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排序方式: 共有263条查询结果,搜索用时 15 毫秒
91.
Magorzata Gbylik-Sikorska Anna Gajda Ewelina Nowacka-Kozak Beata ebkowska-Wieruszewska Andrzej Posyniak 《Molecules (Basel, Switzerland)》2021,26(5)
In this work, for the first time, Ultra-High-Performance Liquid Chromatography–Tandem Mass Spectrometry (UHPLC–MS/MS) method was developed for qualitative and quantitative analysis of veterinary antibiotics (cephalosporins, diaminopyrimidines, fluoro(quinolones), lincosamides, macrolides, penicillins, pleuromutilins, sulfonamides, tetracyclines, and sulfones) in hen eggshells. The sample preparation method is based on a liquid–liquid extraction with a mixture of metaphosphoric acid, ascorbic acid, EDTA disodium salt dihydrate, and acetonitrile. The chromatographic separation was performed on Luna® Omega Polar C18 10 column in gradient elution mode and quantitated in an 8 min run. Validation such as linearity, selectivity, precision, recovery, matrix effect, limit of quantification (LOQ), and limit of detection (LOD) was found to be within the acceptance criteria of the validation guidelines of the Commission Decision 2002/657/EC and EUR 28099 EN. Average recoveries ranged from 81–120%. The calculated LOQ values ranged from 1 to 10 µg/kg, the LOD values ranged from 0.3 to 4.0 µg/kg, depending on analyte. The developed method has been successfully applied to the determination of antibacterial compounds in hen eggshell samples obtained from different sources. The results revealed that enrofloxacin, lincomycin, doxycycline, and oxytetracycline were detected in hen eggshell samples. 相似文献
92.
Rodriguez-Docampo Z Pascu SI Kubik S Otto S 《Journal of the American Chemical Society》2006,128(34):11206-11210
Structural and thermodynamic data are presented on the binding properties of anion receptors containing two covalently linked cyclopeptide subunits that bind sulfate and iodide anions with micromolar affinity in aqueous solution. A synchrotron X-ray crystal structure of the sulfate complex of one receptor revealed that the anion is bound between the peptide rings of the biscyclopeptide. Intimate intramolecular contacts between the nonpolar surfaces of the proline rings of the individual receptor moieties in the complex suggest that hydrophobic interactions within the receptor that do not directly involve the guest contribute to complex stability. This finding is supported by a microcalorimetric analysis of the solvent dependence of complex stability, which showed that increasing the water content of the solvent has only a weak influence on the Gibbs energy of binding. Hence, the increasing amount of energy required for desolvating the binding partners in solutions containing more water is almost compensated by the increasingly favorable hydrophobic interactions. Further observations that suggest that guest-induced intra-receptor interactions contribute to guest binding are (i) anion binding of a monomeric cyclopeptide lacking the covalent linkage between the two rings leads to the formation of 2:1 complexes; (ii) in the crystal structure of the 2:1 iodide complex of this monotopic receptor, a similar arrangement of the two cyclopeptide rings has been found as in the sulfate complex of the biscyclopeptide; (iii) complex formation of the monomeric cyclopeptide in aqueous solution is highly cooperative with a large stability constant corresponding to the formation of the 2:1 complexes from relatively instable 1:1 complexes; (iv) the monomeric cyclopeptide forms only 1:1 anion complexes in DMSO where hydrophobic interactions do not take place; and (v) introducing polar hydroxy groups on the proline rings of the monomeric cyclopeptide disrupts cooperativity causing the formation of only 1:1 complexes even in aqueous solution. Taken together these observations demonstrate that, in addition to direct receptor-substrate interactions, noncovalent interactions between the two subunits of such biscyclopeptides contribute significantly to anion complex stability. Reinforcement of molecular recognition through intra-receptor interactions should be an attractive new strategy to boost host-guest affinities. 相似文献
93.
Piotr Mucha Jaroslaw Ruczynski Michal Dobkowski Ewelina Backtrog Piotr Rekowski 《Electrophoresis》2019,40(2):336-342
Systemin (Sys) is an 18‐aa plant peptide hormone involved in the regulation of plant's defensive response. Sys is considered as a fast‐spreading systemic wound signal. We developed a simple and rapid CE method to monitor the spreading of Sys peptides through tomato plant. A 1,2,3‐triazole‐linked AZT‐systemin conjugate was designed as a model to study the possibility of translocating small cargo molecules 3'‐Azido‐2',3'‐dideoxythymidine by systemin. The Sys peptides (Sys, N‐propiolyl Sys, and AZT‐systemin conjugate) were injected into the stem and leaves of mature tomato plant. Its transportation throughout the plant tissue was traced by CE. The peptides were clearly visible in the crude tomato exudates and an optimum separation was achieved in 25 mM phosphate “buffer” at pH 2.5 and a voltage of 20 kV using uncoated fused silica capillary. CE analysis showed that Sys peptides are well separated from tomato plant exudates ingredients and are stable in tomato stem and leaf exudates for up to 24 h. CE study revealed that the Sys peptides are effectively spreading throughout tomato stem and leaves and the peptides could be directly detected in the crude plant matrixes. The translocation was strongly inhibited by sodium azide. The results showed that the established CE method can be used to characterize plant peptides spreading under plant physiological conditions. 相似文献
94.
The influence of process parameters on the epoxidation of 2-buten-1-ol with 30 % hydrogen peroxide using the titanium silicalite
TS-1 is presented. The studies were carried out under autogenic pressure (in autoclave). Methanol was used as a solvent in
this process. The influence of temperature, 2-buten-1-ol to hydrogen peroxide mole ratio, amount of solvent used, TS-1 catalyst
content, and reaction time on the reaction was studied. The major product of this reaction, 2,3-epoxybutan-1-ol (EB), has
numerous applications in chemical industry. 相似文献
95.
We found that difluorocarbene generated from chlorodifluoromethane with 50% aqueous sodium hydroxide reacts with lipophilic tertiary amines 1a-g giving difluoromethyltrialkylammonium chlorides 2a-g in high yields. Similarly, difluoromethyltrialkylammonium iodides 3h-l, nitrates 4h-k, or isothiocyanates 5i,j were synthesized from hydrophilic tertiary amines 1h-l and the corresponding sodium or potassium salts. The process is catalytic with respect to the base used. 相似文献
96.
Natalia Festinger Adrianna Lemiesz Ewelina Skowron Sylwia Smarzewska Witold Ciesielski 《Electroanalysis》2021,33(4):854-863
A sensitive square-wave voltammetry (SWV) method based on basal-plane pyrolytic graphite electrode (BPPGE) and edge-plane pyrolytic graphite electrode (EPPGE) was developed to determine the concentration of the pesticide mandipropamid (MAN) in spiked river water and grape juice samples. Under optimal experimental conditions, the SWV response of EPPGE and BPPGE was linear over the concentration ranges of 0.7 to 9.0 μmol L−1 and 0.5 to 10.0 μmol L−1, respectively. The method was successfully used to determine MAN in spiked samples with good recovery. Cyclic voltammetry (CV) was conducted to understand the mechanism underlying the electrode process of MAN. 相似文献
97.
Thermal Decomposition of Carbamoyl Meldrum's Acids: A Starting Point for the Preparation of 1,3‐Oxazine Derivatives 下载免费PDF全文
Sławomir Makowiec Ewelina Najda Karolina Janikowska 《Journal of heterocyclic chemistry》2015,52(1):205-210
The ability to undergo [4 + 2] versus [2 + 2] cycloaddition was under investigation for ketenes thermally generated from carbamoyl Meldrum's acid. Usually, 1,3‐oxazino‐5‐carbamoylo‐4,6‐diones are formed when carbamoyl Meldrum's acid reacts with imine. However, in some cases, a reaction takes an unexpected course, leading to the formation of tetraponerines alkaloids derivatives or cyclic iminoethers. 相似文献
98.
Szymon Bocian Ewelina Dziubakiewicz Bogusław Buszewski 《Journal of separation science》2015,38(15):2625-2629
A set of seven home‐made silica based bonded phases with different functional groups was investigated. Their zeta potential data in methanol and acetonitrile as well as in methanol/water and acetonitrile/water solution were obtained by using a Zetasizer. The influence of polar functional groups on a zeta potential was investigated. The results show that the amines incorporated in the structure of chemically bonded phases of reversed‐phase materials are protonated during chromatographic analysis, resulting in changes of the zeta potential from negative to positive values. Acetonitrile causes more negative values and methanol provides positive (or less negative) values of the zeta potential. 相似文献
99.
Ewelina Zatorska 《Mathematical Methods in the Applied Sciences》2011,34(2):198-212
We study a nonlocal modification of the compressible Navier–Stokes equations in mono‐dimensional case with a boundary condition characteristic for the free boundaries problem. From the formal point of view, our system is an intermediate between the Euler and Navier–Stokes equations. Under certain assumptions, imposed on initial data and viscosity coefficient, we obtain the local and global existence of solutions. Particularly, we show the uniform in time bound on the density of fluid. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
100.
Rodriguez-Docampo Z Eugenieva-Ilieva E Reyheller C Belenguer AM Kubik S Otto S 《Chemical communications (Cambridge, England)》2011,47(35):9798-9800
Using dynamic combinatorial disulfide chemistry we have developed a new generation of neutral synthetic receptors for anions, based on a macrobicyclic peptide structure. These receptors show an exceptional affinity and selectivity for sulfate ions in aqueous solution [log K(a) = 8.67 in 41 mol% (67 volume%) acetonitrile in water]. The high affinity depends on a delicate balance between rigidity and flexibility in the structure of the receptor. 相似文献