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31.
A global optimization strategy, based upon application of a genetic algorithm (GA), is demonstrated as an approach for determining the structures of molecules possessing significant conformational flexibility directly from gas-phase electron diffraction data. In contrast to the common approach to molecular structure determination, based on trial-and-error assessment of structures available from quantum chemical calculations, the GA approach described here does not require expensive quantum mechanical calculations or manual searching of the potential energy surface of the sample molecule, relying instead upon simple comparison between the experimental and calculated diffraction pattern derived from a proposed trial molecular structure. Structures as complex as all-trans retinal and p-coumaric acid, both important chromophores in photosensing processes, may be determined by this approach. In the examples presented here, we find that the GA approach can determine the correct conformation of a flexible molecule described by 11 independent torsion angles. We also demonstrate applications to samples comprising a mixture of two distinct molecular conformations. With these results we conclude that applications of this approach are very promising in elucidating the structures of large molecules directly from electron diffraction data. 相似文献
32.
Ahmed El Abed Etienne Gaudin Jacques Darriet 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(10):i138-i140
Single crystals of Sr4Mn2.09Cu0.91O9 have been grown by flux synthesis and the structure, closely related to the hexagonal perovskite 2H, was solved from single‐crystal X‐ray data in space group P321. The structure of Sr4Mn2CuO9 is composed of chains of face‐sharing polyhedra with a sequence of two octahedra and one trigonal prism. The octahedra are filled by Mn atoms and the Cu atoms are randomly distributed at the centres of the square faces of the trigonal prism. A stacking fault is observed within one of the two chains, which can be attributed to a shifting of the chain along the c axis. 相似文献
33.
An efficient and mild method for the reduction of aromatic nitro and azido groups on solid support using Al/NiCl2·6H2O and Al/NH4Cl is described. This solid phase reduction technique has been applied towards the synthesis of DNA binding pyrrolo[2,1-c][1,4]benzodiazepine antitumour antibiotics. 相似文献
34.
Ahmed M. El-Sayed Esmat M. A. Hamzawy 《Monatshefte für Chemie / Chemical Monthly》2006,137(9):1119-1125
Summary. The magnetic and microstructure properties of Fe2O3–0.4NiO–0.6ZnO–B2O3 glass system, which was subjected to heat treatment in order to induce a magnetic crystalline phase (Ni0.4Zn0.6-Fe2O4 crystals) within the glass matrix, were investigated. DSC measurement was performed to reveal the crystallization temperature
of the prepared glass sample. The obtained samples, produced by heat treatment at 765°C for various times (1, 1.5, 2, and
3 h), were characterized by X-ray diffraction, IR spectra, transmission electron microscopy, and vibrating sample magnetometer.
The results indicated the formation of spinel Ni–Zn ferrite in the glass matrix. Particles of the ferrite with sizes ranging
from 28 to 120 nm depending on the sintering time were observed. The coercivity values for different heat-treatment samples
were found to be in the range from 15.2 to 100 Oe. The combination of zinc content and sintering times leads to samples with
saturation magnetization ranging from 12.25 to 17.82 emu/g. 相似文献
35.
Ahmed M. Mayouf Yong-Tae Park 《Journal of photochemistry and photobiology. A, Chemistry》2002,150(1-3):115-123
The UV photolysis of 2′-chloro-4-R-benzanilides in acetonitrile solution under nitrogen atmosphere leads to the formation of intramolecular photocyclization products (9-R-phenanthridin-6(5)-one, 20–30%), along with the minor photoreduction, photo-Fries, and intramolecular photosubstitution products. The photoreaction of 2′-chloro-4-R-benzanilide in acetonitrile solution containing 10% of water or aqueous sodium hydroxide however gives 2-(4-R-phenyl)-1,3-benzoxazole as a major product (30%) along with the photoreduction, photo-Fries and intramolecular photocyclization products. 相似文献
36.
M. R. Ahmed A. M. Demidov S. A. Al-Najjar M. A. Al-Amili 《Journal of Radioanalytical and Nuclear Chemistry》1974,23(1-2):199-203
Gamma-spectra of stainless-steel and soil samples have been measured to study the potentialities of using spectroscopy of γ-radiation from the inelastic scattering of reactor fast neutrons for element analysis of various materials. Contents of iron, chromium, nickel, manganese, silicon and titanium in stainless steel, and those of silicon, iron, aluminium, magnesium, calcium, sodium, titanium, carbon and oxygen in soil have been determined by comparing the spectra of the samples with those of specially prepared mixtures of elements. The distinguishing features, advantages, and limitations of this method are discussed. 相似文献
37.
El-Sayed A. Hegazy Ahmed M. Dessouki A. M. Rabie I. Ishigaki 《Journal of polymer science. Part A, Polymer chemistry》1984,22(12):3673-3685
Some properties of the membranes obtained by preirradiation grafting of acrylic acid onto poly(tetrafluoroethylene-perfluorovinyl ether) copolymer (PFA) films have been investigated. The dimensional change caused by grafting and swelling behavior, water uptake, electrical conductivity, and mechanical properties of the grafted films were found to increase as the grafting proceeds. The influence of the preparation conditions (such as preirradiation dose, monomer concentration, grafting temperature, and film thickness) on those properties was studied. These properties were found to be dependent mainly on the degree of grafting regardless of grafting conditions, except at higher monomer concentration (>40 wt %). The electric conductivity and mechanical properties for the membranes obtained at higher AAc concentrations were lower than those obtained at lower ones. Analysis by x-ray microscopy of the grafted films revealed that the grafting begins at the part close to the film surface and proceeds into the central part with progressive diffusion of monomer to give finally homogeneous distribution of the electrolytes in the whole bulk of the polymer. The membranes show good electrochemical and mechanical properties which make them acceptable for practical use as cation-exchange membranes. 相似文献
38.
M. E. Ahmed M. S. Ibrahim Y. M. Temerk A. M. Kawde 《Monatshefte für Chemie / Chemical Monthly》1996,127(6-7):609-619
Summary A systematic comparative study of the adsorption and association of 3-methylxanthine (3MXan) and 7-methylxanthine (7MXan) at mercury-solution interfaces in acidic, neutral, and alkaline buffer solutions was undertaken by ac voltammetry. At bulk concentrations of3MXan above a threshold value, the stacking interactions between vertically oriented molecules lead to a slow reorientation aroundE
ecm, and the molecules adopt a perpendicular orientation. The association of the adsorbed molecules of7MXan is hindered and is not effective in promoting base-base stacking interactions. The enhanced surface activity of3MXan compared to7MXan results from the different position of the methyl group in the purine moiety. The effect of some divalent metal ions on adsorption stages and association of the investigated compounds has been studied. The results indicate that the complexation of methylated xanthine enhances the stacking interactions and hence would be expected to facilitate the formation of perpendicularly stacked layers ofM(II)-MXan complexes on the electrode surface. The adsorption parameters of the investigated compounds have been computed in absence and presence of Cu(II) at differentpH values. The results are compared with the behaviour of xanthine.
Vergleichende Untersuchungen zur Adsorption und Assoziation von 3-Methylxanthin und 7-Methylxanthin an einer geladenen Grenzfläche
Zusammenfassung Eine systematische vergleichende Untersuchung zur Adsorption und Assoziation von 3-Methylxanthin (3MXan) und 7-Methylxanthin (7MXan) an Quecksilber-Lösungs-Grenzflächen wurde mittels AC-Voltammetrie in sauren, neutralen und basischen Pufferlösungen durchgeführt. Oberhalb einer bestimmten Grenzkonzentration kommt es durch Wechselwirkungen zwischen vertikal angeordneten3MXan-Molekülen zu einer langsamen Reorientierung umE ecm, was zu einer senkrechten Anordnung führt. Die Assoziation von7MXan ist gehindert und erlaubt daher keine ausgeprägten Basen-Basen-Wechselwirkungen. Die erhöhte Oberflächenaktivität von3MXan gegenüber7MXan resultiert aus der unterschiedlichen Stellung der Methylgruppe am Purinrest. Der Effekt einiger zweiwertiger Metallionen auf die Adsorption und Assoziation von3MXan und7MXan wurde untersucht. Die Ergebnisse weisen auf eine Begünstigung der Wechselwirkungen hin; die Bildung von vertikal angeordneten Grenzschichten an der Elektrodenoberfläche sollte daher im Fall vonM(II)-Mxan-Komplexen erleichtert sein. Die Adsorptionsparameter der untersuchten Verbindungen wurden in Abwesenheit und in Gegenwart von Cu(II) bei verschiedenenpH-Werten berechnet. Die Ergebnisse werden mit dem Verhalten von Xanthin verglichen.相似文献
39.
Determination of camptothecin in biological fluids using reversed-phase high-performance liquid chromatography with fluorescence detection 总被引:1,自引:0,他引:1
Camptothecin, a plant alkaloid with antitumor activity, is a potent and rapidly acting inhibitor of DNA synthesis. The objective of this study was to develop a sensitive high-performance liquid chromatographic (HPLC) method for the detection and estimation of the camptothecin concentration in biological fluids. Using HPLC coupled with fluorescence detection, at an excitation wavelength of 370 nm and an emission wavelength of 434 nm, we found that the lower limits of detection for camptothecin in aqueous, plasma and urine samples were 0.5, 1 and 10 ng/ml, respectively. The ideal mobile phase used was methanol-10 mM potassium phosphate (75:25, v/v, pH 4.0). To determine the utilization of the method in a biological system, we studied the pharmacokinetics of camptothecin in mice. Elimination of camptothecin from mice blood was triphasic and followed first-order kinetics. The half-life of camptothecin in mouse blood was 25.7 min. Our studies indicate that HPLC with fluorescence detection for the determination of camptothecin in different media is a simple, rapid, sensitive and reproducible method. 相似文献
40.
Marine brown alga Dictyota dichotoma (Dictyotaceae) collected from Karachi coast of Arabian Sea yielded two new (1 and 2) enones (dolastane-diterpenoids) named: dichotenone-A and -B along with an olide (loliolide, 3) as a new source. Their structures have been characterized with the aid of 2D-NMR spectroscopic techniques. 相似文献