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81.
Bomholt J Moth-Poulsen K Harboe M Karlson AO Qvist KB Bjørnholm T Stamou DG 《Langmuir : the ACS journal of surfaces and colloids》2011,27(3):866-869
The aggregation of casein micelles (CMs) induced by milk-clotting enzymes is a process of fundamental importance in the dairy industry for cheese production; however, it is not well characterized on the nanoscale. Here we enabled the monitoring of the kinetics of aggregation between single CMs (30-600 nm in diameter) by immobilizing them on a glass substrate at low densities and subsequently imaging them with fluorescence microscopy. We validated the new method by a quantitative comparison to ensemble measurements of aggregation. Single-particle statistics allowed us to observe for the first time several heterogeneities in CM aggregation. We observed two types of CM growth: a slow increase in the size of CMs and a stepwise increase attributed to interactions between aggregates preformed in solution. Both types of growth exhibit a lag phase that was very heterogeneous between different CMs, suggesting significant differences in their composition or structure. Detailed size histograms of CMs during aggregation also revealed the presence of two distinct subpopulations with different growth amplitudes and kinetics. The dependence of these distinct nanoscale processes/parameters on aggregation conditions is not accessible to bulk measurements that report only ensemble-average values and may prove important to an in-depth understanding of CM aggregation. 相似文献
82.
Nguyen S Alund SJ Hiorth M Kjøniksen AL Smistad G 《Colloids and surfaces. B, Biointerfaces》2011,88(2):664-673
The present study investigated the surface coating of charged liposomes by three different types of pectin (LM, HM and amidated pectin) by particle size determinations and zeta potential measurements. The pectins and the pectin coated liposomes were visualized by atomic force microscopy. The adsorption of pectin onto positive liposomes yielded a reproducible increase in particle size and a shift of the zeta potential from positive to negative side for all three pectin types, whereas the adsorption of pectin onto negative liposomes did not render any significant changes probably due to electrostatic repulsion. The positive liposomes coated with HM-pectin gave the largest pectin coated particles with the least negative zeta potential, while the opposite was observed for the LM-pectin coated positive liposomes. Furthermore, results from dynamic light scattering revealed narrow size distributions, indicating that the degree of aggregation was low for the pectin coated liposomes. As liposomes are able to encapsulate drugs and pectin has been found to be mucoadhesive, these pectin coated liposomes may be potential drug delivery systems. 相似文献
83.
In our group, we work on the surface modification of cellulose nanocrystals. During this work, we have encountered reproducibility
issues when the same reactions were performed on nanocrystals from different hydrolysis batches, indicating a variable surface
composition. Given the inherent purity of the nanoparticles themselves, this issue was believed to be due to the presence
of adsorbed species at the surface of the nanocrystals blocking reactive sites. To investigate this in detail, nanocrystals
from several batches were extracted with different solvents. The effect of these extractions on the surface composition of
the nanowhiskers was investigated, followed by its effect on the Surface-Initiated Ring-Opening Polymerization (SI-ROP) of
ε-caprolactone. The extracted impurities were analysed by NMR (1H and 13C) and MS, showing a variety of adsorbed species which can be removed by solvent extraction. A Soxhlet extraction using ethanol
before the reaction was shown to be the most effective in removing adsorbed low molecular weight organic compounds produced
during the hydrolysis, resulting in improved reproducibility between reactions using nanocrystals from different batches,
as confirmed by FTIR, elemental analysis and XPS. Extraction with ethanol should thus be performed before all reaction as
these adsorbed species can be expected to interfere with all surface modification reactions. 相似文献
84.
A binary mixture of ciprofloxacin hydrochloride (CIP) and metronidazole (MET) was determined by five simple and accurate methods, without prior separation. In the first method, CIP was determined by second derivative spectrophotometric method ((2)D) by measuring the amplitude at 282 nm (zero ordinate value of MET). On the other hand, the determination of MET was based on isosbestic point technique, where the total content of the mixture was determined at 294.5 nm (isosbestic point), then the content of MET could be calculated by subtraction. The second method was first derivative ratio spectrophotometric method ((1)DD) where the total amplitude at 261 and 285 nm and the amplitude at 295.5 nm were selected to simultaneously determine CIP and MET in binary mixture, respectively. The third method was based on dual wavelength analysis, in which two wavelengths were selected, at which the absorbances of the other component were the same. The fourth method depends on using Q-analysis method (absorbance ratio) which involves the formation of Q-absorbance equation using the respective absorptivity values at 294.5 nm (isosbestic point) and 281.5 nm (λ(max) of CIP). The fifth method is partial least-squares (PLS) chemometric technique for determination of CIP and MET. The developed methods were successfully applied to the analysis of CIP and MET in laboratory prepared mixtures and tablets with good recoveries and their validation was carried out following the International Conference on Harmonization (ICH) guidelines. The results obtained were statistically compared with each other showing no significant difference with respect to accuracy and precision. 相似文献
85.
Takwa M Larsen MW Hult K Martinelle M 《Chemical communications (Cambridge, England)》2011,47(26):7392-7394
Based on molecular modelling, the enzyme Candida antarctica lipase B was redesigned as a catalyst for the ring opening polymerization of D,D-lactide. Two mutants with 90-fold increased activity as compared to the wild-type enzyme were created. In a preparative synthesis of poly(D,D-lactide) the mutants greatly improved the rate and the degree of polymerization. 相似文献
86.
Beel R Kobialka S Schmidt ML Engeser M 《Chemical communications (Cambridge, England)》2011,47(11):3293-3295
SOMO catalysis has lately obtained large interest as a new and powerful version of enantioselective organocatalysis which includes radical steps initiated by a one-electron oxidation. The intermediate enamine radical cation has been postulated, but has not been observed directly so far. This communication now reports the direct detection of this key intermediate. 相似文献
87.
88.
89.
Marianne Spittel 《Fresenius' Journal of Analytical Chemistry》1966,217(2):125-126
Ohne Zusammenfassung 相似文献
90.