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241.
A key contact in the active site of an aminoglycoside phosphotransferase enzyme (APH(3')-IIIa) is a pi-pi stacking interaction between Tyr42 and the adenine ring of bound nucleotides. We investigated the prevalence of similar Tyr-adenine contacts and found that many different protein systems employ Tyr residues in the recognition of the adenine ring. The geometry of these stacking interactions suggests that electrostatics play a role in the attraction between these aromatic systems. Kinetic and calorimetric experiments on wild-type and mutant forms of APH(3')-IIIa yielded further experimental evidence of the importance of electrostatics in the adenine binding region and suggested that the stacking interaction contributes approximately 2 kcal/mol of binding energy. This type of information concerning the forces that govern nucleotide binding in APH(3')-IIIa will facilitate inhibitor design strategies that target the nucleotide binding site of APH-type enzymes. 相似文献
242.
Sculimbrene BR Morgan AJ Miller SJ 《Journal of the American Chemical Society》2002,124(39):11653-11656
Peptide-based catalysts have been found that catalyze the enantiodivergent phosphorylation of a meso myo-inositol-derived triol (1). The sequential screening of random peptide libraries, followed by the evaluation of a focused library, led to the identification of two peptides (2 and 24) that are complementary in producing enantiomeric D-myo-inositol-1-phosphate and D-myo-inositol-3-phosphate derivatives. The catalysts were then used to complete efficient total syntheses of both D-I-1P and D-I-3P in optically pure form. Additional information is gleaned from relative rate experiments that unambiguously show the catalysts to afford enantioselection through rate accelerative pathways with respect to simple achiral alkylimidazole catalysts. Furthermore, solvent effect studies show that the two enantiodivergent catalysts exhibit different tolerances of polar media. The systematic discovery of site-selective catalysts establishes a basis for future studies of chiral catalysts that differentiate unique functional groups in polyfunctional molecules. 相似文献
243.
Adam W García H Diedering M Martí V Olivucci M Palomares E 《Journal of the American Chemical Society》2002,124(41):12192-12199
The photochemical denitrogenation of the cyclopentene-annelated DBH-type azoalkanes 1 has been examined in solution as a function of bridgehead substitution and temperature. For all derivatives, namely, the unsubstituted 1a(H/H), monomethyl 1b(Me/H) dimethyl 1c(Me/Me), monophenyl 1d(Ph/H), and diphenyl 1e(Ph/Ph), the temperature-dependent ratio of syn and anti housanes 2 provides experimental support for a competition between the singlet (high temperature) and triplet (low temperature) reaction channels in the direct photolysis. The syn/anti ratio of the housanes 2 depends on the extent and type of bridgehead substitution; the amount of the anti diastereomer (retention) follows the order Ph > Me > H, and double substitution is more effective than single. This stereochemical memory is interpreted in terms of the mass effect (inertia) of the substituents and steric interaction (size) between the substituents at the bridgehead and the methylene bridge during the deazetation step of the transient diazenyl diradical conformations (1)DZ (exo-ax) and (1)DZ (exo-eq). These conformers are impulsively generated upon decay of the (1)(n,pi)-excited azoalkane, a trajectory assessed through computational work. The new mechanistic feature disclosed by the unprecedented anti stereoselectivity (retention) is the intervention of a puckered 1,3-cyclopentanediyl singlet diradical (1)DR as product bifurcation step, whose conformational relaxation to the planar species (loss of stereochemical memory) is encumbered by bridgehead substitution. 相似文献
244.
Bornheim A Lipeles E Pappas SP Shapiro A Sun WM Weinstein AJ Masek G Paar HP Mahapatra R Morrison RJ Briere RA Chen GP Ferguson T Tatishvili G Vogel H Adam NE Alexander JP Bebek C Berkelman K Blanc F Boisvert V Cassel DG Drell PS Duboscq JE Ecklund KM Ehrlich R Gibbons L Gittelman B Gray SW Hartill DL Heltsley BK Hsu L Jones CD Kandaswamy J Kreinick DL Magerkurth A Mahlke-Krüger H Meyer TO Mistry NB Nordberg E Palmer M Patterson JR Peterson D Pivarski J Riley D Sadoff AJ Schwarthoff H 《Physical review letters》2002,88(23):231803
We report a new measurement of the Cabibbo-Kobayashi-Maskawa parameter Vub made with a sample of 9.7 x 10(6) BB- events collected with the CLEO II detector. Using heavy quark theory, we combine the observed yield of leptons from semileptonic B decay in the end-point momentum interval 2.2-2.6 GeV/c with recent CLEO II data on B-->X(s)gamma to find Vub = (4.08+/-0.34+/-0.44+/-0.16+/-0.24)x10(-3), where the first two uncertainties are experimental and the last two are from theory. 相似文献
245.
We report a highly accurate phase-based technique for measuring arbitrarily long optical distance with subnanometer precision. The method employs a Michelson interferometer with a pair of harmonically related light sources, one cw and the other low coherence. By slightly detuning (~2 nm) the center wavelength of the low-coherence source between scans of the target sample, we can use the phase relationship between the heterodyne signals of the cw and the low-coherence light to measure the separation between reflecting interfaces with subnanometer precision. As this technique is completely free of 2pi ambiguity, an issue that plagues most phase-based techniques, it can be used to measure arbitrarily long optical distances without loss of precision. We demonstrate one application of this technique, the high-precision determination of the differential refractive index. 相似文献
246.
Heise H Sakellariou D Meriles CA Moulé A Pines A 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,156(1):146-151
In a recent publication we presented a method to obtain highly resolved NMR spectra in the presence of an inhomogeneous B(0) field with the help of a matched RF gradient. If RF gradient pulses are combined with "ideal" 90 degrees pulses to form inhomogeneous z rotation pulses, the line broadening caused by the B(0) gradient can be refocused, while the full chemical shift information is maintained. This approach is of potential use for NMR spectroscopy in an inhomogeneous magnetic field produced by an "ex-situ" surface spectrometer. In this contribution, we extend this method toward two-dimensional spectroscopy with high resolution in one or both dimensions. Line narrowing in the indirect dimension can be achieved by two types of nutation echoes, thus leading to depth-sensitive NMR spectra with full chemical shift information. If the nutation echo in the indirect dimension is combined with a stroboscopic acquisition using inhomogeneous z-rotation pulses, highly resolved two-dimensional correlation spectra can be obtained in matched field gradients. Finally, we demonstrate that an INEPT coherence transfer from proton to carbon spins is possible in inhomogeneous B(0) fields. Thus, it is possible to obtain one-dimensional (13)C NMR spectra with increased sensitivity and two-dimensional HETCOR spectra in the presence of B(0) gradients of 0.4 mT/cm. These schemes may be of some value for ex-situ NMR analysis of materials and biological systems. 相似文献
247.
The methyltrioxorhenium(MTO)-catalyzed oxidation of silanes to silanols and the epoxidation of various olefins by aqueous 85% H(2)O(2) proceed in high yields and excellent product selectivities (no disiloxanes, diols) in the presence of the zeolite NaY. The oxidative species is located inside the 12-A supercages. This prevents the bimolecular condensation of the silanol to disiloxane by steric means and the Lewis-acid assisted hydrolysis of the epoxide to the diol. 相似文献
248.
Janusz Gwozdziewicz Krzysztof Kurdyka Adam Parusinski 《Proceedings of the American Mathematical Society》1999,127(4):1057-1064
We prove that every polynomial of degree has at most zeros on the curve . As a consequence we deduce that the existence of a uniform bound for the number of zeros of polynomials of a fixed degree on an analytic curve does not imply that this curve belongs to an o-minimal structure.
249.
With the help of drug design, 8-, 9-, 10- and 11-methylacronycines 24, 25, 26 and 27 have been selected and synthesised to improve the cytostatic potency of acronycine ( 31 ). The condensation of phloroglucinol with 6-methylanthranilic acid gave 7-hydroxy-1,9-dimethyldibenzo[b,j][1,7]phenanthroline-8,14(5H,13H)-dione ( 32 ) as the main product. 相似文献
250.
Mieczysław Jaroniec Adam W. Marczewski 《Monatshefte für Chemie / Chemical Monthly》1984,115(8-9):997-1012
A generalizedLangmuir equation is proposed for describing the monolayer adsorption of single gases on heterogeneous solids. The special cases of this equation are:Langmuir-Freundlich, Tóth andFreundlich type adsorption isotherms. The energy distribution function corresponding to this equation produces all types of simple energy distributions, i.e., symmetrical distribution, decreasing exponential distributions and asymmetrical distributions showing widening in the directions of low and high adsorption energies.
Gasadsorption auf energetisch heterogenen Feststoffen, 1. Mitt.: Eine generalisierteLangmuir-Gleichung und ihre Energieverteilung
Zusammenfassung Es wird eine generalisierteLangmuir-Gleichung zur Beschreibung der einlagigen Adsorption eines einzelnen Gases auf heterogenen Feststoffen beschrieben. Die speziellen Fälle dieser Gleichung sind:Langmuir-Freundlich-, Tóth- undFreundlich-Typ der Adsorptionsisotherme. Die Energieverteilung, die dieser Gleichung entspricht, gibt alle Typen einer einfachen Verteilung wieder, z. B. eine symmetrische Verteilung, eine abfallende exponentielle Verteilung und asymmetrische Verteilungen mit Erweiterung in die Richtung von niedriger oder hoher Adsorptionsenergie.相似文献