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排序方式: 共有224条查询结果,搜索用时 15 毫秒
51.
J. Major H. Schneider A. Seeger A. Siegle H. Stoll I. Billard M. Koch U. Lauff 《Journal of Radioanalytical and Nuclear Chemistry》1995,190(2):481-491
Correlated measurement of the lifetime and of the lineshape of the 511 keV annihilation radiation of positrons (age-momentum correlation, AMOC) has become a powerful tool for investigating reactions of positron or positronium in condensed matter as a function of time. The beam-based
+ AMOC method installed at the Stuttgart relativistic positron beam facility (E
kin
e+
4MeV) offers substantial advantages over the conventional E coincidence technique resulting in a fast data taking due to the unity detection efficiency of the
+ scintillator which provides the start signal for the lifetime measurement. In this paper the application of AMOC to positronium chemistry is illustrated by the study of spin conversion of positronium in the system methanol/HTEMPO as a function of HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a paramagnetic solute) concentration. The same technique can be applied to other systems and other reactions in positronium chemistry, e. g., oxidation or complex formation. 相似文献
52.
Major H Castro-Perez J Nicholson JK Wilson ID 《Rapid communications in mass spectrometry : RCM》2003,17(1):76-80
The metabolic fate of 4-bromoaniline (4-BrA) was investigated following intraperitoneal administration to the rat at 50 mg kg(-1), using high-performance liquid chromatography/time-of-flight tandem mass spectrometry (HPLC/TOF-MS/MS). Up to five metabolites were detected in urine that correspond to isomeric pentose conjugates (possibly ribosides) of a hydroxysulphate of 4-BrA. This identification is supported by further studies where the water used in the reversed-phase solvent system was replaced with deuterated water in order to confirm that the number of exchangeable protons present in the metabolites was consistent with the proposed structures. 相似文献
53.
George H. Major Vincent Fernandez Neal Fairley Matthew R. Linford 《Surface and interface analysis : SIA》2022,54(3):262-269
The Gaussian–Lorentzian sum (GLS) and product (GLP) functions remain important in X-ray photoelectron spectroscopy (XPS) peak fitting. Here, we present a detailed view of these functions, comparing them with each other and with the Voigt function (the “LA(m)” function). First, we show the GLS, GLP, and LA(m) functions as a function of their mixing parameters, m, which reveals differences between them. We then illustrate the use of these functions to fit a series of spectra acquired at different pass energies (resolutions). Next, we show the underlying Gaussian and Lorentzian components of a series of GLS and GLP functions as a function of m, which confirms that the GLS is a simple linear combination of Gaussian and Lorentzian functions. However, one of the two functions used to make the GLP can be very wide, that is, at its extremes, one of these functions has infinite width. We then discuss a plot of the areas of the GLS, GLP, and LA(m) functions as a function of m, which reveals the expected, linear increase in area of the GLS, but nonlinear changes in the areas of the other two functions. Finally, to better understand them, we fit these functions to each other. These results indicate that the GLS and GLP better match the LA(m) function at lower and higher values of the mixing parameter, respectively. 相似文献
54.
55.
Combined QM/MM simulations have been carried out to investigate the origin of the carbon acidity enhancement in the alanine racemization reaction catalyzed by alanine racemase (AlaR). The present studies show that enhancement of carbon acidity of alpha-amino acids by cofactor pyridoxal 5'-phosphate, PLP, with an unusual, unprotonated pyridine is due to solvation effects, in contrast to the intrinsic electron-withdrawing stabilization by the pyridinium ion to form a quinonoid intermediate. Alanine racemase further lowers the alpha-proton acidity and provides an overall 14-17 kcal/mol transition state stabilization. Our computational results are consistent with the hypothesis that the use of the unusual form of PLP cofactor in AlaR is to raise the free energy of the intermediate, thereby increasing the reprotonation rate and enhancing the enzyme selectivity for racemization. 相似文献
56.
Combined quantum mechanical/molecular mechanical simulations have been carried out to investigate the origin of the carbon acidity enhancement in the alanine racemization reaction catalyzed by alanine racemase (AlaR). The present study shows that the enhancement of carbon acidity of alpha-amino acids by the cofactor pyridoxal 5'-phosphate (PLP) with an unusual, unprotonated pyridine is mainly due to solvation effects, in contrast to the intrinsic electron-withdrawing stabilization by the pyridinium ion to form a quinonoid intermediate. Alanine racemase further lowers the alpha-proton acidity and provides an overall 14-17 kcal/mol transition-state stabilization. The second key finding of this study is that the mechanism of racemization of an alanine zwitterion in water is altered from an essentially concerted process to a stepwise reaction by formation of an external aldimine adduct with the PLP cofactor. Finally, we have used a centroid path integral method to determine the intrinsic kinetic isotope effects for the two proton abstraction reactions, which are somewhat greater than the experimental estimates. 相似文献
57.
Péter Major 《Probability Theory and Related Fields》2006,134(3):489-537
Let a sequence of iid. random variables ξ
1, . . . ,ξ
n
be given on a space
with distribution μ together with a nice class
of functions f(x
1, . . . ,x
k
) of k variables on the product space
For all f ∈
we consider the random integral J
n,k
(f) of the function f with respect to the k-fold product of the normalized signed measure
where μ
n
denotes the empirical measure defined by the random variables ξ
1, . . . ,ξ
n
and investigate the probabilities
for all x>0. We show that for nice classes of functions, for instance if
is a Vapnik–Červonenkis class, an almost as good bound can be given for these probabilities as in the case when only the
random integral of one function is considered. A similar result holds for degenerate U-statistics, too.
Supported by the OTKA foundation Nr. 037886 相似文献
58.
Dugdale SB Watts RJ Laverock J Major Z Alam MA Samsel-Czekała M Kontrym-Sznajd G Sakurai Y Itou M Fort D 《Physical review letters》2006,96(4):046406
The Fermi surface topology of the shape-memory alloy Ni0.62Al0.38 has been determined using Compton scattering. A large area of this Fermi surface can be made to nest with other areas by translation through a vector of approximately 0.18[1,1,0](2pi/a), which corresponds to the wave vector associated with martensitic precursor phenomena such as phonon softening and diffuse streaking in electron diffraction patterns. This observation is compelling evidence that these phenomena are driven by the enhanced electron-lattice coupling due to the Fermi surface nesting. 相似文献
59.
60.
M. Hampele A. Kratzer K. Maier J. Major K. -H. Münch Th. Pfiz 《Hyperfine Interactions》1994,87(1):1043-1047
A new analog integrating detection technique for time resolvedSR measurements at high intensity pulsed muon beams has been developed. Data acquisition is realized by digitizing the integrated anode signal for each burst by means of a fast transient recorder. Details of setup and performance are described. 相似文献