共查询到20条相似文献，搜索用时 78 毫秒
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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. 相似文献
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Brooks PR 《Science (New York, N.Y.)》1976,193(4247):11-16
Beams of oriented molecules have been used to directly study geometrical requirements in chemical reactions. These studies have shown that reactivity is much greater in some orientations than others and demonstrated the existence of steric effects. For some reactions portions of the orientation results are in good accord with traditional views of steric hindrance, but for others it is clear that our chemical intuition needs recalibrating. Indeed, the information gained from simultaneously orienting the reactants and observing the scattering angle of the products may lead to new insights about the detailed mechanism of certain reactions. Further work must be done to extend the scope and detail of the studies described here. More detailed information is needed on the CH(3)I reaction and the CF(3)I reaction. The effects of alkyl groups of various sizes and alkali metals of various sizes are of interest. In addition, reactions where a long-lived complex is formed should be studied to see if orientation is important. Finally, it would be of interest to apply the technique to the sort of reactions that led to our interest in the first place: the S(N)2 displacements in alkyl halides where the fascinating Walden inversion occurs. 相似文献
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Anions of many types, both organic and inorganic, farmiliar and exotic, can be generated in the gas phase by rational chemical synthesis in a flowing afterglow apparatus. Once formed, the rates, products, and mechanisms of their reactions with neutral species of all kinds can be studied, not only at room temperature but at higher energies in a drift field. These completely unsolvated ions undergo a large number of reactions that are analogous to those they undergo in solution, as well as some that are less familiar. New types of ions, for which there are no counterparts in solution, can be produced and their chemical reactions explored. 相似文献
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Leone SR 《Science (New York, N.Y.)》1985,227(4689):889-895
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. 相似文献
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Hase WL 《Science (New York, N.Y.)》1994,266(5187):998-1002
Computer simulations and animations of the motion of atoms as a chemical reaction proceeds give a detailed picture of how the reaction occurs at a microscopic level. This information is particularly useful for testing the accuracy of statistical models, which are used to calculate various attributes of chemical reactions. Such simulations and animations, in concert with experimental and ab initio studies, have begun to provide a microscopic picture of the intimate details of a particular class of gas-phase ion-molecule bimolecular reactions known as S(N)2 nucleophilic substitution. In these reactions, a nucleophile is displaced from a molecule by another nucleophile. The dynamical model of S(N)2 reactions that emerges from the computer studies, and its relation to statistical theories, is discussed here. 相似文献
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Boyce M Bryant KF Jousse C Long K Harding HP Scheuner D Kaufman RJ Ma D Coen DM Ron D Yuan J 《Science (New York, N.Y.)》2005,307(5711):935-939
Most protein phosphatases have little intrinsic substrate specificity, making selective pharmacological inhibition of specific dephosphorylation reactions a challenging problem. In a screen for small molecules that protect cells from endoplasmic reticulum (ER) stress, we identified salubrinal, a selective inhibitor of cellular complexes that dephosphorylate eukaryotic translation initiation factor 2 subunit alpha (eIF2alpha). Salubrinal also blocks eIF2alpha dephosphorylation mediated by a herpes simplex virus protein and inhibits viral replication. These results suggest that selective chemical inhibitors of eIF2alpha dephosphorylation may be useful in diseases involving ER stress or viral infection. More broadly, salubrinal demonstrates the feasibility of selective pharmacological targeting of cellular dephosphorylation events. 相似文献
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Studies of the spectra of cyclopolysilanes have led to several significant conclusions. These molecules possess both easily ionized Si-Si bonding electrons and low energy delocalized antibonding orbitals, and can therefore serve either as electron donors or acceptors. This model explains the similarity in properties between cyclopolysilanes and aromatic hydrocarbons. Further investigations of cyclopolysilanes seem likely to provide the key to understanding of controversial questions of bonding in metalloid compounds even as studies of their carbon analogs, the cyclic and cage hydrocarbons, have been crucial to present knowledge of organic stereochemistry and reaction mechanisms. The reactions of cyclopolysilanes are not only interesting in themselves, but have opened the way to the synthesis of complex polysilanes and thus to whole new areas of study. Improved methods of synthesis and isolation are needed, but the number and kinds of compounds that can be prepared seem almost limitless. Perhaps a polymetal chemistry comparable in breadth and variety to carbon chemistry is now developing. 相似文献
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Water catalysis of a radical-molecule gas-phase reaction 总被引：1，自引：0，他引：1
Vöhringer-Martinez E Hansmann B Hernandez-Soto H Hernandez H Francisco JS Troe J Abel B 《Science (New York, N.Y.)》2007,315(5811):497-501
There has been considerable speculation about the role of water and water complexes in chemical gas-phase reactions, including the conjecture that water may act as a molecular catalyst through its ability to form hydrogen bonds. Here, we present kinetic studies in which the effect of water on the rate of the reaction between hydroxyl radicals and acetaldehyde has been measured directly in Laval nozzle expansions at low temperatures. An increasing enhancement of the reaction rate by added water was found with decreasing temperatures between 300 and 60 kelvin. Quantum chemical calculations and statistical rate theory support our conclusions that this observation is due to the reduction of an intrinsic reaction barrier caused by specific water aggregation. The results suggest that even single water molecules can act as catalysts in radical-molecule reactions. 相似文献
10.
DC Clary 《Science (New York, N.Y.)》1998,279(5358):1879-1882
It is now possible to use rigorous quantum scattering theory to perform accurate calculations on the detailed state-to-state dynamics of chemical reactions in the gas phase. Calculations on simple reactions, such as H + D2 --> HD + D and F + H2 --> HF + H, compete with experiment in their accuracy. Recent advances in theory promise to extend such accurate predictions to more complicated reactions, such as OH + H2 --> H2O + H, and even to reactions of molecules on solid surfaces. New experimental techniques for probing reaction transition states, such as negative-ion photodetachment spectroscopy and pump-probe femtosecond spectroscopy, are stimulating the development of new theories. 相似文献
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Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction. 相似文献
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Veblen DR Banfield JF Guthrie GD Heaney PJ Ilton ES Livi KJ Smelik EA 《Science (New York, N.Y.)》1993,260(5113):1465-1472
Crystal defects and chemical reactions occurring at scales beyond the resolution of light microscopes have major effects on the chemical and physical properties of rocks and minerals. High-resolution imaging, diffraction, and chemical analysis in the transmission electron microscope have become important methods for exploring mineral defect structures and reaction mechanisms and for studying the distribution of phases resulting from reactions. These techniques have shown that structural disorder is common in some rock-forming minerals but rare in others. They have also established mechanisms by which many reactions occur at the atomic cluster scale. These data thus provide an atomistic basis for understanding the kinetics of geological reactions. Furthermore, apparent major-element, minor-element, and trace-element chemistry of minerals can be influenced by submicroscopic inclusions or intergrowths, which commonly form as products of solid-state reactions. 相似文献
14.
The planning of alternative routes for the synthesis of complex organic molecules has been facilitated by the formulation of guiding strategies that can be applied to a broad range of problems. Analysis of organic synthesis can be carried out in the retrosynthetic direction, opposite to the actual process of chemical synthesis, or bidirectionally, that is, as a combined retrosynthetic and synthetic search. An interactive computer program is described which utilizes the general strategies of retrosynthetic analysis and an appropriate database to generate pathways of chemical intermediates for chemical synthesis of a particular target structure. Computer graphics and standard chemical structures are used for man-machine communication. 相似文献
15.
Wörner HJ Bertrand JB Fabre B Higuet J Ruf H Dubrouil A Patchkovskii S Spanner M Mairesse Y Blanchet V Mével E Constant E Corkum PB Villeneuve DM 《Science (New York, N.Y.)》2011,334(6053):208-212
Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection. At longer delays, we observe the onset of statistical dissociation dynamics. The present results demonstrate that high-harmonic spectroscopy could become a powerful tool to highlight electronic dynamics occurring along nonadiabatic chemical reaction pathways. 相似文献
16.
《Science (New York, N.Y.)》1980,209(4452):89-95
Chemical synthesis is the ultimate source of most of the new materials that serve society. Progress in chemical synthesis depends on comparable advances in other branches of chemistry, such as in analysis, methods for determining the intimate details of chemical structures and of reaction mechanisms, and theories of chemical structure and reactions. Recent progress in some of these areas of chemistry is described. 相似文献
17.
Moskun AC Jailaubekov AE Bradforth SE Tao G Stratt RM 《Science (New York, N.Y.)》2006,311(5769):1907-1911
Highly energized molecules normally are rapidly equilibrated by a solvent; this finding is central to the conventional (linear-response) view of how chemical reactions occur in solution. However, when a reaction initiated by 33-femtosecond deep ultraviolet laser pulses is used to eject highly rotationally excited diatomic molecules into alcohols and water, rotational coherence persists for many rotational periods despite the solvent. Molecular dynamics simulations trace this slow development of molecular-scale friction to a clearly identifiable molecular event: an abrupt liquid-structure change triggered by the rapid rotation. This example shows that molecular relaxation can sometimes switch from linear to nonlinear response. 相似文献
18.
Enzymatic modification of transfer RNA 总被引：12，自引：0，他引：12
D S?ll 《Science (New York, N.Y.)》1971,173(994):293-299
The molecular events leading to the synthesis of mature tRNA are only now becoming amenable to experimental study. In bacterial and mammalian cells tRNA genes are transcribed into precursor tRNA. These molecules, when isolated, contain additional nucleotides at both ends (20) of the mature tRNA and lack most modified nucleosides. Presumably, specific nucleases ("trimming" enzymes) cut the precursor to proper tRNA size. The C-C-A nucleotide sequence of the amino acid acceptor end common to all tRNA's does not seem to be coded by tRNA genes (30), and may be added to the trimmed molecules by the tRNA-CMP-AMP-pyrophosphorylase (71). Modifications at the polynucleotide level of the heterocyclic bases or the sugar residues give rise to the modified nucleosides in tRNA. Although newly available substrates have allowed the detection of more of the enzymes involved in these reactions, there is still no knowledge about the sequence of modification or trimming events leading to the synthesis of active tRNA. Progress in these studies may not be easy because enzyme preparations free of nucleases or other tRNA modifying enzymes are required. The role of the modified nucleosides in the biological functions of tRNA is still unknown. Possibly pseudouridine is required for ribosome mediated protein synthesis; some other modified nucleosides in tRNA are not required for this reaction, but may enhance its rate. What might be the role of the large variety of modified nucleosides in tRNA? One is tempted to speculate that such nucleosides are important in other cellular processes in which tRNA is thought to participate such as virus infection, cell differentiation, and hormone action (2, 3). Mutants in a number of tRNA-modifying enzymes are needed in order to extend our knowledge of their purpose and of tRNA involvement in other biological processes. But unless tRNA-modifying enzymes specific for a particular tRNA species exist, no simple selection procedure can be devised. Possibly some of the regulatory mutants of amino acid biosynthesis may prove to affect tRNA-modifying enzymes (72). Transfer RNA's are macromolecules well suited for the study of nucleic acid-protein interactions. The tRNA molecules are structurally very similar, and they interact with a large number of enzymes or protein factors (2, 3). Each aminoacyl-tRNA synthetase, for instance, very precisely recognizes a set of cognate isoacceptor tRNA's (2, 73). The availability of the tRNA- modifying enzymes adds another dimension to the problem of the nature of specific recognition of tRNA by proteins. There are some tRNA-modifying enzymes, such as the uracil-tRNA methylase, which may recognize all tRNA species, while others, such as the isopentenyl-tRNA transferase, probably recognize only a selected set of tRNA molecules, even with different amino acid accepting capacities. With well-characterized RNA precursor and tRNA molecules we can hope to delineate those features of primary, secondary, and tertiary structure involved in the specific interactions of tRNA with these enzymes. 相似文献
19.
Transannular chemical reactions are unparalleled in their ability to generate high degrees of stereochemical and architectural complexity in a single transformation. However, the successful application of this approach in synthesis depends on the ability to predict and control the outcome of the transannular reaction. Use of a chiral catalyst in this context represents an attractive, yet unused, strategy. This report describes a catalytic, asymmetric transannnular Diels-Alder (TADA) reaction that affords polycyclic products in high enantiomeric excess. This catalyst system can also alter the inherent diastereoselectivity of cyclizations with substrates containing chiral centers. Additionally, the catalytic enantioselective TADA has been used as the key step in a total synthesis of the sesquiterpene 11,12-diacetoxydrimane; this route may provide a general approach to the polycyclic carbon framework shared by many terpene natural products. 相似文献