全文获取类型
收费全文 | 348830篇 |
免费 | 34576篇 |
国内免费 | 225篇 |
专业分类
383631篇 |
出版年
2018年 | 8442篇 |
2017年 | 8276篇 |
2016年 | 7146篇 |
2015年 | 4316篇 |
2014年 | 4992篇 |
2013年 | 6834篇 |
2012年 | 10058篇 |
2011年 | 15002篇 |
2010年 | 11038篇 |
2009年 | 10719篇 |
2008年 | 12821篇 |
2007年 | 14531篇 |
2006年 | 7322篇 |
2005年 | 8054篇 |
2004年 | 7474篇 |
2003年 | 7522篇 |
2002年 | 7287篇 |
2001年 | 17381篇 |
2000年 | 17629篇 |
1999年 | 13317篇 |
1998年 | 3871篇 |
1997年 | 4144篇 |
1996年 | 3870篇 |
1995年 | 3492篇 |
1994年 | 3474篇 |
1993年 | 3572篇 |
1992年 | 10402篇 |
1991年 | 10348篇 |
1990年 | 9812篇 |
1989年 | 9666篇 |
1988年 | 8994篇 |
1987年 | 8366篇 |
1986年 | 7562篇 |
1985年 | 7413篇 |
1984年 | 5839篇 |
1983年 | 5090篇 |
1982年 | 3627篇 |
1981年 | 3183篇 |
1980年 | 3035篇 |
1979年 | 5355篇 |
1978年 | 4115篇 |
1977年 | 3750篇 |
1976年 | 3301篇 |
1975年 | 3789篇 |
1974年 | 3951篇 |
1973年 | 3894篇 |
1972年 | 3979篇 |
1971年 | 3686篇 |
1970年 | 2829篇 |
1969年 | 2742篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
R M Dixon B T Golding S Mwesigye-Kibende D N Ramakrishna Rao 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1985,311(1152):531-544
The vitamin B12 coenzyme adenosylcobalamin assists the enzymic catalysis of molecular rearrangements of the type (formula; see text) in which the migrating group X can be OH, NH2 or a suitable substituted carbon atom such as C(=CH2)CO2H. This paper discusses evidence for the participation of organic radicals as intermediates in these reactions. Theoretical and model studies supporting the intermediacy of radicals in the reactions catalysed by the enzymes diol dehydratase and alpha-methyleneglutarate mutase are described. For the model studies, alkyl radicals, alkylcobaloximes (alkyl represents, for example, ethoxycarbonyl substituted, but-3-enyl and cyclopropylmethyl) and also dihydroxyalkylcobalamins have been investigated. The Co-C alpha-C beta angle of 125 degrees in adenosylcobalamin is shown to be an 'especial' angle by analysis of the crystal structures of R- and S-2,3-dihydroxypropylcobalamin. 相似文献
952.
Mamta Singhvi Dipti Joshi Shalaka Gaikaiwari Digambar V. Gokhale 《Indian journal of microbiology》2010,50(1):97-100
Method for production and regeneration of Lactobacillus delbrueckii protoplasts are described. The protoplasts were obtained by treatment with a mixture of lysozyme and mutanolysin in protoplast
buffer at pH 6.5 with different osmotic stabilizers. The protoplasts were regenerated on deMan, Rogosa and Sharpe (MRS) with
various osmotic stabilizers. Maximum protoplast formation was obtained in protoplast buffer with sucrose as an osmotic stabilizer
using a combination of lysozyme (1 mg/ml) and mutanolysin (10 μg/ml). Maximum protoplast regeneration was obtained on MRS
medium with sucrose (0.5 M) as an osmotic stabilizer. The regeneration medium was also applicable to other species of lactobacilli
as well. This is, to our knowledge, the first report on protoplast formation and efficient regeneration in case of L. delbrueckii. 相似文献
953.
It is generally perceived that landscape patterns or textures in a given protected area are spatially stationary. The findings
of this study suggest that this common perception is only partially correct. Over the course of 52 years, equilibrium in landscape
shifting was detected using digital data for the Lassen Volcanic National Park (USA). Vertical aerial photographs taken of
the park in 1941 were geo-referenced with the digital orthophoto quarter quadrangle (DOQQ) images of the same area from 1993
to identify landscape compositions and to measure change. Spatial analysis was used to observe pattern changes over time.
The results suggested that landscape development maintained equilibrium while patches were in various stages of a successional
sequence. The total area of each landscape component held steady, although over time patches throughout the landscape changed—a
shifting-mosaic steady state (SMSS). These findings reflect the limitations of contemporary environmental conservation theory.
They also suggest the importance of considering landscape change in policies that currently govern park planning and management. 相似文献
954.
955.
A N Dedenkov N T Ra?khlin I M Kvetno? E S Kurilets I P Balmasova 《Biulleten' eksperimental'no? biologii i meditsiny》1986,102(10):491-493
The ability of serotonin, melatonin and beta-endorphin synthesis by secreting granules of natural killer cells (NK cells) was established by immunohistochemical method. It was shown that cytotoxic effect of NK cells can be to a certain extent related to the presence of secreting granules. A possible direct contribution of NK cell-synthesized hormones to the natural killer antitumour effect is discussed. 相似文献
956.
A simple and rapid method is described for the purification of supercoiled PM2 DNA by affinity chromatography on columns of H1 histone covalently coupled to agarose. The method does not require the use of intercalating agents or ultracentrifugation procedures. Under the conditions most appropriate for purification, elution is carried out in a single step with buffered 0.7 M NaCl after the sample has been loaded onto the column in buffered 0.2 M NaCl. The DNA eluted at the higher salt concentration consists of supercoiled closed circular DNA at greater than 90% purity independently of the ratio of supercoiled to nicked circular DNA in the input mixture. 相似文献
957.
Microtubules induced by the binding of GTP or of a non-hydrolysable analog of GTP onto the exchangeable site of tubulin appear very similar according to electron microscopy and polymerisation kinetics criteria. However, we show here that the exchangeable sites or “E” sites of the tubulin subunits remain available for nucleotide exchange inside the GMP-PCP-microtubules contrary to the “E” sites inside the GDP-microtubules. Moreover, under specific conditions, GMP-PCP induces the polymerisation of tubulin into a bidimensional, pseudocrystallin structure. Such a “crystallisation” is inhibited by GTP and GDP. 相似文献
958.
There are reported the results of certain measurements made on 158 human cadavers of both sexes. A series of main parameters are given, such as the distance between the superior mesenteric vein and on the one hand its confluence with other vessels and on the other hand the pancreas and the horizontal part of the duodenum: the distance between the superior mesenteric vein together with the portal vein, and the inferior caval vein; the diameters of the portal trunk and the confluence angle between the roots of the portal vein. The results reflect the possibility to perform a troncular portocaval anastomosis in at least 90 per cent of the cases. The authors' attention was, however, directed towards the anatomy of the superior mesenteric vein, as this vessel is preferred by some surgeons in the achievement of the portocaval anastomosis. 相似文献
959.
Abstract— An enzyme radiochemical assay for p -octopamine, m -octopamine (norphenylephrine) and phenylethanolamine based on the N -methylation of these amines by the enzyme phenylethanolamine N -methyl transferase ( S -adenosyl- l -methionine: phenylethanolamine N -methyl transferase (EC 2.1.1.28) has been developed. [3 H]Methyl- S -adenosyl- l -methionine was used as methyl donor. The reaction products are converted to their dansyl derivatives and separated by TLC in three different solvent systems prior to liquid scintillation counting. The method exhibits a sensitivity of less than 10 pg for each amine and is suitable for the measurement of endogenous p -octopamine levels in mammalian brain. The highest levels of p -octopamine were found in the hypothalamus (3.4 ng/g) but despite the sensitivity of the assay, neither phenylethanolamine nor m -octopamine could be detected. After MAO inhibition, however, both of these amines were found to be present. p -Octopamine was increased substantially in all brain regions following the administration of an MAO inhibitor, whereas pretreatment with reserpine produced a significant decrease in the hypothalamus. 相似文献
960.