摘要:

AbstractThe anomeric effect of the functional groupsX= CN, CCH, COOH, COO−, OCH3, NH2, and NH+3has been studied withab initiotechniques. Geometry effects upon rotation around the central CO bond inXCH2OCH3have been compared in the various compounds. The energy differences between the conformers with a gauche and trans (XCOC) arrangement were calculated at the 6‐31G* level in the fully optimized 4‐21G geometries. Energy differences calculated at the 4‐21G level appeared not to be reliable, especially for the groups X that contain non‐sp3hybridized atoms. The 6‐31G* energy differences indicate a normal anomeric effect forX= COO−, OCH3, and NH2(g+) (ca. 13 kJ/mol) and a small anomeric effect forX= COOH, CN, and CCH (ca. 6 kJ/mol). ForX= NH2(t) and NH+3a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the groupXis varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angl

ISSN:0192-8651    

DOI:10.1002/jcc.540110702

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAs part of a study of factors controlling biological redox reactions of nicotinamide cofactors [nicotinamide adenine dinucleotide (phosphate) NAD(P)H], we have investigated the effect on a model reaction of the conformational state (cisortrans) of the carboxamide side chain, using quantum chemical methods. The reaction is that for the enzyme dihydrofolate reductase between the NADPH analogue, 1‐methyl‐dihydronicotinamide, and the protonated forms of the folate and dihydrofolate substrate analogues, pyrazine and dihydropyrazine. Some calculations on pterin and dihydropterin substrate analogues were also carried out in order to gauge the effects of inter‐ring coupling. The influence of carboxamide side‐chain conformation of nicotinamide on the energetics of the hydride‐ion transfer, and on the structures of the transition states and stable intermolecular‐interaction complexes, are examined as a function of the orientation of approach of the reactants. These approach geometries include those corresponding to the observed binding of cofactor and either substrate or inhibitor in the enzyme active site. Reactant, product, reactants‐complex, and transition‐state geometries were optimized at the semiempirical AM1 level, whileab initioSCF/STO‐3G and SCF/3‐21G single‐point calculations were carried out at the AM1 optimized geometries for all species, as well as full geometry optimizations for isolated reactants and products. The results show that reactants‐complex and transition‐state energies are lower for thetransconformer of dihydronicotinamide than for thecisconformer, due to more favorable H‐bonding or electrostatic interactions with the protonated substrate. Also, consideration of the structural parameters, including reaction coordinate bond lengths, ring geometries, and charge distributions, indicate that thetranstransition states are more product‐like than those for thecis.For the (trans) approaches corresponding to the enzymic orientation for substrate, the intermolecular interaction for the folate reaction lacks the stabilizing influence of the formal H‐bond which is present for the dihydrofolate reaction, and consequently the reactants‐complex and

ISSN:0192-8651    

DOI:10.1002/jcc.540110703

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractTwo‐dimensional Nuclear Magnetic Resonance (2DFTNMR) is presently the most powerful tool to determine protein structures in solution. Peak assignment (an interpretation of the two‐dimensional spectra that leads to the individuation of pairs of Hydrogen atoms that are involved in an NOE peak) is a cornerstone of such use of 2DFTNMR. Manual peak assignment of a protein often requires months of work by a specialized equipe. An automation of this task could speed up the protein study process, or alternatively allow to study previously unmanageable proteins. This article describes PEPTO, an expert system for the interpretation of sets of 2DFTNMR spectra on proteins. The present version of the program deals with spectra obtained from NOESY and COSY experiments. Tests of PEPTO on simulated spectra of five proteins with known assignments are also described and dicus

ISSN:0192-8651    

DOI:10.1002/jcc.540110704

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractComplexes of alkali and alkaline earth cations with organic compounds are modeled by describing ionligand interaction energies with pair potentials and intraligand as well as interligand energies with the MM2 potential. New pair potentials for the interaction of Li+, Na+, K+, Mg2+, and Ca2+ions are derived on the basis of 30,000ab initiointeraction energy values with 70 selected model ligand molecules. Various problems of the combination of these two basically different potentials are discussed. An application for the K+complex of 18‐crown‐6 is presented. For more flexible ligands the introduction of three‐body correction terms of the pair potentials seems nece

ISSN:0192-8651    

DOI:10.1002/jcc.540110705

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA computer program is developed to compute distance polynomials of graphs containing up to 200 vertices. The code also computes the eigenvalues and the eigenvectors of the distance matrix. It requires as input only the neighborhood information from which the program constructs the distance matrix. The eigenvalues and eigenvectors are computed using the Givens‐Householder method while the characteristic polynomials of the distance matrix are constructed using the codes developed by the author before. The newly developed codes are tested out on many graphs containing large numbers of vertices. It is shown that some cyclic isospectral graphs are differentiated by their distance polynomials although distance polynomials themselves are in general not unique structural invariant

ISSN:0192-8651    

DOI:10.1002/jcc.540110706

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA van der Waals‐type parameter set has been developed for hydrogen bonding in the MM2(85) force field to reproduce the structures and binding energies of complexes (H2O)2, (CH3OH)2, H3N …︁ H2O, H3N …︁ HOCH3, (CH3)3N …︁ H2O, and HCONH2…︁ H2O reported in the literature. The MM2(85) force field modified in this way was then applied to the title compound to explain its unexpected stability, unusual properties of its hydroxyl group, and solvent effect on the reaction mechanism in the condensation of Kryptofix 22 ® wit

ISSN:0192-8651    

DOI:10.1002/jcc.540110707

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe MM3 force field has been extended to include alkenes. Forty‐five compounds were examined, and structures, conformational equilibria, heats of formation, and rotational barriers, were calculated. For a smaller representative group, the vibrational spectra and entropies were also calculated. In general, these quantities, except for the vibrational spectra, agree with available data to approximately within experimental error. The vibrational frequencies for a set of eight well‐assigned structures were calculated to a root‐mean‐square error of

ISSN:0192-8651    

DOI:10.1002/jcc.540110708

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe MM3 molecular mechanics program has been extended to conjugated systems. A VESCF method is applied to the pi‐system to calculate bond orders, from which various stretching and torsional parameters are obtained. The procedure gives somewhat better results than the analogous MM2 calculations. It has been applied to a study of 81 compounds of aromatic and other conjugated hydrocarbons, as well as 45 alkenes and unconjugated polyenes. The structures calculated are generally in good agreement with experiment, and the heats of formation of these compounds can be calculated with a rms value of 0.62 kcal/mol, which may be compared with the average experimental error of 0.61 kcal/mol. In addition, vibrational frequencies for five representative conjugated model structures are calculated, with an rms value of 46 cm−1, and from these, other properties such as entropy can be calcula

ISSN:0192-8651    

DOI:10.1002/jcc.540110709

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractAb initioreaction path calculations for the cation radical vinylcyclobutane rearrangement at the MP2/ 6‐31G*//3‐21G level reveal a concerted,srreaction path with an activation energy of 9.4 kcal/mol. The vinylcyclobutane cation radical itself, at both the MP2 and MP3 levels of theory has predominant olefin cation radical character but with modest stretching of one of the adjacent ring carbon—carbon

ISSN:0192-8651    

DOI:10.1002/jcc.540110710

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

ISSN:0192-8651    

DOI:10.1002/jcc.540110701

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY