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31.
Amorín M Castedo L Granja JR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(22):6543-6551
Here we report the preparation and structural characteristics of self-assembling peptide tubelets composed of 32-membered rings formed of alternating alpha-amino acids and cis-3-aminocyclohexanecarboxylic acids. The tubelets possess a partial hydrophobic core environment, provided by the projection of the cyclohexane C2 methylene moiety into the lumen, and a Van der Waals pore diameter of about 7 A. 相似文献
32.
The synthesis of 2,8-dimethyl 3,9-dichloro 6-sila[5.5] spiro undecatetra 2.4,8,10-ene is described. The thermal decomposition of the adduct formed by the reaction of this product with methyl acetylene dicarboxylate is studied. From the characterization of the copyrolysis products, two possible mechanisms are proposed which imply the formation of digonal silicon such as 2-silaallene and 2-silaketene. 相似文献
33.
The chemical behaviour of sila-2 butadienes, formed as transient intermediates either by thermolysis or by photolysis of various 1-vinylsilacyclobutanes, was studied with respect to hydroxylated compounds of different pKa values Two mechanisms can explain the nature of the products obtained on the co-thermolysis of the cyclic compounds with phenol, one with 1-silacyclobut-1-ene intermediate and the other involving an allylic silicenium cation. In both hypothetical mechanisms, the 2-silabutadienes behave as a conjugated system since they lead either to (2+2) cycloaddition or to (1,2)- and (1,4)-electrophilic addition. This evidence for a conjugation phenomenon through a silicon atom is supported by the calculation of the delocalisation energies of butadiene and 2-silabutadiene. 相似文献
34.
Strategically designed sample composition (SSC) is a new technique that decreases the number of analytical determinations needed in routine screening to as few as the number of original sample specimens while providing information that is specific to them. Although this new technique has been applied to environmental studies, this paper describes its first application to food safety studies. Contamination of milk samples with polychlorinated biphenyls (PCBs) was chosen as a case study to show the usefulness and potential of the SSC technique with a fast analytical procedure that involves saponification of the samples and solid-phase microextraction of the PCBs. A total of 20 sample specimens can be analyzed in 11 determinations with excellent predictions of the positive samples and the concentration levels of the contaminants. The robustness of the strategy was investigated and demonstrated. 相似文献
35.
M. Angel Mateos Rocío Gálvez F. Javier Manuel de Villena José M. Pingarrón 《Mikrochimica acta》1995,120(1-4):339-349
An electroanalytical study of the herbicide propazine's reduction process in micellar solutions and oil-in-water emulsions is reported. The anionic surfactant sodium pentanesulphonate was chosen as the most suitable. The differential pulse polarograms of micellar solutions had two reduction peaks below pH 2.0, whereas only one peak was obtained above pH 2.O. Ethyl acetate was chosen as the organic solvent to form propazine emulsions. Unlike in micellar solutions, the DPP polarograms of propazine emulsions showed only one peak even at pH < 2.0, suggesting that propazine hydrolysis was hindered in the emulsified medium. The limiting current is diffusion-controlled and the electrode process is irreversible. Propazine can be determined by differential pulse polarography over the 1.0 × 10–1 – 1.0 × 10–1moll–1 and 1.0 × 10–15 – 4.0 × 10–1 moll–1 concentration ranges and the limit of detection was 2.8 × 10–1 moll–1. Of the potential interferents simazine, methoprotryne and terbutryn (alls-triazines), thiram (a dithiocarbamate), dinoseb (nitrophenolic), and heptachlor (chlorinated cyclo-diene herbicide), only the first two were significant (10% error for equimolar concentrations). The method was applied to the determination of propazine in spiked drinking water. At a concentration level of 2.0 × 10–1 moll–1 a recovery of 94 ± 6% was obtained, after tenfold concentration on Sep-Pak. 相似文献
36.
Silva L Coutinho A Fedorov A Prieto M 《Journal of photochemistry and photobiology. B, Biology》2003,72(1-3):17-26
Nystatin is a polyene antibiotic frequently applied in the treatment of topical fungal infections. In this work, a 7-nitrobenz-2-oxa-1,3-diazole (NBD) hexanoyl amide derivative of nystatin was synthesized and its detailed photophysical characterization is presented. The average conformation of the labelled antibiotic in tetrahydrofuran, ethanol and methanol was determined by intramolecular (tetraene to NBD) fluorescence resonance energy transfer measurements. At variance with the literature [Can. J. Chem. 63 (1985) 77-85], it was concluded that there is no need to invoke a solvent-dependent conformational equilibrium between extended and closed conformers of the antibiotic, because the mean tetraene-to-NBD separating distance was found to remain constant (approximately 18 A) in all the solvents studied. In addition, the large solvent dependence of the fluorescence anisotropy observed for the non-derivatized nystatin, was rationalized on the basis of the prolate ellipsoidal geometry of the molecule. It was concluded that the rod shaped and amphipathic antibiotic remains monomeric in different solvents within the concentration range studied (2-20 microM). 相似文献
37.
Manuel Urbano Cuadrado Gonzalo Cerruela García Irene Luque Ruiz Miguel Ángel Gómez-Nieto 《Journal of mathematical chemistry》2006,40(1):15-27
A method for the treatment of long-dimensional chemical data arrays is presented in this work with the aim of maximising classification models. The method is based on the construction of fingerprints and the subsequent generation of a similarity matrix. The similarity calculation has been modified through a scaling process to take into account different significance shown by the variables. The method was applied to spectral measurements of wines and several aspects were studied, namely: threshold considered in the construction of fingerprints and patterns, weighting factor used for scaling, normalisation method, etc. The application of both Principal Components Analysis and Soft-Independent Modelling of Class Analogies to the similarity matrices gave better classifications of the information than those obtained using original data. 相似文献
38.
M. Jesús Rodríguez M. Isabel Fernndez Ana M. Gonzlez‐Noya Marcelino Maneiro Rosa Pedrido Miguel Vzquez Bruno Donnadieu Manuel R. Bermejo 《无机化学与普通化学杂志》2005,631(11):2161-2166
M(HL)(H2O)n complexes have been obtained by the electrochemical reaction of Fe, Co, Ni, Cu, Zn and Cd anodes with the potentially pentadentate and trianionic asymmetrical Schiff base 3‐aza‐N‐{2‐[1‐aza‐2‐(5‐nitro‐2‐hydroxylphenyl)‐vinyl]phenyl}‐4‐(5‐nitro‐2‐hydroxyphenyl)but‐3‐enamide (H3L), containing a hard amido donor atom. The complexes have been characterized by elemental analysis, mass spectrometry, IR and 1H NMR spectroscopies, magnetic measurements and molar conductivities. Co(HL)(H2O) ( 2 ) has been found to rearrange in DMF solution into a crystallographically solved octahedral complex, CoL1(H2O)2 ( 7 ) [where H2L1 is the symmetrical Schiff base ligand N,N′‐(1,2‐phenylene)‐bis(5‐nitro‐3‐hydroxysalicylidenimine)]. A hydrolysis mechanism is discussed to explain this rearrangement. 相似文献
39.
Matilde Fondo Ana M. García Deibe Noelia Ocampo Manuel R. Bermejo Jesús Sanmartín 《无机化学与普通化学杂志》2005,631(11):2041-2045
The heptadentate Schiff base H3L reacts with cobalt(II) acetate in methanol to form the discrete dinuclear complex Co2L(OAc)2(OMe)(H2O)2 ( 1 ·2H2O). The reaction of 1 ·2H2O with NMe4OH·5H2O in methanol gives rise to displacement of the acetate by methanolate groups, yielding Co2L(OMe)3(H2O) ( 2 ·1H2O). Recrystallizations of the Schiff base, 1 ·2H2O and 2 ·H2O in different solvents, produce single crystals of H3L, 1 ·2.5H2O and 2 ·2MeOH, respectively. The crystal structures of 1 ·2.5H2O and 2 ·2MeOH show the cobalt atoms double bridged by and endogenous phenol oxygen atom and an exogenous methanolate oxygen donor, giving rise to Co2O2 cores with Co···Co distances of ca. 2.87 Å. 相似文献
40.
Mercedes Villacampa Manuel Martínez Gregorio Gonzlez-Trigo M Nica M. Sllhuber 《Journal of heterocyclic chemistry》1992,29(6):1541-1544
The synthesis of 6β-hydroxy- and 6β,7β-dihydroxy-8-alkyl-8-azabicyclo[3.2.1]octane-3-spiro-5′-hydantoins was stereoselectively achieved by Bucherer-Bergs reaction of the corresponding ketones. An α configuration on C3 was proposed for all hydantoins on the basis of spectral data. 相似文献