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1.
Ramnial T McKenzie I Gorodetsky B Tsang EM Clyburne JA 《Chemical communications (Cambridge, England)》2004,(9):1054-1055
One-electron oxidation of N-heterocyclic carbenes (NHCs) has been carried out using oxidising agents such as tetracyanoethylene (TCNE) and ferrocenium [Cp(2)Fe](+); the formation of carbene radical cations is postulated. 相似文献
2.
The atomistic origin of fatigue failure in micron-sized silicon devices is not fully understood. Two series of density-functional theory calculations on cubic diamond Si explore the effect of surface bond formation on crack healing in systems which exhibit strong surface reconstruction. Both series introduce a separation between Si(100) layers (i.e., the crack) and allow the ions to relax to their minimum-energy configuration. The initial surface ionic positions are either bulk terminated or 2 x 1 reconstructed. A plot of the energy versus the introduced separation reveals that once the surfaces reconstruct, the crack is no longer able to return to the equilibrium configuration. Rather, the healed crack interface contains defects which places the flawed energy minimum at a finite strain of 3% and an increased energy of 1.13 Jm2 relative to the equilibrium configuration. The irreversible plastic deformation supports the mechanism proposed by Kahn et al. [Science 298 1215 (2002)] that invokes mechanically induced subcritical cracking to explain the delayed onset of failure. 相似文献
3.
Bulloch EM Jones MA Parker EJ Osborne AP Stephens E Davies GM Coggins JR Abell C 《Journal of the American Chemical Society》2004,126(32):9912-9913
(6S)-6-Fluoroshikimate has antimicrobial activity. The molecular basis of this effect had not been identified, but there was speculation that (6S)-6-fluoroshikimate is first converted in vivo into 2-fluorochorismate, which then could inhibit 4-amino-4-deoxychorismate synthase (ADCS). 2-Fluorochorismate was prepared from E-fluorophosphoenolpyruvate and erythose-4-phosphate by the sequential reactions of DAHP synthase, dehydroquinate synthase, dehydroquinase, shikimate dehydrogenase, EPSP synthase, and chorismate synthase. Inhibition studies on ADCS showed that it was inhibited rapidly and irreversibly by 2-fluorochorismate. Electrospray mass spectrometry of the inactivated enzyme showed an additional mass of 198 +/- 10 Da. A novel peptide of 1087.6 Da was identified in the HPLC trace for the tryptic digest of 2-fluorochorismate-inactivated ADCS. Sequencing of this peptide by MS/MS showed that the peptide corresponded to residues 272-279 with a modification of 206.1 Da on Lys-274. This observation is particularly exciting in the context of a recent proposal for the catalytic mechanism of ADCS. 相似文献
4.
Proteases are key regulators of many physiological and pathological processes [1,2], and are recognized as important and tractable drug candidates. Consequently, knowledge of protease substrate recognition and specificity promotes identification of biologically relevant substrates, helps elucidating a protease's biological function, and the design of specific inhibitors. Traditional methods for establishing substrate recognition profiles involve the identification of the scissile bond within a given protein substrate by proteomic methods such as Edman degradation. Then, synthetic peptide variants of this sequence can be screened in an iterative fashion to arrive at more optimized substrates. Even though it can be fruitful, this iterative strategy is biased toward the original substrate sequence and it is also tremendously cumbersome. Furthermore, it is not amenable to high throughput analysis. In 1993, Matthew & Wells presented a method for the use of monovalent "substrate phage" libraries for discovering peptide substrates for proteases, in which more than 10(7) potential substrates can be tested concurrently [3]. A library of fusion proteins was constructed containing randomized substrate sequences placed between a binding domain and the gene III coat protein of the filamentous phage, M13, which displays the fusion protein and packages the gene coding for it inside. Each fusion protein was displayed as a single copy on filamentous phagemid particles (substrate phage). This method allows one to rapidly survey the substrate recognition and specificity of individual or closely related members of proteases. Over the past decade, substrate phage screening has shown terrific utility in rapidly determining protease specificity and characterization of substrate recognition profile of proteases. In some cases, the structural insights of the catalytic domain were obtained from comparison of substrate specificity among closely related family of proteases [4-6]. The number of proteases (from various classes) characterized by this approach testifies to its power. Since the initial development of substrate phage library, different versions of the substrate phage cloning vectors have been constructed to further improve the utility of substrate phage display. This review will provide an overview of the construction of substrate phage display libraries, screening of substrate phage libraries, examples of application, summary and future directions. 相似文献
5.
Deleterious gases such as CO and H(2)S can cause degradation of steel by reacting with the metal surface. Here we consider whether alloying the steel surface might be able to inhibit these damaging surface reactions by raising the barriers to molecular dissociation. We employ first-principles density functional theory techniques to investigate the elementary reaction pathways and barriers for CO and H(2)S on FeAl and Fe(3)Si surfaces and compare them with pure Fe surfaces (as a model for steel). We find that H(2)S dissociates on iron surfaces much more easily than CO does. Although FeAl surfaces raise the barriers for H(2)S dissociation, they significantly lower the barriers for CO dissociation. On the other hand, Fe(3)Si surfaces raise the barriers for CO dissociation, but they are as vulnerable as Fe surfaces to H(2)S dissociation. Our findings suggest that alloying iron with Al or Si is unlikely to simultaneously increase its resistance to the initial stages of chemical degradation by CO and H(2)S. 相似文献
6.
The through-shell borane reduction and methyllithium addition to benzaldehyde (1), benzocyclobutenone (2), and benzocyclobutenedione (3) incarcerated inside a hemicarcerand (4) with four tetramethylenedioxy bridges are reported. All guests could be reduced and methylated. Selective monoreduction and monomethylation were observed for 3. In the methyllithium addition to 4[symbol: see text]3, the initially formed lithium alcoholate underwent a Moore rearrangement. The reactivity of the incarcerated guests toward methyllithium increased in the order 1 < 2 < 3 and toward borane in the order 1 < 2 approximately equal 3. Guest reactivity was correlated with the inner-phase location of the reacting carbonyl group in the preferred guest inner-phase orientation. The latter was determined from the X-ray structures of 4[symbol: see text]1, 4[symbol: see text]2, and 4[symbol: see text]3, from molecular mechanical calculations, and from the hemicarcerand-induced upfield shift of the guest proton resonances. In the methyllithium and n-butyllithium addition to 4[symbol: see text]1 and 4[symbol: see text]3 at elevated temperatures, selective cleavage of a host's spanner or tetramethylenedioxy bridge, respectively, was observed. The cleavage of one spanner also took place in the methyllithium addition to the 1-methyl-2-pyrrolidinone hemicarceplex. These scission reactions are initiated by the initially formed lithium alcoholates, which show enhanced basicity and nucleophilicity in the inner phase as compared to the bulk phase. Mechanisms for the host scission reactions are discussed. 相似文献
7.
Lukas AS Bushard PJ Weiss EA Wasielewski MR 《Journal of the American Chemical Society》2003,125(13):3921-3930
The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me(2)N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI(+)-Ph-NI(-), 6ANI(+)-Ph-NI(-), and XAn(+)-6ANI-Ph-NI(-) singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield (3)NI. The radical ion pair distances within the dyads are about 11-12 A, whereas those in the triads are about approximately 16-19 A. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The (3)NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of ln 2J versus r(DA), the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 +/- 0.1. Since both 2J and k(CR), the rate of radical ion pair recombination, are directly proportional to V(2), the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with beta = 0.5 +/- 0.1 A(-)(1). This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions. 相似文献
8.
A question that has interested Dr. J.J. Kirkland is addressed: what is the nature of the silanols that cause tailing to persist at low pH in reversed-phase chromatography? Chromatograms for a cationic dye, 1,1'-didodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI), were studied at varying pH using an Agilent SB-C8 column and 80% ACN/water for six DiI concentrations ranging from 0.9 to 316 microM. The chromatograms showed increased retention and tailing from pH 1 to 5, as expected. Simulations of the chromatograms agreed well with experiment for a bi-Langmuir isotherm with weak (C8) and strong (silica) adsorption sites. The simulation parameters revealed that the number of strong adsorption sites decreases by 40% from pH 1 to 5, which indicates that the silanols causing tailing are in the SiOH, not the SiO-, form. This seems paradoxical because tailing increases with increasing pH. The simulation parameters reveal that this increased tailing from pH 1 to 5 owes to doubling of the partition coefficient for DiI to the strong adsorption site, which more than compensates for the decreasing number of sites. We attribute this increased partition coefficient to increased long-range coulombic interactions with the increasingly abundant SiO- groups at higher pH, which boosts DiI's partition coefficient for both the C8 and SiOH sites. The picture thus emerges that for DiI, higher pH causes increased tailing because the SiO- groups exacerbate tailing that actually originates from adsorption to SiOH groups. 相似文献
9.
The stability and structure of non-covalent complexes of various peptides contatining basic amino acid residues (Arg, Lys) with metalloporphyrins were studied in a quadrupole ion trap mass spectrometer. The complexes of heme and three other metalloporphyrins with a variety of basic peptides and model systems were formed via electrospray ionization (ESI) and their stability was probed by energy-variable collision-induced dissociation (CID). A linear dependence for basic peptides and model compounds/metalloporphyrin complexes was observed in the plots of stability versus degrees of freedom and was used to evaluate relative bond strength. These results were then compared with previous data obtained for complexes of metalloporphyrins with His-containing peptides and peptides containing no basic amino acids. The binding strengths of Lys-containing peptide complexes in the gas phase was found to be almost as strong as that of Arg-containing complexes. Both systems showed stronger binding than His- containing peptides studied previously. To probe the structure of Arg and Lys non-covalent complexes (charge solvation versus salt bridges), two techniques, CID and ionmolecule reactions, were used. CID experiments indicate that the gas-phase complexes are most likely formed by charge solvation of the central metal ion in the metalloporphyrin by basic side chains of Arg or Lys. Results from the ionmolecule reaction studies are consistent with the charge solvation structure as well. 相似文献
10.
Switching between three different adsorbed states was achieved using potential control to separately induce electrochemical behaviour associated with changes in binding and orientation of the thiophene and pyridine moieties of 2-(2'-thienyl)pyridine adsorbed to Au(111) in neutral and basic solution. 相似文献