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31.
An improved method for the analysis of major antioxidants of Hibiscus esculentus Linn 总被引:1,自引:0,他引:1
Major antioxidants of aqueous ethanol extract from Lady's Finger (Hibiscus esculentus Linn) were systematically investigated in this study. Firstly, high-performance liquid chromatography (HPLC) was applied to identify antioxidant peaks in a sample by spiking the sample extract with 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radical, which was prepared from manganese dioxide and ABTS. Secondly, in order to identify the elution period of major antioxidant peaks, the antioxidant capacities of different fractions from solid-phase extraction (SPE) were measured, and the chromatograms of fractions were also recorded. Lastly, multiple mass spectrometry (MS(n)) was used to elucidate the possible chemical structures of antioxidants, and nuclear magnetic resonance (NMR) was further applied for structure confirmation. The major antioxidant compounds in lady's finger were identified to be quercetin derivatives and (-)-epigallocatechin using HPLC-MS and HPLC-MS(n) (n = 2-4) techniques. It was found that about 70% of total antioxidant activity was contributed by four quercetin derivatives. The structures of major antioxidants, which were isolated by semi-preparative RP-HPLC with two tandem C18 columns, were further confirmed using UV-vis absorption spectroscopy and 13C NMR spectra. Quercetin 3-O-xylosyl (1' --> 2') glucoside, quercetin 3-O-glucosyl (1' --> 6') glucoside, quercetin 3-O-glucoside and quercetin 3-O-(6'-O-malonyl)-glucoside were first identified and characterized as major antioxidants in lady's finger. 相似文献
32.
The second harmonic generation of SV shear waves at isotropic solid-solid interfaces is experimentally studied. The amplitude of shear waves is measured for the interfaces of glass-air, glass-iron, glass-copper, and glass-aluminum. The measured angular relation of amplitude of the second harmonic wave is compared with theory and the agreement is reasonably good. The influence of the physical state of the interface on second harmonic generation is also observed. It is found that the second harmonic generation is sensitive to the interface state. 相似文献
33.
用熔融共混法制备了不同组分的聚苯乙烯(PS)与聚氧化乙烯(PEO)共混物(PS/PEO).在玻璃转变温度Tg及其以上温区,利用相对能量耗散技术研究了该共混物的动力学行为.结果发现,在能量耗散-温度曲线上出现了两个弛豫型的耗散峰(α峰和α′峰).分析表明,α峰是与PS玻璃转变有关的特征耗散峰; α′峰则对应于一种“液-液转变”.两者的弛豫时间τ都不满足Arrhenius关系.此外,还研究了组分对这两个弛豫型耗散峰的影响,并给予了定性的解释.
关键词:
相对能量耗散
玻璃转变
力学弛豫 相似文献
34.
Qi-Rui Li Weimin Sheng 《Calculus of Variations and Partial Differential Equations》2013,48(1-2):41-66
In this paper we consider the problem of finding closed hypersurfaces with prescribed Weingarten curvature in a large class of Riemannian manifolds. Under some sufficient conditions, we obtain an existence result by establishing a priori estimates and standard degree theory for the admissible solutions to the prescribed curvature equation. We mainly show that the primary technique developed in Urbas, Wang and the second author’s work in 2004 is flexible enough to be used in more general ambient geometry. 相似文献
35.
A synthesis of racemic thalidomide ( 1 ) was described and the important formal [3+3] cycloaddition strategy was a keystep. The total yield of thalidomide ( 1 ) was 18% in five steps from known 3 . 相似文献
36.
A novel amperometric nitric oxide sensor with a wide linear range, low detection limit and fast response time was developed.
The sensor was fabricated using a poly-brilliant cresyl blue (PBCB)/Nafion film modified glassy carbon electrode (GCE). The
PBCB on the working GCE surface dramatically improves the oxidation response of nitric oxide and lowers the required potential
for the two-step oxidation of NO. Otherwise, Nafion coated onto the film electrode surface does not only improve the selectivity
of the sensor, but also further lowers the active energy of the direct three-electron oxidation of NO to nitrate. The effect
of the preanodic time and the film thickness of the electropolymerization on the nitric oxide response as well as the volume
of Nafion coated onto the surface and the potential for amperometric detection were optimized. This novel sensor has been
applied to the determination of NO released from rat liver cells, and the result is satisfactory.
Correspondence: Sheng Shui Hu, Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China 相似文献
37.
Ultra-cold atoms provide ideal platforms for interferometry.The macroscopic matter-wave property of ultra-cold atoms leads to large coherent length and long coherent time,which enable high accuracy and sensitivity to measurement.Here,we review our efforts to improve the performance of the interferometer.We demonstrate a shortcut method for manipulating ultra-cold atoms in an optical lattice.Compared with traditional ones,this shortcut method can reduce the manipulation time by up to three orders of magnitude.We construct a matter-wave Ramsey interferometer for trapped motional quantum states and significantly increase its coherence time by one order of magnitude with an echo technique based on this method.Efforts have also been made to enhance the resolution by multimode scheme.Application of a noise-resilient multi-component interferometer shows that increasing the number of paths could sharpen the peaks in the time-domain interference fringes,which leads to a resolution nearly twice compared with that of a conventional double-path two-mode interferometer.With the shortcut method mentioned above,improvement of the momentum resolution could also be fulfilled,which leads to atomic momentum patterns less than 0.6hkL. To identify and remove systematic noises,we introduce the methods based on the principal component analysis (PCA) that reduce the noise in detection close to the 1/√2 of the photon-shot noise and separate and identify or even eliminate noises.Furthermore,we give a proposal to measure precisely the local gravity acceleration within a few centimeters based on our study of ultracold atoms in precision measurements. 相似文献
38.
Xin‐Gang Xie Dr. Xun‐Wei Wu Dr. Hing‐Ken Lee Dr. Xiao‐Shui Peng Prof. Dr. Henry N. C. Wong Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(23):6933-6941
Plakortone B is a naturally occurring bicyclic[3.3.0]furanolactone compound with attractive bioactivities. Although the relative configuration of plakortone B’s central core had been established by NMR spectroscopic methods, the absolute configuration of its four stereocenters remained unknown. In the present paper, all four possible isomers of plakortone B were synthesized and one of these molecules was found to be identical with the natural plakortone B on the basis of 1H, 13C NMR spectra and specific rotation comparisons. Thus, the absolute configuration of the natural plakortone B was determined to be (3S,4S,6R,10R). 相似文献
39.
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