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101.
A novel methodology for 'reverse-docking' a cationic peptide-based organocatalyst to a rigid anionic transition state (TS) model for the conjugate addition of azide to alpha,beta-unsaturated carbonyl substrates is described. The resulting docking poses serve as simplified TS models for enantioselective catalysis. Molecular mechanics-based scoring and ranking of the docking poses, followed by clustering and structural analysis, reveal a clear energetic preference for docking to the S-enantiomeric azidation TS model, in agreement with experiment. Clear energetic trends emerged from docking the catalyst to both enantiomers of all six azidation TS models of this study. Structural analysis of the most favorable pose suggests a mechanism for enantioselective catalysis that is consistent with principles of molecular recognition, catalysis, and experimental data. 相似文献
102.
Tingmin Wang Jerald S. Bradshaw Janet C. Curtis Peter Huszthy Reed M. Izatt 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(2):113-122
Significant - interaction is found in the complexes of (S, S)-dimethylpyridino-18-crown-6 with (R)- and (S)-[-(1-naphthyl)ethyl]ammonium perchlorate. This finding is supported by the1H NOESY NMR spectral technique, greater chemical shift changes of aromatic protons in both host and guest molecules upon complexation, and by molecular mechanics calculations. Because of the flexibility of the ligand, the tripod hydrogen bonding causes13C relaxation times of all periphery carbons to decrease without significant selectivity. Rotational energy barrier calculations of the methyl groups of the complexed ligand also show that the (S, S)-host-(R)-guest is the more stable complex. 相似文献
103.
Günter Ehrlich Thomas Nestler Rainer Voigtmann Willi Moldenhauer 《Mikrochimica acta》1989,98(4-6):249-258
To compare concentration profiles in solids in an objective manner, a multi-dimensional generalization of the comparison of two means by Student'st-test is proposed. The approach is based on piecewise modelling the profiles using rougha priori information, estimating the model parameters and their covariance matrix and comparing the estimated parameters by use of multidimensional test variables. The piecewise modelling is treated in the most important cases in such a way that the partial functions are straight lines or cubic polynoms (splines) with continuity at all knots in each case. It enables us to compare whole profiles as well as the most relevant parts of them. Two ways of estimating the covariance matrix as a generalization of the variance estimation are discussed. The approach is useful for any kind of line profiles if rough information on the profile type is available. As an example SIMS depth-profiles of aluminum obtained from thin-layer systems after different technological heat treatments are considered. These profiles are modeled by three straight lines. 相似文献
104.
《Journal of Molecular Structure》1995,343(1-3):125-132
A computer method has been developed which is an alternative to molecular dynamics in the sense that it pictures conformational motion. It simulates propagation in conformational movement based on conformational interconversion phenomena. The method starts with the knowledge of the topology of the conformational potential energy hyper-surface, which is described by the minima and the transition states. The topology is obtained by the recently described software
. The simulation of conformational motion is based on the Boltzmann statistics of movement between the minima and the transition states at a given temperature. The method is illustrated for methyl--glucopyranoside and -
-galNAc(1-3)[-
-Fuc(1-2)]Gal-O-Me molecules. Conformational transitions of hydroxyl groups as well as glycosidic linkages are discussed. 相似文献
105.
The performance of a recently developed method to generate representative atomistic models of amorphous polymers has been investigated. This method, which is denoted SuSi, can be defined as a random generator of energy minima. The effects produced by different parameters used to define the size of the system and the characteristics of the generation algorithm have been examined. Calculations have been performed on poly(L,D-lactic) acid (rho = 1.25 g/cm3) and nylon 6 (rho = 1.084 g/cm(3)), which are important commercial polymers. 相似文献
106.
D. M. Turdybekov K. M. Turdybekov E. V. Burdel'naya A. Zh. Turmukhambetov S. M. Adekenov Yu. V. Gatilov 《Chemistry of Natural Compounds》2003,39(1):19-21
The structure of lappacontine hydrobromide hydrate was solved by an x-ray structure analysis. 相似文献
107.
O. V. Shishkin 《Russian Chemical Bulletin》1997,46(9):1510-1512
The molecular geometries and inversion barriers of the rings in 2-oxo-1,2,3,4-tetrahydropyridine and its alkyl-substituted
(Me, Et, Pri, or But) and phenyl-substituted derivatives were calculated by the molecular mechanics method. The introduction of substituents has
no substantial effect on the equilibrium conformation of the heterocycle (a distorted sofa). For 4-alkyl- and 3-alkyl-substituted
derivatives (except for 4-Me and 4-Et derivatives), an axial orientation of the alkyl group is more favorable. The phenyl
substituents have equatorial and axial orientations at postions 4 and 3, respectively.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1584–1586, September, 1997. 相似文献
108.
From the de Broglie matter wave hypothesis and Planck’s energy quantization law, and assuming conservation of energy in the
absorption of a photon and its consequent conversion to kinetic energy of motion by a material particle initially at rest,
one can deduce a simple mathematical relationship between the wavelength λ (or frequency ν), of the photon absorbed by the
particle at rest, and the resulting de Broglie matter wave length, λD, of the particle with kinetic energy of motion of mv2/2. The relationship so deduced, λD∝√λ, suggests that visible wavelengths of light, from about 4000 ?, in the violet, to beyond about 7000 ?, in the red, on
absorption by an electron at rest, lead to material electron wavelengths, λD, of the order of the size of the electron transfer proteins seen in the photosynthetic reaction centers of photosynthesizing
organisms, at about a size of 50–100 ?. In addition to understanding the mechanism of photosynthesis as a material wave mediated
phenomenon, further areas of importance of the relations pointed out in this paper are in the design of experiments to gain
a deeper understanding of the basic tenets of wave mechanics, and in the use of tunable lasers to probe various properties
of material waves, and to precisely control their properties for applications including lithography. 相似文献
109.
二分岔现象(pitchfork bifurcation)是非平衡非线性动力系统中最典型和最简单的分岔现象.1972年Schlogl提出了如下一个化学反应模型: A+2x?3X,X?B (1)假定组份A和B的浓度可由外界控制为恒定,在某些条件下该模型可呈现出二分岔现象.自那以后,Schlogl模型已成为人们研究非平衡相变时最广泛采用的理论模型,该模型对推动非平衡非线性现象的研究起了十分重要的作用.但从物理-化学的角度来看,该模型存在着如下一些问题:首先该模型涉及一个三分子自催化反应步骤,这种反应步骤在现实反应系统 相似文献
110.
Teik-Cheng Lim 《Journal of mathematical chemistry》2003,33(3-4):279-285
The van der Waals forces for non-bonded interaction can be expressed either by the Exponential-6 or by the Lennard-Jones(m-n) potential functions, whereby m > n. Hitherto a relationship exists between the Exponential-6 and the Lennard-Jones(12-6) potential functions, with a scaling factor = 13.772 at or near the equilibrium and = 12.0 for long range interaction. This paper attempts to develop relationships between Exponential-6 and a more generalized Lennard-Jones(m-n). Analysis reveals that the relationship exists only when n = 6 and that two sets of scaling factors (as functions of index m) applies for the relationship between Exponential-6 and the Lennard-Jones(m-6), whereby m > 6. 相似文献