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排序方式: 共有118条查询结果,搜索用时 15 毫秒
71.
Mester Z Willie S Yang L Sturgeon R Caruso JA Fernández ML Fodor P Goldschmidt RJ Goenaga-Infante H Lobinski R Maxwell P McSheehy S Polatajko A Sadi BB Sanz-Medel A Scriver C Szpunar J Wahlen R Wolf W 《Analytical and bioanalytical chemistry》2006,385(1):168-180
A new selenized yeast reference material (SELM-1) produced by the Institute for National Measurement Standards, National Research
Council of Canada (INMS, NRC) certified for total selenium (2,059±64 mg kg−1), methionine (Met, 5,758±277 mg kg−1) and selenomethionine (SeMet, 3,431±157 mg kg−1) content is described. The ±value represents an expanded uncertainty with a coverage factor of 2. SeMet and Met amount contents
were established following a methanesulfonic acid digestion of the yeast using GC-MS and LC-MS quantitation. Isotope dilution
(ID) calibration was used for both compounds, using 13C-labelled SeMet and Met. Total Se was determined after complete microwave acid digestion based on ID ICP-MS using a 82Se spike or ICP-OES spectrometry using external calibration. An international intercomparison exercise was piloted by NRC
to assess the state-of-the-art of measurement of selenomethione in SELM-1. Determination of total Se and methionine was also
attempted. Seven laboratories submitted results (2 National Metrology Institutes (NMIs) and 5 university/government laboratories).
For SeMet, ten independent mean values were generated. Various acid digestion and enzymatic procedures followed by LC ICP-MS,
LC AFS or GC-MS quantitation were used. Four values were based on species-specific ID calibration, one on non-species-specific
ID with the remainder using standard addition (SA) or external calibration (EC). For total selenium, laboratories employed
various acid digestion procedures followed by ICP-MS, AFS or GC-MS quantitation. Four laboratories employed ID calibration,
the remaining used SA or EC. A total of seven independent results were submitted. Results for methionine were reported by
only three laboratories, all of which used various acid digestion protocols combined with determination by GC-MS and LC UV.
The majority of participants submitted values within the certified range for SeMet and total Se, whereas the intercomparison
was judged unsuccessful for Met because only two external laboratories provided values, both of which were outside the certified
range. 相似文献
72.
Damian Clancy Sang Taphou Mendy 《Methodology and Computing in Applied Probability》2011,13(3):603-618
Probably the simplest model for endemic infection is the susceptible-infected-susceptible (SIS) logistic model. Long-term
behaviour of this model prior to disease extinction is described by the quasi-stationary distribution. This quasi-stationary
distribution has been the subject of much previous work, including derivation of a variety of approximations, using both standard
distributional forms and specialized approximating formulae. The aim of this paper is to carry out a systematic comparison
between approximations. As well as comparing previously available approximations, we derive several new variants. Taking into
account both accuracy (measured using total variation distance) and simplicity, and denoting by R
0 the basic reproduction number, our main findings are: (a) in the subcritical region R
0 < 1 a geometric distribution approximation is preferred; (b) in the supercritical region R
0 ≫ 1 a beta-binomial distribution is preferred. Both of these preferred approximations are new. 相似文献
73.
Robert S. Clancy Scott McCullough 《Proceedings of the American Mathematical Society》1998,126(11):3299-3305
In this paper we solve a mapping problem for a particular class of Hilbert modules over an algebra multipliers of a diagonal Nevanlinna-Pick (NP) kernel. In this case, the regular representation provides a multiplier norm which induces the topology on the algebra. In particular, we show that, in an appropriate category, a certain class of Hilbert modules are projective. In addition, we establish a commutant lifting theorem for diagonal NP kernels.
74.
Supramolecular control of charge transport in molecular wires 总被引:1,自引:0,他引:1
Grozema FC Houarner-Rassin C Prins P Siebbeles LD Anderson HL 《Journal of the American Chemical Society》2007,129(44):13370-13371
75.
Chen Z Stepanenko V Dehm V Prins P Siebbeles LD Seibt J Marquetand P Engel V Würthner F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(2):436-449
The self-assembly of a new, highly fluorescent perylene bisimide dye 2 into pi stacks, both in solution and condensed phase, has been studied in detail by NMR spectroscopy, vapor pressure osmometry (VPO), UV/Vis and fluorescence spectroscopy, differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and X-ray diffraction. The NMR and VPO measurements revealed the formation of extended pi-pi stacks of the dye molecules in solution. The aggregate size determined from VPO and DOSY NMR measurements agree well with that obtained from the concentration and temperature-dependent UV/Vis spectral data by employing the isodesmic model (equal K model). In the condensed state, dye 2 possesses a hexagonal columnar liquid crystalline (LC) phase as confirmed by X-ray diffraction analysis. The columnar stacking of this dye has been further explored by atomic force microscopy (AFM). Well-resolved columnar nanostructures of the compound are observed on graphite surface. A color-tunable luminescence from green to red has been observed upon aggregation which is accompanied by an increase of the fluorescence lifetime and depolarization. The observed absorption properties can be explained in terms of molecular exciton theory. The charge transport properties of dye 2 have been investigated by pulse radiolysis-time resolved microwave conductivity measurements and a 1D charge carrier mobility up to 0.42 cm(2) V(-1) s(-1) is obtained. Considering the promising self-assembly, semiconducting, and luminescence properties of this dye, it might serve as a useful functional material for nano(opto)electronics. 相似文献
76.
Ronan M. Bellabarba Gerald P. Clancy Pedro T. Gomes Ana M. Martins Leigh H. Rees Malcolm L. H. Green 《Journal of organometallic chemistry》2001,640(1-2):93-112
The dimethylphosphino substituted cyclopentadienyl precursor compounds [M(C5Me4CH2PMe2)], where M=Li+ (1), Na+ (2), or K+ (3), and [Li(C5H4CR′2PMe2)], where R′2=Me2 (4), or (CH2)5 (5), [HC5Me4CH2PMe2H]X, where X−=Cl− (6) or PF6− (7) and [HC5Me4CH2PMe2] (8), are described. They have been used to prepare new metallocene compounds, of which representative examples are [Fe(η-C5R4CR′2PMe2)2], where R=Me, R′=H (9); R=H and R′2=Me2 (10), or (CH2)5 (11), [Fe(η-C5H4CMe2PMe3)2]I2 (12), [Fe{η-C5Me4CH2P(O)Me2}2] (13), [Zr(η-C5R4CR′2PMe2)2Cl2], where R=H, R′=Me (14), or R=Me, R′=H (15), [Hf(η-C5H4CMe2PMe2)2]Cl2] (16), [Zr(η-C5H4CMe2PMe2)2Me2] (17), {[Zr(η-C5Me4CH2PMe2)2]Cl}{(C6F5)3BClB(C6F5)3} (18), [Zr{(η-C5Me4CH2PMe2)2Cl2}PtI2] (19), [Mn(η-C5Me4CH2PMe2)2] (20), [Mn{(η-C5Me4CH2PMe2B(C6F5)3}2] (21), [Pb(η-C5H4CMe2PMe2)2] (23), [Sn(η-C5H4CMe2PMe2)2] (24), [Pb{η-C5H4CMe2PMe2B(C6F5)3}2] (25), [Pb(η-C5H4CMe2PMe2)2PtI2] (26), [Rh(η-C5Me4CH2PMe2)(C2H4)] 29, [M(η,κP-C5Me4CH2PMe2)I2], where M=Rh (30), or Ir, (31). 相似文献
77.
78.
79.
The feasibility of close coupling techniques for bifurcation processes is investigated using a simple model. Density contours and a flux map illustrate the bifurcation process. The relevance of this study to the theory of three-dimensional chemical reactions is discussed. 相似文献
80.
Zervosen A Herman R Kerff F Herman A Bouillez A Prati F Pratt RF Frère JM Joris B Luxen A Charlier P Sauvage E 《Journal of the American Chemical Society》2011,133(28):10839-10848
Boronic acids bearing appropriate side chains are good inhibitors of serine amidohydrolases. The boron usually adopts a tetrahedral conformation, bound to the nucleophilic serine of the active site and mimicking the transition state of the enzymatic reaction. We have solved the structures of complexes of a penicillin-binding protein, the DD-peptidase from Actinomadura sp. R39, with four amidomethylboronic acids (2,6-dimethoxybenzamidomethylboronic acid, phenylacetamidomethylboronic acid, 2-chlorobenzamidomethylboronic acid, and 2-nitrobenzamidomethylboronic acid) and the pinacol ester derived from phenylacetamidomethylboronic acid. We found that, in each case, the boron forms a tricovalent adduct with Oγ of Ser49, Ser298, and the terminal amine group of Lys410, three key residues involved in the catalytic mechanism of penicillin-binding proteins. This represents the first tricovalent enzyme-inhibitor adducts observed by crystallography. In two of the five R39-boronate structures, the boronic acid is found as a tricovalent adduct in two monomers of the asymmetric unit and as a monocovalent adduct with the active serine in the two remaining monomers of the asymmetric unit. Formation of the tricovalent complex from a classical monocovalent complex may involve rotation around the Ser49 Cα-Cβ bond to place the boron in a position to interact with Ser298 and Lys410, and a twisting of the side-chain amide such that its carbonyl oxygen is able to hydrogen bond to the oxyanion hole NH of Thr413. Biphasic kinetics were observed in three of the five cases, and details of the reaction between R39 and 2,6-dimethoxybenzamidomethylboronic acid were studied. Observation of biphasic kinetics was not, however, thought to be correlated to formation of tricovalent complexes, assuming that the latter do form in solution. On the basis of the crystallographic and kinetic results, a reaction scheme for this unexpected inhibition by boronic acids is proposed. 相似文献