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排序方式: 共有139条查询结果,搜索用时 78 毫秒
1.
The application of the simulated ab-initio molecular orbital (SAMO) method — a new non-empirical technique — to the band structure of polymers is reported. The method uses full ab-initio results on small molecules to construct for polymers molecular orbitals which simulate those obtained from full ab-initio treatments. Results are compared with those obtained from published ab-initio and semi-empirical calculations. 相似文献
2.
Gao H Petzold CJ Leavell MD Leary JA 《Journal of the American Society for Mass Spectrometry》2003,14(8):916-924
A rapid and accurate method of quantifying positional isomeric mixtures of phosphorylated hexose and N-acetylhexosamine monosacchrides by using gas-phase ion/molecule reactions coupled with FT-ICR mass spectrometry is described. Trimethyl borate, the reagent gas, reacts readily with the singly charged negative ions of phosphorylated monosaccharides to form two stable product ions corresponding to the loss of one or two neutral molecules of methanol from the original adduct. Product distribution in the ion/molecule reaction spectra differs significantly for isomers phosphorylated in either the 1- or the 6-position. As a result, the percents of total ion current of these product ions for a mixture of the two isomers vary with its composition. In order to determine the percentage of each isomer in an unknown mixture, a multicomponent quantification method is utilized in which the percents of total ion current of the two product ions for each pure monosaccharide phosphate and the mixture are used in a two-equation, two-unknown system. The applicability of this method is demonstrated by successfully quantifying mock mixtures of four different isomeric pairs: Glucose-1-phosphate and glucose-6-phosphate; mannose-1-phosphate and mannose-6-phosphate; galactose-1-phosphate and galactose-6-phosphate; N-acetylglucosamine-1-phosphate and N-acetylglucosamine-6-phosphate. The effects of mixture concentrations and ion/molecule reaction conditions on the quantification are also discussed. Our results demonstrate that this assay is a fast, sensitive, and robust method to quantify isomeric mixtures of phosphorylated monosaccharides. 相似文献
3.
M. Trinkel W. Trettnak F. Reininger R. Beneš P. O Leary O. S. Wolfbeis 《International journal of environmental analytical chemistry》2013,93(1-4):237-251
Abstract An optical sensor for the determination of ammonia in water based on ion pairing has been investigated. A pH-sensitive dye is immobilized as an ion pair in a silicone matrix. The colour of the dye changes from yellow to blue depending on the concentration of ammonia in the sample solution. This change is reversible. The concentration of ammonia can be determined by measuring the transmittance at a given wavelength. All measurements were performed with a dual-beam optical meter. The measurement range was from 5.9 × 10?7 to 1 × 10?3 M (0.01 to 17 mg/l) in 0.1 M phosphate buffer of pH 8. The detection limit was 10 μg/l. The response times at a flow rate of 2.5 ml/min were 4 min for t90 and 10 min for t100 at a change from 41.9 to 82.5 μM ammonia and 12 min for t90 and 48 min for t100 at a change from 160 to 0 μM ammonia. The operational lifetime of the ammonia sensor was limited to a period of a few days only. A continuous decrease in baseline signal and relative signal change was observed over the whole measurement. The storage stability was more than 10 months (dry). With respect to possible application of the ammonia sensor to environmental analysis, the influence of pH, typical interferences, such as amines and various detergents on the sensor response was investigated. No interference due to pH was observed in the range from pH 5 to pH 9. With methyl- and ethylamine the response was not completely reversible. The sensor was affected by cationic detergents, but not by anionic or neutral detergents. 相似文献
4.
5.
An electrospray ionization mass spectrometry based assay was developed for kinetic measurements and inhibitor screening of yeast hexokinase. There is considerable discrepancy in the literature as to the accuracy of kinetic data obtained for hexokinase. In the assay described herein, the product, glucose 6-phosphate was directly monitored by ion trap mass spectrometry and quantified using an internal standard, 2 deoxy-glucose 6-phosphate. The kinetic parameters, K(M) and V(max) for the two substrates were determined without using a coupling enzyme as is normally employed in the traditional spectrophotometric assay for systems lacking a chromophore. In addition, hexokinase was successfully immobilized onto an amino-link gel, and a mock library was screened against the immobilized enzyme for the identification of possible inhibitors. After comparing the mass spectra of the library before and after incubation, trehalose 6-phosphate, ADP, and oxidized glutathione were differentiated from other weak or non-inhibitors. Inhibition behavior of ADP with respect to ATP was further evaluated with the ESI-MS assay and the value of K(i) was determined. This ESI-MS assay was demonstrated to be both accurate and precise for determining kinetic constants and for identifying enzyme inhibitors. 相似文献
6.
N. S. Kelsall C. E. Svensson S. Fischer D. E. Appelbe R. A. E. Austin D. P. Balamuth G. C. Ball J. A. Cameron M. P. Carpenter R. M. Clark M. Cromaz M. A. Deleplanque R. M. Diamond P. Fallon D. F. Hodgson R. V. F. Janssens D. G. Jenkins G. J. Lane C. J. Lister A. O. Macchiavelli C. D. O’Leary D. G. Sarantities F. S. Stephens D. C. Schmidt D. Seweryniak K. Vetter J. C. Waddington R. Wadsworth D. Ward A. N. Wilson A. V. Afanasjev S. Frauendorf I. Ragnarsson 《The European Physical Journal A - Hadrons and Nuclei》2003,20(1):131-132
High-spin states have been studied in 72Kr and 72Br using the 40Ca + 40Ca and 36Ar + 40Ca reactions at 164 and 145 MeV, respectively. The properties and configurations of the high-spin bands observed have been interpreted using unpaired cranked Nilsson-Strutinsky (CNS), and for 72Kr, paired cranked relativistic Hartree-Bogoliubov (CRHB) calculations. In 72Kr a new band has been identified that has the properties expected for the doubly aligned S-band configuration. In 72Br the previously known bands have been extended to higher spin. This has lead to a re-interpretation of the configurations.Received: 31 October 2002, Published online: 24 February 2004PACS:
21.10.Re Collective levels - 23.20.Lv
transitions and level energies - 27.50. + e
相似文献
7.
Leary SM Brex PA MacManus DG Parker GJ Barker GJ Miller DH Thompson AJ 《Magnetic resonance imaging》2000,18(4):455-459
1H magnetic resonance spectroscopy (MRS) provides a unique tool to detect and quantify brain metabolites. In multiple sclerosis it can be used to investigate axonal loss or dysfunction through measurement of N-acetyl aspartate (NAA), a neuronal marker. Previous studies in adults have reported variable effects of aging on metabolite concentrations but have predominantly focused on changes in the elderly. This study has examined a younger adult age group to provide a reference database more applicable to the multiple sclerosis population. Single voxel (1)H MRS was carried out in 44 subjects between 22 and 62 years of age. Sixteen subjects underwent repeat examination after one year. Absolute concentrations of NA (the sum of NAA and N-acetyl aspartate glutamate), NAA, creatine/phosphocreatine (Cr), choline containing compounds (Cho) and myo-inositol (mI) were measured. NA, NAA and mI concentrations did not correlate with age but there were significant correlations between age and Cr (r = 0.43, p = 0.004) and Cho (r = 0.38, p = 0. 011) concentrations. No significant differences in metabolite concentrations were seen over one year. This study provides evidence that age-related changes of metabolite concentrations occur even in a young to middle aged adult population. This emphasizes the need to perform absolute quantification of metabolite concentrations rather than ratios and the importance of age-matching in (1)H MRS studies of multiple sclerosis. 相似文献
8.
9.
Lieza M. Danan Zhihao Yu Peter J. Ludden Weitao Jia Kevin L. Moore Julie A. Leary 《Journal of the American Society for Mass Spectrometry》2010,21(9):1633-1642
Human tyrosylprotein sulfotransferases catalyze the transfer of a sulfuryl moiety from the universal sulfate donor PAPS to
the hydroxyl substituent of tyrosine residues in proteins and peptides to yield tyrosine sulfated products and PAP. Tyrosine
sulfation occurs in the trans-Golgi network, affecting an estimated 1% of the tyrosine residues in all secreted and membrane-bound
proteins in higher order eukaryotes. In this study, an effective LC-MS-based TPST kinetics assay was developed and utilized
to measure the kinetic properties of human TPST-2 and investigate its catalytic mechanism when G protein-coupled CC-chemokine
receptor 8 (CCR8) peptides were used as acceptor substrates. Through initial rate kinetics, product inhibition studies, and
radioactive-labeling experiments, our data strongly suggest a two-site ping-pong model for TPST-2 action. In this mechanistic
model, the enzyme allows independent binding of substrates to two distinct sites, and involves the formation of a sulfated
enzyme covalent intermediate. Some insights on the important amino acid residues at the catalytic site of TPST-2 and its covalent
intermediate are also presented. To our knowledge, this is the first detailed study of the reaction kinetics and mechanism
reported for human TPST-2 or any other Golgi-resident sulfotransferase. 相似文献
10.
Sandhya DwevediG. Markandeyulu P.R. OhodnickiAlex Leary M.E. McHenry 《Journal of magnetism and magnetic materials》2011,323(15):1929-1933
A systematic investigation of the influence of different types of annealing on the magnetoimpedance (MI) effect in melt-spun (Co1−xFex)89Zr7B4 [x=0, 0.025, 0.05] and (Co0.88Fe0.12)78.4Nb2.6Si9B9Al ribbons has been carried out in the frequency range 500 kHz-13 MHz and under dc magnetic fields (Hdc) up to 80 Oe. In the stress annealed ribbons, the strain-induced transverse anisotropy is seen to result in large MI. Magnetic domains were investigated in the ribbons through magnetic force microscopy. 相似文献