全文获取类型
收费全文 | 105150篇 |
免费 | 7211篇 |
国内免费 | 4084篇 |
出版年
2024年 | 223篇 |
2023年 | 1723篇 |
2022年 | 2469篇 |
2021年 | 3364篇 |
2020年 | 3435篇 |
2019年 | 4697篇 |
2018年 | 3908篇 |
2017年 | 2876篇 |
2016年 | 2932篇 |
2015年 | 3466篇 |
2014年 | 6313篇 |
2013年 | 8019篇 |
2012年 | 4976篇 |
2011年 | 6379篇 |
2010年 | 5496篇 |
2009年 | 5286篇 |
2008年 | 5405篇 |
2007年 | 5543篇 |
2006年 | 4956篇 |
2005年 | 4494篇 |
2004年 | 4150篇 |
2003年 | 3297篇 |
2002年 | 2857篇 |
2001年 | 1963篇 |
2000年 | 1539篇 |
1999年 | 1631篇 |
1998年 | 1510篇 |
1997年 | 1241篇 |
1996年 | 1171篇 |
1995年 | 1124篇 |
1994年 | 1045篇 |
1993年 | 863篇 |
1992年 | 828篇 |
1991年 | 704篇 |
1990年 | 580篇 |
1989年 | 505篇 |
1988年 | 471篇 |
1987年 | 396篇 |
1986年 | 373篇 |
1985年 | 469篇 |
1984年 | 672篇 |
1983年 | 456篇 |
1982年 | 527篇 |
1981年 | 407篇 |
1980年 | 344篇 |
1979年 | 297篇 |
1978年 | 236篇 |
1977年 | 208篇 |
1976年 | 181篇 |
1974年 | 124篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Masayoshi Yamaguchi Reiko Makino Noriaki Shimokawa 《Molecular and cellular biochemistry》1996,165(2):145-150
992.
Isolation of a gene essential for biosynthesis of the lipopeptide antibiotics plipastatin B1 and surfactin in Bacillus subtilis YB8 总被引:12,自引:0,他引:12
Bacillus subtilis YB8 was found to produce the lipopeptide antibiotics surfactin and plipastatin B1. A gene, lpa-8, required for the production of both lipopeptides was cloned from strain YB8. When this gene was inactivated in strain YB8,
neither surfactin nor plipastatin B1 was produced. However, the defective strain transformed with an intact lpa-8 gene had restored ability to produce both peptides. Nucleotide sequence analysis of the region essential for the production
of the peptides revealed the presence of a large open reading frame. The deduced amino acid sequence of lpa-8 (224 amino acid residues) showed sequence similarity to that of sfp (from surfactin-producing B. subtilis), lpa-14 (from iturin A- and surfactin-producing B. subtilis), psf-1 (from surfactin-producing Bacillus pumilus), gsp (from gramicidin-S-producing Bacillus brevis), and entD (from siderophore-enterobactin-producing Escherichia coli), which are able to complement a defect in the sfp gene and promote production of the lipopeptide antibiotic surfactin. The sequence similarity among these proteins and the
product similarity of cyclic peptides suggests that they might be involved in the biosynthesis or secretion of the peptides.
Received: 14 July 1995 / Accepted: 22 December 1995 相似文献
993.
Summary During the growth of Kluyveromyces marxianus var. marxianus ATCC 10022 on lactose, peaks of glucose, but not -galactosidase activity, were detected iroculture medium. Harvested and washed whole cells produced glucose and galactose from lactose, or ortho-nitro-phenol from the chromogenic substrate ortho-nitro-phenyl--D-galactopyranoside (ONPG), indicating that -galactosidase is physically associated with cells. ONPG hydrolysis by whole cells presented a monophasic kinetics (Km 36.6 mM) in lactose exponential growth phase cells, but a biphasic kinetics (Km 0.2 and 36.6 mM) in stationary growth phase cells. Permeabilization with digitonin or disruption of cells from both growth phases led to monosite ONPG hydrolysis (Km 2.2 to 2.5 mM), indicating that =galactosidase is not located in the periplasm. In addition, the energy inhibitors fluoride or arsenate, as well as the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) prevented ONPG hydrolysis by whole cells. These findings indicate that energy coupled transmembrane transport is the rate-limiting step for intracellular ONPG cleavage. The taxonomic and physiologic implications of the exclusive intracellular location of -galactosidase of K. marxianus var. marxianus ATCC 10022 are discussed. 相似文献
994.
Low temperature field emission electron microscopy was used to determine the location of free water in soybean seeds. Frozen, hydrated soybean seeds were fractured, the water etched from the fractured surface, and then part of the etched surface was refractured. The resulting surface, which contained a freeze-fractured face as well as a freeze-etched face was coated with platinum and viewed on the cryostage of a low temperature field emission electron microscope. Two surfaces could be viewed simultaneously to determine the location of water in the seed tissue. Viewing the fractured surface gave an indication of the extent of hydration of the tissue. Viewing the etched surface detailed the macro- and microanatomy of the tissue. Viewing the intersection between the fractured and etched surfaces allowed observation of the environment of partially etched cells and organelles. The technique avoids artifacts associated with chemical fixation, dehydration, and critical-point drying, procedures that affect the water content of the seed. The technique does not affect the degree of hydration of the seed and can be used to localize water in the inter- and intracellular environment of the seed. This technique could find wide application in studies of water relationships of seeds during development, maturation, and imbibition. 相似文献
995.
996.
997.
998.
Synthesis of methyl
-Ribose was converted into methyl
and this, on tin-mediated allylation, gave a mixture of the 2-O-allyl and 3-O-allyl derivatives which were separated by chromatography. The more polar isomer was characterised as the 3-O-allyl derivative after conversion via
(which was also synthesised from
) into the known
. Methyl
was converted into methyl
via methyl
. 相似文献
Full-size image
Full-size image
Full-size image
Full-size image
Full-size image
Full-size image
999.
Oscar Varela Patricia A. Zunszain Daniel O. Cicero Ricardo F. Baggio Daniel R. Vega María T. Garland 《Carbohydrate research》1996,280(2):187
The conformation in 2H2O of 4-thio-l-lyxono-1,4-lactone (1) was studied by nuclear magnetic resonance spectroscopy, by means of homonuclear (J1H,1H) and heteronuclear (J1H,13C) coupling constants. The couplings were directly measured by a two-dimensional heteronucleus-coupled ω1 hetero-half-filtered proton-proton correlation (HETLOC) experiment, which does not require 13C isotopic enrichment. In solution, the thiolactone ring of 1 adopts preferentially the E3 conformation, and its hydroxymethyl group populates mainly the gt rotamer. The X-ray diffraction data of a single crystal of 1 indicates that also in the solid state the thiolactone ring adopts an E3 conformation, with a puckering somewhat larger than that observed for aldono-1,4-lactones and furanose rings. The molecules are linked by hydrogen bonds, which form chains. Particularly, O-5 is fully engaged as donor and acceptor in hydrogen bonding and the rotameric conformation of the hydroxymethyl group of 1 is fixed in the tg form. 相似文献
1000.
The content of reduced glutathione and of glutathione disulfide as well as the activities of glutathione reductase, glutathione peroxidase, glutathione S-transferases, catalase and superoxide dismutases were determined in human hepatoma Hep 3B cells in relation to free-radical toxicity in order to appreciate the defense capacities of these cells compared to data on normal hepatocytes. When Hep 3B cells were exposed to lindane, a known inducer of free-radical production, superoxide dismutase activity appeared as the best-adapted cellular parameter for early detection of the resulting free-radical toxicity.Abbreviations AAS
atomic absorption spectrometry
- CDNB
1-chloro-2,4-dinitrobenzene
- DMEM
Dulbecco's modified Eagle medium
- GPx
glutathione peroxidase
- G.Red
glutathione reductase
- GSH
reduced glutathione
- GSSG
glutathione disulfide
- GST
glutathione S-transferases
- Prot
proteins
- SOD
superoxide dismutase 相似文献