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41.
Picquart M. López T. Gómez R. Torres E. Moreno A. Garcia J. 《Journal of Thermal Analysis and Calorimetry》2004,76(3):755-761
Sol-gel zirconia was characterized using high-resolution thermogravimetry (Hi-Res TG) and differential scanning calorimetry
(DSC) and compared with X-ray diffraction, Raman scattering, and UV-Vis spectroscopy. ZrO2-x(OH)2x·yH2O annealed below 400°C show typical behavior of amorphous material. As the annealing temperature is increased, the tetragonal
and monoclinic phases crystallize. Typical Hi-Res TG curve shows that the samples are continuously dehydrated in a long temperature
range, between room temperature and 600°C. The total mass loss relative to the initial mass is of about 29%. The DSC analysis
coupled with TG and structural information, indicate that the exothermic processes about 355 and 447°C can be related to the
nucleation process of the formation of tetragonal zirconia, with bulk crystallization at 447°C.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
42.
Josefina Garcia Robert Greenhouse Joseph M Muchowski Jose Antonio Ruiz 《Tetrahedron letters》1985,26(15):1827-1830
2-Alkyl-3-cyanoindoles are obtained when 1-alkylmethyl-2-chloro-(or 2-phenylsulfonyl)-3-phenylsulfonylindoles are reacted with excess azide ion (90°/DMF). The reaction is considered to occur by a fragmentation recombination process in which the Schiff's base is of central importance. This proposal is supported by the formation of 2-substituted indole-3-carboxylates from aldehydes and the α-phenylsulfonyl--aminophenylacetic acid ester derivative . 相似文献
43.
F. M. Bautista J. M. Campelo A. Garcia D. Luna J. M. Marinas A. A. Romero M. R. Urbano 《Reaction Kinetics and Catalysis Letters》1997,62(1):47-54
The vapor-phase catalytic alkylation of phenol with methanol and dimethyl carbonate on a series of differently prepared CrPO4 (Cr/P=1) and CrPO4-AlPO4 (CrAIP) catalysts, has been studied at different temperatures (473–673 K). The reaction is first order in phenol, giving
a mixture of O- and C-alkylated products (C-alkylation taking place preferentially at theortho-position). Moreover, dimethyl carbonate is a better methylating agent than methanol. 相似文献
44.
Aguilar CN Favela-Torres E Viniegra-González G Augur C 《Applied biochemistry and biotechnology》2002,102(1-6):407-414
Undesirable protease production by Aspergillus niger Aa-20 in submerged culture and solid-state culture was evaluated using different concentrations of tannic acid as sole carbon
source in a model system designed for tannase production. Protease production was found to be dependent on the culture system
used (submerged culture or solid-state culture) and on the initial tannic acid concentration. Expression of protease activity
in submerged culture was higher (up to 10 times) than activity obtained in solid-state culture, using identical culture medium
composition. In submerged culture, the lowest final protease activity (0.13 IU) was obtained with the highest tannic acid
concentration, while in solid-state culture protease activity was not affected by changes in initial substrate concentration.
Absence of detectable proteolytic activity in solid-state culture is related to high production of tannase enzyme. Hence,
the use of solid-state culture for fungal enzyme production may allow for higher and more stable enzyme titers present in
culture extracts. 相似文献
45.
Enzymes can be used in nonpolar reaction media to modify waterinsoluble substrates. A variety of pretreatments, applied to
the enzyme prior to introduction to the nonpolar media, can improve enzyme activity. However, the various pretreatments have
not been studied using directly comparable conditions, nor have they been applied simultaneously to test for interactive effects.
This work evaluates pretreatment of lipase with various classes of additives. The pretreated lipase is used to catalyze esterification
between citronellol and acetic acid in a medium of n-hexane. The effectiveness of a particular pretreatment is presented in terms of relative performance (RP), which is equal
to the number of times faster the pretreated lipase catalyzes the reaction relative to untreated lipase. The individual and
interactive effects of the pretreatment factors were studied and compared. Buffer salts had a much stronger performance-enhancing
effect than nonbuffer salts; pretreatment with 90% (w/w) sodium phosphate yielded lipase with an RP of approx 64. A strong
interaction was found between the treatments with sodium phosphate and pH adjustment. These treatments may mitigate the inhibitory
effect of acetic acid. Activating effects of phase interfaces and active-site protectants are shown to be complementary to
other treatments, demonstrating that they likely act by distinct mechanisms. 相似文献
46.
Cayuela E Jalón FA Manzano BR Espino G Weissensteiner W Mereiter K 《Journal of the American Chemical Society》2004,126(22):7049-7062
Diastereomerically pure complexes of formula CpRuCl(PP) and CpRuH(PP) with chiral ferrocenyl diphosphines were prepared and the selectivity of proton-transfer processes over the monohydride compounds with different acids was studied. With 1 equiv of HBF(4) the cis-dihydrogen and trans-dihydride complexes were formed while with 3 equiv of CF(3)CO(2)H the trans-dihydride derivative was the only product. However, the use of 1 equiv of CF(3)CO(2)H led to a dihydrogen bonded complex with an extremely short RuH...HO(2)CF(3) interaction that exhibits proton-hydride exchange. Using the labeled acid CF(3)CO(2)D, a stereoselective transference of the deuteron was demonstrated that implies the previous epimerization of the monohydride and the subsequent attack of the acid in the position previously occupied by the hydride. 相似文献
47.
The intergrowth of 1,10-decanedicarboxylic acid and urea give infinite hydrogen-bonded chains of the guest included in the hexagonal urea host. A deuterium high-resolution solid-state NMR study of the selectively deuterated intergrowth compound 1,10-decanedicarboxylic acid/hydrogenated urea at variable temperature in the range 90 < or = T < or = 300 K was performed on a single crystal. The analysis of the second moment as a function of temperature is shown to be compatible with the known phase transition occurring near T(c) = 203 K. Moreover, the spectra indicate that the orientational disorder is strong, and is compared to an axial uniform disorder. For this purpose, the general equation for the second moment of a system with uniform two-dimensional axial orientational disorder is given, and a method to take into account the non-uniform excitation of the pulse sequence is proposed. 相似文献
48.
M. E. Garcia G. M. Pastor K. H. Bennemann 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):293-295
The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized Hg n clusters, we determine the size dependence of the bond character and the ionization potentialI p (n). For neutral Hg n clusters we obtain a transition from van der Waals to covalent behaviour at the critical sizen c ~10–20 atoms. Results forI p (Hg n ) withn≤20 are in good agreement with experiments, and suggest that small Hg n + clusters can be viewed as consisting of a positive trimer core Hg 3 + surrounded byn?3 polarized neutral atoms. 相似文献
49.
The reaction of several α-amino acids and peptides (containing Gly, L-Ala, L-Leu, L- or DL-Phe, and/or L- or D-Val) with air-diluted nitrogen oxides has been studied to roughly mimic the N-nitrosation of peptide bonds that the contaminated urban air might produce in pulmonary tissues. Most N-protected α-amino acids give practically quantitative yields of N-nitroso derivatives. N-Protected dipeptides afford either dinitrosated peptides, mixtures of di- and mononitrosated compounds, selectively mononitrosated products, or no reaction at all, depending mainly on steric effects. The same trends are observed for some higher peptides. The (po1y)nitrosated Peptides, which retain the chirality of the starting materials, have been characterized by 1H and 13C NMR spectroscopy and are cleaved by pyrrolidine and amino esters under mild conditions to give (new) amides or peptides plus diazo derivatives. 相似文献
50.
A preconcentration and determination methodology for vanadium at trace levels in parenteral solutions was developed. Cloud point extraction was successfully employed for the preconcentration of vanadium prior to inductively coupled plasma atomic optical emission spectrometry (ICP-OES) coupled to a flow injection (FI) system. The vanadium was extracted as vanadium-2-(5-bromo-2-pyridylazo)-5-diethylaminophenol [V-(5-Br-PADAP)] complex, at pH 3.7 mediated by micelles of the nonionic surfactant polyoxyethylene (5.0) nonylphenol (PONPE 5.0). The extracted surfactant-rich phase (100 mul) was mixed with 100 mul of ethanol and this final volume injected into ICP-OES for the vanadium determination. Under these conditions, the 50 ml sample solution preconcentration allowed raising an enrichment factor of 250-fold; however, it was possible to obtain a theoretical enrichment factor of 500-fold. The lower limit of detection (LOD) obtained under the optimal conditions was 16 ng l(-1). The precision for 10 replicate determinations at the 2.0 mug l(-1) V level was 2.3% relative standard deviation (RSD), calculated with the peak heights. The calibration graph using the preconcentration system for vanadium was linear with a correlation coefficient of 0.9996 at levels near the detection limits up to at least 50 mug l(-1). The method was successfully applied to the determination of vanadium in parenteral solution samples. 相似文献