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  • Articles  (114)
  • 2000-2004  (114)
  • Chemistry and Pharmacology  (114)
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  • Articles  (114)
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  • Chemistry and Pharmacology  (114)
  • Physics  (114)
  • 1
    Publication Date: 2004-06-01
    Description: ▪ Abstract  Proton-coupled electron transfer (PCET) reactions involve the concerted transfer of an electron and a proton. Such reactions play an important role in many areas of chemistry and biology. Concerted PCET is thermochemically more favorable than the first step in competing consecutive processes involving stepwise electron transfer (ET) and proton transfer (PT), often by ≥1 eV. PCET reactions of the form [Formula: see text] can be termed hydrogen atom transfer (HAT). Another PCET class involves outersphere electron transfer concerted with deprotonation by another reagent, [Formula: see text]. Many PCET/HAT rate constants are predicted well by the Marcus cross relation. The cross-relation calculation uses rate constants for self-exchange reactions to provide information on intrinsic barriers. Intrinsic barriers for PCET can be comparable to or larger than those for ET. These properties are discussed in light of recent theoretical treatments of PCET.
    Print ISSN: 0066-426X
    Electronic ISSN: 1545-1593
    Topics: Chemistry and Pharmacology , Physics
    Published by Annual Reviews
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  • 2
    Publication Date: 2004-06-01
    Description: ▪ Abstract  The methods of single-molecule fluorescence spectroscopy and microscopy have been recently utilized to explore the mechanism of action of several members of the kinesin and myosin biomolecular motor protein families. Whereas ensemble averaging is removed in single-molecule studies, heterogeneity in the behavior of individual motors can be directly observed, without synchronization. Observation of translocation by individual copies of motor proteins allows analysis of step size, rate, pausing, and other statistical properties of the process. Polarization microscopy as a function of nucleotide state has been particularly useful in revealing new and highly rotationally mobile forms of particular motors. These experiments complement X-ray and biochemical studies and provide a detailed view into the local dynamical behavior of motor proteins.
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    Electronic ISSN: 1545-1593
    Topics: Chemistry and Pharmacology , Physics
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  • 3
    Publication Date: 2004-06-01
    Description: ▪ Abstract  Methods based on self-assembly, self-organization, and forced shape transformations to form synthetic or semisynthetic enclosed lipid bilayer structures with several properties similar to biological nanocompartments are reviewed. The procedures offer unconventional micro- and nanofabrication routes to yield complex soft-matter devices for a variety of applications for example, in physical chemistry and nanotechnology. In particular, we describe novel micromanipulation methods for producing fluid-state lipid bilayer networks of nanotubes and surface-immobilized vesicles with controlled geometry, topology, membrane composition, and interior contents. Mass transport in nanotubes and materials exchange, for example, between conjugated containers, can be controlled by creating a surface tension gradient that gives rise to a moving boundary or by induced shape transformations. The network devices can operate with extremely small volume elements and low mass, to the limit of single molecules and particles at a length scale where a continuum mechanics approximation may break down. Thus, we also describe some concepts of anomalous fluctuation-dominated kinetics and anomalous diffusive behaviours, including hindered transport, as they might become important in studying chemistry and transport phenomena in these confined systems. The networks are suitable for initiating and controlling chemical reactions in confined biomimetic compartments for rationalizing, for example, enzyme behaviors, as well as for applications in nanofluidics, bioanalytical devices, and to construct computational and complex sensor systems with operations building on chemical kinetics, coupled reactions and controlled mass transport.
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    Topics: Chemistry and Pharmacology , Physics
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  • 4
    Publication Date: 2004-06-01
    Description: ▪ Abstract  Recent progress in the development of semiclassical methods to describe quantum effects in molecular dynamics is reviewed. Focusing on rigorous semiclassical methods that are based on the initial-value representation of the semiclassical propagator, we discuss several promising schemes that have been developed in the past few years to extend the applicability of semiclassical approaches to complex molecular systems. In particular, integral-filtering techniques and forward-backward methods are surveyed. Furthermore, recently proposed approaches that allow the semiclassical description of nonadiabatic molecular dynamics are discussed. The potential and efficiency of these methods is illustrated by selected applications.
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    Electronic ISSN: 1545-1593
    Topics: Chemistry and Pharmacology , Physics
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  • 5
    Publication Date: 2004-06-01
    Description: ▪ Abstract  After providing some background material to establish the interest content of this subject, we summarize the many different ways in which water can be prepared in the amorphous state, making clear that there seems to be more than one distinct amorphous state to be considered. We then give some space to structural and spectroscopic characterization of the distinct states, recognizing that whereas there seems to be unambiguously two distinct states, there may be in fact be more, the additional states mimicking the structures of the higher-density crystalline polymorphs. The low-frequency vibrational properties of the amorphous solid states are then examined in some detail because of the gathering evidence that glassy water, while difficult to form directly from the liquid like other glasses, may have some unusual and almost ideal glassy features, manifested by unusually low states of disorder. This notion is pursued in the following section dealing with thermodynamic and relaxational properties, where the uniquely low excess entropy of the vitreous state of water is confirmed by three different estimates. The fact that the most nearly ideal glass known has no properly established glass transition temperature is highlighted, using known dielectric loss data for amorphous solid water (ASW) and relevant molecular glasses. Finally, the polyamorphism of glassy water, and the kinetic aspects of transformation from one form to the other, are reviewed.
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  • 6
    Publication Date: 2004-06-01
    Description: ▪ Abstract  Recent applications of neutron reflectometry to the study of wet interfaces are described. An outline is given of the basic principles that allow the techniques to determine composition and structure in a variety of situations. These are the adsorption of surfactant molecules at air/liquid and solid/liquid interfaces, the shape of the segment-density profiles of different types of polymer, including block copolymers and polyelectrolytes, adsorption in mixed surfactant and polymer/surfactant systems, and interfacial systems of biophysical interest.
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  • 7
    Publication Date: 2004-06-01
    Description: ▪ Abstract  This is the story of a native-born American who came as a postdoc to the country of his parents, Germany. There, by good fortune, he could participate in the revival and the rebuilding of the physical sciences following the ravishments of the Second World War, becoming at the age of 38, the director of a Max-Planck-Institut in Göttingen. Working under nearly ideal conditions, he carried out basic research using molecular beams. Aided by many active, youthfully impulsive, yet perceptive and imaginative, students and experienced knowledgeable guest scientists from many countries, he enjoyed exciting adventures into unknown landscapes in the fields of molecular gas-phase interactions and solid-surface phenomena and, most recently, in the realms of quantum liquids and solids.
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    Topics: Chemistry and Pharmacology , Physics
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  • 8
    Publication Date: 2004-06-01
    Description: ▪ Abstract  Progress in the study of a new class of chemically bound compounds of noble-gas atoms is reviewed. The focus is on rare-gas molecules of the form HNgY, where Ng is a noble-gas atom and Y is an electronegative group, prepared by photolysis of HY in the rare-gas matrix. Other related types of new molecules of noble-gas atoms are discussed as well. Topics discussed in this review include: (a) The nature of bonding and the energetic stability of the compounds. (b) The vibrational spectroscopy of the molecules, and its role in identification of the species. (c) The mechanism and dynamics of photochemical formation of HNgY in the matrix, and the pathways for thermal and infrared (IR)-induced decomposition. Specifically, attention is given to the issue of “direct” formation following photolysis of HY versus “delayed” formation involving H atom diffusion. (d) Molecules of the lighter rare gases Ar, Ne, and He, focusing on the experimentally prepared HArF and on theoretical predictions suggesting the existence of other molecules. (e) The most-recently discovered photochemically induced insertion compounds of Ng into hydrocarbons, such as HXeCCH. (f) Clusters of HNgY with other molecules. The possible existence of neat aggregates and crystals of HNgY. The reviewed state-of-the-art suggests this field is at an early stage of development with major open questions bearing on the surprising properties of the molecules and on the formation mechanisms. These are part of the challenge for the future.
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  • 9
    Publication Date: 2004-06-01
    Description: ▪ Abstract  This review discusses the recent progress that has been made in the application of computer simulations to study crystal nucleation in colloidal systems. We discuss the concept and the numerical methods that allow for a quantitative prediction of crystal-nucleation rates. The computed nucleation rates are predicted from first principles and can be directly compared with experiments. These techniques have been applied to study crystal nucleation in hard-sphere colloids, polydisperse hard-sphere colloids, weakly charged or slightly soft colloids, and hard-sphere colloids that are confined between two-plane hard walls.
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  • 10
    Publication Date: 2004-06-01
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