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951.
The relative stabilities of guanine–cytosine (G–C) DNA base pairs are theoretically investigated with a focus on the keto–enol tautomerism as well as on the cis–trans enol isomerism by using both ab initio method and the density functional theory. The G–C pairs of the keto tautomers turn out to be in general more stable than those of the enol tautomers, 9H-guanine pair, 9KK (see the notation below), being the most stable one. The stability of the G–C pairs appears to be affected by various factors including the keto–enol tautomerization, cis–trans enol isomerization, and the steric hindrance between two tautomeric pairs. Although the B3LYP calculations tend to underestimate the binding energies, in addition, it is shown that the binding energy of G–C pairs interacting through hydrogen bonds can be reasonably calculated with the DFT method, contrary to the base pairs with stacked configurations. 相似文献
952.
953.
The contents of twelve minor and trace elements in liver, ascites, and blood of sarcoma-180 tumor-bearing mice were determined by instrumental neutron activation analysis at various tumor growing stages. Biological samples were irradiated by reactor neutrons and subsequently subject to direct analysis using a high-resolution HPGe -spectrometer. A statistically significant difference for the concentration of Na and Cl in all tumor samples relative to those in a normal control group is found. During the tumor growing period, concentration of Mg, K, Fe, Se, Rb, Cu, Zn, and Mn in liver and ascites of tumored mice vary with respect to the tumor size. Each elemental concentration and its biological function in the tumored mice is discussed and possible correlation to humans is analyzed. 相似文献
954.
研究了水溶性钯-膦配合物PdCl2(TPPTS)2催化1-己烯加氢反应,考察了两相催化体系中添加表面活性剂CTAB,膦配体TPPTS,以及溶液pH值等对反应的影响,证明了1-己烯加氢反应中有一诱导期.反应中涉及两种催化活性物种.一种是由PdCl2(TPPTS)2还原生成的低氧化态催化物种,它在一定的CTAB浓度下,可以保持相当的稳定性.另一种是被TPPTS或H2还原形成的钯黑.根据PdCl2和TPPTS反应过程的31PNMR研究,对PdCl2(TPPTS)2分子中Pd被还原和TPPTS被氧化提出了解释 相似文献
955.
Neal Tai-sheng Chung 《高分子科学》2000,(5):459-463
Ladderlike polychloropropylsilsesquioxane (CP-T) was first synthesized by "stepwise coupling polymerization".The molecular weight of CP-T is as high as 1.7×10~5. It is a tough, elastic reactive ladderlike polymer and has good heat-resistant and adhesive properties. CP-T can further react with many nucleophilic reagents to generate new ladderlike reactivepolysilsesquioxanes, which can be used as potential precursors in the synthesis of functional ladderlike polymers. 相似文献
956.
The biosynthesis of hypusine [Nepsilon-(4-amino-2-hydroxybutyl)-lysine] occurs in the eIF-5A precursor protein through two step posttranslational modification involving deoxyhypusine synthase which catalyzes transfer of the butylamine moiety of spermidine to the epsilon-amino group of a designated lysine residue and subsequent hydroxylation of this intermediate. This enzyme is exclusively required for cell viability and growth of yeast (Park, M.H. et al., J. Biol. Chem. 273: 1677-1683, 1998). In an effort to understand structure-function relationship of deoxyhypusine synthase, posttranslational modification(s) of the enzyme by protein kinases were carried out for a possible cellular modulation of this enzyme. And also twelve deletion mutants were constructed, expressed in E. coli system, and enzyme activities were examined. The results showed that deoxyhypusine synthase was phosphorylated by PKC in vitro but not by p56lck and p60c-src. Treatment with PMA specifically increased the relative phosphorylation of the enzyme supporting PKC was involved. Phosphoamino acid analysis of this enzyme revealed that deoxyhypusine synthase is mostly phosphorylated on serine residue and weakly on threonine. Removal of Met1-Glu10 (deltaMet1-Glu10) residues from amino terminal showed no effect on the catalytic activity but further deletion (deltaMet1-Ser20) caused loss of enzyme activity. The enzyme with internal deletion, deltaGln197-Asn212 (residues not present in the human enzyme) was found to be inactive. Removal of 5 residues from carboxyl terminal, deltaLys383-Asn387, retained only slight activity. These results suggested that deoxyhypusine synthase is substrate for PKC dependent phosphorylation and requires most of the polypeptide chains for enzyme activity except the first 15 residues of N-terminal despite of N- and C-terminal residues of the enzyme consist of variable regions. 相似文献
957.
Chien Chung Shen-Ming Liu Cheng-Chang Chan 《Journal of Radioanalytical and Nuclear Chemistry》1992,161(2):495-502
Environmental samples of beach sand taken at northern Taiwan have been monitored using a coincident gamma-ray spectrometer. The concentration of134Cs, under intense interference from natural and other artificial radioactivity, is determined as low as 0.2 Bq/kg dry. The accumulation of trace levels of134Cs originates from the discharge of nuclear power plants nearby. 相似文献
958.
959.
Chang YT Choi J Ding S Prieschl EE Baumruker T Lee JM Chung SK Schultz PG 《Journal of the American Chemical Society》2002,124(9):1856-1857
A facile synthesis of a combinatorial ceramide library and their activities in the NF-kappaB pathway and in apoptosis induction/prevention were demonstrated. A novel NF-kappaB activating molecule was discovered among ceramide containing beta-galactose, and the structural requirements of ceramides for apoptosis induction was elucidated. 相似文献
960.
Krishnan PS Veeramani S Vora RH Chung TS Uchimura S Sugitani H 《Journal of chromatography. A》2002,977(2):207-212
A methodology was developed for the determination of molecular weight aveages of polyamic acid ionic salt (PAS) by size-exclusion chromatography (SEC). Polystyrene standards were used for calibration and THF-DMF 1:1 by volume containing 0.06 M LiBr and 0.06 M H3PO4 was used as the mobile phase. The proposed methodology was found to be reproducible. 相似文献