排序方式: 共有82条查询结果,搜索用时 62 毫秒
41.
McNaughton L Hernández G LeMaster DM 《Journal of the American Chemical Society》2003,125(13):3813-3820
Proton spin relaxation induced by the triplet ground state of O(2) in the zinc-containing diamagnetic analogue of sperm whale deoxymyoglobin has been measured as a function of oxygen concentration. As no covalent binding of oxygen to the metal occurs in the zinc species, the relaxation effects of O(2) on the protein (1)H resonances arise exclusively via much weaker noncovalent interactions. The relaxation effects at the amide proton sites are found to be highly localized and are derived almost exclusively from O(2) binding at the four previously identified xenon binding sites. Relative binding constants of 1.0, 0.08, 0.07, and 0.23 were determined for the Xe 1, Xe 2, Xe 3, and Xe 4 sites, respectively. In combination with earlier measurements of the kinetics of the heme binding of oxygen, these equilibria measurements enable a more detailed analysis of models characterizing O(2) entry and egress. A correlation is established between the fraction of O(2) which enters the Fe(2+)-binding site via rotation of the distal histidine side chain (so-called "histidine gate") and the experimentally observable O(2) (or CO) lifetime in the Xe 1 site. A physiological role for these secondary oxygen binding sites is proposed in enhancing the efficiency of the O(2) association reaction by rendering more favorable its competition with water binding in the distal heme pocket. 相似文献
42.
Adriane G. Simanke Raquel S. Mauler Griselda B. Galland 《Journal of polymer science. Part A, Polymer chemistry》2002,40(4):471-485
The effect of the copolymerization temperature and amount of comonomer in the copolymerization of ethylene with 1,3‐cyclopentadiene, dicyclopentadiene, and 4‐vinyl‐1‐cyclohexene and the rac‐Et[Ind]2ZrCl2–methylaluminoxane metallocene system was studied. The amount of comonomer present in the reaction media influenced the catalytic activity. Dicyclopentadiene was the most reactive comonomer among the cyclic dienes studied. In general, copolymers synthesized at 60 °C showed higher catalytic activities. Ethylene–dicyclopentadiene copolymers with high comonomer contents (>9%) did not show melting temperatures. 1,3‐Cyclopentadiene dimerized into dicyclopentadiene during the copolymerization, giving a terpolymer of ethylene, cyclopentadiene, and dicyclopentadiene. A complete characterization of the products was carried out with 1H NMR, 13C NMR, heteronuclear chemical shift correlation, differential scanning calorimetry, and gel permeation chromatography. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 471–485, 2002; DOI 10.1002/pola.10133 相似文献
43.
Menegário AA Smichowski P Tonello PS Polla G Oliveira EP Santelli RE 《Analytica chimica acta》2008,625(2):131-136
l-proline was immobilized on controlled pore glass to study the ability of this material for the separation and preconcentration of Sb(III) and Sb(V). The substrate was packed in a minicolumn and incorporated in a flow injection system. The effluents of the on-line solid phase extraction (before and after elution) were directly coupled to the hydride generation inductively coupled plasma optical emission spectrometry system. The effect of pH, sample (and eluent) volume, flow rates of sample loading and elution on separation of Sb(III) e Sb(V) were evaluated. Our experiments demonstrated that Sb(V) was not retained and it was selectively determined during the loading step, while retained Sb(III) was determined after elution. The proposed system was also used for the selective preconcentration of Sb(III). In this case, a preconcentration factor of 11 and a limit of detection of 90 ng L−1 for Sb(III) were achieved when 8 mL of sample were loaded into the column. The speciation analysis of inorganic Sb in river water and effluent samples was performed using the proposed method. The values obtained for total Sb (obtained by sum of Sb(III) and Sb(V)) were in good agreement with expected values. Recoveries of Sb(III) and Sb(V) in the river water Standard Reference Material 1640 (from National Institute of Standard and Technology) and spiked river waters were between 83 and 111%. 相似文献
44.
Fabiana de C. Fim Jonathan M. Guterres Nara R. S. Basso Griselda B. Galland 《Journal of polymer science. Part A, Polymer chemistry》2010,48(3):692-698
The synthesis of polyethylene/graphite nanocomposites by in situ polymerization was achieved using the catalytic system Cp2ZrCl2 (bis(cyclopentadienyl)zirconium(IV) dichloride)/methylaluminoxane (MAO). Graphite with nano dimensions, previously treated with MAO, was added into the reactor as filler at percentages of 1, 2, and 5% (w/w). XRD analysis showed that the chemical and thermal treatments employed preserve the structure of the graphite sheets. The formation of graphite nanosheets and nanocomposites was confirmed by TEM and AFM. TEM micrographics showed that the polyethylene grew between the graphene nanosheets, giving intercalated and exfoliated graphite nanocomposites. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 692–698, 2010 相似文献
45.
Verónica R. Elías Marcos I. Oliva Eliana G. Vaschetto Griselda A. Eimer Silvia P. Silvetti 《Journal of magnetism and magnetic materials》2010,322(21):3438-3442
Mesoporous molecular sieves of MCM-48 type were loaded with iron by the wet impregnation method, using Fe(III) nitrate or Fe(II) sulfate aqueous solutions as Fe sources, to obtain a magnetic porous composite. The iron loaded materials were characterized by XRD, N2 adsorption and DRUV-vis and compared with the Si-MCM-48 host. Their magnetic properties were studied by measuring the hysteresis loops up to 1.5 T at different temperatures (5-300 K) and by magnetization vs. temperature curves following the conventional zero field cooling (ZFC) and field cooling (FC) protocols. Materials with high structure regularity and surface area are obtained, which exhibit a mixed paramagnetic and superparamagnetic behavior, arising in isolated iron ions inserted in the host framework, and in small iron oxide clusters or nanoparticles forming inside the pores, respectively. Larger hematite particles (8-13 nm) grown on the external surface provide a quite small ferromagnetic contribution to the hysteresis loop. 相似文献
46.
Griselda Barrera Galland Jo o H. Z. dos Santos Fernanda C. Stedile Paula Palmeira Greco Adriano D. Campani 《Journal of molecular catalysis. A, Chemical》2004,210(1-2):149-156
(nBuCp)2ZrCl2 was grafted on a series of modified silica and evaluated in ethylene/1-hexene copolymerization and their performance was compared with the homogenous system and with that resulting from its immobilization on bare silica. Silica was modified by polymethylhydrosiloxane (PMHS), Me3SiCl, Ph3SiOH, SnCl4, isodrin and aldrin. (nBuCp)2ZrCl2 grafted on PMHS-modified silica afforded the catalyst with the highest activity. Comonomer incorporation, melting point and polydispersity was shown to be dependent on the catalyst nature. Bimodality was observed in the case of ethylene homopolymerization employing PMHS-silica-based catalysts. 相似文献
47.
Griselda Barrera Galland Luciano Forgiarini Da Silva Adriana Nicolini 《Journal of polymer science. Part A, Polymer chemistry》2005,43(20):4744-4753
A systematic study of the influence of the α‐olefin size, the catalyst stereospecificity and the reaction temperature was done on the catalytic activity and tacticity of poly‐α‐olefins from 1‐hexene to 1‐octadecene. The metallocenes used were rac‐Et[Ind2]ZrCl2 ( 1 ) and Me2C[Cp(9‐Flu)]ZrCl2 ( 2 ) to obtain isotactic and syndiotactic polyolefins. Some catalysts giving atactic polymers were also used in order to study all the possible 13C NMR pentades. Catalytic activities increased and isotacticity and syndiotacticity decreased with temperature, but no real trend was found with the α‐olefin size. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4744–4753, 2005 相似文献
48.
Hainaut Donatien Deelstra Griselda 《Methodology and Computing in Applied Probability》2019,21(4):1337-1375
Methodology and Computing in Applied Probability - We propose a new approach for bivariate financial time series modelling which allows for mutual excitation between shocks. Jumps are triggered by... 相似文献
49.
Carlos L. P. Carone Geraldo L. Crossetti Nara R. S. Basso Álvaro G. O. Moraes João H. Z. dos Santos Rubens Bisatto Griselda B. Galland 《Journal of polymer science. Part A, Polymer chemistry》2007,45(22):5199-5208
The catalyst DADNi(NCS)2 (DAD = (ArN?C(Me)? C(Me)?ArN); Ar = 2,6‐C6H3), activated by methylaluminoxane, was tested in ethylene polymerization at temperatures above 25 °C and variable Al/Ni ratio. The system was shown to be active even at 80 °C and when supported on silica. However, catalyst activity decreased. The catalyst system was also tested in ethylene and 10‐undecen‐1‐ol copolymerization at different ethylene pressures. The best activities were obtained at low polar monomer concentration (0.017 mol/L), using triisopropylaluminum (Al‐i‐Pr3) to protect the polar monomer. The incorporation of the comonomer increased with the increase of polar monomer concentration. According to 13C NMR analyses, all the resulting polyethylenes were highly branched and the polar monomer incorporation decreased as ethylene pressure increased. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5199–5208, 2007 相似文献
50.