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51.
52.
A new reagentless system for sulphite (or sulphur dioxide) determination is reported based on the use of an organic conducting polymer, polyaniline, and its absorbance variation at 550 nm, depending on the sulphite concentration. After chemical polymerisation of aniline a very thin film of polyaniline is obtained. Although the response is not fully reversible, each film can be used for at least 10 measurements for low analyte concentrations (up to 0.5 mg l−1) and five measurements for higher sulphite concentrations. Moreover, the reproducibility, ease of preparation and low cost of the films, permit the use of a new disposable system for each measurement. When the change in absorbance at 550 nm was measured for 210 s (stabilisation time), the system showed a linear response, which ranged from 0.025 to 1.50 mg l−1 of sulphite. A theory with regard to the reaction mechanism between the polyaniline films and sulphite is also proposed. The system was applied to sulphite determination in wine samples and the results were in agreement with those obtained by the Official Method of Analysis (iodometric titration). 相似文献
53.
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
54.
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
55.
Alexandre A.M. Lapis Jackson D. Scholten Marcos N. Eberlin 《Tetrahedron letters》2006,47(38):6775-6779
Intermolecular hydroamination or hydroarylation reactions of norbornene and cyclohexadiene performed with catalytic amounts of Brönsted or Lewis acid in ionic liquids were found to provide higher selectivity and yields than those performed in classical organic solvents. The ionic liquid increases the acidity of the media and stabilizes ionic intermediates through the formation of supramolecular aggregates. 相似文献
56.
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
57.
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
58.
The effects of anionic, non-ionic, and cationic surfactants and thallium(I) nitrate on the paper-substrate room-temperature phosphorescence (RTP) of eleven phenothiazine derivatives are described. Enhancement factors within the range 1.5–10 are observed after addition of sodium dodecylsulfate, dodecyltrimethylammonium chloride and thallium(I) nitrate. The influence of surfactant on phosphor characteristics as well as effects of moisture are discussed. Limits of detection were in the range 0.2–2 ng. Results are also reported for p-aminobenzoic acid, carbazole and o-terphenyl. 相似文献
59.
60.