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排序方式: 共有785条查询结果,搜索用时 31 毫秒
161.
古代食谱重建是目前国际考古学研究的热点之一.随着研究的深入,传统基于骨骼提取出骨胶原蛋白进行分析的方法逐渐凸显出一定的局限性.尤其是在考察先民食物结构和营养等级等方面,针对蛋白大分子获得的同位素信息,无法排除一些内在或外在因素的影响,因而常会引导我们做出错误的判断.因此,发展一种能进一步提高古代食谱研究准确率和分辨率的... 相似文献
162.
Houchao Xu Prof. Dr. Bernd Goldfuss Prof. Dr. Jeroen S. Dickschat 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(38):9758-9762
A systematic computational study addressing the entire chemical space of guaianes in conjunction with an analysis of all known compounds shows that 1,3-hydride shifts are rare events in guaiane biosynthesis. As demonstrated here, 1,3-hydride shifts towards guaianes can only be realized for two stereochemically well defined out of numerous possible stereoisomeric skeletons. One example is given by the mechanism of guaia-4(15)-en-11-ol synthase from California poplar, an enzyme that yields guaianes with unusual stereochemical properties. The general results from DFT calculations were experimentally verified through isotopic-labeling experiments with guaia-4(15)-en-11-ol synthase. 相似文献
163.
The temperature variation of the Debye-Waller factors of Cs+ and Br− ions in CsBr powder has been studied using powder X-ray diffraction. The integrated intensities of the Bragg peaks at different
temperatures have been obtained. These results have been verified by structure factor least squares refinement program. Theoretical
shell model lattice dynamical calculations have been done using a 7-parameter model in the harmonic approximation and the
values compared with the present X-ray measurements. The observed intensities have been corrected for first order thermal
diffuse scattering. 相似文献
164.
K. W. D. Ledingham 《Hyperfine Interactions》2006,171(1-3):69-81
High intensity lasers interacting with solid and gas targets can produce energetic beams of electrons, protons, heavy ions,
photons and neutrons. There is the potential for producing “table top” sources of radiation for many different applications. 相似文献
165.
R. C. Viscovini F. C. Cruz A. Scalabrin D. Pereira 《International Journal of Infrared and Millimeter Waves》2004,25(2):257-275
In this work we present results of an investigation on Doppler limited infrared absorbing transitions of CHD2OH by optoacoustic detection. It is an alternative and attractive technique to be applied to this methanol isotopomer, in comparison to Fourier transform spectroscopy. Using a waveguide CO2 laser of 290 MHz tunability on each line, we were able to observe 60 IR absorptions, most of them of large offset. The data will be useful in the theoretical analysis of this molecule, as well as in the generation of far-infrared (FIR) laser radiation in optically pumped molecular lasers. 相似文献
166.
A new method is described for a precise and simultaneous determination of the rate of production and dissolution of biosilica in marine waters, using isotopic dilution technique. No HF or F2 is required for chemical preparations as the change in isotopic composition is measured on silica producing SiO2− ions. The seawater sample flask is spiked with 30Si(OH)4 (<10% of increase in situ concentration) and incubated in in situ conditions. At the end of incubation, changes of the 30Si:28Si ratios in particulate and liquid phases are measured by using a thermal ionisation mass spectrometer Finnigan THQ. The relative analytical precision of the isotopic ratio measurements is <0.5%. The limit of detection of the change in isotopic ratio during incubation is 0.02 atom%. The overall repeatability determined on eight subsamples (average production: 0.23 μM day−1; average dissolution: 0.07 μM day−1) is ±0.02 and ±0.01 μM day−1 for production and dissolution, respectively. Using mass and isotopic balances of the particulate and dissolved phases in the incubation flask, the best estimates for production and dissolution rates are calculated iteratively. This method was applied to 112 samples of marine waters (production, range: 0.00-2.38 μM day−1; dissolution, range: 0.00-1.18 μM day−1). 相似文献
167.
Two Bacterial Diterpene Synthases from Allokutzneria albata Produce Bonnadiene,Phomopsene, and Allokutznerene
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Lukas Lauterbach Jan Rinkel Prof. Dr. Jeroen S. Dickschat 《Angewandte Chemie (International ed. in English)》2018,57(27):8280-8283
Two diterpene synthases from Allokutzneria albata were studied for their products, resulting in the identification of the new compound bonnadiene from the first enzyme. Although phylogenetically unrelated to fungal phomopsene synthase, the second enzyme produced a mixture of phomopsene and a biosynthetically linked new compound, allokutznerene, as well as spiroviolene. Both enzymes were subjected to in‐depth mechanistic studies involving isotopic labelling experiments, metal‐cofactor variation, and site‐directed mutagenesis. Oxidation products of phomopsene and allokutznerene are also discussed. 相似文献
168.
169.
Bertolini T Rubino M Lubritto C D'Onofrio A Marzaioli F Passariello I Terrasi F 《Journal of mass spectrometry : JMS》2005,40(8):1104-1108
A systematic analysis of efficiency, reproducibility and accuracy of cryogenic purification of CO(2) from air samples for isotopic analyses is presented. The technical characteristics of the cryogenic line are given in detail. To study the cryogenic process, three different operating parameters are considered: flow rate of the gas entering the line, pressure of the gas in the line, and CO(2)-trap shape. Experimental results demonstrate that efficiency, reproducibility and accuracy strongly depend on the CO(2)trap shape. Moreover, a dependence of reproducibility and accuracy on the flow rate of the gas is found, but not on its pressure. High precision (< or =0.02 per thousand for delta(13)C and < or =0.05 per thousand for delta(18)O) and good accuracy (<0.09 per thousand for delta(13)C and <0.14 per thousand for delta(18)O) is achieved after applying the N(2)O correction. 相似文献
170.
In a recent article, Kiyosawa [J. Solution Chem. 33, 323 (2004)] reports that the freezing points of isotopic mixtures of ordinary water and 17O enriched water show an unexpectedly large linear dependence on the concentration of H217O. Surprisingly, the constant of proportionality to the H217O concentration is nearly five times larger than that of H218O found in earlier studies by Kiyosawa [J. Solution Chem. 20, 583 (1991)]. We show that the H217O result is not consistent with other data or models. For example, a recent determination of the triple point temperature
dependence on isotopic composition in naturally and artificially depleted waters [White et al. in Temperature, Its Measurement and Control in Science and Industry, Vol. 7, D. C. Ripple, Ed., AIP CP 684, 221–226 (2003)] is consistent with the H218O and D2O results from Kiyosawa (1991) [White and Tew in Report of the 22nd Meeting of the Consultative Committee for Thermometry, Document CCT/03-21, BIPM, Severes, France, 2003] but is inconsistent with the H217O results from Kiyosawa (2004). Additionally, the results from Kiyosawa (1991) are close to what would be found in ideal solutions
for those isotopic forms, whereas the H217O proportionality from Kiyosawa (2004) is about 10 times larger than similarly predicted. One possible explanation is that
the original 17O enriched water sample contained a small amount of D2O, and the sample, if available, should be subject to isotopic analysis to help resolve these inconsistencies. 相似文献