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181.
Efforts to develop a quantitative understanding of molecular recognition rely on the additivity of individual intermolecular interactions, and cooperativity represents one of the major potential stumbling blocks. A chemical double-mutant cycle has been used to experimentally measure cooperativity between functional group interactions within a complex framework. The interaction between two aromatic groups varies by 0.2 +/- 0.4 kJ mol(-1) in synthetic H-bonded complexes that differ by 8-13 kJ mol(-1) in overall stability. In these systems, the free energies associated with individual intermolecular interactions can therefore be reliably treated in an additive fashion. The results suggest that alternative explanations should be considered for cooperative phenomena observed in other systems, and a rationale based on the population of partially bound states in flexible molecules is proposed to account for the enthalpic chelate effect and enthalpy-entropy compensation. 相似文献
182.
José A. Rodriguez 《Theoretical chemistry accounts》2002,107(3):117-129
The surface chemistry of oxides is relevant for many technological applications: catalysis, photoelectrolysis, electronic-device
fabrication, prevention of corrosion, sensor development, etc. This article reviews recent theoretical works that deal with
the surface chemistry of oxides. The account begins with a discussion of results for the adsorption of CO and NO on oxides,
systems which have been extensively studied in the literature and constitute an ideal benchmark for testing the quality of
different levels of theory. Then, systematic studies concerned with the behavior of adsorbied alkali metals and sulfur-containing
molecules are presented. Finally, a correlation between the electronic and chemical properties of mixed-metal oxides is analyzed
and basic principles for designing chemically active oxides are introduced. Advances in theoretical methods and computer capabilities
have made possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces.
Still many problems in this area remain as a challenge, and the approximate nature of most theoretical methods makes necessary
a close coupling between theory and experiment. Following this multidisciplinary approach, the importance of band-orbital
interactions for the reactivity of oxide surfaces has become clear. Simple models based on band-orbital mixing can explain
trends found for the interaction of many adsorbates with oxide surfaces. These simple models provide a conceptual framework
for modifying or controlling the chemical activity of pure oxides and for engineering mixed-metal oxides. In this respect,
theoretical calculations can be very useful for predicting the best ways for enhancing the reactivity of oxide systems and
reducing the waste of time, energy and materials characteristic of an empirical design.
Received: 21 June 2001 / Accepted: 8 October 2001 / Published online: 1 February 2002 相似文献
183.
A sensitive LC-MS-MS method for the direct determination and quantification of 15 fluorobenzoic acids (FBAs) was developed. FBAs are used as conservative tracers for hydrological modelling of water flow and in studies of pesticides and other xenobiotic compounds. The use of FBAs is discussed in relation to other tracers (bromide, chloride, uranine). The method covers mono-substituted fluorobenzoic acid, difluorobenzoic acid, trifluorobenzoic acid, and tetrafluorobenzoic acid. The general detection limit in ground water was 1 microg/l using electrospray ionisation and 20 microg/l using atmospheric pressure chemical ionisation. Analysis time was less than 10 min, small sample volumes were needed and no clean-up was required. 相似文献
184.
A highly enantioselective (up to 97.5% ee) and diastereoselective (95:5 dr trans/cis) Cu(I)-catalyzed cyclopropanation of alkenes using phenyliodonium ylide generated in situ from iodosobenzene and methyl nitroacetate is reported. The cyclopropanation took place with high enantioselectivity for a wide range of alkenes, and the reaction was performed at room temperature. 1-Nitrocyclopropyl esters are versatile building blocks to access the corresponding cyclopropane amino esters and aminocyclopropanes in two and three steps, respectively, from commercially available products. 相似文献
185.
del Río JC Gutiérrez A Martínez AT 《Rapid communications in mass spectrometry : RCM》2004,18(11):1181-1185
A series of non-wood plant fibers, namely kenaf, jute, sisal and abaca, have been analyzed upon pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) of the whole material. The pyrolysis products mainly arise from the carbohydrate and lignin moieties of the fibers. The lignin-derived phenols belonged to the p-hydroxyphenylpropanoid (H), guaiacylpropanoid (G) and syringylpropanoid (S) structures, and showed a high S/G ratio of between 2.0 and 5.4, the highest corresponding to kenaf. Among the lignin-derived phenols released, small amounts of sinapyl and coniferyl acetates (in both cis- and trans-forms) were identified for the first time upon Py-GC/MS of lignocellulosic materials. Acetylation of the sinapyl and coniferyl alcohols was at the gamma-position of the side chain. The release of these alcohols derived from intact acetylated lignin units upon pyrolysis seems to indicate that the native lignin in the fibers selected for this study is at least partially acetylated. Sinapyl (and coniferyl) acetates have recently been suggested to be authentic lignin precursors involved in the polymerization of lignin along with the normal sinapyl and coniferyl alcohols. Py-GC/MS will offer a convenient and rapid tool for analyzing naturally acetylated lignins, as well as to screen plant materials for the presence of acetylated units in lignin. 相似文献
186.
Emilio San-Fabián José Pérez-Jordá Federico Moscardó 《Theoretical chemistry accounts》1990,77(3):207-212
Summary The correlation energy of two- and four-isoelectronic series, a representative example for which the existing spin-density functionals fails, is calculated using the Colle and Salvetti method, considering mono- and multideterminantal wave functions. The results are in agreement with experimental data, and show the potentiality of this method when it is applied to wave functions including the most relevant configurational features. Also, results for the ionization energies and electron affinities of first- and second-row atoms are reported. 相似文献
187.
Adamantane oxazolidine derivatives have been prepared. The corresponding mono and biradicals have molecular shapes well suited to the study of the motion anisotropy in solution. 相似文献
188.
Siebert HC Born K André S Frank M Kaltner H von der Lieth CW Heck AJ Jiménez-Barbero J Kopitz J Gabius HJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(2):388-402
The branched pentasaccharide chain of ganglioside GM1 is a prominent cell surface ligand, for example, for cholera toxin or tumor growth-regulatory homodimeric galectins. This activity profile via protein recognition prompted us to examine the binding properties of peptides with this specificity. Our study provides insights into the mechanism of molecular interaction of this thus far unexplored size limit of the protein part. We used three pentadecapeptides in a combined approach of mass spectrometry, NMR spectroscopy and molecular modelling to analyze the ligand binding in solution. Availability of charged and hydrophobic functionalities affected the intramolecular flexibility of the peptides differently. Backfolding led to restrictions in two cases; the flexibility was not reduced significantly by association of the ligand in its energetically privileged conformations. Major contributions to the interaction energy arise from the sialic acid moiety contacting Arg/Lys residues and the N-terminal charge. Considerable involvement of stacking between the monovalent ligand and aromatic rings could not be detected. This carbohydrate binding strategy is similar to how an adenoviral fiber knob targets sialylated glycans. Rational manipulation for an affinity enhancement can now be directed to reduce the flexibility, exploit the potential for stacking and acquire the cross-linking capacity of the natural lectins by peptide attachment to a suitable scaffold. 相似文献
189.
Kaale E Govaerts C Hoogmartens J Schepdael AV 《Rapid communications in mass spectrometry : RCM》2005,19(20):2918-2922
Aminoglycoside antibiotics are widely used to treat serious Gram-negative and Gram-positive bacterial infections. The lack of a UV chromophore presents a problem in the analysis of aminoglycosides. Derivatization with 1,2-phthalic dicarboxaldehyde (OPA) in the presence of a thiol made it possible to introduce a UV chromophoric thioisoindole moiety. A qualitative mass spectrometry study was carried out to confirm the molecular identity of the products formed. The conditions described earlier to derivatize gentamicin and kanamycin yielded products in which all primary amino groups are fully derivatized. On the other hand, with tobramycin and amikacin, there was also formation of incompletely derivatized products that contained one thioisoindole group less than the fully derivatized product. This study has therefore brought an additional insight into the nature of the OPA-aminoglycoside derivatives studied. 相似文献
190.
Daniel R. Coulter Ranty H. Liang Salvador Di Stefano Jovan Moacanin Amitava Gupta 《Chemical physics letters》1982,87(6):594-598
Transient emission studies following pulse radiolysis of solid poly(1-vinyl naphthalene) show existence of excited monomers and two excimers. Quenching experiments indicate that excimers are not formed directly by recombination of ions but probably by trapping of migrating monomeric excitation in performed traps whose density is approximately one in 103. 相似文献