全文获取类型
收费全文 | 1918篇 |
免费 | 62篇 |
国内免费 | 9篇 |
专业分类
化学 | 1458篇 |
晶体学 | 6篇 |
力学 | 40篇 |
数学 | 315篇 |
物理学 | 170篇 |
出版年
2024年 | 1篇 |
2023年 | 21篇 |
2022年 | 105篇 |
2021年 | 92篇 |
2020年 | 75篇 |
2019年 | 56篇 |
2018年 | 39篇 |
2017年 | 32篇 |
2016年 | 62篇 |
2015年 | 74篇 |
2014年 | 47篇 |
2013年 | 114篇 |
2012年 | 122篇 |
2011年 | 157篇 |
2010年 | 84篇 |
2009年 | 77篇 |
2008年 | 129篇 |
2007年 | 138篇 |
2006年 | 107篇 |
2005年 | 103篇 |
2004年 | 83篇 |
2003年 | 77篇 |
2002年 | 73篇 |
2001年 | 19篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 3篇 |
1997年 | 10篇 |
1996年 | 13篇 |
1995年 | 11篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 7篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有1989条查询结果,搜索用时 15 毫秒
971.
Marina Piscopo Gian Carlo Tenore Rosaria Notariale Viviana Maresca Maria Maisto Francesca de Ruberto 《Natural product research》2020,34(18):2607-2611
AbstractIn this paper we report for the first time a method for the extraction of the protein fraction from Feijoa sellowiana Berg. fruit and its electrophoretic analysis. In addition we analyzed the protein fraction for its antioxidant activity and its effectiveness against different Gram-positive and Gram-negative bacteria both as American type culture collection (ATCC) standard and clinically isolated strains. Both antimicrobial and antioxidant activity resulted stronger respect to that previously obtained for the acetonic extract from the same fruit. Further, we study both activities also on the product of in vitro gastrointestinal digestion of F. sellowiana fruit proteins. The results showed that in vitro gastrointestinal digestion increased 10 fold the antioxidant activity, while the antimicrobial activity, tested only on ATCC strains, resulted from 2 to 4 fold increased. MTT assays showed the non-toxicity of these proteins both before and after digestion. 相似文献
972.
Francesca Rigano Marianna Oteri Giuseppe Micalizzi Domenica Mangraviti Paola Dugo Luigi Mondello 《Journal of separation science》2020,43(9-10):1773-1780
In a previous article, Rigano et al. established a new linear retention index system for the identification of triacylglycerols by liquid chromatography methods only on the basis of the retention behavior and independently from many experimental parameters. In that work, a database of 209 compounds was built, but only 54 of them, typical of vegetable oils, were confirmed by mass spectrometry. The aim of the present research is to extend the applicability of the novel approach to more complex samples, such as fish lipid extracts, and assess the complementarity between mass spectromtery and retention information to achieve univocal identification. With this purpose, a new software was implemented to make the identification process easy and automatic as in gas chromatography‐mass spectrometry where the retention index filter is added in the spectral search to discriminate between compounds with similar mass spectrometry spectra. A total of 69 species were identified and, thanks to their baseline separation obtained by an ultra high performance liquid chromatography method, a semiquantification was also performed. The species under investigation were Dicentrarchus labrax, coming from aquaculture and the wild. Some differences in their native lipid composition were observed, probably related to a different diet. A major number of samples would be necessary to confirm such a preliminary finding. 相似文献
973.
Dr. Martina Torelli Prof. Francesca Terenziani Dr. Alessandro Pedrini Dr. Francesca Guagnini Dr. Ilaria Domenichelli Prof. Chiara Massera Prof. Enrico Dalcanale 《ChemistryOpen》2020,9(2):261-268
The eligibility of tetraquinoxaline cavitands (QxCav) as molecular grippers relies on their unique conformational mobility between a closed (vase) and an open (kite) form, triggered in solution by conventional stimuli like pH, temperature and ion concentration. In the present paper, the mechanochemical conformational switching of ad hoc functionalized QxCav covalently embedded in an elastomeric polydimethylsiloxane and in a more rigid polyurethane matrix is investigated. The rigid polymer matrix is more effective in converting mechanical force into a conformational switch at the molecular level, provided that all four quinoxaline wings are covalently connected to the polymer. 相似文献
974.
Francesca Munari Carlo G. Barracchia Cinzia Franchin Francesca Parolini Stefano Capaldi Alessandro Romeo Luigi Bubacco Michael Assfalg Giorgio Arrigoni Mariapina D'Onofrio 《Angewandte Chemie (International ed. in English)》2020,59(16):6607-6611
In the brain of individuals with Alzheimer's disease, the regulatory protein ubiquitin is found conjugated to different lysine residues of tau protein assembled into pathological paired helical filaments. To shed light on the hitherto unexplored ubiquitination‐linked conformational transitions of tau, the availability of in vitro ubiquitin conjugation methods is of primary importance. In our work, we focused on the four‐repeat domain of tau and assembled an enzymatic machinery formed by UBE1, Ubc13, and CHIP enzymes. The enzymatic reaction resulted in monoubiquitination at multiple sites, reminiscent of the ubiquitination pattern observed in vivo. We further exploited chemoselective disulfide coupling reactions to construct three tau regioisomers with site‐specific monoubiquitination. Protein aggregation experiments revealed that the multiple enzyme‐derived products were unable to convert into amyloid fibrils, while the semisynthetic conjugates exhibited diverse capability to form filaments. This study contributes novel insight into the effects of a key post‐translational modification on aberrant protein self‐assembly. 相似文献
975.
Maria Pia Lenza Iker Oyenarte Dr. Tammo Diercks Jon Imanol Quintana Dr. Ana Gimeno Dr. Helena Coelho Ana Diniz Dr. Francesca Peccati Sandra Delgado Alexandre Bosch Dr. Mikel Valle Dr. Oscar Millet Prof. Dr. Nicola G. A. Abrescia Dr. Asís Palazón Dr. Filipa Marcelo Dr. Gonzalo Jiménez-Osés Prof. Dr. Jesús Jiménez-Barbero Dr. Ana Ardá Dr. June Ereño-Orbea 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(52):23971-23979
The glycan structures of the receptor binding domain of the SARS-CoV2 spike glycoprotein expressed in human HEK293F cells have been studied by using NMR. The different possible interacting epitopes have been deeply analysed and characterized, providing evidence of the presence of glycan structures not found in previous MS-based analyses. The interaction of the RBD 13C-labelled glycans with different human lectins, which are expressed in different organs and tissues that may be affected during the infection process, has also been evaluated by NMR. In particular, 15N-labelled galectins (galectins-3, -7 and -8 N-terminal), Siglecs (Siglec-8, Siglec-10), and C-type lectins (DC-SIGN, MGL) have been employed. Complementary experiments from the glycoprotein perspective or from the lectin's point of view have permitted to disentangle the specific interacting epitopes in each case. Based on these findings, 3D models of the interacting complexes have been proposed. 相似文献
976.
Novel carbon nanotube composites by grafting reaction with water-compatible redox initiator system 总被引:1,自引:0,他引:1
Giuseppe Cirillo Tommaso Caruso Silke Hampel Diana Haase Francesco Puoci Manfred Ritschel Albrecht Leonhardt Manuela Curcio Francesca Iemma Vyacheslav Khavrus Mandy Grobosch Nevio Picci 《Colloid and polymer science》2013,291(3):699-708
Multi-walled carbon nanotubes were chemically functionalized with methacrylic acid and methacrylated bovine serum albumin by free radical grafting reaction to obtain novel nanocomposites. The nanotubes were synthesized by aerosol-assisted chemical vapor deposition, and then the monomers were directly grafted by the action of hydrogen peroxide/ascorbic acid redox pair which allows operating in water-compatible and eco-friendly environment without the generation of any toxic reaction by-product. A multi-technique approach was used to evaluate the effectiveness of the grafting process employing Fourier transform infrared, Raman, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analyses. Considering the high importance of methacrylate polymers and bovine serum albumin, the proposed nanocomposites could be of great applicability in biomedical and pharmaceutical fields. 相似文献
977.
Accorsi S Barra AL Caneschi A Chastanet G Cornia A Fabretti AC Gatteschi D Mortalo C Olivieri E Parenti F Rosa P Sessoli R Sorace L Wernsdorfer W Zobbi L 《Journal of the American Chemical Society》2006,128(14):4742-4755
Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch. 相似文献
978.
Marco Rossato Francesca Favaretto Marnie Granzotto Marika Crescenzi Alessandra Boscaro Angelo Di Vincenzo Federico Capone Edoardo Dalla Nora Eva Zabeo Roberto Vettor 《Molecules (Basel, Switzerland)》2022,27(6)
Extracellular ATP exerts important functions as an extracellular signaling molecule via the activation of specific P2 purinergic receptors (P2X and P2Y). We investigated the expression of the different P2 receptors and their possible functional activation in human adipocytes in primary culture. We performed molecular expression analysis of the P2 receptors in human mature adipocytes; examined their functional activation by different nucleotides evaluating [Ca2+]i modifications and IL-6 secretion, and determined the ability of adipocytes to release ATP in the extracellular medium. Human adipocytes express different P2X and P2Y receptors. Extracellular ATP elicited a rise in [Ca2+]i via the activation of P2X and P2Y receptor subtypes. Human adipocytes spontaneously released ATP in the extracellular medium and secreted IL-6 both at rest and after stimulation with ATP. This stimulatory effect of ATP on IL-6 secretion was inhibited by pre-incubation with apyrase, an ATP metabolizing enzyme. These results demonstrate that human adipocytes express different P2X and P2Y receptors that are functionally activated by extracellular nucleotides. Furthermore, human adipocytes spontaneously release ATP, which can act in an autocrine/paracrine fashion on adipocytes, possibly participating in the regulation of inflammatory cytokine release. Thus, P2 purinergic receptors could be a potential therapeutic target to contrast the inflammatory and metabolic complications characterizing obesity. 相似文献
979.
Francesca Serra 《Liquid crystals》2016,43(13-15):1920-1936
ABSTRACTThe use of nematic liquid crystals for directed assembly of particles and for the creation of multistable systems depends on the ability to control the topological defects and the distortions of the director field. These are not only driven by topological constraints and by anchoring energies but also by the curvature field created by the bounding surfaces. This review explores the interaction between defects, topology, inclusions and curvature in nematics. I will introduce the relationship between curvature and the Frank elastic energy in nematics, and then I will give an overview of specific examples that show how this coupling can create unexpected behaviours, such as lock-and-key interactions, anchoring transitions on curved surfaces and memory effects. 相似文献
980.
Bombelli C Borocci S Diociaiuti M Faggioli F Galantini L Luciani P Mancini G Sacco MG 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10271-10274
The condensation of calf thymus DNA into the cholesteric-like psi-phase was observed by circular dichroism in liposome suspensions formulated with specific cationic gemini surfactants. The stereochemistry of the gemini spacer, the presence of specific functional groups, and the covalent link between the headgroups are fundamental issues in the condensation of DNA. Transmission electron microscopy images showed a multilamellar morphology, which corresponds with condensation. 相似文献