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991.
Tushar K. Chakraborty V. Ramakrishna Reddy S. Uday Kumar S. Kiran Kumar Archna Mathur Neena Gupta 《Tetrahedron》2004,60(38):8329-8339
Conformational analysis of vasoactive intestinal peptide (VIP) receptor binding inhibitor Leu1-Met2-Tyr3-Pro4-Thr5-Tyr6-Leu7-Lys81 by various NMR techniques and constrained molecular dynamics (MD) simulation studies revealed that the molecule had a turn structure involving its Tyr3-Pro4-Thr5-Tyr6 moiety with intramolecular hydrogen bond between Tyr6NH→Tyr3CO. In order to mimic the structure of 1, peptidomimetic analogs 2-4 were synthesized using conformationally constrained scaffolds of 3,4-dideoxy furanoid sugar amino acids (2S,5R)-ddSaa1 5 and its enantiomer (2R,5S)-ddSaa2 6. All these analogs displayed well defined three-dimensional structures akin to that found in 1. Peptides 2 and 3, which differed only in the sugar amino acid stereochemistry, show propensity of structures with identical intramolecular hydrogen bonds between ThrNH→MetCO. A similar structure with a hydrogen bond between TyrNH→MetCO was observed in 4. 相似文献
992.
Rosa M. Gomila Antonio Frontera David Quiñonero Antonio Costa 《Tetrahedron letters》2004,45(51):9387-9391
Several porphyrin dimers have been newly designed and synthesized to construct assemblies with 1,4-diazabicyclo[2.2.2]octane (DABCO) as a bidentate binding ligand. Semi empirical (AM1) and ab initio calculations have been used to study the assemblies generated by the organization of dimers and DABCO, including the computation of 1H NMR complexation-induced chemical shifts using the ab initio/GIAO methodology. The diagnostic capacity of the theoretical method has been applied to explain experimental results and geometrical features of the complexes. 相似文献
993.
Tikhonova I. A. Shubina E. S. Dolgushin F. M. Tugashov K. I. Teplitskaya L. N. Filin A. M. Sivaev I. B. Petrovskii P. V. Furin G. G. Bregadze V. I. Epstein L. M. Shur V. B. 《Russian Chemical Bulletin》2003,52(3):594-600
The reaction of cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3 (1) with the polyhedral [B12H11SCN]2– anion in THF at 20 °C affords the {[(o-C6F4Hg)3](B12H11SCN)}2– (4) and {[(o-C6F4Hg)3]2(B12H11SCN)}2– (5) complexes. Complex 5 was isolated as the tetrabutylammonium salt. X-ray diffraction analysis showed that this complex has a bent-sandwich structure in which the [B12H11SCN]2– anion is located between the planes of two molecules 1 and is coordinated to both these molecules through B—H—Hg bridges and S—Hg bonds. The stability constants of complexes 4 and 5 in THF (20 °C), which were determined from the IR spectroscopic data, are 16 L mol–1 and 992 L2 mol–2, respectively. 相似文献
994.
We have examined two-dimensional electrophoresis (2-DE) gel maps of polypeptides from the Gram-negative bacterium Methylococcus capsulatus (Bath) and found the same widespread trains of spots as often reported in 2-DE gels of polypeptides of other Gram-negative bacteria. Some of the trains of polypeptides, both from the outer membrane and soluble protein fraction, were shown to be generated during the separation procedure of 2-DE, and not by covalent post-translational modifications. The trains were found to be regenerated when rerunning individual polypeptide spots. The polypeptides analysed giving this type of trains were all found to be classified as stable polypeptides according to the instability index of Guruprasad et al. (Protein Eng. 1990, 4, 155-161). The phenomenon most likely reflects conformational equilibria of polypeptides arising from the experimental conditions used, and is a clear drawback of the standard 2-DE procedure, making the gel picture unnecessarily complex to analyse. 相似文献
995.
Yu‐Hsiang Hu Chuh‐Yung Chen 《Journal of polymer science. Part A, Polymer chemistry》2002,40(21):3692-3702
The bulk polymerization of methyl methacrylate initiated with 2‐pyrrolidinone and n‐dodecyl mercaptan (R‐SH) has been explored. This polymerization system showed “living” characteristics; for example, the molecular weight of the resulting polymers increased with reaction time by gel permeation chromatographic analysis. Also, the polymer was characterized by Fourier transform infrared spectroscopy, 1H NMR, and 13C NMR techniques. The polymer end with the iniferter structures was found. By the initial‐rate method, the polymerization rate depended on [2‐pyrrolidinone]1.0 and [R‐SH]0. Combining the structure analysis and the polymerization‐rate expression, a possible mechanism was proposed. n‐Dodecyl mercaptan served dual roles—as a catalyst at low conversion and as a chain‐transfer agent at high conversion. Finally, the thermal properties were studied, and the glass‐transition temperature and thermal‐degradation temperature were, respectively, 25 and 80–100 °C higher than that of the azobisisobutyronitrile system. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3692–3702, 2002 相似文献
996.
Jacek Kozioł 《Monatshefte für Chemie / Chemical Monthly》1991,122(1-2):47-50
Summary Stability constants and enthalpies of formation were determined forEDA complexes of 2,4,6-trinitropyridine-1-oxide (TNPO) with some pyridine bases. Energies of transition,hv
CT
, calculated by the SCF MO CI-1 PPP method fit well the corresponding experimental data. Based on these results, geometries for the systems under study are proposed.
Spektroskopische und theoretische Untersuchung von molekularen Komplexen des 2,4,6-Trinitropyridin-1-oxids mit einigen Pyridinderivaten
Zusammenfassung Es wurden Stabilitätskonstanten und Bildungsenthalpiewerte einigerEDA-Komplexe von 2,4,6-Trinitropyridin-1-oxid mit ausgewählten Pyridinbasen bestimmt. Dabei wurde eine gute Übereinstimnmung zwischen den Anregungsenergienhv CT aus den UV-VIS-Spektren und den mit der SCF-MO-CI-1 PPP-Methode errechneten Werten gefunden. Es werden für die untersuchten Komplexe entsprechende Strukturen vorgeschlagen.相似文献
997.
2,2-Difluoro-1,2,3-trimethyl-1,3-diaza-2λ5-phospholidine The title compound 1 is obtained in about 50% yield by oxidative fluorination of 1,2,3-trimethyl-1,3-diaza-2λ3-phospholidine, 6 , with IF5. 1 is characterized by its nmr, mass, and ir spectra. 相似文献
998.
D. V. Shishkin E. R. Mukhamed’yarova N. Z. Baibulatova V. A. Dokichev Yu. V. Tomilov 《Chemistry of Natural Compounds》2007,43(3):293-297
Diastereomers of N-(2-(1-adamantyl)-2-hydroxyethyl)cytisine were synthesized by reduction of N-(2-(1-adamantyl)-2-oxoethyl)cytisine with NaBH4. Their structures were established using x-ray structure analysis.
__________
Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 244–247, May–June, 2007. 相似文献
999.
4,4′‐hexafluoroisopropylidene‐2,2‐bis‐(phthalic acid anhydride) (1) was reacted with L ‐methionine (2) in acetic acid and the resulting N,N′–(4,4′‐hexafluoroisopropylidenediphthaloyl)‐bis‐L ‐methionine (4) was obtained in high yield. The direct polycondensation reaction of this diacid with several aromatic diols such as bisphenol A (5a), phenolphthalein (5b), 1,4‐dihydroxybenzene (5c), 4,4′‐dihydroxydiphenyl sulfide (5d), 4,6‐dihydroxypyrimidine (5e), 4,4′‐dihydroxydiphenyl sulfone (5f) and 2,4′‐dihydroxyacetophenone (5g) was carried out in a system of thionyl chloride and pyridine. Expecting that the reaction with thionyl chloride in pyridine might involve alternative intermediates different from an acyl chloride, the polycondensation at a higher temperature favorable for the reaction of the expected intermediate with nucleophiles was attempted, and a highly thermally stable poly(ester‐imide) was obtained by carrying out the reaction at 80°C. All of the above polymers were fully characterized by 1H‐NMR, 19F‐NMR FT‐IR spectroscopy, elemental analysis and specific rotation. Some structural characterization and physical properties of these optically active poly(ester‐ imide)s are reported. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
1000.
Esko Taskinen 《Structural chemistry》1998,9(6):411-418
The spatial structures of a number of mono- and disubstituted 1,1-dimethoxycyclohexanes (cyclohexanone dimethyl acetals) were studied by 13C NMR spectroscopy. In the monosubstituted acetals, substituents (Me, Et, i-Pr, and MeO) on C-2 are axially oriented, contrary to their normal, equatorial orientation on C-3 and C-4. Besides the spectroscopic study, the relative thermodynamic stabilities of the cis-trans isomers of a few 2,X-dialkyl (X = 3, 4, 5, or 6) derivatives of the parent cyclohexanone dimethyl acetal were determined by acid-catalyzed chemical equilibration in MeOH solution. In the most stable isomeric form, the 2-substituent is axial and the other equatorial. In the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derivative, where the 13C NMR shift data point to a predominance of the diaxial form. In general, the enthalpy difference between the isomeric forms is ca. 9 kJ mol–1, while the entropy term favors the less stable isomer by 4 to 16 J K–1 mol–1. In the 2,6-dimethyl derivatives, however, the trans form is favored by only 0.8 kJ mol–1 in G
m
at 298.15 K. The main findings of the experimental work are in good agreement with ab initio calculations. 相似文献