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11.
Kuznetsov V. V. Kutepov S. A. Makhova N. N. Lyssenko K. A. Dmitriev D. E. 《Russian Chemical Bulletin》2003,52(3):665-673
A new method was developed for the synthesis of 6-substituted 1,5-diazabicyclo[3.1.0]hexanes and 7-substituted 1,6-diazabicyclo[4.1.0]heptanes by condensation of N-monohalotrimethylene- and N-monohalotetramethylenediamines with carbonyl compounds in the presence of bases. X-ray diffraction studies and quantum-chemical B3LYP/6-31G* calculations demonstrated that the conformations of the resulting bicyclic systems are stabilized by stereoelectronic interactions. As a result, a boat conformation prevails in 1,5-diazabicyclo[3.1.0]hexanes, whereas the energies of chair, half-chair, and boat conformations of 1,6-diazabicyclo[4.1.0]heptanes are equalized. 相似文献
12.
E. L. Kristallovich A. G. Eshimbetov V. M. Promyslov N. D. Chuvylkin L. I. Belen'kii L. V. Molchanov Kh. M. Shakhidoyatov 《Chemistry of Heterocyclic Compounds》2003,39(11):1516-1520
Quantum-chemical calculations and IR spectroscopy were used to study the conformations as well as the energy and spectral characteristics of 2,3-trimethylene- and 2,3-pentamethylene-3,4-dihydro-4-quinazolinones. The shift of -electron density from the heterocyclic system to the carbonyl group and, thus, the proton affinity of the oxygen atom of this group increase with expansion of the bond angle at the nitrogen atom in going from a five-membered to seven-membered ring. 相似文献
13.
通过IR方法,观测并讨论了在不同条件下制备的超高分子量PE(WHMPE)凝胶膜中旁式(G构象)构象与超拉伸性的关系,提出了G构象浓度及分布对UHMPE凝胶膜超拉伸性影响的新概念,同时也讨论了它与链缠结之间的联系,发现:反映G构象的相对浓度的比值σ=I1303I1352随制膜浓度的增大而增加,在临界浓度C0(0.4~0.5g@100ml)附近所形成的膜,具有超高拉伸性(λ=200),其G构象的变化与Matsuo由比浓粘度提出的缠结链改变相对应;热处理温度比热处理时间对σ值的影响更显著,在一定温度下约4分钟后σ值基本不变;在σ值最低的温度范围拉伸,凝胶膜具有良好的超高拉伸性. 相似文献
14.
Germer A Mügge C Peter MG Rottmann A Kleinpeter E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):1964-1973
The solution-state conformations of N,N',N"-triacetyl chitotriose (1) and other potential chitinase inhibitors 2-4 were studied using a combination of NMR spectroscopy (NOESY) and molecular mechanics calculations. Determination solely of the global energy minimum conformation was found to be insufficient for an agreement with the NMR results. An appropriate consistency between the NMR experimental data and theoretical calculations was only reached by assessing the structures as population-weighted average conformers based on Boltzmann distributions derived from the calculated relative energies. Analogies, but also particular differences, between the synthetic compounds 2-4 and the naturally-occurring N,N',N"-triacetyl chitotriose were found. Furthermore, the conformation of compounds 1 and 2 when bound to hevamine was also studied using transferred NOESY experiments and the binding process was found to impart a level of conformational restriction on the ligands. The preferred conformation as determined for 1 in the bound state to hevamine belonged to one of the conformational families found for the compound when free in solution, although full characterisation of the bound-state conformations was impeded due to severe signal overlap. Saturation transfer difference NMR experiments were also employed to analyse the binding epitopes of the bound compounds. We thus determined that it is mainly the acetyl amido groups of the trisaccharide and the heterocyclic moiety which are in close contact with hevamine. 相似文献
15.
A newly developed unbiased structural optimization method, named dynamic lattice searching (DLS), is proposed as an approach for conformational analysis of atomic/molecular clusters and used in understanding the energy landscape of large clusters. The structures of clusters are described in terms of the number of basic tetrahedron (BT) units they contain. We found that the hit numbers of different structural motifs in DLS runs is proportional to the number of BTs. A parameter T(max) is defined to limit the maximal number of atoms moved in a structural transition. Results show that T(max) is a key parameter for modulating the efficiency of the DLS method and has a great influence on the hit number of different motifs in DLS runs. Finally, the effect of potential range on the conformational distribution of the (Morse)(98) cluster is also discussed with different potential-range parameters. 相似文献
16.
17.
18.
广义的荧光猝灭系指所有能使荧光强度降低的物理和化学过程,但通常涉及的主要有动态猝灭和静态猝灭,二者均遵从Stern-Volmer方程,但由于猝灭作用本质不同,它们在荧光寿命变化、温度效应及吸收光谱等方面表现出差异可资区别.本文通过稳态荧光强度变化和荧光衰减速率的比较研究了具有不同烷基链长的十四烷基苄基二甲基氯化铵(Zeph)和苄基三甲基溴化铵(TMBA)对芘的荧光猝灭,并基于电导实验结果以及猝灭剂全反式构象从理论上计算了Zeph和TMBA的猝灭速率常数,讨论了荧光猝灭的性质和长链分子的构型。 相似文献
19.
Elizabeth P.G. Arêas José A.G. Arêas James Hamburger Warner L. Peticolas Paulo S. Santos 《Journal of colloid and interface science》1996,180(2):578
A remarkably high viscosity has been induced in protein aqueous solutions by the addition of certain structurally related organic solvents. The effect has been observed for lysozyme aqueous solutions containing tetramethylurea (TMU), dimethylsulfoxide (DMSO), dimethylformamide, and hexamethylphosphortriamide. The effect has also been induced in ferrocytochrome c aqueous solutions by TMU. Critical concentrations for both the protein and organic solvent were verified for the onset of the viscosity increase. A common feature of the solvents which were able to induce the effect is a dipolar moiety (C=O, S → O and P → O) and a nonpolar region represented by the methyl groups. The resulting fluids show an extremely restricted flow and a typical non-Newtonian, pseudoplastic behavior. Use was made of1H nuclear magnetic resonance (NMR) and Raman spectroscopy to characterize protein structural modifications and of13C NMR to investigate changes in relaxation times and chemical shifts in the solvent/water solutions. A systematic rheological characterization of the systems was undertaken for some of the solvents, and unusual patterns of viscous effects were identified for the solvent/water systems both with and without protein. The process was found to be at least partially reversible, as concluded from the recovered original solution rheological characteristics and the original protein1H NMR spectrum, after eliminating the organic solvent by ultrafiltration. The whole process was characterized as consisting of two mutually independent stages. The first involves an extensive conformational transition of the polypeptide backbone, from a predominantly α-helical to increased random coiled and β-sheet structures, with the occurrence of nonorthodox protein secondary structures at regions above the solvent critical point. The second stage consists of short-lived interchain contacts leading to an entanglement of the macromolecular system as a whole. A microphase reversion in the organic solvent/water mixture, supported by13C NMR and rheological results, is proposed as the driving force causing the observed behavior. 相似文献
20.
Conformations of 2-methoxytetrahydropyran as a model for the six-membered ring in aldopyranosides have been calculated by the PCILO method using the algorithm of the conjugated gradient to optimize the geometry. The calculated geometry of the fourteen basic forms of 2-methoxytetrahydropyran was found to be in agreement with the available data obtained by X-ray diffraction of pyranosides. The results indicate differences in the geometry of 2-methoxytetrahydropyran resulting from the change of the axial vs. equatorial position of the methoxyl group. These changes are particularly meaningful in the values of bond angles and they are in agreement with the anomeric and exoanomeric effects. The experimentally found differences in the energies of an axial (4
C
1) and equatorial (1
C
4) conformer, G = 2.9–3.7 kJ/mol, and the dipole moment, = 1.20 ± 0.05 D (1D = 3.33 10–30mAs) agree well with the calculated values E = 3.18 kJ/mol and <> = 1.18 D which, in turn, suggest that the axial conformer is preferred over the equatorial one by a ratio a:e = 78:22. 相似文献