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991.
992.
Kim HG Lee CW Yun S Hong BH Kim YO Kim D Ihm H Lee JW Lee EC Tarakeshwar P Park SM Kim KS 《Organic letters》2002,4(22):3971-3974
[formula: see text] A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone moiety can readily be converted into an aromatic pi-system (hydroquinone) upon reduction, the nanomechanical molecular cyclophane system exhibits a large flapping motion like a molecular flipper from the electrochemical redox process. The conformational changes upon reduction and oxidation are caused by changes of nonbonding interaction forces (devoid of bond formation/breaking) from the edge-to-face to face-to-face aromatic interactions and vice versa, respectively. 相似文献
993.
Kang KR Kim JS Chung SI Park MH Kim YW Lim D Lee SY 《Experimental & molecular medicine》2002,34(6):489-495
Deoxyhypusine synthase catalyzes the first step in the posttranslational synthesis of an unusual amino acid, hypusine, in the eukaryotic translation initiation factor 5A (eIF-5A) precursor protein. We earlier observed that yeast recombinant deoxyhypusine synthase was phosphorylated by protein kinase C (PKC) in vitro (Kang and Chung, 1999) and the phosphorylation rate was synergistically increased to a 3.5-fold following treatment with phosphatidylserine (P.Ser)/diacylglycerol (DAG)/ Ca(2+), suggesting a possible involvement of PKC. We have extended study on the phosphorylation of deoxyhypusine synthase in vivo in different cell lines in order to define its role on the regulation of eIF5A in the cell. Deoxyhypusine synthase was found to be phosphorylated by endogenous kinases in CHO, NIH3T3, and chicken embryonic cells. The highest degree of phosphorylation was found in CHO cells. Moreover, phosphorylation of deoxyhypusine synthase in intact CHO cells was revealed and the expression of phosphorylated deoxyhypusine synthase was significantly diminished by diacyl ethylene glycol (DAEG), a PKC inhibitor, and enhanced by phorbol 12-myristate 13-acetate (PMA) or Ca(2+)/DAG. Endogenous PKC in CHO cell and cell lysate was able to phosphorylate deoxyhypusine synthase and this modification is enhanced by PMA or Ca(2+) plus DAG. Close association of PKC with deoxyhypusine synthase in the CHO cells was evident in the immune coprecipitation and was PMA-, and Ca(2+)/phospholipid dependent. These results suggest that phosphorylation of deoxyhypusine synthase was PKC-dependent cellular event and open a path for possible regulation in the interaction with eIF5A precursor for hypusine synthesis. 相似文献
994.
Novel silicon-based alternating copolymers: synthesis,photophysical properties,and tunable EL colors
Hwan Kyu Kim Kyung Lim Paik Nam Seob Baek Youngil Lee Katsumi Yoshino 《Macromolecular Symposia》2003,192(1):135-150
We synthesized novel silicon-based alternating copolymers for tunable electroluminescent (EL) colors by Heck synthetic method. Their thermal, photophysical and electroluminescent properties were studied. Most of them exhibited a blue-green EL color at the operating voltage of lower than 12 V. Unusually, we observed the white EL color from a EL device based on SiPhPVK. From photophysical studies and the time-resolved PL spectroscopies, it might be attributed to the formation of stabilized excited state in SiPhPVK. Furthermore, in order to reduce the operating voltage of their LED with increasing the electron affinity of the main chain in silicon-based alternating copolymers, we synthesized the silicon-based copolymers containing electron transporting oxadiazole units in main chain. We also studied their photophysical and electroluminescent properties. 相似文献
995.
Huang JH Luci JJ Lee TY Swenson DC Jensen JH Messerle L 《Journal of the American Chemical Society》2003,125(7):1688-1689
Reduction of the organoditantalum allene complex (eta-C5Me4R)2Ta2(mu-X)X3(mu-eta1,eta3-C3H4) (R = Me (Cp*), Et; X = Cl, Br) with sodium amalgam leads to the propynylidene complex (eta-C5Me4R)2Ta2(mu-H)2X2(mu-HCCCH) by a formal double 1,3-C-H activation of the allene ligand. The solid-state molecular structure contains a planar HCCCH ligand bridging, in parallel coordination mode, the two tantalum atoms, with the HCCCH and Ta atoms coplanar. Key structural features are a Ta-Ta distance of 2.8817(7) A, propynylidene C-C-C angle of 153.7(13) degrees , C-C distance of 1.370(8) A, Ta-C(central) distance of 2.194(9) A, and Ta-C(terminal) distance of 1.970(9) A. Molecular orbital calculations on the complex at the RHF/SBK(d) and B3LYP/LanL2dz levels of theory demonstrate that the propynylidene ligand is best viewed formally as an allenediylidene(4-) ligand bonded to two d0 tantalum atoms via two Ta=C(terminal) double bonds and an unusual three-center, two-electron bridge bond involving both tantalum atoms and a lone pair on the planar, tetracoordinate central carbon. There is no net Ta-Ta bonding based on the orbital analysis. 相似文献
996.
Phytochemical investigation of the methanol extract of Euchresta formosana resulted in the isolation of thirty-four compounds. Compounds 1, 3-12, 15, 27, 29 and 32-24 were isolated from this species for the first time. These compounds were identified by spectral analyses and tested for antiplatelet aggregation and anti-HIV activities. Among these compounds, tectorigenin (1), 3',4',5-trihydroxyisoflavone (3), and euchretin F (19) were the most effective antiplatelet aggregation compounds; they inhibited both AA- (arachidonic acid) and collagen-induced platelet aggregation. Meanwhile, flemiphyllin (B), quercetin (13), euchretin M (23), and formosanatin C (26) inhibited HIV replication in H9 lymphocyte cells. 相似文献
997.
A series of 43 stilbene derivatives that showed cytotoxicity against human lung carcinoma (A549) was analyzed using comparative molecular field analysis (CoMFA) for defining the hypothetic pharmacophore model. The polyoxylated stilbenes were found to be active inhibitors of tubulin polymerization. Several cis-stilbenes are structurally similar to combretastatins. However, the trans-stilbenes are assumed to be close to resveratrol found in grapes and have been reported to be potential cancer chemopreventive agents by modulating the initiation, promotion, and progression of the carcinogenic process. With several synthesized compounds that were evaluated for antitumor cytotoxicity against human lung tumor cells (A549), the stilbene derivatives were subjected to CoMFA. To perform systematic molecular modeling of these compounds, a conformational search was carried out based on the precise dihedral angle analysis of the lead compound (1p). The X-ray crystallographic structure of combretastatin A-1 was also used for defining the active conformers of the compounds. After determining the energy-minimized conformers of the lead compound (1p), CoMFA was performed using five different alignments. The three dimensional (3D)-quantitative structure-activity relationship study resulted in reasonable cross-validated, conventional r(2) values equal to 0.640 and 0.958, respectively. 相似文献
998.
Cholesterol-based hydrogen-bonded liquid crystals 总被引:1,自引:0,他引:1
Hydrogen bonding is a powerful tool for assembling molecules and building new liquid crystalline structures. In this study, non-symmetric dimesogens were prepared by intermolecular hydrogen bonding between rationally designed H-bond donor (3-cholesteryloxycarbonylpentanoic acid) and acceptor (4-(pyridine-4-ylmethyleneimino)phenyl 4-alkoxybenzoate) moieties. Their liquid crystalline properties were investigated by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. Cholesteric and smectic phases were observed. As for the covalently linked dimesogens, several types of smectic periodicities occur for these H-bonded cholesteryl compounds depending on the molecular parameters. 相似文献
999.
The vibrationally resolved mass analyzed threshold ionization (MATI) spectra of jet-cooled 7-azaindole have been recorded by ionizing via four different intermediate levels. The adiabatic ionization energy of this molecule is determined to be 65 462±5 cm−1, which is greater than that of indole by 2871 cm−1. The vibrational spectra of 7-azaindole in the S1 and D0 states have been successfully assigned by comparing the measured frequencies with those of indole as well as the predicted values from the ab initio calculations. Detailed analysis on the MATI spectra shows that the structure of the cation is somewhat different from that of this species in the neutral S1 state. 相似文献
1000.
Imide-siloxane block copolymer/silica hybrid membranes: preparation, characterization and gas separation properties 总被引:1,自引:0,他引:1
Ho Bum Park Jang Ki Kim Sang Yong Nam Young Moo Lee 《Journal of membrane science》2003,220(1-2):59-73
Imide-siloxane block copolymer/silica hybrid membranes with covalent bonds were prepared via sol–gel reaction. The structural informations of these hybrid membranes were obtained by using Fourier transform-infrared spectrometry (FT-IR), 29Si nuclear magnetic resonance (29Si NMR), XPS and thermogravimetric analysis (TGA). The gas separation properties of the hybrid membranes were also investigated in terms of organosiloxane (PDMS) or silica content at various temperatures. In the hybrids, the addition of PDMS phase increased the permeabilities of gases such as He, CO2, O2, and N2, indicating that the gas transport occurred mainly through rubbery organic matrix. Meanwhile, the PDMS phase contributed the decreased gas selectivities to nitrogen but the reduction in selectivities was very small in comparison with other siloxane containing polymeric membranes. This might be due to the restriction of chain mobility by the existence of inorganic component such as silica network in the hybrids. Additionally, the increase of silica content in these hybrid membranes considerably retarded the falling-off of gas selectivity at elevated temperature. The increase of silica content in hybrid membranes resulted in well-formed silica networks and hence these inorganic components restricted the plasticization of organic matrix by the thermal segmental motion of organic components, leading to preventing the large decrease of the gas selectivity. 相似文献