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991.
Petra A W Van Den Berg Koert Grever Arie Van Hoek Willem J H Van Berkel Antonie J W G Visser 《Journal of Chemical Sciences》2007,119(2):123-133
Conformational heterogeneity of the FAD cofactor in p-hydroxybenzoate hydroxylase (PHBH) was investigated with time-resolved polarized flavin fluorescence. For binary enzyme/substrate
(analogue) complexes of wild-type PHBH and Tyr222 mutants, crystallographic studies have revealed two distinct flavin conformations;
the ‘in’ conformation with the isoalloxazine ring located in the active site, and the ‘out’ conformation with the isoalloxazine
ring disposed towards the protein surface. Fluorescence-lifetime analysis of these complexes revealed similar lifetime distributions
for the ‘in’ and ‘out’ conformations. The reason for this is twofold. First, the active site of PHBH contains various potential
fluorescence-quenching sites close to the flavin. Fluorescence analysis of uncomplexed PHBH Y222V and Y222A showed that Tyr222
is responsible for picosecond fluorescence quenching free enzyme. In addition, other potential quenching sites, including
a tryptophan and two tyrosines involved in substrate binding, are located nearby. Since the shortest distance between these
quenching sites and the isoalloxazine ring differs only little on average, these aromatic residues are likely to contribute
to fluorescence quenching. Second, the effect of flavin conformation on the fluorescence lifetime distribution is blurred
by binding of the aromatic substrates: saturation with aromatic substrates induces highly efficient fluorescence quenching.
The flavin conformation is therefore only reflected in the small relative contributions of the longer lifetimes. 相似文献
992.
B. Lesiak A. Kosiński R. Nowakowski L. Kövér J. Tóth D. Varga I. Cserny A. Sulyok G. Gergely 《Surface and interface analysis : SIA》2007,39(10):798-804
In elastic peak electron spectroscopy (EPES), the nearest vicinity of elastic peak in the low kinetic energy region reflects electron inelastic and quasielastic processes. Incident electrons produce surface excitations, inducing surface plasmons, with the corresponding loss peaks separated by 1–20 eV energy from the elastic peak. In this work, X‐ray photoelectron spectroscopy (XPS) and helium pycnometry are applied for determining surface atomic composition and bulk density, whereas atomic force microscopy (AFM) is applied for determining surface morphology and roughness. The component due to electron recoil on hydrogen atoms can be observed in EPES spectra for selected primary electron energies. Simulations of EPES predict a larger contribution of the hydrogen component than observed experimentally, where hydrogen deficiency is observed. Elastic peak intensity is influenced more strongly by surface morphology (roughness and porosity) than by surface excitations and quasielastic scattering of electrons by hydrogen atoms. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
993.
S. Vatsala V. Bansal D. K. Tuli M. M. Rai S. K. Jain S. P. Srivastava A. K. Bhatnagar 《Chromatographia》1994,38(7-8):456-460
Summary Hydrazine, an oxygen scavenger in boiler water, was derivatised to the corresponding acetone azine and determined at the ng ml–1 level by gas chromatography. Morpholine, a corrosion inhibitor used in steam boilers, was estimated either directly (if >2.0 g ml–1) or by quantitative preconcentration (0.1 ng – 2.0 g ml–1). To obtain symmetrical peaks for these amines, the column packing was coated with KOH. Use of a nitrogen-specific detector improved accuracy of estimation of hydrazine and morpholine, giving a RSD of 1.9–3.6%. Chromatographic analysis of these amines in boiler feed water and steam condensate samples collected from boilers servicing a petroleum refinery is described. Environmental safety regulations calls for monitoring of hydrazine and the methods developed can easily be adapted for this purpose. 相似文献
994.
iPP/sPP, iPP/rPP, iPP/PVDF and iPP/PA-6 blends, and their β-nucleated forms were prepared in the present study. The components of iPP/sPP and iPP/rPP blends are compatible in the molten state. The phase structure of the melt of iPP/PVDF and iPP/PA-6 blends is heterogeneous. The melting and crystallisation characteristics as well as the structure and polymorphic composition of these blends were studied by polarised light microscopy (PLM) and differential scanning calorimetry (DSC). When semicrystalline polymers are added to iPP, the most important factor of the formation a blend with β-crystalline phase is the α-nucleation effect of the second polymer. In the case of polymers with an α-nucleating effect, the temperature range of their crystallisation should be lower than that of β-iPP. β-nucleated iPP/PVDF and iPP/PA-6 blends are extreme examples showing that completely β-iPP matrix can not form even in the presence of a highly effective β-nucleant, because of the strong α-nucleating ability and higher crystallisation temperature range of PVDF and PA-6. We found that the β-crystallisation tendency of random propylene copolymers can be enhanced by adding an iPP homopolymer. 相似文献
995.
Korolev A. M. Eremenko L. T. Meshikhina L. V. Eremenko I. L. Aleksandrov G. G. Konovalova N. P. Lodygina V. P. 《Russian Chemical Bulletin》2003,52(8):1859-1863
New procedures were developed for the synthesis of 3,3-bis(hydroxymethyl)oxetane dinitrate (1) by O-nitration of the corresponding glycol (3) or its mononitrate (6), which were prepared by the reactions of 2,2-bis(hydroxymethyl)propane-1,3-diol (pentaerythritol) (2) mono- (4) and dinitrates (5), respectively, with alkali. A new method was devised for the synthesis of compounds 4 and 5 by the reaction of tetraol 2 with concentrated HNO3 in dichloroethane. The structures of compounds 1 and 6 were established by X-ray diffraction analysis. 相似文献
996.
S. V. Klement’eva N. F. Cherepennikova V. V. Semenov A. I. Kirillov M. A. Lopatin O. V. Kuznetsova Yu. A. Kurskii A. A. Zaitsev L. G. Klapshina V. E. Duglas G. A. Domrachev 《Russian Chemical Bulletin》2007,56(11):2214-2224
A method for the synthesis of polyphosphosiloxane by the thermal condensation of an equimolar mixture of trimethyl phosphate
and (3-aminopropyl)triethoxysilane at 200 °C was developed. The reaction affords ethanol and polyphosphosiloxane-{Si(OEt)[(CH2)3NR1R2]-O-P(O)(OMe)-O}n-(R1 = H, Me; R2 = Me), whose composition and structure were confirmed by 1H, 13C, and 31P NMR spectroscopy, IR spectroscopy, and elemental analysis. The scheme of polymerization involving the intermediate formation
of methyl-and dimethylphosphoric acids and their condensation with ethoxysilanes was proposed. The calcination of the obtained
polyphosphosiloxane in vacuo at 350 °C results in the elimination of the amino groups and alkoxide substituents, and a spatially cross-linked polymer
is formed as an amorphous powder. Its further thermolysis at 600 and 1000 °C gives crystalline phosphosilicates Si5O(PO4)6 or SiP2O7. Their amorphous and crystalline samples were characterized by IR spectroscopy, X-ray diffraction analysis, and solid-state
13C and 31P spectroscopy.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2138–2148, November, 2007. 相似文献
997.
The electronic structure of the benzaldehyde molecule has been studied by X-ray emission spectroscopy. The gas-phase O-K- and C-K-spectra of this compound have been obtained. MNDO quantum-chemical calculations have been carried out. The structure of the MO's of benzaldehyde has been compared with those of benzene and formaldehyde molecules. The character of the p-p interaction of the phenyl and formyl fragments has been considered. The contribution of the latter to the highest occupied molecular orbitals of the-system has been shown to be small.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1033–1037, June, 1994. 相似文献
998.
2-Trifluoromethyl-4H-thiochromene-4-thione obtained from 2-trifluoromethyl-4H-thiochromen-4-one and P2S5 reacts with aromatic amines, hydrazine hydrate, phenylhydrazine, and hydroxylamine at the C(4) atom of the chromene ring
to give the corresponding anils, azine, hydrazones, and oxime of thiochromone. 2-Trifluoromethyl-4H-thiochromen-4-one is oxidized by hydrogen peroxide in AcOH into 4-oxo-2-trifluoromethyl-4H-thiochromene 1,1-dioxide and reduced by NaBH4 to 2-trifluoromethyl-4H-thiochromen-4-ol or cis-2-(trifluoromethyl)thiochroman-4-ol. When treated with hydrazine hydrate, thiochromen-4-one gives 3(5)-(2-mercaptophenyl)-5(3)-trifluoromethylpyrazole.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 504–509, March, 2006. 相似文献
999.
Sergey I. Vdovenko Igor I. Gerus Natalia V. Lutenko Valery P. Kukhar Jacek Wjcik 《Journal of Molecular Structure》2007,840(1-3):125-132
Spatial structure of six β-substituted enones, with common structure R1O–CR2CH–COCF3, were R1 = C2H5, R2 = H (ETBO); R1 = R2 = CH3 (TMPO); R1 = C2H5, R2 = C6H5 (ETPO); R1 = C2H5, R2 = 4- O2NC6H4 (ETNO); R1 = C2H5, R2 = C(CH3)3 (ETDO) were investigated by 1H and 19F NMR, infrared spectroscopy and AM1 calculations. NMR spectra revealed that enones (MBO), (ETBO) and (TMPO) are exclusively (3E) isomers, whereas in (ETPO), (ETNO) and especially in (ETDO) the percentage of (3Z) isomers is significant and depends on the nature of solvents. Conformational behaviour of studied enones are determined by the rotation around of CC double bond, C–C and C–O single bonds (correspondingly trifluoroacetyl and alkoxy groups), and (EZZ) conformer being the most stable in all cases. IR spectra revealed that with the exception of (ETDO) (EZZ) conformer is most populated in all cases. Bulky substituents like phenyl or tert-butyl group at β-position of enone result in the equilibrium mainly between (EZZ) and (ZZZ) forms, whereas β-hydrogen and β-methyl substituents determine the equilibrium between (EZZ) and (EEZ) or (EZE) conformers. 相似文献
1000.
A new chelating polymeric sorbent has been developed using Merrifield chloromethylated resin anchored with quinoline-8-ol (HQ). The modified polymeric resin was characterized by FT-IR spectroscopy and elemental analysis. The HQ anchored resin showed superior binding affinity for U(VI) over Th(IV) and La(III). The influence of various physicochemical parameters on the recovery of U(VI) were optimized by both static and dynamic methods. The phase exchange kinetic studies performed for U(VI) revealed that <5 min was sufficient for reaching equilibrium metal ion sorption. The maximum sorption capacity of HQ anchored resin for U(VI) was found to be 120.30 mg g−1 of resin which is higher than other solid phase extraction sorbents reported so far excepting N,N-dibutyl, N′-benzoyl thiourea sorbed Amberlite XAD-16. The developed HQ anchored polymeric resin is highly selective as none of the extraneous species were found to have any deleterious effect. Solid phase extraction (SPE) studies performed using HQ anchored polymeric resin offered enrichment factor of 100 and the lowest concentration below which recoveries become non-quantitative is 5 μg l−1. The accuracy of the developed SPE method in conjunction with Arsenazo III procedure was tested by analyzing marine sediment (MESS-3) and soil (IAEA–Soil 7) reference materials. Furthermore, the above procedure has been successfully employed for the analysis of real soil and sediment samples. 相似文献