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941.
Abstract
The complex (1-Cyclopropyl-6-fluoro-1,4-dihydro-8-methoxy-4-(oxo-\κO)-3-quinolinecarboxylato-κO3) difluoro-boron crystallizes in the monoclinic space group P21/c, a = 11.43400(10) ?, b = 9.16420(10) ?, c = 13.8745(2) ?, β = 106.6470(10)o, V = 1392.89(3) ?3, and Z = 4; which is a resonance hybrid with an almost tetrahedral boron which produces a 6-membered ring, where the boron atom is slightly bent out of plane. 相似文献942.
It is still a major challenge to simultaneously isolate artemisinin and its precursors, especially dihydroartemisinic acid and artemisinic acid, from herbal Artemisia annua. A rapid, economical and automatical chromatographic separation process to isolate and purify artemisinin, dihydroartemisinic acid and artemisinic acid at the same time on a preparative scale was developed. The procedure included solvent extraction of ground Artemisia annua leaves by refluxing and purification of crude extract by preparative reverse-phase high-performance liquid chromatography (RP-HPLC). Fractions containing artemisinin and its precursors were collected and identified by gas chromatography and mass spectrometry. High purity of artemisinin, dihydroartemisinic acid and artemisinic acid was obtained by preparative HPLC with a C(18) column and 60% acetonitrile in water as the mobile phase. The techniques described here are useful tools for the preparative-scale isolation of artemisinin and its precursors in a fast, cost-effective and environmental friendly manner. 相似文献
943.
Er-na Xun Xiao-li Lv Wei Kang Jia-xin Wang Hong Zhang Lei Wang Zhi Wang 《Applied biochemistry and biotechnology》2012,168(3):697-707
The lipase from Pseudomonas fluorescens (Lipase AK, AKL) was immobilized onto the magnetic Fe3O4 nanoparticles via hydrophobic interaction. Enzyme loading and immobilization yield were determined as 21.4?±?0.5?mg/g and 49.2?±?1.8?%, respectively. The immobilized AKL was successfully used for resolution of 2-octanol with vinyl acetate used as acyl donor. Effects of organic solvent, water activity, substrate ratio, and temperature were investigated. Under the optimum conditions, the preferred isomer for AKL is the (R)-2-octanol and the highest enantioselectivity (E?=?71.5?±?2.2) was obtained with a higher enzyme activity (0.197?±?0.01???mol/mg/min). The results also showed that the immobilized lipase could be easily separated from reaction media by the magnetic steel and remained 89?% of its initial activity as well as the nearly unchanged enantioselectivity after five consecutive cycles, indicating a high stability in practical operation. 相似文献
944.
为了提高TiO2薄膜的光致亲水性及其持久性, 将聚乙二醇(PEG)2000 引入TiO2溶胶中, 利用提拉法在载玻片上制备出含PEG的TiO2薄膜样品, 通过测试样品紫外光照下水滴接触角的变化, 考察不同浓度PEG对TiO2薄膜光致亲水性能的影响; 并通过测试光照后的亲水薄膜样片暗处放置不同时间后接触角的变化, 比较含PEG的TiO2薄膜和纯TiO2薄膜样品的亲水持久性. 结果表明: PEG作为一种非离子型长链分子, 其适量的添加可促进TiO2薄膜的光致亲水性及其持久性. 基于薄膜样品的傅里叶变换红外(FTIR)光谱和紫外-可见漫反射光谱(UV-Vis DRS)结果, 认为PEG作为空穴捕获剂有助于TiO2薄膜中Ti3+的生成和稳定存在, 进而有助于亲水中心(表面羟基)的形成. 此研究对于TiO2光自洁涂料的应用很有意义, 也为研究TiO2的光激发瞬态行为提供了一个可能的简易方法. 相似文献
945.
采用顶空固相微萃取-气相色谱-质谱法分离和鉴定芝麻香精中挥发性成分,用归一化法测定其相对含量。为使固相微萃取达到更高的效率,选用50/30μm DVB-CAR-PDMS的固相萃取头,萃取温度及时间为60℃和1h。两种芝麻香精中分别鉴定出31和28种香气成分,其中含量较高的几类物质分别为吡嗪类、呋喃类、吡啶类、酮类和酚类物质。1号香精中的主要香气化合物为:1,6-二氢-咪唑并[4,5-d]咪唑、甲基吡嗪、3,5-二甲基苯酚、糠醛、2,6-二甲基-4-氨基吡啶和5-甲基-2-呋喃甲醛。2号香精中的主要香气化合物为:2,3-二甲基吡嗪、2-糠酸糠酯、4-氨基吡啶、3,5-二甲基苯酚、乙酰基吡嗪和糠醛。 相似文献
946.
对枸杞子中的水溶性多糖成分进行了分离提取及组成分析。采用水提、醇沉等方法提取枸杞子中的多糖成分,用Sevage法除去蛋白质后,用凝胶色谱法进行多糖的纯化,制备3-甲基-1-苯基-2-吡唑啉酮(PMP)衍生物进行液相色谱分析。结果表明,枸杞多糖由鼠李糖、阿拉伯糖、木糖、葡萄糖、半乳糖5种单糖组成,其含量分别为5.28%,17.87%,38.29%,17.82%,20.74%。 相似文献
947.
948.
A multi-period mean–variance portfolio selection problem with an uncertain exit time is one of the nonseparable dynamic optimization problems as the principle of optimality of dynamic programming no longer applies. In this paper, we introduce a mean-field formulation to tackle this multi-period nonseparable problem directly without introducing an embedding scheme. Moreover, we shed light on the efficient feature of the mean-field formulation when dealing with the issue of dynamic nonseparability. 相似文献
949.
A mixed linear quadratic (MLQ) optimal control problem is considered. The controlled stochastic system consists of two diffusion processes which are in different time horizons. There are two control actions: a standard control action \(u(\cdot )\) enters the drift and diffusion coefficients of both state equations, and a stopping time \(\tau \) , a possible later time after the first part of the state starts, at which the second part of the state is initialized with initial condition depending on the first state. A motivation of MLQ problem from a two-stage project management is presented. It turns out that solving an MLQ problem is equivalent to sequentially solve a random-duration linear quadratic (RLQ) problem and an optimal time (OT) problem associated with Riccati equations. In particular, the optimal cost functional can be represented via two coupled stochastic Riccati equations. Some optimality conditions for MLQ problem is therefore obtained using the equivalence among MLQ, RLQ and OT problems. In case of seeking the optimal time in the family of deterministic times (even through somewhat restrictive, such seeking is still reasonable from practical standpoint), we give a more explicit characterization of optimal actions. 相似文献
950.
This paper combines copula functions with GARCH-type models to construct the conditional joint distribution, which is used to estimate Value-at-Risk (VaR) of an equally weighted portfolio comprising crude oil futures and natural gas futures in energy market. Both constant and time-varying copulas are applied to fit the dependence structure of the two assets returns. The findings show that the constant Student t copula is a good compromise for effectively fitting the dependence structure between crude oil futures and natural gas futures. Moreover, the skewed Student t distribution has a better fit than Normal and Student t distribution to the marginal distribution of each asset. Asymmetries and excess kurtosis are found in marginal distributions as well as in dependence. We estimate VaR of the underlying portfolio to be 95% and 99%, by using the Monte Carlo simulation. Then using backtesting, we compare the out-of-sample forecasting performances of VaR estimated by different models. 相似文献