the replicative cycle of plant viroids and virus-associated RNA's.     

Surface molecules identify groups of growing axons

Studies on vertebrate and invertebrate species have established that, during development, axons have the ability to choose particular paths over others. The chemical basis of this pathfinding is not clear but biochemical differences between neurons have long been postulated to account for the specificity of neuronal connections. Such subtle molecular differences between different cells in a single tissue are difficult to study with standard biochemical techniques but hybridoma technology has offered a potential solution to this type of problem. This technique has made possible the production of monoclonal antibodies for identifying and characterizing a family of glycoproteins which are expressed on the surface of specific axon bundles during the development of the leech nervous system. The results show that groups of growing axons do indeed carry chemically distinct surface molecules.     

Organic Compounds in Meteorites: They may have formed in the solar nebula, by catalytic reactions of carbon monoxide, hydrogen, and ammonia

Organic compounds in meteorites seem to have formed by catalytic reactions of CO, H2, and NH3 in the solar nebula, at 360 degrees to 400 degrees K and (4 to 10) x 10-6 atm. The onset of these reactions was triggered by the formation of suitable catalysts (magnetite, hydrated silicates) at these temperatures. These reactions may be a source of prebiotic carbon compounds on the inner planets, and interstellar molecules.     

A molecular shift register based on electron transfer

An electronic shift-register memory at the molecular level is described. The memory elements are based on a chain of electron-transfer molecules and the information is shifted by photoinduced electron-transfer reactions. This device integrates designed electronic molecules onto a very large scale integrated (silicon microelectronic) substrate, providing an example of a "molecular electronic device" that could actually be made. The design requirements for such a device and possible synthetic strategies are discussed. Devices along these lines should have lower energy usage and enhanced storage density.     

植物信号分子介导抗病反应的研究进展

植物受到病原菌侵染时会发生一系列防卫反应：通过各种信号反应事件,如抗病信号的产生、传导及互作,最终诱导各种抗病防卫基因的表达和代谢的变化进而产生抗性。在防卫反应过程中,各类信号分子如激素类（水杨酸、茉莉酸、乙烯、脱落酸）,第二信使类(钙离子、活性氧、一氧化氮),低分子肽类(谷胱甘肽),脂类以及糖类发挥着重要作用。最近几年,这些信号分子介导植物抗病反应研究取得重要进展,就此进行了综述,并对今后的研究作出展望。     

SOME REFLECTIONS CONCERNING VALENCE VARIATION AND ATOMIC STRUCTURE

In bringing to a conclusion these few scattered reflections concerning valence variation and atomic structure, may I be permitted to say this: He who is experimentally inclined may henceforth take heart and he need not be quite so timid in interpreting strange chemical reactions on the assumption of possible anomaly in the valence of the reactants. The physicist has no hesitation in speaking of the temporary existence, at the higher temperature employed in spectroscopic work, of such molecules as BaCl, AlO, (HO), CH(+), CH(2), MgF, etc. Is it not possible that under the influence of surface forces somewhat similar valence variation may not be excluded even at lower temperatures? That these strange compounds are thermodynamically unstable may be true. Nevertheless, they may function as the mysterious catalysts, or as initiators of chain reactions.     

选择性雌激素受体调节剂母核结构的芳香烃亲电取代反应

设计了一类新型的氮正离子,探究N-酰基-N-烃氧基氮正离子与不同取代的芳香烃发生分子间的亲电取代反应。探讨了空间效应及电子效应对分子间亲电取代反应的影响,合成了可望作为选择性雌激素受体调节剂新母核的化合物。该类反应的深入研究为其在有机合成和药物合成及修饰方面奠定了理论和试验基础。     

Photochemical Production of Formaldehyde in Earth's Primitive Atmosphere

Formaldehyde could have been produced by photochemical reactions in Earth's primitive atmosphere, at a time when it consisted mainly of molecular nitrogen, water vapor, carbon dioxide, and trace amounts of molecular hydrogen and carbon monoxide. Removal of formaldehyde from the atmosphere by precipitation can provide a source of organic carbon to the oceans at the rate of 10(11) moles per year. Subsequent reactions of formaldehyde in primeval aquatic environments would have implications for the abiotic synthesis of complex organic molecules and the origin of life.