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881.
Sandra Piras Mario Loriga Antonio Carta Giuseppe Paglietti M. Paola Costi Stefania Ferrari 《Journal of heterocyclic chemistry》2006,43(3):541-548
882.
Najmeddine Abdelmoula Hamadi Khemakhem Mario Maglione 《Journal of solid state chemistry》2006,179(12):4011-4019
The perovskite-type oxides Ba1−xLax(1−y)/2Euxy/2Nax/2TiO3 (0?x?0.5 and xy=0.04) were synthesized and characterized by X-ray diffraction as well as dielectric measurements and Raman spectroscopy. The crystal structure of these ceramics has been determined by the Rietveld refinement powder X-ray diffraction data at room temperature. These compounds crystallize at room temperature in tetragonal space group P4mm for 0?x?0.1 and in the cubic group for 0.2?x?0.5. The phase transition temperature TC (or Tm) decreases as x content increases. The degree of diffuseness of the phase transition is more pronounced for higher x content, implying the existence of a composition-induced diffuse phase transition of the ceramics with x?0.1. The evolution of the Raman spectra was studied as a function of various compositions at room temperature. The polarization state was checked by pyroelectric measurements. 相似文献
883.
Six chain-extended analogues of the naturally occurring glycosidase inhibitor salacinol, with ring-heteroatom variation, were synthesized for structure-activity studies with different glycosidase enzymes. The syntheses involved the reaction of PMB-protected D- and L- seleno-, thio-, and iminoarabinitol with a benzylidene- and isopropylidene-protected 1,3-cyclic sulfate, derived from commercially available D-sorbitol, in 1,1,1,3,3,3-hexafluoro-2-propanol containing potassium carbonate. Deprotection of the products afforded the novel selenonium, sulfonium, and iminium analogues of salacinol containing polyhydroxylated, monosulfated, extended acyclic chains of 6-carbons, differing in stereochemistry at the stereogenic centers and ring-heteroatom constitution. Four of these compounds inhibit recombinant human maltase glucoamylase, one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine, with Ki values in the micromolar range, thus providing lead candidates for the treatment of Type 2 diabetes. 相似文献
884.
Daniel A. Lowy Sulay D. Jhaveri Edward E. Foos Leonard M. Tender Mario G. Ancona Arthur W. Snow 《Electrochemistry communications》2006,8(12):1821-1824
Hydrophilic gold nanoclusters were tethered onto gold electrodes modified with mixed 1-octane thiol/1,9-nonane dithiol monolayers. The heterogeneous electron transfer (ET) kinetics of soluble redox species in the supporting electrolyte were investigated at these electrodes by cyclic voltammetry (CV) in the presence and absence of the ion-pairing anions and . The redox species investigated, [Fe(CN)6]3−/4− and [Co(C12H8N2)3]3+/2+ where oppositely charged. The results presented here reveal that the rate of ET for the negatively charged redox species decreases with decreasing ionic charging time constant of the electrolyte. The opposite trend is observed for the positively charged redox species. 相似文献
885.
Bastida A Hidalgo A Chiara JL Torrado M Corzana F Pérez-Cañadillas JM Groves P Garcia-Junceda E Gonzalez C Jimenez-Barbero J Asensio JL 《Journal of the American Chemical Society》2006,128(1):100-116
The emergence of bacterial resistance to the major classes of antibiotics has become a serious problem over recent years. For aminoglycosides, the major biochemical mechanism for bacterial resistance is the enzymatic modification of the drug. Interestingly, in several cases, the oligosaccharide conformation recognized by the ribosomic RNA and the enzymes responsible for the antibiotic inactivation is remarkably different. This observation suggests a possible structure-based chemical strategy to overcome bacterial resistance; in principle, it should be possible to design a conformationally locked oligosaccharide that still retains antibiotic activity but that is not susceptible to enzymatic inactivation. To explore the scope and limitations of this strategy, we have synthesized several aminoglycoside derivatives locked in the ribosome-bound "bioactive" conformation. The effect of the structural preorganization on RNA binding, together with its influence on the aminoglycoside inactivation by several enzymes involved in bacterial resistance, has been studied. Our results indicate that the conformational constraint has a modest effect on their interaction with ribosomal RNA. In contrast, it may display a large impact on their enzymatic inactivation. Thus, the work presented herein provides a key example of how the conformational differences exhibited by these ligands within the binding pockets of the ribosome and of those enzymes involved in bacterial resistance can, in favorable cases, be exploited for designing new antibiotic derivatives with improved activity in resistant strains. 相似文献
886.
Caillet-Saguy C Delepierre M Lecroisey A Bertini I Piccioli M Turano P 《Journal of the American Chemical Society》2006,128(1):150-158
Hemophore HasA is a 19 kDa iron(III) hemoprotein that participates in the shuttling of heme to a specific membrane receptor. In HasA, heme iron has an original coordination environment with a His/Tyr pair as axial ligands. Recently developed two-dimensional protonless (13)C-detected experiments provide the sequence-specific assignment of all but three protein residues in the close proximity of the paramagnetic center, thus overcoming limitations due to the short relaxation times induced by the presence of the iron(III) center. Mono-dimensional (13)C and (15)N experiments tailored for the detection of paramagnetic signals allow the identification of resonances of the axial ligands. These experiments are used to characterize the conformational features and the electronic structure of the heme iron(III) environment. The good complementarity among (1)H-, (13)C-, and (15)N-detected experiments is highlighted. A thermal high-spin/low-spin equilibrium is observed and is related to a modulation of the strength of the coordination bond between the iron and the Tyr74 axial ligand. The key role of a neighboring residue, His82, for the stability of the axial coordination and its involvement in the heme delivery to the receptor is discussed. 相似文献
887.
Poon DK Schubert M Au J Okon M Withers SG McIntosh LP 《Journal of the American Chemical Society》2006,128(48):15388-15389
We have investigated the lysine side chain amines in the 34 kDa catalytic domain from Cellulomonas fimi beta-(1,4)-glycosidase Cex (or CfXyn10A) using 1H-detected 15N heteronuclear correlation NMR spectroscopy. Signals from the 1Hzeta ( approximately 8 ppm) and 15Nzeta ( approximately 35 ppm) of Lys302 in the unmodified enzyme and Lys47 in a trapped cellobiosyl-enzyme intermediate were detected in a 1H-15N HMQC spectrum (pH 6.5 and 30 degrees C). The amine of Lys302 forms a buried ion pair, and that of Lys47 is hydrogen bonded to the cellobioside. Both lysines are positively charged, as unambiguously demonstrated by the splitting of their 15Nzeta signals into quartets (|1JNH| approximately 75 Hz) in a 1H-15N HSQC spectrum recorded without 1H decoupling during 15N evolution. Qualitative insights into the dynamic properties of these lysines are also provided by the deviations of their quartet intensity ratios from that of approximately 3:1:1:3 expected for a highly mobile amine. On the basis of the observed ratios of approximately 1:1:1:1 for the quartet of Lys302 and approximately 0.5:1:1:0.5 for Lys47, the amine of the latter active site residue is most rigidly positioned. Signals from at least 8 and 10 additional positively charged, mobile amines in Cex were observed at 10 degrees C and pH 6.5 and 5.6, respectively. By using conditions of reduced temperature, slightly acidic pH, and low general base concentrations, as well as water flipback pulses to minimize the effects of hydrogen exchange, 1H-15N correlation experiments provide a sensitive route to directly investigate the charge states and dynamic properties of the N-terminal and side chain amines in proteins and protein complexes. 相似文献
888.
Rajadurai C Schramm F Brink S Fuhr O Ghafari M Kruk R Ruben M 《Inorganic chemistry》2006,45(25):10019-10021
A one-dimensional supramolecular head-to-tail N+ -H...N-type hydrogen-bonded chain of the complex [FeII(L)2H](ClO4)3.MeOH [L = 4'-(4'-pyridyl)-1,2':6'1'-bis(pyrazolyl)pyridine] exhibits a reversible, thermally driven spin transition at 286 K with a hysteresis loop of ca. 2 K. 相似文献
889.
890.
Eckert-Maksić M Vazdar M Barbatti M Lischka H Maksić ZB 《The Journal of chemical physics》2006,125(6):64310
The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol(-1), which is higher than the best earlier theoretical estimate of 4.0 kcal mol(-1). This result is confirmed by including into the reference space the orbitals of the CC sigma bonds beyond the standard pi orbital space. It places the present value into the middle of the range of the measured data (1.6-10 kcal mol(-1)). An adiabatic singlet-triplet energy gap of 7.4 kcal mol(-1) between the transition state (1)B(tg) and the first triplet (3)A(2g) state is obtained. A low barrier height for the CBD automerization and a small DeltaE((3)A(2g),(1)B(1g)) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed. 相似文献