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71.
The conversion of methylenedimethyloxonium ylide to ethyl methyl ether was investigated by the semiempirical MNDO method. An activation energy of 51 kJ/mol suggests that this rearrangement is a possible elementary reaction in the well-known MTG process.
. 51 /, , .相似文献
72.
The racemic mixtures of the two epimers of 3,4-diethyl-l,2,3,3a,4,5-hexahydro-canthinone-6 (3,4-diethyl-2,3,3a,4,5,6-hexahydro-6-oxo-1H-indolo-(3,2,1 ,de) (1,5)naphtyridine) have been prepared. They were separated by crystallization of one of the synthesis intermediates. Identification of the configuration was possible by 1h NMR spectroscopy after running a 2D J-resolved spectrum of the “cis”-isomer. 相似文献
73.
André Michel Gérald Villeneuve John DiMaio 《Journal of computer-aided molecular design》1991,5(6):553-569
Summary The molecular basis underlying the divergent receptor selectivity of two cyclic opioid peptides Tyr-c[N
-d-Orn2-Gly-Phe-Leu-] (c-ORN) and [d-Pen2, l-Cys5]-enkephalinamide (c-PEN) was investigated using a molecular modeling approach. Ring closure and conformational searching procedures were used to determine low-energy cyclic backbone conformers. Following reinsertion of amino acid side chains, the narcotic alkaloid 7-[(1R)-1-methyl-1-hydroxy-3-phenylpropyl]-6,14-endoethenotetrahydro oripavine (PEO) was used as a flexible template for bimolecular superpositions with each of the determined peptide ring conformers using the coplanarity and cocentricity of the phenolic rings as the minimum constraint. A vector space of PEO, accounting for all possible orientations for the C21-aromatic ring of PEO served as a geometrical locus for the aromatic ring of the Phe4 residue in the opioid peptides. Although a vast number of polypeptide conformations satisfied the criteria of the opiate pharmacophore, they could be grouped into three classes differing in magnitude and sign of the torsional angle values of the tyrosyl side chain. Only class III conformers for both c-ORN and c-PEN, having tyramine dihedral angles 1 =–150° ± 30° and 2=–155° ± 20°, had significant structural and conformational properties that were mutually compatible while respecting the PEO vector space. Comparison of these properties in the context of the divergent receptor selectivity of the studied opioid peptides suggests that the increased distortion of the peptide backbone in the closure region of c-PEN together with the pendant ,-dimethyl group, combine to generate a steric volume which is absent in c-ORN and that may be incompatible with a restrictive topography of the receptor. The nature and stereo-chemistry of substituents adjacent to the closure region of the peptides could also modulate receptor selection by interacting with a charged () or neutral () subsite. 相似文献
74.
Johannes André 《Journal of Geometry》1992,43(1-2):22-29
Configurational conditions (Schließungsaussagen) of a noncommutative space will be developped from pairs (,
) of digraphs where is a partial digraph of
. In this way we obtain an extensive generalization of Pfalzgraf 's q-simplex-conditions Simq.In Memoriam Hans Zassenhaus 相似文献
75.
Résumé Les caractéristiques électroniques (énergie, charges , géométrie, moment dipolaire, énergie de transition) des quatre formes tautomères de la purine et des phényl-8 N(1)H et N(3)H purines ont été déterminées par la méthode LCAO améliorée. La forme 9 est trouvée la plus stable à l'état isolé. Une interprétation nouvelle des bandes d'absorption est proposée.
Les auteurs remercient le Professeur B. Pullmann (Paris) de l'intérêt qu'il a porté à ce travail, ainsi que Madame H. Feilchenfeld (Jérusalem) pour les résultats inédits qu'elle a bien voulu leur communiquer. 相似文献
Researches on the electronic structure and the characteristics of various tautomer forms of Purine
The electronic characteristics (energy, charge , geometry, dipole moment, transition energy) of the four tautomeric forms of the purine and of the phenyl-8 N(1)H and N(3)H purines have been determined by the improved LCAO method. The form 9 is considered as the most stable at the isolated state. A new interpretation of the absorption bands is proposed.
Zusammenfassung Die elektronischen Charakteristiken (Energie, Ladung , Geometrie, Dipolmoment, Übergangsenergie) der vier Tautomerformen des Purins und der Phenyl-8 N(1)H und N(3)H Purine sind durch die verbesserte LCAO-Methode bestimmt worden. Die Form 9 ist, zum isolierten Stand, als am meisten beständig zu betrachten. Eine neue Erklärung der Absorptionsbänder ist vorgeschlagen.
Les auteurs remercient le Professeur B. Pullmann (Paris) de l'intérêt qu'il a porté à ce travail, ainsi que Madame H. Feilchenfeld (Jérusalem) pour les résultats inédits qu'elle a bien voulu leur communiquer. 相似文献
76.
R.R. Andréa P.C.J. Beentjes D.J. Stufkens A. Oskam 《Journal of organometallic chemistry》1985,288(1):79-88
He(I) and He(II) photoelectron spectra are reported for the 1-aza-1,3-butadienes (R1NCHCHCHR2 denoted by R1,R2-ABD) t-Bu,Me-ABD and i-Pr,Ph-ABD and their tricarbonyliron complexes [Fe(CO)3(R1,R2-ABD)]. Assignments of ionizations from the iron d and ligand orbitals have been made with the aid of He(I)/He(II) intensity ratios and some semi-empirical molecular orbital calculations on the model ligand Me,H-ABD (MNDO) and on the model complex [Fe(CO)3(H,H-ABD)] (CNDO/S).A remarkable feature is the lowering of the ionization energy from the Fe dxz/yz2 orbital with respect to the other d orbitals (dxy/dx2–y2/dz2)6 by about 0.9 eV, an effect which has not been found for the related [Fe(CO)3(1,3-butadiene)] complexes. The involvement of the nitrogen lone pair in the bonding between the R1,R2-ABD and Fe(CO)3 moieties is discussed. 相似文献
77.
Benijts T Günther W Lambert W De Leenheer A 《Rapid communications in mass spectrometry : RCM》2003,17(16):1866-1872
A sonic spray ionization liquid chromatography/mass spectrometry (LC/SSI-MS) procedure combined with off-line solid-phase extraction was optimized for the analysis of 20 endocrine-disrupting chemicals (EDCs) in water samples. Method development included a comparison of the novel sonic spray ionization (SSI) with more traditional ion sources, i.e. pneumatically assisted electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). It was demonstrated that SSI and ESI spectra were very similar, but were more prone to the formation of solvent cluster ions as compared with APCI spectra. This phenomenon was most prominent for SSI and resulted in an increased chemical background in full-scan mass spectra. However, this chemical noise did not affect the overall sensitivity of SSI and ESI. After optimization of LC and MS parameters, the LC/SSI-MS method was validated. Recoveries ranged from 76.3 up to 113.4% for all compounds. Limits of detection (LOD) and quantitation (LOQ) were established between 3.0 and 11.5 ng/L and 9.9 and 38.0 ng/L, respectively. Within-day (n = 5) and between-day (n = 5) reproducibility were investigated at three levels and ranged from 3.3-16.5% and 7.6-19.2%, respectively. Eight-point calibration curves were established and showed linearity for all compounds (r(2) > 0.987) over a linear dynamic range of 10-10 000 ng/L. 相似文献
78.
Maurice Guillot André Marchand François Tcheou H. Le Gall 《Zeitschrift für Physik B Condensed Matter》1982,49(3):221-228
On single crystals of holmium iron garnet (HoIG), magnetic properties have been studied in magnetic field up to 150kOe applied parallel to the main crystallographic directions in the 4.2–300K temperature range. Above 130 K, the magnetization is isotropic and linear magnetic field dependent as previously found in polycrystals and predicted by Néel's ferrimagnetic theory. Nevertheless the paramagnetic Curie temperature is much higher than the polycrystal value. Below 130K, due to the onset of the umbrella structure, the ferrite magnetization presents a non linear field variation with [111] as easy direction. The field evolutions of the anisotropy constants (K
1 andK
2) were calculated. Optical absorption measurements of both 585I
6 and 5I
85I
7 transitions are reported and compared to the results of the literature in terms of inequivalent magnetic sites. 相似文献
79.
1-Deoxy-D-xylulose-5-phosphate (DXP) is a key intermediate in the non-mevalonate pathway to terpenoids in bacteria, and it is the substrate for the enzyme 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXP-R). In order to study the mechanism of DXP-R, we required a flexible synthesis of the substrate which would allow the incorporation of isotopic labels, and the variation of the two stereocentres. Thus 1,4-dihydroxypent-2-yne was selectively reduced to give the E-olefin, and selective phosphorylation of the primary alcohol followed by oxidation of the secondary alcohol gave a substrate suitable for dihydroxylation. Dihydroxylation using stoichiometric OsO4 in the presence of chiral ligands gave protected DXP in high ee. Final hydrogenolysis gave DXP in quantitative yield and high purity. DXP-R was produced by rapid cloning of the dxr gene from Escherichia coli through controlled expression and ion exchange chromatography. The synthetic DXP was fully active in enzyme assays catalysed by recombinant DXP-R. 相似文献
80.
Andrés G. León Ana I. Olives M. Antonia Martín Benito del Castillo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):577-583
The use of cyclodextrins (CDs) in HPLC as mobile phase additives provides a flexible alternative for the separation of chemically
related compounds because these separations can be performed on conventional columns and are economically advantageous over
the use of chiral stationary phases. The present paper describes the influence of the presence of β-cyclodextrin (β-CD) and
2-hydroxypropyl-β-cyclodextrin (HPβ-CD) on the separation of the β-carboline alkaloids norharmane, harmane and harmine. The
nature of the stationary phase (reverse phases C1 and C18) affects the chromatographic separations and also the stability of the inclusion complexes that are developed. The changes
in the proportion of the organic solvents at constant concentration of CDs (3 mM for β-CD and 15 mM for HPβ-CD) modify the
retention factors (k′) for all alkaloids studied. The nature of the organic solvent in the mobile phase also changes the chromatographic parameters.
The logarithm of the capacity factor (k′) is linearly increased with the proportion of water in the hydro-organic mobile phase (ethanolic or methanolic) but the
slopes obtained vary depending on the CD added to the mobile phase. The role of competitive equilibria, i.e., chromatographic
distribution and inclusion complexes formation is discussed.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May, 14–17, 2006 相似文献