全文获取类型
收费全文 | 515篇 |
免费 | 36篇 |
国内免费 | 52篇 |
专业分类
化学 | 547篇 |
晶体学 | 1篇 |
力学 | 1篇 |
综合类 | 18篇 |
数学 | 1篇 |
物理学 | 35篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 18篇 |
2021年 | 28篇 |
2020年 | 45篇 |
2019年 | 34篇 |
2018年 | 40篇 |
2017年 | 16篇 |
2016年 | 40篇 |
2015年 | 22篇 |
2014年 | 26篇 |
2013年 | 12篇 |
2012年 | 24篇 |
2011年 | 23篇 |
2010年 | 27篇 |
2009年 | 32篇 |
2008年 | 30篇 |
2007年 | 26篇 |
2006年 | 23篇 |
2005年 | 18篇 |
2004年 | 19篇 |
2003年 | 17篇 |
2002年 | 8篇 |
2001年 | 9篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有603条查询结果,搜索用时 15 毫秒
541.
Takaaki Kubota Kazutaka Shimbo Michiko Nakano Jun’ichi Kobayashi 《Tetrahedron letters》2006,47(26):4369-4371
Amphezonol A (1), a novel polyhydroxyl linear carbon-chain metabolite, has been isolated from the cultured marine dinoflagellate Amphidinium sp., which was isolated from an Okinawan marine acoel flatworm Amphiscolops sp. The structure of 1 was elucidated by detailed analyses of 2D NMR spectra. Amphezonol A (1) possesses one tetrahydrofuran ring, two tetrahydropyran rings, and twenty-one hydroxyl groups on C60-linear aliphatic chain with one exo-methylene and one methyl branch. Amphezonol A (1) exhibited a modest inhibitory activity against DNA polymerase α. 相似文献
542.
Mathura P. Ramanathan Damodharan Lalithakumari 《Applied biochemistry and biotechnology》1999,80(1):1-12
Organophosphorus insecticides are widely used in agriculture. Despite their biodegradable nature, some are highly toxic and
their residues are found in the environment. Reports on the mineralization of a spectrum of these insecticides by a single
potential strain are scarce. We have isolated a soil isolate, Pseudomonas sp. A3, through enrichment technique, able to degrade methylparathion (MP), malathion, monocrotophos, and Diazinon. The potential
of this strain to mineralize MP as a carbon and/or phosphorus source has been evaluated. On hydrolysis of MP, the aromatic
portion (p-nitrophenol) was used as a carbon and energy source whereas the alkyl moiety (dithiomethylphosphorothioate) was broken down
for the phosphorus source. The results from the experiments involving [U-14C]p-nitrophenol provided the evidence for incorporation of carbon into the cellular constituents and release of CO2 from this insecticide. During the breakdown of MP, nitrite was released as a catabolic by-product. 相似文献
543.
Satoshi Nakamura 《Catalysis Surveys from Asia》2003,7(2-3):157-164
Xylanase is an enzyme that catalyzes the hydrolysis of xylan, a -1,4-linked xylose polymer. Alkaliphilic Bacillus sp. strain 41M-1 secretes a xylanase (xylanase J) that has an alkaline pH optimum. Xylanase J is a multidomain enzyme and consists of two functional domains: a family 11/G catalytic domain and a non-catalytic xylan-binding domain. The xylan-binding domain bound to xylan and enhanced catalytic activity of the adjacent catalytic domain. Mutational analyses revealed some amino acid residues that contribute to catalytic activity, alkaliphily and xylan-binding activity of xylanase J. 相似文献
544.
Garcia-Kirchner O. Bautista-Ramirez Ma. E. Segura-Granados M. 《Applied biochemistry and biotechnology》1998,(1):277-284
Keratinases can be used for the production of potentially important hydrolyzed proteins and chemicals. This study investigated
the keratinolytic activity ofStreptomyces sp on keratinaceous materials like wool. High levels of proteolytic and keratinolytic activity were obtained after 96 h of
culture when two Streptomyces sp strains were grown on basal medium containing mineral salts and 3% (w/v) of defatted wool
as a source of energy, carbon, and nitrogen. The cell-free culture filtrates exhibited rapid proteolytic digestion of keratin
powder. Currently, the authors are testing whether the enzymatic activity obtained is in fact keratinolytic, and not only
an alkaline protease activity. 相似文献
545.
José J. Fernández 《Tetrahedron》2005,61(37):8910-8915
Dactylomelol 1, initially isolated from molluscs of the genus Aplysia, was the first example of a monocarbocyclic diterpene skeleton with a C-6-C-11 cyclisation. This paper reports on the identification and structural elucidation of six new diterpenes, compounds 4-9, isolated from specimens of Laurencia sp., together with dactylomelol, all with this particular carbon backbone that we named dactylomelane. This result is the first to confirm a red alga as the biogenetic origin of this class of compounds. 相似文献
546.
Yu Lei Priti Mulchandani Wilfred Chen Joseph Wang Ashok Mulchandani 《Electroanalysis》2004,16(24):2030-2034
An amperometric microbial biosensor for highly sensitive and selective determination of p‐nitrophenol (PNP) is reported. The biosensor consisted of PNP‐degrader Arthrobacter sp. JS443 immobilized by entrapment in Nafion polymer deposited on the top of the carbon paste electrode transducer. The biosensor was based on the measurement of the oxidation current of the intermediates 4‐nitrocatechol and 1,2,4‐benzenetriol formed by the highly selective oxidation of PNP by Arthrobacter sp. The sensor signal and response time were optimized with applied potential of +0.4 V (vs. Ag/AgCl reference electrode) and 0.03 mg of cells and operating in pH 7.5, 50 mM citrate‐phosphate buffer at room temperature. When operated at optimized conditions, the Arthrobacter sp.‐based biosensor measured as low as 5 nM (0.7 ppb) of PNP. The biosensor demonstrated excellent selectivity with no interference from phenolic compounds such as 2‐nitrophenol, phenol and 3‐chlorophenol but was interfered by 3‐nitrophenol and 3‐methyl‐4‐nitrophenol. It had good precision and intra‐ and inter‐day reproducibility, accuracy and was stable up to 3 days when stored in buffer at 4 °C. When applied for measurement in water from Lake Elsinore, CA, the results obtained were in excellent agreement with the amounts determined spectrophotometrically. 相似文献
547.
The isolation and structure elucidation of belamide A from the marine cyanobacterium Symploca sp. is described. Belamide A is a highly methylated linear tetrapeptide with structural analogy to the important linear peptides dolastatins 10 and 15. Disruption of the microtubule network in A-10 cells was observed at 20 μM and displayed classic tubulin destabilizing antimitotic characteristics. The moderate cytotoxicity of belamide A (IC50 0.74 μM vs HCT-116 colon cancer line) provides new insights into structure-activity relationships for this drug class. 相似文献
548.
Hongwei Liu Teruaki Nishikawa Kazuo Tachibana Hiroshi Nagai Michio Namikoshi 《Tetrahedron letters》2004,45(38):7015-7017
A novel tribenzotetrathiepin alkaloid, named lissoclibadin 1 (1), has been isolated from the ascidian Lissoclinum sp. (cf. L. badium Monniot and Monniot, 1996). The gross structure was assigned on the basis of the spectral data, and one of two possible isomers was selected by the computational modeling study. Lissoclibadin 1 inhibited the growth of the marine bacterium Ruegeria atlantica (15.2 mm at 20 μg/disk). 相似文献
549.
Murae T Takamatsu Y Muraoka R Endoh S Yamauchi N 《Journal of mass spectrometry : JMS》2002,37(2):209-215
Calditocaldarchaeol (neutral tetraether lipid) from Sulfolobus acidocaldarius (acidothermophilic archaea) and intact total lipid from the thermoacidophilic archaea Sulfolobus sp. was examined by electrospray ionization time-of-flight mass spectrometry in the negative-ion mode using high resolution. When the sample was injected as a solution in a 3:1 mixture of methanol (MeOH) and chloroform (CHCl(3)) using an infusion system, the total ether lipid afforded molecular-related ions as [M - H](-) for acidic polar lipids containing a phosphoric or sulfuric group, and as [M + Cl](-) ion for neutral glycolipids. The attachment of chloride was confirmed by the observation of [M + Br](-) ion, instead of [M + Cl](-) ion, when a 3:1 mixture of MeOH and CHBr(3) was used in place of MeOH-CHCl(3) as the solvent. The composition of tetraether neutral glycolipids that are different from each other only in the number of five-membered rings in the isoprenoid chain was determined on the basis of the isotope-resolved mass spectrum of [M + Cl](-) ions. As for acidic tetraether lipids, molecular-related ions [M - H](-)) were not observed when the 3:1 MeOH-CHBr(3) mixture was used as the solvent. These results together afforded a facile method of distinguishing neutral from acidic tetraether lipids in intact total lipids of acidothermophilic archaea. This method was applied to determine the difference of the number of five-membered rings in isoprenyl chains of neutral tetraether glycolipids yielded by the Sulfolobus sp. grown at different temperatures. Discrimination of neutral tetraether glycolipids from acidic tetraether lipids in the total lipids obtained from Thermoplasma sp. was also achieved by this method. 相似文献
550.
M. P. Sobolevskaya V. A. Denisenko A. S. Moiseenko L. S. Shevchenko N. I. Menzorova Yu. T. Sibirtsev N. Yu. Kim T. A. Kuznetsova 《Russian Chemical Bulletin》2007,56(4):838-840
New 3-(4-hydroxybenzyl)piperazine-2,5-dione, together with the known N-[2-(4-hydroxy-phenyl)ethyl]acetamide (N-acetyltyramine), was isolated for the first time from the marine actinobacterium Streptomyces sp. The chemical structures of these compounds were determined by NMR spectroscopy and mass spectrometry. The cytotoxic activities
of the compounds were estimated from their effects on sperm and eggs of the sea urchin Strongylocentrotus intermedius.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 807–809, April, 2007. 相似文献