全文获取类型
收费全文 | 4570篇 |
免费 | 123篇 |
国内免费 | 16篇 |
学科分类
数理化 | 4709篇 |
出版年
2023年 | 50篇 |
2022年 | 138篇 |
2021年 | 132篇 |
2020年 | 109篇 |
2019年 | 97篇 |
2018年 | 82篇 |
2017年 | 91篇 |
2016年 | 186篇 |
2015年 | 146篇 |
2014年 | 139篇 |
2013年 | 245篇 |
2012年 | 260篇 |
2011年 | 286篇 |
2010年 | 171篇 |
2009年 | 149篇 |
2008年 | 279篇 |
2007年 | 270篇 |
2006年 | 222篇 |
2005年 | 238篇 |
2004年 | 162篇 |
2003年 | 156篇 |
2002年 | 112篇 |
2001年 | 49篇 |
2000年 | 47篇 |
1999年 | 44篇 |
1998年 | 32篇 |
1997年 | 31篇 |
1996年 | 52篇 |
1995年 | 44篇 |
1994年 | 37篇 |
1993年 | 34篇 |
1992年 | 26篇 |
1991年 | 27篇 |
1990年 | 28篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 39篇 |
1986年 | 23篇 |
1985年 | 39篇 |
1984年 | 38篇 |
1983年 | 19篇 |
1982年 | 21篇 |
1981年 | 26篇 |
1980年 | 28篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 28篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1858年 | 11篇 |
排序方式: 共有4709条查询结果,搜索用时 0 毫秒
151.
Andrea Ventrella Raffaella Verrone Francesco Longobardi Angela Agostiano Vincenzo Lippolis Michelangelo Pascale Chris M. Maragos Michael Appell Lucia Catucci 《Journal of inclusion phenomena and macrocyclic chemistry》2013,75(3-4):285-292
T-2 and HT-2 toxins are mycotoxins produced by several Fusarium species that are commonly found in various cereal grains, including oats, barley, wheat and maize. Intake estimates indicate that the presence of these mycotoxins in the diet can be of concern for public health. In this work, the inclusion processes occurring between fluorescent anthracene-derivatives of T-2 and HT-2 toxins and different cyclodextrin (CD) molecules were investigated in aqueous solutions by means of UV–Vis absorption, fluorescence emission and dynamic light scattering. Binding constant values and chemico-physical parameters were calculated. It was found that β-CDs give stronger inclusion reactions with both T-2 and HT-2 derivatives, as stated by important emission intensity increments. Such interactions were found to be fundamentally enthalpy-driven. Among β-CDs, the effect of the methylation at hydroxyl groups was tested: as a result, the di-methyl form of β-CD was found to induce the best fluorescence intensity enhancements. 相似文献
152.
Dr. Annalisa Mancini Dr. M. Carla Aragoni Dr. Neil Bricklebank Dr. Carlo Castellano Prof. Francesco Demartin Prof. Francesco Isaia Prof. Vito Lippolis Dr. Anna Pintus Dr. Massimiliano Arca 《化学:亚洲杂志》2013,8(3):639-647
The reactions of 4,5,6,7‐tetrathiocino‐[1,2‐b:3,4‐b′]‐1,3,8,10‐tetrasubstituted‐diimidazolyl‐2,9‐dithiones (R2,R′2‐todit; 1 : R=R′=Et; 2 : R=R′=Ph; 3 : R=Et, R′=Ph) with Br2 exclusively afforded 1:1 and 1:2 “T‐shaped” adducts, as established by FT‐Raman spectroscopy and single‐crystal X‐ray diffraction in the case of complex 1? 2 Br2. On the other hand, the reactions of compounds 1 – 3 with molecular I2 provided charge‐transfer (CT) “spoke” adducts, among which the solvated species 3? 2 I2 ? (1?x)I2 ? x CH2Cl2 (x=0.94) and ( 3 )2 ? 7 I2 ? x CH2Cl2, (x=0.66) were structurally characterized. The nature of all of the reaction products was elucidated based on elemental analysis and FT‐Raman spectroscopy and supported by theoretical calculations at the DFT level. 相似文献
153.
154.
155.
Giancarlo Marconi Sandra Monti Francesco Manoli Alessandra DegliEsposti Andrea Guerrini 《Helvetica chimica acta》2004,87(9):2368-2377
The circular dichroism (CD) spectra of the powerful antimalarian active principle artemisinin (ART; 1 ) from Artemisia annua, and of epiartemisinin (EPI, 2), its C(7) epimer, were studied in solution in the presence and absence of β‐cyclodextrin (β‐cd). A significant inversion of sign in the region of the second electronic transition was detected. The rotational strengths were successfully calculated within the time‐dependent density functional theory (DFT), which enabled us to establish a correlation between the molecular recognition and the biological acitivities of the two isomers. 相似文献
156.
Grald Lelais Peter Micuch Delphine Josien‐Lefebvre Francesco Rossi Dieter Seebach 《Helvetica chimica acta》2004,87(12):3131-3159
The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β3‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β2‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→OtBu etherification (for β2hSer; Schemes 5 and 6), OH→STrt replacement (for β2hCys; Scheme 7), and CH2OH→CH2N3→CH2NH2 transformations (for β2hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (tR), melting points, optical rotations, HPLC on chiral columns, IR, 1H‐ and 13C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis. 相似文献
157.
Covalent Attachment of Cyclic TAT Peptides to GFP Results in Protein Delivery into Live Cells with Immediate Bioavailability 下载免费PDF全文
Dr. Francesco Natale Nina Bohlke Prof. Nediljko Budisa Prof. M. Cristina Cardoso Prof. Christian P. R. Hackenberger 《Angewandte Chemie (International ed. in English)》2015,54(6):1950-1953
The delivery of free molecules into the cytoplasm and nucleus by using arginine‐rich cell‐penetrating peptides (CPPs) has been limited to small cargoes, while large cargoes such as proteins are taken up and trapped in endocytic vesicles. Based on recent work, in which we showed that the transduction efficiency of arginine‐rich CPPs can be greatly enhanced by cyclization, the aim was to use cyclic CPPs to transport full‐length proteins, in this study green fluorescent protein (GFP), into the cytosol of living cells. Cyclic and linear CPP–GFP conjugates were obtained by using azido‐functionalized CPPs and an alkyne‐functionalized GFP. Our findings reveal that the cyclic‐CPP–GFP conjugates are internalized into live cells with immediate bioavailability in the cytosol and the nucleus, whereas linear CPP analogues do not confer GFP transduction. This technology expands the application of cyclic CPPs to the efficient transport of functional full‐length proteins into live cells. 相似文献
158.
Radical anions of nitrobenzothiazoles: EPR study of conjugative properties of benzothiazolyl systems
Francesco Ciminale 《Tetrahedron letters》2004,45(30):5849-5852
The electrochemical reduction in DMSO of the five isomers of nitrobenzothiazole (NBTZ) gave quite persistent radical anions that could be easily characterised by EPR spectroscopy. By contrast, the chemical reduction in alkaline solution, that is by t-BuOK in DMSO or by glucose and MeOK in MeOH, presented some problems with 6- and 4-NBTZ, and in the case of 2-NBTZ did not provide any detectable paramagnetic species. The internal consistency of coupling constants of the nitrobenzothiazole radical anions is in good agreement with the conjugative properties of the various benzothiazolyl systems and allows rectifying a recent EPR characterisation of 6-NBTZ radical anion. 相似文献
159.
In the presence of a highly efficient novel bifunctional organocatalyst at low loadings under mild conditions, enolizable homophthalic anhydrides can be added to a range of aromatic and aliphatic aldehydes to give dihydroisocoumarins, with excellent yields and diastereo- and enantiocontrol (up to 99% ee). 相似文献
160.
Boutin A Coasne B Fuchs AH Galarneau A Di Renzo F 《Langmuir : the ACS journal of surfaces and colloids》2012,28(25):9526-9534
We report experimental nitrogen adsorption isotherms of organics-coated silicas, which exhibit a low-pressure desorption branch that does not meet the adsorption branch upon emptying of the pores. To address the physical origin of such a hysteresis loop, we propose an equilibrium thermodynamic model that enables one to explain this phenomenon. The present model assumes that, upon adsorption, a small amount of nitrogen molecules penetrate within the organic layer and reach adsorption sites that are located on the inorganic surface, between the grafted or adsorbed organic molecules. The number of accessible adsorption sites thus varies with the increasing gas pressure, and then we assume that it stays constant upon desorption. Comparison with experimental data shows that our model captures the features of nitrogen adsorption on such hybrid organic/inorganic materials. In particular, in addition to predicting the shape of the adsorption isotherm, the model is able to estimate, with a reasonable number of adjustable parameters, the height of the low-pressure hysteresis loop and to assess in a qualitative fashion the local density of the organic chains at the surface of the material. 相似文献