全文获取类型
收费全文 | 467篇 |
免费 | 32篇 |
国内免费 | 2篇 |
专业分类
化学 | 411篇 |
晶体学 | 2篇 |
力学 | 8篇 |
数学 | 24篇 |
物理学 | 56篇 |
出版年
2023年 | 3篇 |
2022年 | 10篇 |
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 13篇 |
2018年 | 7篇 |
2017年 | 14篇 |
2016年 | 26篇 |
2015年 | 10篇 |
2014年 | 18篇 |
2013年 | 26篇 |
2012年 | 41篇 |
2011年 | 49篇 |
2010年 | 27篇 |
2009年 | 15篇 |
2008年 | 36篇 |
2007年 | 23篇 |
2006年 | 26篇 |
2005年 | 23篇 |
2004年 | 21篇 |
2003年 | 15篇 |
2002年 | 14篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 1篇 |
排序方式: 共有501条查询结果,搜索用时 0 毫秒
41.
42.
Cloning of the Orange Light‐Producing Luciferase from Photinus scintillans—A New Proposal on how Bioluminescence Color is Determined 下载免费PDF全文
Bruce R. Branchini Tara L. Southworth Danielle M. Fontaine Martha H. Murtiashaw Alex McGurk Munya H. Talukder Rakhshi Qureshi Deniz Yetil Jesse A. Sundlov Andrew M. Gulick 《Photochemistry and photobiology》2017,93(2):479-485
Unlike the enchanting yellow‐green flashes of light produced on warm summer evenings by Photinus pyralis, the most common firefly species in North America, the orange lights of Photinus scintillans are infrequently observed. These Photinus species, and likely all bioluminescent beetles, use the same substrates beetle luciferin, ATP and oxygen to produce light. It is the structure of the particular luciferase enzyme that is the key to determining the color of the emitted light. We report here the molecular cloning of the P. scintillans luc gene and the expression and characterization of the corresponding novel recombinant luciferase enzyme. A comparison of the amino acid sequence with that of the highly similar P. pyralis enzyme and subsequent mutagenesis studies revealed that the single conservative amino acid change tyrosine to phenylalanine at position 255 accounted for the entire emission color difference. Additional mutagenesis and crystallographic studies were performed on a H‐bond network, which includes the position 255 residue and five other stringently conserved beetle luciferase residues, that is proximal to the substrate/emitter binding site. The results are interpreted in the context of a speculative proposal that this network is key to the understanding of bioluminescence color determination. 相似文献
43.
Diego Todescato Danielle Maass Diego Alex Mayer J. Vladimir Oliveira Débora de Oliveira Selene M. A. Guelli Ulson de Souza Antônio Augusto Ulson de Souza 《Applied biochemistry and biotechnology》2017,183(4):1375-1389
Rhodococcus sp. has a broad catabolic diversity and unique enzymatic capabilities, and it is able to adapt under extreme conditions. Thereby, the production of this remarkable bacterium has a great biotechnological and industrial importance. In this sense, we sought to improve the R. erythropolis ATCC 4277 growth through a central composite design, by varying the components of nutrient medium (glucose, malt extract, yeast extract, CaCO3), temperature, and agitation. It was found that the concentrations of glucose and malt extract are not statistically significant, being reduced of 4.0 and 10.0 g L-1 to 2.0 and 5.0 g L?1, respectively. The CaCO3 concentration and temperature were also diminished of 2.0 to 1.16 g L?1and 28 to 23.7 °C, respectively. Optimal growth conditions provided a 240% increase in final biomass concentration, an increment in specific growth rate, and a growth yield coefficient about five times greater. Application of the optimal conditions in biodesulfurization and biodenitrogenation processes showed that desulfurization capability is not associated with optimal growth conditions; however, it was achieved a 47% of nitrogen removal in the assay containing 10% (w/w) of heavy gas oil. 相似文献
44.
Mulet X Kaasgaard T Conn CE Waddington LJ Kennedy DF Weerawardena A Drummond CJ 《Langmuir : the ACS journal of surfaces and colloids》2010,26(23):18415-18423
Three nucleoside lipids have been synthesized: 3'-oleoylthymidine, 3',5'-dioleoylthymidine, and 3'-phytanoylthymidine. Differential scanning calorimetry and X-ray diffraction have been employed to characterize the physical properties of these neat lipids. Polarizing optical microscopy, small-angle X-ray scattering, and cryo-transmission electron microscopy techniques have been used to investigate the phase behavior in aqueous systems. Both oleoyl-based nucleoside lipids adopted a lamellar crystalline phase in the neat form at room temperature, and the phytanoyl derivative exhibited a fluid isotropic phase. Under excess water conditions, the presence of one branched (phytanoyl) or one unsaturated (oleoyl) chain promoted the formation of a liquid-crystalline lamellar phase at physiological temperatures. In contrast, the 3',5'-dioleoylthymidine derivative is nonswelling and does not exhibit lyotropic liquid-crystalline phase behavior. The nucleolipids' propensity for DNA-type binding and recognition has been evaluated by using a monolayer system to measure surface pressure-area isotherms in a Langmuir trough and indicates that the nucleoside base is available for nonspecific hydrogen bonding in the monolayer liquid expanded state for the single-chain nucleolipids but not for the dual-chain amphiphile. 相似文献
45.
Danielle Vellucci Athit Kao Robyn M. Kaake Scott D. Rychnovsky Lan Huang 《Journal of the American Society for Mass Spectrometry》2010,21(8):1432-1445
Protein-protein interaction is one of the key regulatory mechanisms for controlling protein function in various cellular processes.
Chemical cross-linking coupled with mass spectrometry has proven to be a powerful method not only for mapping protein-protein
interactions of all natures, including weak and transient ones, but also for determining their interaction interfaces. One
critical challenge remaining in this approach is how to effectively isolate and identify cross-linked products from a complex
peptide mixture. In this work, we have developed a novel strategy using conjugation chemistry for selective enrichment of
cross-linked products. An azide-tagged cross-linker along with two biotinylated conjugation reagents were designed and synthesized.
Cross-linking of model peptides and cytochrome c as well as enrichment of the resulting cross-linked peptides has been assessed. Selective conjugation of azide-tagged cross-linked
peptides has been demonstrated using two strategies: copper catalyzed cycloaddition and Staudinger ligation. While both methods
are effective, Staudinger ligation is better suited for enriching the cross-linked peptides since there are fewer issues with
sample handling. LC MSn analysis coupled with database searching using the Protein Prospector software package allowed identification of 58 cytochrome
c cross-linked peptides after enrichment and affinity purification. The new enrichment strategy developed in this work provides
useful tools for facilitating identification of cross-linked peptides in a peptide mixture by MS, thus presenting a step forward
in future studies of protein-protein interactions of protein complexes by cross-linking and mass spectrometry. 相似文献
46.
Luu T Medos BJ Graham ER Vallee DM McDonald R Ferguson MJ Tykwinski RR 《The Journal of organic chemistry》2010,75(24):8498-8507
Terminal di-, tri-, tetra-, and pentaynes substituted with a variety of functional groups react with benzyl azide in the presence of CuSO(4)·5H(2)O and ascorbic acid to give derivatives of 4-ethynyl-, 4-butadiynyl-, 4-hexatriynyl-, and 4-octatetraynyl-1,2,3-triazoles in moderate to good yields. These reactions appear to proceed regioselectively, and functionalization occurs exclusively at the terminal alkyne moiety. As well, no evidence of multiple azide additions to the polyyne framework is observed. X-ray crystallographic analysis of nine derivatives is used to document the regioselectivity of the reaction as well as outline structural characteristics of the 1,2,3-triazole products. 相似文献
47.
Ji-Yao Li Biaoxin Chai Weizhen Zhang Xiaobin Wu Chao Zhang Danielle Fritze Zefeng Xia Cam Patterson Michael W Mulholland 《BMC neuroscience》2011,12(1):1-10
Background
Cigarette smoking enhances the risk of stroke. However, the underlying molecular mechanisms are largely unknown. The present study established an in vivo rat secondhand cigarette smoking (SHS) model and examined the hypothesis that SHS upregulates endothelin receptors with increased cerebrovascular contraction via the Raf/extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinases (MAPK) pathway.Results
Rats were exposed to SHS for up to 8 weeks. The cerebral artery vasoconstriction was recorded by a sensitive myograph. The mRNA and protein expressions for endothelin receptors in cerebral arteries were studied by real-time PCR and Western blot. Compared to fresh air exposed rats, cerebral arteries from SHS rats exhibited stronger contractile responses (P < 0.05) mediated by endothelin type A (ETA) receptors. The expressions of mRNA and protein for ETA receptors in the cerebral arteries from SHS rats were higher (P < 0.05) than that in control. SHS did not affect endothelin type B (ETB) receptor-mediated contractions, mRNA or protein levels. The results suggest that SHS upregulates ETA, but not ETB receptors in vivo. After SHS exposure, the mRNA levels of Raf-1 and ERK1/2, the protein expression of phosphorylated (p)-Raf-1 and p-ERK1/2 were increased (P < 0.05). Raf-1 inhibitor, GW5074 suppressed the enhanced ETA receptor-mediated contraction, mRNA and protein levels induced by SHS. In addition, GW5074 inhibited the SHS-caused increased mRNA and phosphorylated protein levels of Raf-1 and ERK1/2, suggesting that SHS induces activation of the Raf/ERK/MAPK pathway.Conclusions
SHS upregulates cerebrovascular ETA receptors via the Raf/ERK/MAPK pathway, which provides novel understanding of mechanisms involved in SHS-associated stroke. 相似文献48.
Huang J Wang AM Shetty A Maitz AH Yan D Doyle D Richey K Park S Pieper DR Chen PY Grills IS 《Magnetic resonance imaging》2011,29(7):993-1001
Objective
To determine the accuracy of magnetic resonance spectroscopy (MRS), perfusion MR imaging (MRP), or volume modeling in distinguishing tumor progression from radiation injury following radiotherapy for brain metastasis.Methods
Twenty-six patients with 33 intra-axial metastatic lesions who underwent MRS (n=41) with or without MRP (n=32) after cranial irradiation were retrospectively studied. The final diagnosis was based on histopathology (n=4) or magnetic resonance imaging (MRI) follow-up with clinical correlation (n=29). Cho/Cr (choline/creatinine), Cho/NAA (choline/N-acetylaspartate), Cho/nCho (choline/contralateral normal brain choline) ratios were retrospectively calculated for the multi-voxel MRS. Relative cerebral blood volume (rCBV), relative peak height (rPH) and percentage of signal-intensity recovery (PSR) were also retrospectively derived for the MRPs. Tumor volumes were determined using manual segmentation method and analyzed using different volume progression modeling. Different ratios or models were tested and plotted on the receiver operating characteristic curve (ROC), with their performances quantified as area under the ROC curve (AUC). MRI follow-up time was calculated from the date of initial radiotherapy until the last MRI or the last MRI before surgical diagnosis.Results
Median MRI follow-up was 16 months (range: 2-33). Thirty percent of lesions (n=10) were determined to be radiation injury; 70% (n=23) were determined to be tumor progression. For the MRS, Cho/nCho had the best performance (AUC of 0.612), and Cho/nCho >1.2 had 33% sensitivity and 100% specificity in predicting tumor progression. For the MRP, rCBV had the best performance (AUC of 0.802), and rCBV >2 had 56% sensitivity and 100% specificity. The best volume model was percent increase (AUC of 0.891); 65% tumor volume increase had 100% sensitivity and 80% specificity.Conclusion
Cho/nCho of MRS, rCBV of MRP, and percent increase of MRI volume modeling provide the best discrimination of intra-axial metastatic tumor progression from radiation injury for their respective modalities. Cho/nCho and rCBV appear to have high specificities but low sensitivities. In contrast, percent volume increase of 65% can be a highly sensitive and moderately specific predictor for tumor progression after radiotherapy. Future incorporation of 65% volume increase as a pretest selection criterion may compensate for the low sensitivities of MRS and MRP. 相似文献49.
Vidoslav Dekić Niko Radulović Rastko Vukićević Biljana Dekić Danielle Skropeta Radosav Palić 《Magnetic resonance in chemistry : MRC》2010,48(11):896-902
Herein, we describe the synthesis and complete assignment of the 1H and 13C NMR chemical shifts of a series of antimicrobial 4‐arylamino‐3‐nitrocoumarin derivatives based on a combination of 1H and 13C NMR, 1H‐1H‐COSY, NOESY, HSQC and HMBC experiments. Conformational effects upon the chemical shifts of the coumarin moiety arising from the anisotropy of the aryl side group are briefly discussed. This study provides the first complete and fully assigned NMR data for this important group of antimicrobial compounds and bridges the gap existing in the literature with regard to NMR structural data for 4‐arylamino‐3‐nitrocoumarins. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
50.
High Sensitive Microsensor Based on Organic‐Inorganic Composite for Two‐Dimensional Mapping of H2O2 by SECM 下载免费PDF全文
Danielle Diniz Justino Isabel Lourenço Torres Rita de Cássia Silva Luz Flavio Santos Damos 《Electroanalysis》2015,27(5):1202-1209
The present work describes the development of a selective, sensitive and stable sensing microsensor for scanning electrochemical microscopy (SECM) to measure H2O2 during electrochemical reduction of oxygen. The microsensor is based on graphene and Poly(3,4‐ethylenedioxythiophene) composite as support to iron (III) hexacyanoferrate (II) (PEDOT/graphene/FeIII4[FeII(CN)6]3 microsensor). The electrochemical properties of the PEDOT/graphene/FeIII4[FeII(CN)6]3 microsensor were investigated by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The PEDOT/graphene/FeIII4[FeII(CN)6]3 microsensor showed an excellent electrocatalytic activity toward hydrogen peroxide (H2O2) reduction with a diminution of the overpotential of about 500 mV in comparison to the process at a bare gold microelectrode. The microsensor presented excellent performance for two dimensional mapping of H2O2 by SECM in 0.1 mol L?1 phosphate buffer solution (pH 7.0). Under optimized conditions, a linear response range from 1 up to 1000 µmol L?1 was obtained with a sensitivity of 0.08 nA L µmol?1 and limit of detection of 0.5 µmol L?1. 相似文献