全文获取类型
收费全文 | 2073篇 |
免费 | 114篇 |
国内免费 | 4篇 |
专业分类
化学 | 1705篇 |
晶体学 | 20篇 |
力学 | 16篇 |
数学 | 205篇 |
物理学 | 245篇 |
出版年
2023年 | 26篇 |
2022年 | 100篇 |
2021年 | 117篇 |
2020年 | 75篇 |
2019年 | 70篇 |
2018年 | 64篇 |
2017年 | 38篇 |
2016年 | 117篇 |
2015年 | 74篇 |
2014年 | 97篇 |
2013年 | 135篇 |
2012年 | 131篇 |
2011年 | 155篇 |
2010年 | 99篇 |
2009年 | 88篇 |
2008年 | 143篇 |
2007年 | 97篇 |
2006年 | 101篇 |
2005年 | 88篇 |
2004年 | 75篇 |
2003年 | 50篇 |
2002年 | 40篇 |
2001年 | 20篇 |
2000年 | 13篇 |
1999年 | 20篇 |
1998年 | 10篇 |
1997年 | 13篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1975年 | 4篇 |
1961年 | 2篇 |
1960年 | 2篇 |
1887年 | 1篇 |
排序方式: 共有2191条查询结果,搜索用时 15 毫秒
941.
Pereira-Meirelles Fatima Ventura Rocha-LeãO Maria Helena Miguez Anna Geraldo Lippel Santť 《Applied biochemistry and biotechnology》1997,(1):73-85
Although Upases have been intensively studied, some aspects of enzyme production like substrate uptake, catabolite repression,
and enzyme stability under long storage periods are seldom discussed in the literature. This work deals with the production
of lipase by a new selected strain ofCandida lipolytica. Concerning nutrition, it was observed that inorganic nitrogen sources were not as effective as peptone, and that oleic acid
or triacylglycerides (TAG) were essential carbon sources. Repression by glucose and stimulation by oleic acid and long chain
TAG (triolein and olive oil) were observed. Extracellular lipase activity was only observed at high levels at late stationary
phase, whereas intracellular lipase levels were constant and almost undetectable during the cultivation period, suggesting
that the produced enzyme was attached to the cell wall, mainly at the beginning of cultivation. The crude lipase produced
by this yeast strain shows the following optima conditions: pH 8.0–10.0, temperature of 55°C. Moreover, this preparation maintains
its full activity for at least 370 d at 5°C. 相似文献
942.
Leonid B. Rubin Tatiana N. Menshonkova Natalia A. Simukova Edward I. Budowsky† 《Photochemistry and photobiology》1981,34(3):339-344
Abstract— The set of final products of thymine conversion induced by high-intensity UV irradiation (λ= 266nm, intensity 1024 -5 × 1029 photons·s−1 ·m−2 , pulse duration 10ns) of the dilute aqueous solution to the first approximation is similar to that formed with ionizing irradiation (γ-irradiation of aqueous solution or autoradiolysis of a solid 2-[14 C]-thymine). The data obtained suggest that high-intensity UV-induced photochemical conversion of thymine involves photoionization and/or photodissociation. These processes pass through the higher excited state(s) populated as a result of the second photon absorption by excited (most probably in the T1 triplet state) thymine molecules. 相似文献
943.
Dr. William J. K. Crone Dr. Natalia M. Vior Dr. Javier Santos‐Aberturas Lukas G. Schmitz Dr. Finian J. Leeper Dr. Andrew W. Truman 《Angewandte Chemie (International ed. in English)》2016,55(33):9639-9643
Bottromycin A2 is a structurally unique ribosomally synthesized and post‐translationally modified peptide (RiPP) that possesses potent antibacterial activity towards multidrug‐resistant bacteria. The structural novelty of bottromycin stems from its unprecedented macrocyclic amidine and rare β‐methylated amino acid residues. The N‐terminus of a precursor peptide (BtmD) is converted into bottromycin A2 by tailoring enzymes encoded in the btm gene cluster. However, little was known about key transformations in this pathway, including the unprecedented macrocyclization. To understand the pathway in detail, an untargeted metabolomic approach that harnesses mass spectral networking was used to assess the metabolomes of a series of pathway mutants. This analysis has yielded key information on the function of a variety of previously uncharacterized biosynthetic enzymes, including a YcaO domain protein and a partner protein that together catalyze the macrocyclization. 相似文献
944.
Debugging Eukaryotic Genetic Code Expansion for Site‐Specific Click‐PAINT Super‐Resolution Microscopy 下载免费PDF全文
Dr. Ivana Nikić Gemma Estrada Girona Jun Hee Kang Giulia Paci Sofya Mikhaleva Christine Koehler Dr. Nataliia V. Shymanska Camilla Ventura Santos Daniel Spitz Dr. Edward A. Lemke 《Angewandte Chemie (International ed. in English)》2016,55(52):16172-16176
Super‐resolution microscopy (SRM) greatly benefits from the ability to install small photostable fluorescent labels into proteins. Genetic code expansion (GCE) technology addresses this demand, allowing the introduction of small labeling sites, in the form of uniquely reactive noncanonical amino acids (ncAAs), at any residue in a target protein. However, low incorporation efficiency of ncAAs and high background fluorescence limit its current SRM applications. Redirecting the subcellular localization of the pyrrolysine‐based GCE system for click chemistry, combined with DNA‐PAINT microscopy, enables the visualization of even low‐abundance proteins inside mammalian cells. This approach links a versatile, biocompatible, and potentially unbleachable labeling method with residue‐specific precision. Moreover, our reengineered GCE system eliminates untargeted background fluorescence and substantially boosts the expression yield, which is of general interest for enhanced protein engineering in eukaryotes using GCE. 相似文献
945.
Water in Ionic Liquids: Correlation between Anion Hydrophilicity and Near‐Infrared Fingerprints 下载免费PDF全文
We show that fingerprints of the different states of water association can be clearly distinguished in the range of the first overtone of water′s symmetric O‐H stretching in the spectra of water‐saturated [EMIm]+‐based ionic liquids with anions of substantially different hydrophilicity, such as hydrophobic [(CF3SO2)2N]?, moderately hydrophilic [CF3SO3]?, and highly hydrophilic [HSO4]?. 相似文献
946.
Olga V. Lukashevich Natalia A. Cherepanova Renata Z. Jurkovska Albert Jeltsch Elizaveta S. Gromova 《BMC biochemistry》2016,17(1):7
Background
Dnmt3a is a DNA methyltransferase that establishes de novo DNA methylation in mammals. The structure of the Dnmt3a C-terminal domain is similar to the bacterial M. HhaI enzyme, a well-studied prokaryotic DNA methyltransferase. No X-ray structure is available for the complex of Dnmt3a with DNA and the mechanistic details of DNA recognition and catalysis by mammalian Dnmts are not completely understood.Results
Mutant variants of the catalytic domain of the murine Dnmt3a carrying substitutions of highly conserved N167, R200, and R202 have been generated by site directed mutagenesis and purified. Their methylation activity, DNA binding affinity, ability to flip the target cytosine out of the DNA double helix and covalent complex formation with DNA have been examined. Substitutions of N167 lead to reduced catalytic activity and reduced base flipping. Catalytic activity, base flipping, and covalent conjugate formation were almost completely abolished for the mutant enzymes with substitutions of R200 or R202.Conclusions
We conclude that R202 plays a similar role in catalysis in Dnmt3a-CD as R232 in M.SssI and R165 in M.HhaI, which could be positioning of the cytosine for nucleophilic attack by a conserved Cys. R200 of Dnmt3a-CD is important in both catalysis and cytosine flipping. Both conserved R200 and R202 are involved in creating and stabilizing of the transient covalent intermediate of the methylation reaction. N167 might contribute to the positioning of the residues from the motif VI, but does not play a direct role in catalysis.947.
Gas chromatography with mass spectrometry for the quantification of ethylene glycol ethers in different household cleaning products 下载免费PDF全文
Marta Pastor‐Belda Natalia Campillo Manuel Hernández‐Córdoba Pilar Viñas 《Journal of separation science》2016,39(12):2292-2299
A rapid and simple procedure is reported for the determination of six ethylene glycol ethers in cleaning products and detergents using gas chromatography with mass spectrometry. The analytes were extracted from 2.0 g samples in acetonitrile (3 mL) and the extract was submitted to a clean‐up step by QuEChERS method, using a mixture containing 0.3 g magnesium sulfate, 0.15 g primary/secondary amine, and 0.05 g C18. The clean acetonitrile extract (1 μL) was injected into the chromatographic system. No matrix effect was observed, so the quantification of the samples was carried out against external standards. Detection limits were in the range 3.0–27 ng/g for the six ethylene glycol ethers. The recoveries obtained, using the optimized procedure, were in the 89.4–118% range, with relative standard deviations lower than 14%. Twenty‐three different household cleaning products, including glass cleaner, degreaser, floor, softeners, and clothes and dishwashing detergents, were analyzed. Large interindividual variations were observed between samples and compounds. 相似文献
948.
Natalia Łukasik Véronique Hubscher-Bruder Sylvia Michel Maria Bocheńska Beata Kamińska 《Supramolecular chemistry》2016,28(7-8):673-685
Pyridine-2,6-carboxamides bearing α- or β-naphthyl- and α- or β-anthryl residues were prepared using simple method from pyridine-2,6-carboxylic acid dichloride and the respective aromatic amines. For the obtained receptors, selective binding of lead(II) and copper(II) was found. Ion–receptor interactions were studied using UV–vis spectroscopy, spectrofluorimetry, 1H NMR and FTIR spectroscopy. The reversible lead(II) and copper(II) binding was discussed in regard of type of aromatic residue and amide bond localisation in aromatic ring, and binding model was proposed. 相似文献
949.
Dmitry Yu. Pushcharovsky Natalia V. Zubkova Igor V. Pekov 《Structural chemistry》2016,27(6):1593-1603
The new tetrahedral complexes revealed in the structures of natural silicates in the last years are analyzed. Several new minerals and structural types widen the structural classification and principles of silicates. The new structures of silicates with mixed anions are formed by stacking of similar units, and thus they can be described in terms of the modular approach and its polysomatic concepts. The difference between the silicate minerals of the Earth’s crust and mantle is considered. Possible mineral transformations occurring in the mantles of the Earth and extrasolar planets are described. 相似文献
950.
Julia I. Schweizer Dr. Markus G. Scheibel Dr. Martin Diefenbach Felix Neumeyer Dr. Christian Würtele Dr. Natalia Kulminskaya Dr. Rasmus Linser Prof. Dr. Norbert Auner Prof. Dr. Sven Schneider Prof. Dr. Max C. Holthausen 《Angewandte Chemie (International ed. in English)》2016,55(5):1782-1786
An experimental and theoretical study of the base‐stabilized disilene 1 is reported, which forms at low temperatures in the disproportionation reaction of Si2Cl6 or neo‐Si5Cl12 with equimolar amounts of NMe2Et. Single‐crystal X‐ray diffraction and quantum‐chemical bonding analysis disclose an unprecedented structure in silicon chemistry featuring a dative Si→Si single bond between two silylene moieties, Me2EtN→SiCl2→Si(SiCl3)2. The central ambiphilic SiCl2 group is linked by dative bonds to the amine donor and the bis(trichlorosilyl)silylene acceptor, which leads to push–pull stabilization. Based on experimental and theoretical examinations a formation mechanism is presented that involves an autocatalytic reaction of the intermediately formed anion Si(SiCl3)3? with neo‐Si5Cl12 to yield 1 . 相似文献