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991.
Valérie Chevalier-Billosta Jean-Paul Joseleau Alain Cochaux Katia Ruel 《Cellulose (London, England)》2007,14(2):141-152
The composition and ultrastructural arrangement of cell wall polymers in wood fibres have determining influence on the properties
of wood derived materials. It is therefore important to improve our understanding of the relationship between fibres organisation,
the modifications induced by pulping treatments, and the resulting paper sheet mechanical properties. The different treatments
to which fibres are subjected during the manufacturing of pulps and papers induce morphological and micro-structural alterations
due to the removal of wall constituents and of microfibrillar elements. The impact of pulping processes on fibres was investigated
at the ultrastructural scale of transmission electron microscopy. Particular attention was given to the effects of beating
in refiners at various intensities on the ultrastructure of fibres. The most characteristic effects consisted of delaminations,
microfibril disorganisation, and even fractures, of varying importance depending on the intensity of the mechanical refining.
The consequences of internal alterations and surface modifications of the fibres were examined in relation to the paper sheet
mechanical properties. Correlations between the type of alteration observed in the fibres and its possible impact on a given
paper mechanical property are suggested. With similar approaches, the effects of drying and recycling were studied. 相似文献
992.
Stereoselectivity of reductive amination of (R)-1-hydroxy-1-phenylpropan-2-one by methylamine was studied. From the four isomers possible, only two are produced by this
reaction. These are marked as (−)-(1R,2S)-ephedrine (desired product) and (+)-(1S,2R)-ephedrine. The reaction stereoselectivity depends both on the type of the catalyst and reaction conditions. The most suitable
type is the supported platinum. However, this catalyst rapidly deactivates. With a decreasing activity of Pt catalyst, the
stereoselectivity decreases. It is also decreased during the production of the second liquid phase (water) in the reaction
mixture. 相似文献
993.
Gabriela Flores-Ramírez Manuel Rivera Alfredo Morales-Pablos Joel Osuna Xavier Soberón Paul Gaytán 《BMC chemical biology》2007,7(1):1
Background
The effect of single and multiple amino acid substitutions in the green fluorescent protein (GFP) from Aequorea victoria has been extensively explored, yielding several proteins of diverse spectral properties. However, the role of amino acid deletions in this protein -as with most proteins- is still unknown, due to the technical difficulties involved in generating combinatorial in-phase amino acid deletions on a target region. 相似文献994.
Electrocatalytic oxidation of thiosulfate on a modified nickel hexacyanoferrate film electrode 总被引:1,自引:0,他引:1
Xingyao Zhou Shengfu Wang Zhengpin Wang Mian Jiang 《Fresenius' Journal of Analytical Chemistry》1993,345(6):424-427
Summary A modified nickel hexacyanoferrate film glassy carbon electrode is prepared by the electrochemical deposition technique. The film is very stable upon voltammetric scanning in the potential range of 1.0 to –0.5 V (vs. SCE) and an oxidation peak occurs at 0.35 V (vs. SCE) (1 mol/l NaNO3). The effects of electrolyte, solvent, coexisting ions and other variables on the voltammetric behaviour of the modified film have been studied. The thickness of the resulting film can be controlled by changing the number of voltammetric cycles and the concentrations of nickel(II) and hexacyanoferrate(III) ions. The film shows catalytic activity towards electrooxidation of thiosulfate with a peak potential +0.5 V (K–-containing media). This oxidation potential of thiosulfate on the modified electrode is shifted negatively by about 550 mV as compared to the naked glassy carbon electrode. For practical application, the modified electrode can be used for the determination of thiosulfate in concentrations from 5.0×10–5 to 1.0×10–1 mol/l. This method has been successfully applied to the determination of thiosulfate in photographic waste effluents. 相似文献
995.
The thermodynamic properties of the mixed aqueous electrolyte of ammonium and alkaline earth metal nitrates have been studied using the hygrometric method at 25?°C. The water activities of these {yNH4NO3+(1?y)Y(NO3)2}(aq) systems with Y ≡ Ba2+, Mg2+ and Ca2+ were measured at total molalities ranging from 0.10 mol?kg?1 to saturation for different NH4NO3 ionic-strength fractions of y=0.20, 0.50 and 0.80. These data allow the calculation of osmotic coefficients. From these measurements, the ionic mixing parameters are determined and used to calculate the solute activity coefficients in the mixtures at different ionic-strength fractions. The results of these ternary solution measurements are compared with those for binary solutions of the alkaline earth nitrates of magnesium, calcium and barium with ammonium nitrates. The behavior of the aqueous electrolyte solutions containing mixtures of barium or calcium or magnesium with ammonium nitrates are correlated and show that ionic interactions are more important for the system containing Mg2+ than for Ca2+ or Ba2+. The trends are mainly due to the effects of the ionic size, polarizability and the hydration of the ions in these solutions. 相似文献
996.
Prithwiraj Byabartta 《Transition Metal Chemistry》2007,32(6):716-726
[Au(C6F5)(tht)], which on reaction with P, O, S-coordinating phosphines in CH2Cl2 medium leads to [Au(C6F5)(X)] [X = PPh3 H, (1a), oMe, (1b), pMe, (1c), mMe, (1d), AsPh3
(2), OPPh3
(3), SPPh3
(4), dppm, dppe, dppa = diphenylphosphino-methane,-ethane,-ammine(5, 6, 7), TPA = 135-tetraaza-7-phosphino adamentane(8), Py4H (9a), 4Bu (9b), 4Ac (9c), tht = tetrahydrothiophen, C6F5 is the pentafluorophenyl ring]. The maximum molecular peak of the corresponding molecule is observed in the ESI mass spectrum.
I.r. spectra of the complexes show –C = C– and C6F5 stretching near at 1610 and 1510, 955, 800 cm−1. The 1H-n.m.r. spectra as well as 31P- (1H)n.m.r. suggest solution stereochemistry, proton movement, phosphorus proton interaction. 13C-n.m.r. spectrum reflect the carbon skeleton in the molecule. In the 1H–1H COSY spectrum of the present complexes and contour peaks in the 1H–13C-HMQC spectrum, assign the solution structure and stereoretentive conformation in each step. 相似文献
997.
Satish M. Annigeri M. P. Sathisha Vidyanand K. Revankar 《Transition Metal Chemistry》2007,32(1):81-87
Binuclear cobalt(II), nickel(II), copper(II) and zinc(II) complexes of general composition [M2L1-2(μ-Cl)Cl2] · nH2O with the Schiff-base ligands (where L1H and L2H are the potential pentadentate ligands derived by condensing 2,6-diformyl-4-methylphenol with 4-amino-3-antipyrine and 2-hydroxy-3-hydrazinoquinoxiline,
respectively) have been synthesized and characterized. Analytical and spectral studies support the above formulation. 1H-NMR and IR spectra of the complexes suggest they have an endogenous phenoxide bridge, with chloride as the exogenous bridge
atom. The electronic spectra of all the complexes are well characterized by broad d–d and a high intensity charge-transfer
transitions. The complexes are chloro-bridged as evidenced by two intense far-IR bands centered around 270–280 cm−1. Magnetic susceptibility measurements show that complexes are antiferromagnetic in nature. The compounds show significant
growth inhibitory activity against fungi Aspergillus niger and Candida albicans and moderate activity against bacteria Bacillus cirroflagellosus and Pseudomonas auresenosa. 相似文献
998.
Chao Song Dan Gao Tianying Yuan Yongli Chen Liping Liu Xiaowu Chen Yuyang Jiang 《中国化学快报》2019,30(5):1038-1042
Cell migration and invasion are critical steps in cancer metastasis, which are the major cause of death in cancer patients. Tumor-associated macrophages(TAMs) and interstitial flow(IF) are two important biochemical and biomechanical cues in tumor microenvironment, play essential roles in tumor progression. However, their combined effects on tumor cell migration and invasion as well as molecular mechanism remains largely unknown. In this work, we developed a microfluidic-based 3 D breast cancer model by co-culturing tumor aggregates, macrophages, monocytes and endothelial cells within 3 D extracellular matrix in the presence of IF to study tumor cell migration and invasion. On the established platform, we can precisely control the parameters related to tumor microenvironment and observe cellular responses and interactions in real-time. When co-culture of U937 with human umbilical vein endothelial cells(HUVECs) or MDA-MB-231 cells and tri-culture of U937 with HUVECs and MDA-MB-231 cells, we found that mesenchymal-like MDA-MB-231 aggregates activated the monocytes to TAM-like phenotype macrophages. MDA-MB-231 cells and IF simultaneously enhanced the macrophages activation by the stimulation of colony-stimulating factor 1(CSF-1). The activated macrophages and IF further promoted vascular sprouting via vascular endothelial growth factor(VEGFα) signal and tumor cell invasion. This is the first attempt to study the interaction between macrophages and breast cancer cells under IF condition. Taken together, our results provide a new insight to reveal the important physiological and pathological processes of macrophages-tumor communication. Moreover, our established platform with a more mimetic 3 D breast cancer model has the potential for drug screening with more accurate results. 相似文献
999.
(2S)- and (2R)-2-Amino-4-bromobutanoic acid were prepared starting from N-Boc-glutamic acid α tert-butyl ester. The double tert-butyl protection was necessary to prevent a partial racemisation during Barton’s radical decarboxylation used to transform the γ-carboxylic group into a bromide. This bromide reacted with different nitrogen, oxygen and sulphur nucleophiles to give nonnatural amino acids characterised by basic or heterocyclic side chains. The title compound was also used to prepare a conformationally constrained peptidomimetic. 相似文献
1000.
Wendt-Pienkowski E Huang Y Zhang J Li B Jiang H Kwon H Hutchinson CR Shen B 《Journal of the American Chemical Society》2005,127(47):16442-16452
Fredericamycin (FDM) A, a pentadecaketide featuring two sets of peri-hydroxy tricyclic aromatic moieties connected through a unique chiral spiro carbon center, exhibits potent cytotoxicity and has been studied as a new type of anticancer drug lead because of its novel molecular architecture. The fdm gene cluster was localized to 33-kb DNA segment of Streptomyces griseus ATCC 49344, and its involvement in FDM A biosynthesis was proven by gene inactivation, complementation, and heterologous expression experiments. The fdm cluster consists of 28 open reading frames (ORFs), encoding a type II polyketide synthase (PKS) and tailoring enzymes as well as several regulatory and resistance proteins. The FDM PKS features a KSalpha subunit with heretofore unseen tandem cysteines at its active site, a KSbeta subunit that is distinct phylogenetically from KSbeta of hexa-, octa-, or decaketide PKSs, and a dedicated phosphopantetheinyl transferase. Further study of the FDM PKS could provide new insight into how a type II PKS controls chain length in aromatic polyketide biosynthesis. The availability of the fdm genes, in vivo characterization of the fdm cluster in S. griseus, and heterologous expression of the fdm cluster in Streptomyces albus set the stage to investigate FDM A biosynthesis and engineer the FDM biosynthetic machinery for the production of novel FDM A analogues. 相似文献