全文获取类型
收费全文 | 853篇 |
免费 | 25篇 |
国内免费 | 2篇 |
专业分类
化学 | 771篇 |
晶体学 | 7篇 |
力学 | 6篇 |
数学 | 50篇 |
物理学 | 46篇 |
出版年
2023年 | 6篇 |
2022年 | 35篇 |
2021年 | 31篇 |
2020年 | 14篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 8篇 |
2016年 | 30篇 |
2015年 | 14篇 |
2014年 | 43篇 |
2013年 | 48篇 |
2012年 | 44篇 |
2011年 | 67篇 |
2010年 | 55篇 |
2009年 | 40篇 |
2008年 | 71篇 |
2007年 | 64篇 |
2006年 | 62篇 |
2005年 | 52篇 |
2004年 | 43篇 |
2003年 | 33篇 |
2002年 | 32篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1975年 | 2篇 |
排序方式: 共有880条查询结果,搜索用时 62 毫秒
761.
Josefa Hernndez-Ruiz Domingo Ruiz-Cano Manuela Giraldo-Acosta Antonio Cano Marino B. Arnao 《Molecules (Basel, Switzerland)》2022,27(5)
Brassicaceae plants are of great interest for human consumption due to their wide variety and nutritional qualities. Of the more than 4000 species that make up this family, about a hundred varieties of 6–8 genera are extensively cultivated. One of the most interesting aspects is its high content of glucosinolates, which are plant secondary metabolites with widely demonstrated anti-oncogenic properties that make them healthy. The most relevant Brassicaceae studies related to food and melatonin are examined in this paper. The role of melatonin as a beneficial agent in seedling grown mainly in cabbage and rapeseed and in the postharvest preservation of broccoli is especially analyzed. The beneficial effect of melatonin treatments on the organoleptic properties of these commonly consumed vegetables can be of great interest in the agri-food industry. Melatonin application extends the shelf life of fresh-cut broccoli while maintaining optimal visual and nutritional parameters. In addition, an integrated model indicating the role of melatonin on the organoleptic properties, the biosynthesis of glucosinolates and the regulatory action of these health-relevant compounds with anti-oncogenic activity is presented. 相似文献
762.
Werner Lwe Stefanie A. Brtter Manuela Weber Peter Luger Joachim Buddrus 《Journal of heterocyclic chemistry》2001,38(2):365-370
Under basic conditions 2,6‐bis(bromomethyl)‐4‐pyrone 8 reacts with tetraethylene glycol to yield the unexpected macrocycle 9 , which is related to the antibiotic Kjellmanianone 10 . We propose that this ring transformation proceeds via the cyclopropyl intermediate d (Scheme 2), which undergoes a ring opening reaction comparable to the Favorskii rearrangement. Also, 8 reacts with methanol/sodium methoxide to yield the 3(2H)‐furanone derivative 11 , the formation of which is suggested to proceed via the intermediate k with a carbenium‐oxonium‐ion subunit (Scheme 3). The structure of the 3(2H)‐furanone derivative was confirmed by X‐ray analysis. 相似文献
763.
764.
765.
766.
M. Manuela Santos J. D. L. Pinto J. B. Ferreira R. A. Guedes de Carvalho 《Journal of solution chemistry》1975,4(1):31-35
The association constant of NaS2O 3 ? complex in a medium of μ=1.0M at 25.0±0.1°C was determined by two techniques: sodium-selective electrode (β1 = 1.48 ± 0.13) and UV spectrophotometry (β1 = 1.25 ± 0.10). There is no evidence, under our experimental conditions, of the existence of the binuclear complex Na2S2O3, which might exist at the very low value of the ratio \(C_{Na_2 s_2 o_3 } /C_{Na} +\) used in our experiments. 相似文献
767.
Manuela Melucci Dr. Massimo Zambianchi Alberto Zanelli Dr. Nadia Camaioni Dr. Massimo Gazzano Dr. Alessandro Bongini Prof. Dr. Giovanna Barbarella Dr. 《Chemphyschem》2007,8(18):2621-2626
A soluble, low‐weight fraction of poly(α‐vinyl,ω‐n‐hexyl‐quaterthiophene), PT4Hex, having n‐hexylquaterthiophenes as side‐chain groups, is prepared by free‐radical polymerization of α‐vinyl,ω‐n‐hexyl‐quaterthiophene and the corresponding properties compared to those of free di‐n‐hexylquaterthiophene (T4Hex). Optical analysis (absorption and emission) and X‐ray diffraction data indicate that in the polyvinyl‐locked architecture the quaterthiophene pendants adopt a cofacial arrangement with a mutual distance close enough for π–π orbitals to overlap (~4 Å). As a consequence of the close chain packing, a shift of the reduction potential of about 0.5 V toward less negative values with respect to free T4Hex, is found for PT4Hex films. Due to its enhanced electron affinity, PT4Hex displays an electron‐acceptor behavior when blended with alkylated and silylated quaterthiophenes acting as donors. 相似文献
768.
Banci L Benvenuti M Bertini I Cabelli DE Calderone V Fantoni A Mangani S Migliardi M Viezzoli MS 《Journal of the American Chemical Society》2005,127(38):13287-13292
It is known that several prokaryotic protein sequences, characterized by high homology with the eukaryotic Cu,ZnSODs, lack some of the metal ligands. In the present work, we have stepwise reintroduced the two missing copper ligands in the SOD-like protein of Bacillus subtilis, through site-directed mutagenesis. The mutant with three out of the four His that bind copper is not active, whereas the fully reconstituted mutant displays an activity of about 10% that of human Cu,ZnSOD. The mutated proteins have been characterized in solution and in the solid state. In solution, the proteins experience conformational disorder, which is believed to be partly responsible for the decreased enzymatic activity and sheds light on the tendency of several human SOD mutants to introduce mobility in the protein frame. In the crystal, on the contrary, the protein has a well-defined conformation, giving rise to dimers through the coordination of an exogenous zinc ion. The catalytic properties of the double mutant, which might be regarded as a step in an artificial evolution from a nonactive SOD to a fully functioning enzyme, are discussed on the basis of the structural and dynamical properties. 相似文献
769.
Teresa M.V.D. Pinho e Melo Ana L. CardosoAntónio M.d'A. Rocha Gonsalves Richard C. StorrJoão Costa Pessoa José A. PaixãoAna M. Beja Manuela Ramos Silva 《Tetrahedron letters》2003,44(34):6409-6412
A phosphorus ylide bearing a 10-phenylsulfonylisoborneol unit reacted with ketenes, generated in situ from acid chlorides and triethylamine, to give allenic compounds. The reaction with methylketene led to asymmetric induction with the selective synthesis of an allene with axial chirality corresponding to an S configuration. The structure of the chiral allene 10-phenylsulfonylisobornyl penta-2,3-dienoate was determined by X-ray crystallography. Chirooptical studies of the chiral allene derivatives were done. 相似文献
770.
Bruno F Curini R Corcia AD Nazzari M Pallagrosi M 《Rapid communications in mass spectrometry : RCM》2002,16(14):1365-1376
An original and highly specific method able to identify and quantify traces of five tetracycline antibiotics (TCAs) in milk and eggs is presented. This method uses a single solid-phase extraction (SPE) cartridge for simultaneous extraction and purification of TCAs in the above matrices. After diluting 5 mL of intact whole milk or 2 g egg samples with Na(2)EDTA-containing water, samples are passed through a 0.5-g Carbograph 4 extraction cartridge. After analyte elution from the SPE cartridge, an aliquot of the final extract is injected into a liquid chromatography/mass spectrometry (LC/MS) instrument equipped with an electrospray ion source and a single quadrupole. MS data acquisition is performed in the positive-ion mode and by a time-scheduled multiple-ion selected ion-monitoring program. With methanol as organic modifier, the in-source collision-induced dissociation (CID) process generated fragment ions able to pick up one methanol molecule. In several cases, these methanol-adduct fragment ions have m/z values higher than those of the protonated molecules. This event is rarely encountered in MS, thus making the analysis of TCAs by this method extremely specific. Compared with a conventional published method, the present protocol extracted larger amounts of TCAs from both milk and egg and decreased the analysis time by a factor of 3. Recovery of TCAs in milk at the 25-ppb level ranged between 81 and 96% with relative standard deviation (RSD) no larger than 9%. Recovery of TCAs in egg at the 50-ppb level ranged between 72 and 92% with RSD no larger than 7%. Estimated limits of quantification(S/N = 10) of the method were 2-9 ppb TCAs in whole milk and 2-19 ppb TCAs in eggs. 相似文献