首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   424篇
  免费   19篇
  国内免费   13篇
化学   430篇
综合类   9篇
物理学   17篇
  2023年   8篇
  2022年   6篇
  2021年   53篇
  2020年   47篇
  2019年   32篇
  2018年   21篇
  2017年   16篇
  2016年   25篇
  2015年   30篇
  2014年   13篇
  2013年   7篇
  2012年   20篇
  2011年   17篇
  2010年   13篇
  2009年   14篇
  2008年   16篇
  2007年   21篇
  2006年   20篇
  2005年   11篇
  2004年   17篇
  2003年   7篇
  2002年   12篇
  2001年   7篇
  2000年   4篇
  1999年   6篇
  1998年   1篇
  1997年   2篇
  1995年   1篇
  1994年   3篇
  1991年   1篇
  1988年   1篇
  1986年   1篇
  1985年   1篇
  1983年   1篇
  1980年   1篇
排序方式: 共有456条查询结果,搜索用时 15 毫秒
1.
In recent years, more and more attention had been paid to the combination of proteins and flavonoids, and several flavonoids had been reported to improve the physicochemical and emulsifying properties of proteins. This study investigated the effects of ultrasonic treatment (450 W for 10 min, 20 min, and 30 min) on the physicochemical properties, antioxidant activity, and emulsifying properties of soy protein isolate (SPI) -hawthorn flavonoids (HF) non-covalent complexes. The results showed that the addition of HF to SPI and 20 min of ultrasound could reduce α-helix and random coil, increase β-sheet and β-turn, and enhance fluorescence quenching. In addition, it decreased the particle size, zeta potential, surface hydrophobicity, and turbidity to 88.43 or 95.27 nm, −28.80 mV, 1250.42, and 0.23, respectively. The protein solubility, free sulfhydryl group, antioxidant activity, emulsifying activity index, and emulsifying stability index all increased to 73.93%, 15.07 μmol/g, 71.00 or 41.91%, 9.81 m2/g, and 67.71%, respectively. Moreover, high-density small and low-flocculation droplets were formed. Therefore, the combined ultrasound treatment and addition of HF to SPI is a more effective method for protein modification compared to ultrasound treatment alone. It provides a theoretical basis for protein processing and application in the future.  相似文献   
2.
采用傅里叶变换红外光谱(FTIR)分析了新鲜野生刺梨果肉、烘干刺梨果肉以及室温存放数日后刺梨果肉的红外光谱特性。为对比新鲜果肉与烘干果肉样品中刺梨黄酮的红外光谱特性,以新鲜野生刺梨果实为研究对象,分别采用了超声辅助溶剂萃取法和超声结合酶辅助半仿生法提取刺梨黄酮。并使用无水乙醇作为超声辅助溶剂萃取法的萃取剂萃取刺梨黄酮,以及使用胃蛋白酶,胰酶和胆汁等模仿胃肠消化的环境,在模拟半仿生条件下提取刺梨黄酮。在两种提取方法中,新鲜果肉和烘干果肉分别反应了0,0.5,1,1.5,2和2.5 h。分别测定了新鲜果肉刺梨黄酮和烘干果肉刺梨黄酮的红外光谱数据。然后,通过分析刺梨黄酮的红外光谱特性,比较了两种方法所提取刺梨黄酮的最佳特征波长组合,得知在相同的反应条件下,反应时间为1.5 h时,超声辅助溶剂萃取法中新鲜果肉与烘干果肉提取物红外光谱对应的透射率强度分别为83.5%和84%;超声结合酶辅助半仿生法中新鲜果肉与烘干果肉提取物红外光谱对应的透射率强度分别为32%和38%。因此,结合郎伯-比尔定律可知,反应时间相同时,对于刺梨黄酮的提取,超声结合酶辅助半仿生法优于超声辅助溶剂萃取法。除此之外,我们发现随着反应时间延长,两种提取方法所获提取物的红外光谱吸收峰强度均呈上升趋势,但是反应2 h后该提取物的红外光谱吸收峰强度趋于平稳。结果表明,刺梨黄酮在3 419 cm-1(羟基O-H伸缩振动)、1 615 cm-1(羰基C═O键伸缩振动)和1 053 cm-1(烷基)处的红外吸收峰对刺梨的新鲜程度具有较好的识别效果;通过比较分析,新鲜果肉与室温存放数日后果肉中的刺梨黄酮物质的红外光谱吸收峰分别与槲皮素和山奈酚吸收峰波数一致;在相同实验条件下,两种提取方法中烘干果肉刺梨黄酮提取物浓度均高于新鲜刺梨果肉。该研究可为刺梨功能化药品和食品的制作及鉴定提供参考。  相似文献   
3.
Polyphenols are secondary plant metabolites playing a major role as potentially functional components. They can also be used for honey authentication. This review gathers the recent literature references about honey extraction procedures, as well as instrumental analysis of phenolic compounds found in honey. Liquid-Liquid extraction is widely used for both extraction and purification purposes, with adequate recovery percentages. However, the use of high solvent volumes is a major disadvantage. More environmentally friendly methods include accelerated solvent extraction, and dispersive and inverse dispersive liquid-liquid microextraction. Solid phase extraction is the most common method for honey polyphenols’ isolation. Polyphenol isolation by a combination of liquid-liquid and solid phase extraction allows good recoveries for a variety of different compounds. High-performance liquid chromatography with ultraviolet or mass spectrometry detectors is by far, the most commonly employed instrumental procedure to separate and quantify polyphenols in honey although capillary electrophoresis has been also successfully used for these purposes. The use of new sorbents, the optimization of current procedures and the development of other simple and rapid analytical techniques are challenges for future analysis of polyphenols found in honey.  相似文献   
4.
Siegesbeckia orientalis L. is an annual herb widely distributed throughout the world and has many medicinal properties. In Chinese traditional system, it is popularly known as Xi-Xian and used for its anti-inflammatory properties. In the present study, two cytotypes (diploid and tetraploid) have been investigated for their secondary metabolites. The different plant parts have been explored in terms of total phenolics, total flavonoids, DPPH radical scavenging acitivity and total antioxidant capacity. Out of different plant parts, leaves have the maximum amount of secondary metabolites and antioxidant potential. HPTLC technique has been applied to quantify six marker compounds in the two cytotypes. Tetraploid cytotype has been compared with diploid cytotype, which shows that tetraploid has the maximum amount of studied secondary metabolites with high antioxidant potential.  相似文献   
5.
Separation of minor compounds especially with similar polarities and structures from complex samples is a challenging work. In the present study, an efficient method was successfully established by macroporous resin column chromatography, medium‐pressure liquid chromatography, and high‐speed countercurrent chromatography for separation of four minor flavonoids from barley seedlings. Macroporous resin column chromatography and medium‐pressure liquid chromatography were used for enrichment of these four flavonoids. High‐pressure liquid chromatography analysis showed the total content of these four flavonoids increased from 2.2% in the crude extract to 95.3% in the medium‐pressure liquid chromatography fraction. It was indicated that the combination of macroporous resin column chromatography and medium‐pressure liquid chromatography could be a practicable strategy for enrichment of minor compounds from complex sample. Then, high‐speed countercurrent chromatography was employed for separation of these four flavonoids using ethyl acetate/n‐butanol/water (0.1% glacial acetic acid) (4:1:5, v/v/v) as solvent system. As a result, four flavonoids including two isomers with purities higher than 98% were obtained. Interestingly, two flavonoids existing in one high‐pressure liquid chromatography peak were also successfully separated. All these indicated high‐speed countercurrent chromatography had great potential for separation of compounds with similar structures and polarities. This study provides a reference for efficient enrichment and separation of minor compounds from complex sample.  相似文献   
6.
In order to improve the permeation and adsorption properties of graphene oxide, heteroatoms and deep eutectic solvent were introduced in this study. After being modified, the structural properties of graphene oxide were improved and the materials were applied to the determination of myricetin and rutin in tea sample by pipette‐tip solid‐phase extraction method. The materials were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X‐Ray diffractomer, energy dispersive spectroscopy, atomic force microscope, and specific surface area by Brunauer–Emmett–Teller N2 adsorption desorption analysis. Meanwhile, they were tested by static and dynamic adsorption. The result showed that the materials after modifying had better adsorption amount for myricetin and rutin than graphene oxide. The calibration graphs of myricetin and rutin in MeOH were linear over 0.10–500.00 µg/mL, and the limits of detection and quantification were in the range of 0.00546–0.0182 µg/mL and 0.00741–0.0247 µg/mL, respectively. A reliable analytical method was developed for recognition targets in tea sample by DES modified nitrogen‐doped graphene oxide with satisfactory extraction recoveries (myricetin 99.77%, rutin 98.14%). It was potential for the rapid purification of myricetin and rutin in tea sample combined with the pipette‐tip solid‐phase extraction.  相似文献   
7.
The current study aimed to investigate the anthocyanins, non-anthocyanins (flavonoids and phenolic acids), and free radicals scavenging potential in the flowers of Rhododendron arboreum using ultra high performance liquid chromatography with ion mobility quadrupole time of flight tandem mass spectrometry. A total of 25 constituents including nine anthocyanins, six phenolic acids, and ten flavonoids were identified in the flower extract. The major anthocyanins identified were cyanidin-3-O-β-galactoside ( 1 ), cyanidin-3-O-α-arabinoside ( 4 ), and cyanidin-3-O-rhamnoside ( 8 ), while quercetin glycosides were the main identified flavonoids in R. arboreum flowers. Additionally, ultra high performance liquid chromatography methods were developed and validated for the quantification of nine compounds (anthocyanins, flavonoid glycosides, and phenolic acids); five of them were quantified using internal standards. The extracts were analyzed for total phenolics (123.6 mg GAE/g), anthocyanin content (1.76% w/w), and evaluated for antioxidant properties against 2,2-diphenyl-1-picrylhydrazyl radical (IC50: 102.06 and 96.92 μg/mL) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) radical cation (112.25 and 45.59 μM TE/g) assays. The profiling of R. arboreum for anthocyanins is reported for the first time. The findings suggest that the flowers are a promising source of bioactive constituents and could be used as functional food, antioxidants, and nutraceuticals.  相似文献   
8.
In this work, the first example of a radical stereodivergent reaction directed towards the stereoselective synthesis of both (R*,R*)- and (R*,S*)-2,2′-biflavanones promoted by samarium diiodide is reported. Control experiments showed that the selectivity of this reaction was exclusively controlled by the temperature. It was possible to generate a variety of 2,2′-biflavanones bearing different substitution patterns at the aromatic ring in good-to-quantitative yields, being both stereoisomers of the desired compounds obtained with total or high control of selectivity. A mechanism that explains both the generation of the corresponding 2,2′-biflavanones and the selectivity is also discussed. The structure and stereochemistry determination of each isomer was unequivocally elucidated by single-crystal X-ray diffraction experiments.  相似文献   
9.
The aim of this work is the phytochemical study of the butanolic extract of the aerial parts of Veronica rosea. Four compounds 14 have been isolated using different chromatographic methods. The structures of these compounds were determined by NMR spectral analysis and mass spectroscopy. The adsorption and anticorrosion effects of this extract were investigated towards the corrosion of copper in 1 M HNO3 aqueous by the weight loss technique and potentiodynamic polarization. The results showed that the butanolic extract is a good inhibitor and the inhibition efficiency increases with increasing of concentration of the inhibitor. The adsorption of this extract on the copper specimen surface was spontaneous and obeyed the Langmuir’s adsorption isotherm. Large value of adsorption equilibrium Constant (K ads = 35 L g?1) was obtained. The polarization experiments confirmed the data obtained by gravimetric weight-loss. Tafel plot of polarization curves indicates that the extract acts as a mixed type inhibitor.  相似文献   
10.
Functional nutrition is a valuable supplementation to dietary therapy. Functional foods are enriched with biologically active substances. Plant polyphenols attract particular attention due to multiple beneficial properties attributed to their high antioxidant and other biological activities. We assessed the effect of grape polyphenols on the life span of C57BL/6 mice and on behavioral and neuroinflammatory alterations in a transgenic mouse model of Parkinson disease (PD) with overexpression of the A53T-mutant human α-synuclein. C57BL/6 mice were given a dietary supplement containing grape polyphenol concentrate (GPC—1.5 mL/kg/day) with drinking water from the age of 6–8 weeks for life. Transgenic PD mice received GPC beginning at the age of 10 weeks for four months. GPC significantly influenced the cumulative proportion of surviving and substantially augmented the average life span in mice. In the transgenic PD model, the grape polyphenol (GP) diet enhanced memory reconsolidation and diminished memory extinction in a passive avoidance test. Behavioral effects of GP treatment were accompanied by a decrease in α-synuclein accumulation in the frontal cortex and a reduction in the expression of neuroinflammatory markers (IBA1 and CD54) in the frontal cortex and hippocampus. Thus, a GP-rich diet is recommended as promising functional nutrition for aging people and patients with neurodegenerative disorders.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号