全文获取类型
收费全文 | 12345篇 |
免费 | 427篇 |
国内免费 | 22篇 |
专业分类
化学 | 8096篇 |
晶体学 | 72篇 |
力学 | 174篇 |
数学 | 2058篇 |
物理学 | 2394篇 |
出版年
2023年 | 106篇 |
2022年 | 245篇 |
2021年 | 321篇 |
2020年 | 262篇 |
2019年 | 224篇 |
2018年 | 241篇 |
2017年 | 269篇 |
2016年 | 587篇 |
2015年 | 445篇 |
2014年 | 482篇 |
2013年 | 980篇 |
2012年 | 741篇 |
2011年 | 875篇 |
2010年 | 553篇 |
2009年 | 483篇 |
2008年 | 566篇 |
2007年 | 513篇 |
2006年 | 393篇 |
2005年 | 345篇 |
2004年 | 287篇 |
2003年 | 265篇 |
2002年 | 261篇 |
2001年 | 135篇 |
2000年 | 109篇 |
1999年 | 121篇 |
1998年 | 110篇 |
1997年 | 116篇 |
1996年 | 106篇 |
1995年 | 101篇 |
1994年 | 81篇 |
1993年 | 79篇 |
1992年 | 80篇 |
1991年 | 76篇 |
1988年 | 70篇 |
1987年 | 79篇 |
1986年 | 72篇 |
1985年 | 96篇 |
1984年 | 96篇 |
1983年 | 83篇 |
1982年 | 74篇 |
1981年 | 72篇 |
1980年 | 86篇 |
1979年 | 76篇 |
1978年 | 85篇 |
1977年 | 86篇 |
1976年 | 64篇 |
1974年 | 69篇 |
1973年 | 75篇 |
1966年 | 64篇 |
1965年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
Raquel González de Orduña Mikael Hult Erica Andreotti Dušan Budjáš Stefan Schönert Marcin Misiaszek 《Journal of Radioanalytical and Nuclear Chemistry》2010,286(2):477-482
A simple technique for pulse shape discrimination in HPGe-detectors of the so-called BEGe type, based on just one parameter obtained from one signal read out, is presented here. This technique allows discriminating between pulses generated when the deposited energy is located within a small region of about 1 mm3 from the pulses generated when the energy is deposited at different locations several mm or cm apart. Two possible applications using this technique are: (i) experiments that look for neutrinoless double β decay in 76Ge, such as GERDA; (ii) γ spectrometry measurements where the Compton continuum can be reduced and the efficiency for cascading γ-rays can remain high. With this active background reduction technique a Compton suppression factor of about 3 was obtained. The detector response may be influenced by the detector size. The detector used for this study had a diameter of 6 cm, a thickness of 2.6 cm and a relative efficiency of 19%. The results obtained with this detector were consistent with the results obtained by Budjá? et al. [J Instrum 4:10, 2009] with a 50% relative efficiency BEGe detector. 相似文献
942.
Filgueira Dde M Guterres LP Votto AP Vargas MA Boyle RT Trindade GS Nery LE 《Photochemistry and photobiology》2010,86(6):1278-1284
The purpose of this study was to verify the occurrence of pigment dispersion in retinal pigment cells exposed to UVA and UVB radiation, and to investigate the possible participation of a nitric oxide (NO) pathway. Retinal pigment cells from Neohelice granulata were obtained by cellular dissociation. Cells were analyzed for 30 min in the dark (control) and then exposed to 1.1 and 3.3 J cm−2 UVA, 0.07 and 0.9 J cm−2 UVB, 20 n m β-PDH (pigment dispersing hormone) or 10 μ m SIN-1 (NO donor). Histological analyses were performed to verify the UV effect in vivo . Cultured cells were exposed to 250 μ m L-NAME (NO synthase blocker) and afterwards were treated with UVA, UVB or β-PDH. The retinal cells in culture displayed significant pigment dispersion in response to UVA, UVB and β-PDH. The same responses to UVA and UVB were observed in vivo . SIN-1 did not induce pigment dispersion in the cell cultures. l-NAME significantly decreased the pigment dispersion induced by UVA and UVB but not by β-PDH. All retinal cells showed an immunopositive reaction against neuronal nitric oxide synthases. Therefore, UVA and UVB radiation are capable of inducing pigment dispersion in retinal pigment cells of Neohelice granulata and this dispersion may be nitric oxide synthase dependent. 相似文献
943.
Myjavcová R Marhol P Křen V Simánek V Ulrichová J Palíková I Papoušková B Lemr K Bednář P 《Journal of chromatography. A》2010,1217(51):7932-7941
Anthocyanins from the fruit Lonicera caerulea L. var. kamtschatica (blueberry honeysuckle, Caprifoliaceae) were studied via (semi)preparative chromatographic fractionation followed by MS and μLC/MS analysis. The extraction procedure was optimized with respect to analytical purposes as well as its potential use for the preparation of nutraceuticals. The highest yield of anthocyanins was obtained using acidified methanol as the extraction medium. A comparable total anthocyanin content was obtained using a mixture of methanol and acetone. However, when Lonicera anthocyanins were in contact with acetone, a condensation reaction occurred to a large extent and related 5-methylpyranoanthocyanins were found. The effect of other extraction media, including ethanol as a "green" solvent, is also discussed. The potential of two fractionation procedures for extract purification differing in their chromatographic selectivity and scale was studied (i.e. using a Sephadex LH-20 gel column and a reversed phase). Fractions obtained by both procedures were used for a detailed analysis. MS and μLC/MS(2) methods were used for monitoring anthocyanin and 5-methylpyranoderivatives content as well as identifying less common and more complex dyes (dimer of cyanidin-3-hexoside, cyanidin-ethyl-catechin-hexosides, etc.). These more complex dyes are most likely formed during fruit treatment. 相似文献
944.
Natacha Lourette Heather Smallwood Si Wu Errol W. Robinson Thomas C. Squier Richard D. Smith Ljiljana Paša-Tolić 《Journal of the American Society for Mass Spectrometry》2010,21(6):930-939
A liquid chromatography-mass spectrometry (LC-MS)-based approach for characterizing the degree of nitration and oxidation of intact calmodulin (CaM) has been used to resolve ~250 CaM oxiforms using only 500 ng of protein. The analysis was based on high-resolution data of the intact CaM isoforms obtained by Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) coupled with an on-line reversed-phase LC separation. Tentative identifications of post-translational modifications (PTMs), such as oxidation or nitration, have been assigned by matching observed protein mass to a database containing all theoretically predicted oxidation products of CaM and verified through a combination of tryptic peptide information (generated from bottom-up analyses) and on-line collisionally induced dissociation (CID) tandem mass spectrometry (MS/MS) at the intact protein level. The reduction in abundance and diversity of oxidatively modified CaM (i.e., nitrated tyrosines and oxidized methionines) induced by macrophage activation has been explored and semiquantified for different oxidation degrees (i.e., no oxidation, moderate, and high oxidation). This work demonstrates the power of the top-down approach to identify and quantify hundreds of combinations of PTMs for single protein target such as CaM and implicate competing repair and peptidase activities to modulate cellular metabolism in response to oxidative stress. 相似文献
945.
Lippow SM Moon TS Basu S Yoon SH Li X Chapman BA Robison K Lipovšek D Prather KL 《Chemistry & biology》2010,17(12):1306-1315
Engineered biosynthetic pathways have the potential to produce high-value molecules from inexpensive feedstocks, but a key limitation is engineering enzymes with high activity and specificity for new reactions. Here, we developed a method for combining structure-based computational protein design with library-based enzyme screening, in which inter-residue correlations favored by the design are encoded into a defined-sequence library. We validated this approach by engineering a glucose 6-oxidase enzyme for use in a proposed pathway to convert D-glucose into D-glucaric acid. The most active variant, identified after only one round of diversification and screening of only 10,000 wells, is approximately 400-fold more active on glucose than is the wild-type enzyme. We anticipate that this strategy will be broadly applicable to the discovery of new enzymes for engineered biological pathways. 相似文献
946.
Peltier P Beláňová M Dianišková P Zhou R Zheng RB Pearcey JA Joe M Brennan PJ Nugier-Chauvin C Ferrières V Lowary TL Daniellou R Mikušová K 《Chemistry & biology》2010,17(12):1356-1366
UDP-galactofuranose (UDP-Galf) is a substrate for two types of enzymes, UDP-galactopyranose mutase and galactofuranosyltransferases, which are present in many pathogenic organisms but absent from mammals. In particular, these enzymes are involved in the biosynthesis of cell wall galactan, a polymer essential for the survival of the causative agent of tuberculosis, Mycobacterium tuberculosis. We describe here the synthesis of derivatives of UDP-Galf modified at C-5 and C-6 using a chemoenzymatic route. In cell-free assays, these compounds prevented the formation of mycobacterial galactan, via the production of short "dead-end" intermediates resulting from their incorporation into the growing oligosaccharide chain. Modified UDP-furanoses thus constitute novel probes for the study of the two classes of enzymes involved in mycobacterial galactan assembly, and studies with these compounds may ultimately facilitate the future development of new therapeutic agents against tuberculosis. 相似文献
947.
It is well known that free radicals are responsible for oxidative stress and cause numerous health disorders. As a result, the study of molecules that can scavenge free radicals is significant. One of the most important classes of free radical scavengers are carotenoids (CAR). In this work, the effectiveness of the CAR in terms of the radical adduct formation (RAF) reaction is studied using density functional theory calculations (in polar and non-polar environments). The reactions between four CAR [β-carotene (BC), zeaxanthin (ZEA), canthaxanthin (CANTA) and astaxanthin (ASTA)] with eight different radicals (?OH, ?OOH, ?CH3, ?O–CH3, ?OO–CH3, ?SH, ?O–CH2–CH=CH2, and ?OO–CH2–CH=CH2), as well as substantial further reactions involved in the radical chain propagation, are analyzed. According to our results, the RAF reactions are controlled to a larger extent by the nature of the free radical than by the particular CAR they are reacting with. Thermochemistry calculations predict that each CAR molecule is able to scavenge at least two free radicals, which would lead to the termination of the radical chain process. Epoxy and diepoxy CAR species can be formed, being epoxy molecules as good free radical scavengers as their parent CAR. ASTA and CANTA are predicted to be less reactive, when reacting through RAF mechanism, than BC and ZEA. 相似文献
948.
949.
950.
Suitability of a pyridinium ionic liquid as a solvent in desulfurization has been analyzed. (Liquid + liquid) equilibria for ternary systems composed by 1-hexyl-3,5-dimethyl pyridinium {bis[trifluoromethylsulfonyl]imide, thiophene, and three hydrocarbons representative of fuel (n-heptane, 2,2,4 trimethylpentane, and toluene) have been determined at T = 298.15 K and atmospheric pressure. High solubility of thiophene in the ionic liquid and also of toluene have been found, being this solvent practically immiscible with 2,2,4 trimethylpentane and heptane. Equilibrium data of these systems have been well correlated with UNIQUAC equations finding the highest deviations for the ternary system involving toluene. NRTL model drove to worse results being considered as not suitable model to correlate the experimental results. 相似文献