排序方式: 共有34条查询结果,搜索用时 0 毫秒
1.
Two 2,6‐Dioxabicyclo[3.3.1]nonan‐3‐ones from Phragmanthera capitata (Spreng.) Balle (Loranthaceae) 下载免费PDF全文
Bruno N. Lenta Joél T. Ateba Jean R. Chouna Makoah N. Aminake Flore Nardella Gabriele Pradel Beate Neumann Hans Georg Stammler Catherine Vonthron‐Sénécheau Silvère Ngouela Norbert Sewald 《Helvetica chimica acta》2015,98(7):945-952
Phytochemical investigation of the leaves of Phragmanthera capitata collected on Cassia spectabilis tree led to the isolation of two natural lactones, rel‐(1R,5S,7S)‐7‐[2‐(4‐hydroxyphenyl)ethyl]‐2,6‐dioxabicyclo[3.3.1]nonan‐3‐one ( 1 ) and 4‐{2‐[rel‐(1R,3R,5S)‐7‐oxo‐2,6‐dioxabicyclo[3.3.1]non‐3‐yl]ethyl}phenyl 3,4,5‐trihydroxybenzoate ( 2 ) together with the known compounds betulinic acid ( 3 ), dodoneine ( 4 ), quercetin 3‐O‐α‐L ‐rhamnopyranoside ( 5 ), quercetin 3‐O‐α‐L ‐arabinofuranoside ( 6 ), quercetin ( 7 ), betulin ( 8 ), lupeol ( 9 ), and sitosterol ( 10 ). Their structures were established by means of modern spectroscopic techniques, and the relative configuration of compound 1 was confirmed by X‐ray analysis. Compounds 1 and 2 were tested in vitro for their antiplasmodial activity against the Plasmodium falciparum chloroquine sensitive‐strains NF54 and 3D7. Compound 2 exhibited good antiplasmodial activity against both strains with IC50 of 2.4 and 4.9 μM , respectively, while compound 1 was inactive. 相似文献
2.
Ngouela S Ndjakou BL Tchamo DN Zelefack F Tsamo E Connolly JD 《Natural product research》2005,19(1):23-27
A prenylated xanthone has been isolated from the seeds of Symphonia globulifera together with the known compounds sitosterol and oleanolic acid. The structure of the new compound was elucidated by spectroscopic methods. 相似文献
3.
We show both theoretically and experimentally in an optical fiber system that a set of incoherent nonlinear waves irreversibly evolves to a specific equilibrium state, in which the individual wave packets propagate with identical group velocities. This intriguing process of velocity locking can be explained in detail by simple thermodynamic arguments based on the kinetic wave theory. Accordingly, the selection of the velocity-locked state is shown to result from the natural tendency of the isolated wave system to approach the state that maximizes the nonequilibrium entropy. 相似文献
4.
In this work we study a reaction–diffusion problem with delay and we make an analysis of the stability of solutions by means of bifurcation theory. We take the delay constant as a parameter. Special conditions on the vector field assure existence of a spatially nonconstant positive equilibrium Uk , which is stable for small values of the delay. An increase of the delay destabilizes the equilibrium of Uk and leads to super or subcritical Hopf bifurcation. 相似文献
5.
Séverine Bernard Catherine Cabuzel Silvère Paul Nuiro Alain Pietrus 《Acta Appl Math》2018,155(1):85-98
This paper deals with variational inclusions of the form \(0 \in K-f(x)\) where \(f : \mathbb{R}^{n} \rightarrow \mathbb{R} ^{m}\) is a semismooth function and \(K\) is a nonempty closed convex cone in \(\mathbb{R}^{m}\). We show that the previous problem can be solved by a Newton-type method using the Clarke generalized Jacobian of \(f\). The results obtained in this paper extend those obtained by Robinson in the famous paper (Robinson in Numer. Math. 19:341–347, 1972). We provide a semilocal method with a superlinear convergence that is new in the context of semismooth functions. Finally, numerical results are also given to illustrate the convergence. 相似文献
6.
7.
Ngouamegne ET Fongang RS Ngouela S Boyom FF Rohmer M Tsamo E Gut J Rosenthal PJ 《Chemical & pharmaceutical bulletin》2008,56(3):374-377
From the ethyl acetate extract of the stem bark of Endodesmia calophylloides (Guttiferae), a novel friedelane triterpenoid named endodesmiadiol (1), as well as the known compounds friedelin (2), canophyllol (3), canophyllal (4), cerin (5), morelloflavone (6), volkensiflavone (7), 8-deoxygartanin (8), 3 beta-acetoxyoleanolic acid (9) and 1,8-dihydroxy-3-isoprenyloxy-6-methylxanthone (10) have been isolated. The structures of these compounds were established by spectroscopic analysis, and the relative configuration of endodesmiadiol (1) was confirmed by X-ray diffraction. The antiplasmodial activity of the isolated compounds was evaluated against the W2 strain of Plasmodium falciparum which is resistant to chloroquine and other antimalarial drugs. All the compounds were found to be active with IC50 values ranging from 7.2 to 23.6 microM. The IC50 of endodesmiadiol was found to be 11.8 microM. 相似文献
8.
Biotechnological Production of Xylitol: Enhancement of Monosaccharide Production by Post-Hydrolysis of Dilute Acid Sugarcane Hydrolysate 总被引:1,自引:0,他引:1
Boutros Fouad Sarrouh Ricardo de Freitas Branco Silvio Silvério da Silva 《Applied biochemistry and biotechnology》2009,153(1-3):163-170
Dilute-acid hydrolysis pretreatment of sugarcane bagasse resulted in release of 48% (18.4 g/L) of the xylan in the hemicellulose fraction into the hydrolysate as monomeric xylose. In order to enhance the recuperation of this monomer, a post-hydrolysis stage consisted of thermal treatment was carried out. This treatment resulted in an increase in xylose release of 62% (23.5 g/L) of the hemicellulose fraction. Original and post-hydrolysates were concentrated to the same levels of monomeric xylose in the fermentor feed. During the fermentation process, cellular growth was observed to be higher in the post-hydrolysate (3.5 g/L, Y x/s?=?0.075 g cells/g xylose) than in the original hydrolysate (2.9 g/L, Y x/s?=?0.068 g cells/g xylose). The post-treated hydrolysate required less concentration of sugars resulting in a lower concentration of fermentation inhibitors, which were formed primarily in the dilute acid hydrolysis step. Post-hydrolysis step led to a high xylose–xylitol conversion efficiency of 76% (0.7 g xylitol/g xylose) and volumetric productivity of 0.68 g xylitol/L h when compared to 71% (0.65 g xylitol/g xylose and productivity of 0.61 g xylitol/L h) for the original hemicellulosic hydrolysate. 相似文献
9.
Pedro Rosario José Luis Martínez José Miguel Silván 《Accreditation and quality assurance》2008,13(9):493-499
In analytical chemistry, proficiency testing usually consists in tests that laboratories conduct under routine conditions
and report the result to the PT provider who then converts the result to a score which helps the participant to assess the
accuracy of the result. The aim of this work is to show PT providers, accreditations bodies, and participating laboratories
that different scoring results can be achieved depending on the evaluation system selected. The influence of different evaluation
techniques on the results of an interlaboratory comparison for determination of gold in precious metals alloys was investigated.
Results from 19 participating laboratories were evaluated by means of the three procedures: (1) classical statistical approach—outliers
detection; (2) robust methods—(2A) robust procedure and (2B) ISO 13528; and (3) fitness for purpose. Evaluation of the same
PT data revealed very interesting issues depending on the different scoring systems that were used and the robustness of the
statistical methods used for detecting outliers. As a general rule, laboratories with scoring Z > 2 offered clearly poorer performance in robust approaches than classical ones. In order to support this first evidence,
we evaluated a second data set with results from 24 laboratories (mercury from soil samples) by means of the four mentioned
approaches. Selection and comparison of different scoring systems must be done very carefully, because sometimes they are
not the best approach for studying the data population or the more appropriate one for evaluating the distribution of the
data. Finally it should be taken into account that sometimes the robust scoring systems are not always suitable for evaluating
the results of some PT schemes. 相似文献
10.
Koel Saha Jhilly Dasgupta Sudip Chakraborty Felipe Antonio Fernandes Antunes Jaya Sikder Stefano Curcio Julio Cesar dos Santos Hassan A. Arafat Silvio Silvério da Silva 《Cellulose (London, England)》2017,24(8):3191-3207
Ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM]oAc) was employed for the pretreatment of sugarcane bagasse (SCB) and extraction of lignin, a potentially valuable by-product of the biofuel industry. Response surface methodology based on central composite design was exploited and thereby an empirical model, exhibiting a coefficient of determination, R2, of 0.9890, was established to optimize lignin recovery. In particular, a maximum lignin yield, equal to 90.1%, was calculated at the optimal pretreatment conditions, namely time: 120 min, temperature: 140 °C, and ionic liquid to bagasse ratio equal to 20:1 (wt/wt). The presence of guaiacyl and syringyl rings in lignin was confirmed by Fourier transform infrared spectroscopy (FTIR); whereas UV–Vis spectrophotometry showed that both p-coumaric acid and ferulic acid were contained in the lignin. Thermal analysis indicated a maximum decomposition rate of 2%/°C at 265 °C while Gel permeation chromatography analysis revealed that the molecular weight (Mw) of recovered lignin was equal to 1769 g/mol. Comparison of FTIR spectra of pretreated and untreated bagasse showed a negligible presence of lignin in the pretreated samples. Maximum delignification of bagasse after pretreatment was thus ensured. Thermal stability of the ionic liquid towards recyclability was proven by thermogravimetric analysis. The present study established adequate performance of neat and recycled ([EMIM]oAc) with regard to lignin recovery from SCB. 相似文献