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排序方式: 共有257条查询结果,搜索用时 15 毫秒
111.
Glass RS Gruhn NE Lorance E Singh MS Stessman NY Zakai UI 《Inorganic chemistry》2005,44(16):5728-5737
Stannylated dinuclear iron dithiolates (mu-SSnMe(2)CH(2)S)[Fe(CO)(3)](2), (mu-SCH(2)SnMe(2)CH(2)S) [Fe(CO)(3)](2), and (mu-SCH(2)SnMe(3))(2)[Fe(CO)(3)](2), which are structurally similar to the active site of iron-only hydrogenase, were synthesized and studied by gas-phase photoelectron spectroscopy. The orbital origins of ionizations were assigned by comparison of He I and He II photoelectron spectra and with the aid of hybrid density functional electronic structure calculations. Stannylation lowers the ionization energy of sulfur lone pair orbitals in these systems owing to a geometry-dependent interaction. The Fe-Fe sigma bond, which is the HOMO in all these systems, is also substantially destabilized by stannylation due to a previously unrecognized geometry-dependent interaction between axial sulfur lone pair orbitals and the Fe-Fe sigma bond. Since cleaving the Fe-Fe sigma bond is a key step in the mechanism of action of iron-only hydrogenase, these newly recognized geometry-dependent interactions may be utilized in designing biologically inspired hydrogenase catalysts. 相似文献
112.
Chuan Lin Shaheen A. Al-Muhtaseb James A. Ritter 《Journal of Sol-Gel Science and Technology》2003,28(1):133-141
Very fine nickel hydroxide and oxide xerogel powders were prepared using a new sol-gel synthesis procedure in which nickel ethoxide was produced through the reaction of nickel chloride, as a precursor, with sodium ethoxide in dehydrated ethanol, followed by the hydrolysis of nickel ethoxide with ammonia and drying the resulting hydrogel under subcritical pressures to form the xerogel. The effects of thermal treatment on the surface area, pore volume, crystallinity and particle structure of the resulting xerogels were investigated and found to have significant effects on all of these properties. Overall, the xerogel remained amorphous as Ni(OH)2 space up to 200°C, with little change in the surface area and pore volume. At 250°C, the Ni(OH)2 began to decompose and form crystalline NiO with the uniformity of the crystals increasing with an increase in temperature. The surface area and pore volume decreased sharply when increasingthe temperature beyond 250°C; this was the temperature where maximums of about 270 m2/g and 0.33 cm3/g were exhibited by this composite amorphous Ni(OH)2 and crystalline NiO xerogel powders. At the higher calcination temperatures, very uniform NiO crystals with average crystallite sizes of 1.7 nm and 14.5 nm were obtained at 400 and 600°C, respectively. 相似文献
113.
Summary 1,2-Diphenyl-2-(benzoylhydrazino)ethanol (DBE) and 1,2-diphenyl-2-(p-nitrobenzoylhydrazino)ethanol (DPNBE) form complexes of general formulae [M(H2L)2]X2 and [M2L2] (where M = Co or Ni, H2L and L are the neutral and dibasic forms of the ligands and X = Cl or NO3). The analytical, spectral, magnetic and physical studies show the complexes to be of two types, namely monomeric cationic species prepared in neutral media and neutral binuclear oxygen-bridged species formed in alkaline solution. The ligands are tridentate, coordinating through carbonyl (or enolic) oxygen, azomethine nitrogen and alcoholic oxygen atoms. 相似文献
114.
Khashayar Karimian Farajolah Mohanazadeh Shaheen Rezai 《Journal of heterocyclic chemistry》1983,20(4):1119-1121
Covalent immobilization of the thiazole nucleus on a chloromethylated polystyrene copolymer results in the formation of a catalytically active thiazolium salt, capable of repeated acyloin and benzoin condensations. The nature of the solvent and the substituents on the thiazolium nucleus has been found to have marked effects of the rate of catalysis. 相似文献
115.
Kalauni SK Choudhary MI Khalid A Manandhar MD Shaheen F Atta-ur-Rahman Gewali MB 《Chemical & pharmaceutical bulletin》2002,50(11):1423-1426
From the leaves of Sarcococca coriacea two new steroidal alkaloids, epoxynepapakistamine-A [(20S)-20-(N-methylamino)-3beta-(tigloylamino)-5alpha-pregna-16alpha,17alpha-epoxy-2beta,4beta-di-O-acetate] (1), and epoxysarcovagenine-D [(20S)-20-(N-methylamino)-3beta-(tigloylamino)-5alpha-pregna-2-en-16alpha,17alpha-epoxy-4-one] (2), and two known compounds funtumafrine C [(20S)-20-(N,N-dimethylamino)-5alpha-pregna-3-one] (3) and N-methylfuntumine (4) were isolated. Their structures were elucidated on the basis of their spectral properties. The compounds 1, 3 and 4 were found to have cholinesterase inhibitory activity when tested for the inhibition of electric eel acetylcholinesterase and horse serum butyrylcholinesterase. They inhibited both enzymes in a concentration dependent fashion. 相似文献
116.
A hypothesis is presented for explaining the experimentally determined anomalous D/H ratio observed in the samples from the naturally occurring fission reaction in the Oklo phenomenon. No other explanation has been given, to our best knowledge, for the large difference between the measured D/H ratio in the Oklo samples and the expected values in a fission neutron spectrum. A multicomponent system consisting of hydrogen, deuterium, tritium and helium nuclei is considered. An analytical solution is derived and solved using as boundary conditions the experimentally determined value of the D/H ratio. The solution of the rate equations for hydrogen and deuteron concentrations, assuming a pure fission process without a deuteron sink term, yields a D/H ratio of 445 ppm for a reaction in which the fluence of neutrons is 1021 n/cm2. This exceeds the experimentally observed value of 127 ppm, and the naturally occurring value of 150 ppm. Solving the same rate equations accounting for a deuterium sink term using a hypothesis of deuteron disintegration, and the experimentally observed value of 127 ppm yields a deuteron disintegration constant of 7.47·10–14 s–1. Deutron disintegration would provide a neutron source, in addition to the fission neutrons, driving a subcritical chain reaction over an extended period of time. Relationship of the presented hypothesis to the Vlasov theory of an annihilation meteorite impact explosion explaining the experimentally observed anomalous235U/238U ratio, and to the suggestion of deuteron disintegration as a possible explanation of some observations of deuterium dissociation in palladium and titanium electrodes is discussed. The tritium and helium-3 rate equations are further solved under the deuteron disintegration hypothesis and the relationship of the present work to the work by JONES et al. is discussed. 相似文献
117.
M.Iqbal Choudhary SyedGhulam Musharraf MahmudTareqHassan Khan Doaa Abdelrahman Massod Parvez Farzana Shaheen Atta‐ur‐Rahman 《Helvetica chimica acta》2003,86(10):3450-3460
The microbial transformation of (+)‐isolongifolen‐4‐one ( 4 ) by a number of fungi by means of a standard two‐stage fermentation technique afforded (7R)‐12‐hydroxyisolongifolen‐4‐one ( 5 ), (7S)‐13‐hydroxyisolongifolen‐4‐one ( 6 ), (11R)‐11‐hydroxyisolongifolen‐4‐one ( 7 ), (10R)‐10‐hydroxyisolongifolen‐4‐one ( 8 ), and (9R)‐9‐hydroxyisolongifolen‐4‐one ( 9 ) (Scheme). All five metabolites were found to be new, and metabolites 6 and 9 showed potent tyrosinase inhibitory activity (Table 1). The metabolites and their derivatives were characterized on the basis of spectroscopic and single‐crystal X‐ray‐diffraction techniques. 相似文献
118.
BinaShaheen Siddiqui ZhanatZhymabekovna Karzhaubekova GauharShahmanovna Burasheva NurgylAdaibaevna Sultanova 《Helvetica chimica acta》2006,89(5):962-970
Studies on the chemical constituents of the aerial parts of Kalidium foliatum led to the isolation of three new olean‐12‐ene‐23,28‐dioic acids in their either partly or fully esterified forms (Me or glucosyl (Glc) esters), which were named kalidiumoside A ( 1 ), kalidiumin ( 2 ), and kalidiumoside B ( 3 ). Their structures were elucidated through spectral studies including 2D‐NMR experiments (HMQC, HMBC, COSY, NOESY) and J‐resolved spectra. Also isolated were the two known compounds dianic acid and dianoside F, which were identified through comparison of their spectroscopic data with those reported in the literature. 相似文献
119.
Choudhary MI Sultan S Khan MT Yasin A Shaheen F Atta-ur-Rahman 《Natural product research》2004,18(6):529-535
Fermentation of (+)-androst-4-ene-3,17-dione (1) with Curvularia lunata for 10 days yielded five oxidative and reductive metabolites, androsta-1,4-diene-3,17-dione (2), 17beta-hydroxyandrosta-1,4-dien-3-one (3), 11alpha-hydroxyandrost-4-ene-3,17-dione (4), 11alpha,17beta-dihydroxyandrost-4-en-3-one (5) and 15alpha-hydroxyandrosta-1,4-dien-17-one (6). The structures of these metabolites were elucidated on the basis of spectroscopic techniques. These microbially transformed products were assayed against the clinically important enzymes, tyrosinase and prolyl endopeptidase. 相似文献
120.
M.Iqbal Choudhary KrishnaPrasad Devkota SarfrazAhmad Nawaz Farzana Shaheen Atta‐ur‐Rahman 《Helvetica chimica acta》2004,87(5):1099-1108
Bioassay‐guided phytochemical investigation of Sarcococca hookeriana has resulted in the isolation and structure elucidation of five new pregnane‐type steroidal alkaloids: (?)‐hookerianamide A (=(2β,3β,4β,20S)‐20‐(dimethylamino)‐3‐[(3‐methylbut‐2‐enoyl)amino]‐5α‐pregn‐16‐ene‐2,4‐diol; 1 ), (+)‐hookerianamide B (=(2α,3β,4β,20S)‐4‐acetoxy‐20‐(dimethylamino)‐3‐[(3‐methylbut‐2‐enoyl)amino]‐5α‐pregnan‐2‐ol; 2 ), (?)‐hookerianamide C (=(2β,3β,20S)‐2‐acetoxy‐20‐(dimethylamino)‐3‐[(3‐methylbut‐2‐enoyl)amino]‐5α‐pregnane; 3 ), (?)‐hookerianamine A (=(3β,20S)‐20‐(dimethylamino)‐3‐(methylamino)‐5α‐pregn‐14‐ene; 4 ), and (+)‐phulchowkiamide A (=(3β,20S)‐20‐(methylamino)‐3‐[(2‐methylbut‐2‐enoyl)amino]‐5α‐pregn‐2‐en‐4‐one; 5 ). These compounds, as well as the two chemically derived acetyl derivatives 6 and 7 , displayed cholinesterase inhibition in a concentration‐dependent manner. 相似文献