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排序方式: 共有416条查询结果,搜索用时 31 毫秒
401.
Sara Valsecchi Marianna Rusconi Stefano Polesello 《Analytical and bioanalytical chemistry》2013,405(1):143-157
Bioaccumulation of PFAS in aquatic organisms is an environmental problem of growing concern around the world. This problem has been tackled by regulatory bodies by proposing EQS for biota in EU water bodies and tolerable daily intake for food. The introduction of regulatory limits requires the availability of harmonised and validated analytical methods of sufficient sensitivity. This paper reviews recent advances in analytical methods for analysis of PFAS in aquatic organisms. The methods available for biota analysis are mostly based on three extraction procedures: ion-pair extraction, solvent liquid extraction, and alkaline digestion. The resulting extracts are then subjected to different clean-up or enrichment steps on solid adsorbents, for example graphitized carbon black, C18, and WAX phases. All methods reviewed in this work give reliable results but are partially validated only, because of the lack of certified reference materials and regular interlaboratory exercises. The few interlaboratory exercises performed on real unspiked samples did not afford satisfactory results for PFAS other than PFOS, especially for matrices with high lipid content, for example mussels. The reasons for those partially negative results have been identified, and can mainly be attributed to calibration procedures and availability and purity of standards. The urgent need for certified reference materials for this type of analysis is emphasized. 相似文献
402.
Novel acridine spirocompounds, spiro[dihydroacridine-9′(10′H),5-imidazolidine]-2-thiones have been prepared by the spontaneous cyclization of 1-substituted 3-(acridin-9-ylmethyl)thioureas, which were obtained from 1-(acridin-9-yl)methanamine, N-(acridin-9-ylmethyl)propan-1-amine, and N-(acridin-9-ylmethyl)benzylamine and alkyl/aryl isothiocyanates, as continuation of our previous studies on new acridine spirocycles. Imidazolidine-2-thiones thus obtained were subsequently transformed with mesitylnitrile oxide to imidazolidine-2-one analogues, some of which partly reopened to the corresponding (acridin-9-ylmethyl)ureas. An unusual spirocyclization via a CH carbanion instead of the N-1 nitrogen has been found for 3-(acridin-9-ylmethyl)-1-(acridin-9-yl)thioureas possessing two acridine rings. Structural modifications in positions 1, 3, and 4 of the spiro ring together with 1H, 13C, and 15N NMR spectroscopy and X-ray crystallography have been employed to rationalize a general propensity of various 9-substituted acridines to undergo easy spirocyclization. 相似文献
403.
Assadi-Porter FM Tonelli M Maillet E Hallenga K Benard O Max M Markley JL 《Journal of the American Chemical Society》2008,130(23):7212-7213
We present a robust method for monitoring the binding of ligands to the heterodimeric (T1R2+T1R3) human sweet receptor (a family 3 GPCR receptor). The approach utilizes saturation transfer difference (STD) NMR spectroscopy with receptor proteins expressed on the surface of human epithelial kidney cells. The preparation investigated by NMR can contain either live cells or membranes isolated from these cells containing the receptor. We have used this approach to confirm the noncompetitive binding of alitame and cyclamate to the receptor and to determine that greatly reduced receptor binding affinity compared to wild-type brazzein explains the lack of sweetness of brazzein mutant A16C17. This approach opens new avenues for research on the mechanism of action of the sweet receptor and for the design of new noncalorigenic sweeteners. 相似文献
404.
2-Bromoethylamines are converted to the corresponding formate esters in the presence of DMF. Both primary and secondary bromides are smoothly transformed to the esters in satisfactory yields. The reaction mechanism involves the formation of an aziridinium ion, which upon reaction with DMF forms a Vilsmeier-type intermediate that is further hydrolyzed to the corresponding formates. Participation of the β-amino group appears to control not only the regioselectivity but also the stereoselectivity of the reaction. Application of the reaction conditions to chiral substrates indicated that non-rearranged products are formed with retention of configuration at the reacting center. 相似文献
405.
Justyna Mrozek Emilia Sikorska Agnieszka Skwierawska Joanna Malicka Jerzy Karolczak Bogdan Banecki Wiesław Wiczk 《Journal of inclusion phenomena and macrocyclic chemistry》2008,62(3-4):269-278
Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). In this paper we describe the influence of Leu chirality of cyclic [Leu]enkephalins on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for both cyclic [Leu]enkephalins. Moreover, d-configuration of Leu residue causes an increase of the binding constant of cyclic enkephalin compared to l-analogue. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Trp or Leu residue, a side and/or bottom association complexes are formed. 相似文献
406.
Prukała D Prukała W Koczorowski R Khmelinskii IV Sikorska E Sikorski M 《Rapid communications in mass spectrometry : RCM》2008,22(3):409-416
Electron ionisation (EI) mass spectra and electrospray ionisation (ESI) mass spectra at different cone voltages of a series of isomeric methyl- and dimethylalloxazines are discussed, and compared with those of lumichrome, and 1- and 3-methyllumichrome. Examination of ESI mass spectra taken at a higher cone voltage and the use of isotope-labelled methanol allow us to discuss the fragmentation pathways of [M+H]+ and [M-H](-) ions. The fragmentation pathways of all of the compounds and the characteristic fragment ions formed in EI-MS are compared with published data. The influence of methyl and dimethyl substituents in the benzene ring on the fragmentation pathways leading to the loss of 43 and 45 Da upon both electron and electrospray ionisation is described. 相似文献
407.
Marianna Budai Pl Grf Andreas Zimmer Katalin Ppai Imre Klebovich Krisztina Ludnyi 《Journal of photochemistry and photobiology. A, Chemistry》2008,198(2-3):268-273
Fluoroquinolone antibacterial agents are among the drugs most commonly causing phototoxic side effects. The phototoxicity may be originated in formation of reactive oxygen species upon ultraviolet exposure. Researches aiming the liposomal encapsulation of fluoroquinolones, expecting an increase in their therapeutic index, enhance the importance of studies on physicochemical properties and photostability of liposomal preparations. We studied the photodegradation of ciprofloxacin, ofloxacin and lomefloxacin by mass spectrometry upon various doses of UV irradiation. Lomefloxacin, the most phototoxic fluoroquinolone among them, was encapsulated into small unilamellar and multilamellar liposomes. Impact of vesicle structure and lipid composition – the presence of unsaturated fatty acid containing dioleoyl-phosphatidylcholine in dipalmitoyl-phosphatidylcholine liposomes – on the lomefloxacin photolysis was investigated; the structure of the main photoproducts was identified by mass spectrometry. It was found that the presence and type of lipids influence the ways of photodegradation process. 相似文献
408.
Aequationes mathematicae - Let X, Y be linear spaces over a field $${\mathbb {K}}$$ . Assume that $$f :X^2\rightarrow Y$$ satisfies the general linear equation with respect to the first... 相似文献
409.
Favretto D Cosmi E Ragazzi E Visentin S Tucci M Fais P Cecchetto G Zanardo V Viel G Ferrara SD 《Analytical and bioanalytical chemistry》2012,402(3):1109-1121
A number of metabolic abnormalities have been observed in pregnancies complicated by intrauterine growth restriction (IUGR).
Metabolic fingerprinting and clinical metabolomics have recently been proposed as tools to investigate individual phenotypes
beyond genomes and proteomes and to advance hypotheses on the genesis of diseases. Non-targeted metabolomic profiling was
employed to study fetal and/or placental metabolism alterations in IUGR fetuses by liquid chromatography high-resolution mass
spectrometry (LC-HRMS) analysis of cord blood collected soon after birth. Samples were collected from 22 IUGR and 21 appropriate
for gestational age (AGA) fetuses. Birth weight differed significantly between IUGR and AGA fetuses (p < 0.001). Serum samples were immediately obtained and deproteinized by mixing with methanol at room temperature and centrifugation;
supernatants were lyophilized and reconstituted in water for analysis. LC-HRMS analyses were performed on an Orbitrap mass
spectrometer linked to a Surveyor Plus LC. Samples were injected into a 1.0 × 150-mm Luna C18 column. Spectra were collected
in full-scan mode at a resolution of approximately 30,000. Data were acquired over the m/z range of 50–1,000, with measurements performed in duplicate. To observe metabolic variations between the two sets of samples,
LC-HRMS data were analyzed by a principal component analysis model. Many features (e.g., ionic species with specific retention
times) differed between the two classes of samples: among these, the essential amino acids phenylalanine, tryptophan, and
methionine were identified by comparison with available databases. Logistic regression coupled to a receiver-operating characteristic
curve identified a cut-off value for phenylalanine and tryptophan, which gave excellent discrimination between IUGR and AGA
fetuses. Non-targeted LC-HRMS analysis of cord blood collected at birth allowed the identification of significant differences
in relative abundances of essential amino acids between IUGR and AGA fetuses, emerging as a promising tool for studying metabolic
alterations. 相似文献
410.
Five pure CD derivatives synthesized in our laboratory were used as chiral selectors in the presence of copper(II) ion. Three enantiomeric pairs of amino acids were submitted to separation experiments in CE, by exploiting the ligand exchange mechanism. The results obtained in the investigated systems, together with those of the analogous systems previously studied, clearly show the usefulness of this technique in chiral separations. By comparing the ligand exchange CE results with potentiometric results, either reported elsewhere or studied here for the first time (system Cu/CDampy/tyrosine), it has been possible to rationalise the separation results. The importance of the availability of pure selectors, and to characterise them both spectroscopically and thermodynamically is discussed. 相似文献