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21.
The reaction of Cp(2)ZrCl(2) with 2 equiv of BuLi at -78 degrees C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)C[triple bond]CAr(f) (R(1), Ar(f) = (CH(2))(4)Me, p-C(6)F(4)H; Me, p-C(6)F(4)H; Ph, C(6)F(5)), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar(f) substituents preferentially adopt the 3,4-positions (beta beta) of the zirconacyclopentadiene ring. With Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (alpha beta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alpha alpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp(2)ZrC(4)-2,5-Ph(2)-3,4-(C(6)F(5))(2), which resulted from the coupling of PhC[triple bond]C(C(6)F(5)), was converted to the corresponding thiophene by reaction with S(2)Cl(2), and to an arene by reaction with MeO(2)CC[triple bond]CCO(2)Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF(3)C(6)H(4))C[triple bond]C(p-MeC(6)H(4)) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C(6)F(5))C[triple bond]CCH(2)](2)CH(2) with Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alpha alpha). Zirconocene coupling of the diyne (C(6)F(5))C[triple bond]C-1,4-C(6)H(4)-C[triple bond]C(C(6)F(5)) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.  相似文献   
22.
The insoluble resins synthesized by attaching imidazoles to poly(chloromethylstyrene-co-divinylbenzene) effectively hydrolyzed albumin with half-life as short as 20 min at pH 7 and 25 degrees C. Thus, peptide hydrolysis was accomplished with imidazole in an artificial system for the first time. The imidazole-based artificial proteinases manifested optimum activity at pH 7-8. The proteolytic activity of the imidazole-based artificial proteinases exceeded that of previously reported organic artificial proteinases including catalytic antibodies. High proteolytic activity was observed when imidazole was attached to the resin through the C-2 atom instead of the N atom. The catalytic activity was greatly reduced when the content of imidazole was lowered. This indicates catalytic cooperation of at least two proximal imidazole moieties attached to the resin. Possible mechanisms for the effective protein hydrolysis by the proximal imidazoles are presented.  相似文献   
23.
A highly potent anti-MRSA sesquiterpenoid has been isolated from Ulmus davidiana var. japonica, which has been traditionally used to treat infectious diseases in Korea. This naturally occurring antibiotic was identified as mansonone F (1). This compound has been found to be highly active specifically against MRSA and showed an MIC range of 0.39-3.13 microg/ml which is comparable to that of vancomycin.  相似文献   
24.
A variety of protein isolation and purification techniques for ribonucleoprotein (RNP) complexes were investigated for their compatibility with downstream analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Ribosomal proteins from Escherichia coli 70S ribosomes were obtained using methods such as phenol extraction and precipitation by organic solvents or acids. Under optimal conditions, more than 90% of the expected ribosomal proteins were detected in a single MALDI-MS experiment. The most effective approach combined ribosome denaturation by buffer exchange with acid precipitation of the ribosomal ribonucleic acids. An improved acid precipitation approach, involving the sequential additions of acetic and trifluoroacetic acid, yielded more complete protein coverage while minimizing loss of ion signal from lower molecular weight proteins. With phenol extraction, substantial gains in ion abundance of higher molecular weight proteins are noted, although some of the lower molecular weight proteins were not efficiently extracted. These results illustrate several effective approaches for protein isolation from protein complexes such as RNPs that are MALDI-MS compatible, and these approaches should extend the use of MALDI-MS for proteomics-based analyses of other protein-nucleic acid complexes.  相似文献   
25.
A series of novel tetrahomodioxacalix[4]biscrowns with crown-2, crown-3, crown-4, crown-5, and crown-6 units were synthesized. Conformations of each product are dependent on the base used and their conformation stabilities. All conformations were proven by NMR spectra and/or X-ray crystal structures. The 1,3-alternate homodioxacalix[4]biscrown-4 (4b) shows the best selectivity for K+, whereas the 1,3-alternate homodioxacalix[4]crown-5 (5) does for Cs+. Those selectivities are attributable to electrostatic interaction between the metal ion and the crown ring, as well as a pi-metal complexation. However, the C-1,2-alternate conformation does not take the metal ions regardless of the crown species as a result of steric hindrance from the methylene bridge of an ArCH2Ar unit.  相似文献   
26.
Brain activation has been used to understand brain-level events associated with cognitive tasks or physical tasks. As a quantitative measure for brain activation, we propose entropy in place of signal amplitude and beta value, which are widely used, but sometimes criticized for their limitations and shortcomings as such measures. To investigate the relevance of our proposition, we provided 22 subjects with physical stimuli through elbow extension-flexion motions by using our exoskeleton robot, measured brain activation in terms of entropy, signal amplitude, and beta value; and compared entropy with the other two. The results show that entropy is superior, in that its change appeared in limited, well established, motor areas, while signal amplitude and beta value changes appeared in a widespread fashion, contradicting the modularity theory. Entropy can predict increase in brain activation with task duration, while the other two cannot. When stimuli shifted from the rest state to the task state, entropy exhibited a similar increase as the other two did. Although entropy showed only a part of the phenomenon induced by task strength, it showed superiority by showing a decrease in brain activation that the other two did not show. Moreover, entropy was capable of identifying the physiologically important location.  相似文献   
27.
Gil  Dongkyun  Suh  Boeon  Kim  Cheal 《Journal of fluorescence》2021,31(6):1675-1682
Journal of Fluorescence - We synthesized an original reversible colorimetric chemosensor PDJ...  相似文献   
28.
Abstract

To improve the compatibility of styrene-maleic anhydride copolymer/low density polyethylene (SMA/LDPE) blends, LDPE grafted with 2-hydroxyethyl methacrylate-isophorone diisocyanate (LDPE-g-HI) was prepared and blended with SMA of which anhydride was converted to carboxylic acid (SMAAc). The infrared spectra of LDPE-g-HI established the presence of isocyanate group. In the blend morphology, some adhesions between the two phases and much finer dispersions were observed in the SMAAc/LDPE-g-HI blends, indicating that chemical reactions took place during the melt blending. The lower heat capacity change at the glass transition temperature demonstrated that chemical bonds were produced in the SMAAc/LDPE-g-HI blends. From the results of the rheological test, it was found that strong positive deviation from the mixing rule occurred in viscosity for the SMAAc/LDPE-g-HI blends, concerning with good adhesion and finer dispersions. In the measurement of tensile property, the improved mechanical properties for the SMAAc/LDPE-g-HI blends were shown.  相似文献   
29.
30.

An experimental study was performed to investigate the effect of surface coating on the critical heat flux for downward facing boiling on the outer surface of a hemispherical vessel. Steady-state boiling experiments were conducted in the subscale boundary layer boiling (SBLB) facility using test vessels with metallic microporous coatings to obtain the local boiling curves and the local critical heat flux (CHF) limits. Similar heat transfer performance was observed for microporous aluminum and microporous copper coatings. When compared to the corresponding data without coatings, the boiling curves for the coated vessels were found to shift upward and to the right. This meant that the CHF limit was higher with surface coating and that the minimum film boiling temperatures were located at higher wall superheats. In particular, the microporous coatings were found to enhance the local CHF values appreciably at all angular locations explored in the experiments. Results of the present study showed that the microporous aluminum coating was very durable. Even after many cycles of steady state boiling, the vessel coating remained rather intact, with no apparent changes in color or structure. Although similar heat transfer performance was observed for microporous copper coatings, the latter were found to be much less durable and tended to degrade after several cycles of boiling.  相似文献   
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