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991.
给出Logistic,Gompertz,Usher和Von Bertalanffy 4种增长曲线的拐点、最大加速点和最大减速点的表达式.利用R语言编写程序,结合拟牛顿算法与随机优化算法,对微信月活跃用户数(WMAUs)的发展趋势进行分析与拟合.结果表明,4条曲线的拟合值在最大加速点到最大减速点时间段内基本吻合,但在初期和后期有所差别.Logistic曲线在初期的预测值偏大,而后期偏小,偏差相对较大;Von Bertalanffy曲线的偏差最小,且在饱和值、拐点、最大加速点、最大减速点及拟合优度的估计值方面都优于另外3种曲线;Usher和Gompertz曲线居于Logistic与Von Bertalanffy曲线之间,对后期的预测值有些保守.最后,基于价值理念的观点分析了影响WMAUs变动的主要因素. 相似文献
992.
993.
A graphene monolithic column was fabricated in a capillary using π-electron-rich poly(N-vinylcarbazole-divinylbenzene) as the supporter through in situ one-step polymerization for the enrichment of trace benzodiazepines in biological samples. This new three-dimensional monolith showed uniformity and a continuous column bed; more importantly, it retained the unique properties of graphene that are typically associated with individual graphene sheets. Based on the large delocalized π-electron system, graphene forms π–π stacking interactions with benzodiazepines and benzene rings of poly(N-vinylcarbazole-divinylbenzene), which not only enhance the extraction performance for benzodiazepines compared to the neat polymer but also provide chemical stability of the graphene monolith. Moreover, several factors likely to affect the extraction, including ionic strength, sample pH, sample volume, and eluant volume were studied in detail. The optimized method gave a linear range of 0.005–1?ng?mL?1, and detection limits of 1.12–2.35?ng?L?1. Finally, the graphene monolith was successfully applied to the separation and enrichment of benzodiazepines from urine and hair samples coupled with high-performance liquid chromatography–mass spectrometry. The recoveries were in the range of 78.6–85.6% for urine and 87.2–94.3% for hair with relative standard deviations of 3.4–6.9 and 2.9–8.3%, respectively. 相似文献
994.
Crystal Structures of Bacterial (6‐4) Photolyase Mutants with Impaired DNA Repair Activity 下载免费PDF全文
Kalinga Bowatte Dennis Kwiatkowski Esther Mittmann Heba Qasem Norbert Krauß Xiaoli Zeng Zhong Ren Patrick Scheerer Xiaojing Yang Tilman Lamparter 《Photochemistry and photobiology》2017,93(1):304-314
PhrB from Agrobacterium fabrum is the first prokaryotic photolyase which repairs (6‐4) UV DNA photoproducts. The protein harbors three cofactors: the enzymatically active FAD chromophore, a second chromophore, 6,7‐dimethyl‐8‐ribityllumazine (DMRL) and a cubane‐type Fe‐S cluster. Tyr424 of PhrB is part of the DNA‐binding site and could provide an electron link to the Fe‐S cluster. The PhrBY424F mutant showed reduced binding of lesion DNA and loss of DNA repair. The mutant PhrBI51W is characterized by the loss of the DMRL chromophore, reduced photoreduction and reduced DNA repair capacity. We have determined the crystal structures of both mutants and found that both mutations only affect local protein environments, whereas the overall fold remained unchanged. The crystal structure of PhrBY424F revealed a water network extending to His366, which are part of the lesion‐binding site. The crystal structure of PhrBI51W shows how the bulky Trp leads to structural rearrangements in the DMRL chromophore pocket. Spectral characterizations of PhrBI51W suggest that DMRL serves as an antenna chromophore for photoreduction and DNA repair in the wild type. The energy transfer from DMRL to FAD could represent a phylogenetically ancient process. 相似文献
995.
Ling Yan Liang Qiao Ji Ji Yixin Li Xuefei Yin Ling Lin Xiaohui Liu Jun Yao Yi Wang Bin Liu Kun Qian Baohong Liu Pengyuan Yang 《Analytica chimica acta》2017
Mass spectrometry (MS)-based proteome profiling is essential for molecular diagnostics in modern biomedical study. To date, sample preparation including protein extraction and proteolysis is still very challenging and lack of efficiency. Recently tips-based sample preparation protocols exhibit strong potentials to achieve the goal of “a proteome in an hour”. However, in-tip proteolysis is still rarely reported and far from ideal for dealing with complex bio-samples. In this work, nanoreactors encapsulated micropipette tips were demonstrated as high performance devices for fast (∼minutes) and multiplexing proteolysis to assist the profiling of cancer cells proteome. Nanoporous silica materials with controlled pore size and surface chemistry were prepared as nanoreactors and encapsulated in micropipette tips for efficient in situ proteolysis. The as-constructed device showed desirable sensitivity (LOD of 0.204 ± 0.008 ng/μL and LOQ of 0.937 ± 0.055 ng/μL), selectivity, stability (two months under −20 °C), reusability (at least 10 times), and little memory effect in MS based bottom-up proteomic analysis. It was used for comprehensive protein mapping from cancer cell lines. The number of identified proteins was increased by 18%, 22%, 52%, and 52% dealing with HepG2, F56, MCF7, and HCCLM3 cancer cells, compared to traditional in-solution proteolysis based bottom-up proteomic strategy. With the enhanced performance, our work built a novel, efficient and miniaturized platform for facile proteomic sample preparation, which is promising for advanced biomarkers discovery in biomedical study. 相似文献
996.
Double Michael addition of nitromethane to divinyl ketones: A remarkably positive effect of additive
Yongqi Yao Yingying Liu Ling Ye Feng Chen Xinying Li Zhigang Zhao Xuefeng Li 《Tetrahedron》2017,73(16):2311-2315
An efficient double Michael addition of nitromethane to divinyl ketones was established in good to high yields (75–99%). A wide range of cyclohexanones were obtained with excellent diastereocontrol (up to >20:1 dr) and enantioinduction (91–99% ee) in a one-pot fashion. The involvement of basic additive significantly enhanced the reactivity of this cascade sequence. 相似文献
997.
A highly selective HBT-based “turn-on” fluorescent probe for hydrazine detection and its application
Zhen Chen Xinxin Zhong Wangbo Qu Ting Shi Heng Liu Hanping He Xiuhua Zhang Shengfu Wang 《Tetrahedron letters》2017,58(26):2596-2601
As one of the important industrial chemicals, hydrazine (N2H4) can be inhaled through the skin, leading to many serious health issues. In this paper, we constructed a novel turn-on fluorescent probe HBTM for N2H4 detection based on ESIPT and ICT mechanism by incorporating the methyl dicyanvinyl group to 2-(2′-hydroxylphenyl) benzothiazole (HBT) fluorophore. The probe showed the following advantages: high sensitivity with detection limit of 2.9 × 10?7 M, high selectivity over other related interfering species, wide linear range of 0–140 μM and pH value adaptation. Moreover, the probe could detect N2H4 on paper strips and image N2H4 in living cells. 相似文献
998.
Leccinum rugosiceps is an edible mushroom belonging to genus Leccinum of Boletaceae. Its fruiting bodies are richer in nutrients than many vegetables and fruit. The model of support vector machine was established for the discrimination of L. rugosiceps from regions based on rapid and low-cost ultraviolet and infrared spectroscopies. The mid-level data fusion was performed by support vector machine. Compared to a single spectroscopic technique, mid-level data fusion provided higher accuracy by selecting the most significant variance from data matrixes based on partial least squares discriminant analysis. The accuracy of the classification of samples in the calibration and test sets were 85.00 and 94.74%, higher than separate measurements by ultraviolet or infrared spectroscopy. This approach has applications for authentication and quality assessment of L. rugosiceps. 相似文献
999.
Zhenyu?Cheng Qihui?Shen Huashi?Yu Dandan?Han Fangli?Zhong Yingjie?YangEmail author 《Mikrochimica acta》2017,184(12):4587-4595
The authors describe an electrochemical sensor for hydrogen peroxide (H2O2). It was constructed by consecutive, selective modification of a glassy carbon electrode (GCE) with Prussian Blue (PB), layered molybdenum disulfide (MoS2), and reduced graphene oxide (rGO). The properties of the modified GCE were characterized via high-resolution transmission electron microscopy, UV-vis spectroscopy and X-ray diffraction. The electrochemical properties of the electrode were studied using cyclic voltammetry and electrochemical impedance spectroscopy. The sensor exhibits excellent electrocatalytic activity for the reduction of hydrogen peroxide in comparison to GCEs modified with MoS2-rGO or PB only. Response is linear in the 0.3 μM to 1.15 mM H2O2 concentration range at a working analytical voltage of 0.1 V, with a 0.14 μM detection limit. The electrochemical sensitivity is 2883.5 μA·μM?1·cm?2, and response is fast (<10 s). The sensor is selective, stable and reproducible. This is attributed to the efficient electron transport properties of the MoS2-rGO composite and the high loading with PB. 相似文献
1000.
A series of metal–organic frameworks built from a propionate-functionalized purine-containing ligand 3-(6-oxo-6,9-dihydro-1H-purin-1-yl)propanoic acid (H2L), {[La(HL)3(H2O)2]·2H2O}n (1), {[Ce(HL)3(H2O)2]·4H2O}n (2), [Co(HL)2(H2O)2]n (3), {[Cd(L)(H2O)]·0.5H2O}n (4) and {[Pb(HL)(C2O4)0.5(H2O)]·2H2O}n (5), was synthesized and characterized. Isostructural 1 and 2 have polymeric chain structures further linked into 3-D porous supramolecular frameworks with 1-D open channels through complicated interchain hydrogen bonding interactions. At 77 K and 1 bar, the dehydrated porous materials 1 and 2 show adsorption behaviors with maximum nitrogen uptakes of 14 and 23 mL g?1, respectively. Complexes 3–5 are 2-D coordination polymers but have different topological structures. Metallohelicate 3 has (4,4) nets composed of left- and right-handed metal–organic helices sharing the common metal centers, but metallohelicate 4 possesses (4·82) topology and 5 has 63-topological structure. In 3 and 5, the polymeric layers are further assembled through regular interlayer hydrogen bonding interactions to form 3-D supramolecular frameworks. Additionally, the thermostabilities of 1–5 as well as the magnetism of 3 were also investigated. 相似文献