全文获取类型
收费全文 | 16744篇 |
免费 | 828篇 |
国内免费 | 22篇 |
专业分类
17594篇 |
出版年
2023年 | 114篇 |
2022年 | 279篇 |
2021年 | 437篇 |
2020年 | 261篇 |
2019年 | 270篇 |
2018年 | 415篇 |
2017年 | 441篇 |
2016年 | 506篇 |
2015年 | 699篇 |
2014年 | 778篇 |
2013年 | 1099篇 |
2012年 | 1210篇 |
2011年 | 1102篇 |
2010年 | 656篇 |
2009年 | 599篇 |
2008年 | 693篇 |
2007年 | 702篇 |
2006年 | 616篇 |
2005年 | 514篇 |
2004年 | 495篇 |
2003年 | 414篇 |
2002年 | 373篇 |
2001年 | 341篇 |
2000年 | 310篇 |
1999年 | 249篇 |
1998年 | 135篇 |
1997年 | 111篇 |
1996年 | 100篇 |
1995年 | 122篇 |
1994年 | 108篇 |
1993年 | 100篇 |
1992年 | 223篇 |
1991年 | 215篇 |
1990年 | 205篇 |
1989年 | 197篇 |
1988年 | 171篇 |
1987年 | 169篇 |
1986年 | 149篇 |
1985年 | 178篇 |
1984年 | 173篇 |
1983年 | 117篇 |
1982年 | 115篇 |
1981年 | 109篇 |
1980年 | 98篇 |
1979年 | 153篇 |
1978年 | 114篇 |
1977年 | 99篇 |
1974年 | 120篇 |
1973年 | 93篇 |
1972年 | 102篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Effect of thymosin alpha 1 on the antitumor activity of tumor-associated macrophage-derived dendritic cells 总被引:4,自引:0,他引:4
We have previously suggested that thymosin 1 (thy1), an immunomodulating thymic hormone, can activate tumor-associated macrophages to a tumoricidal state in a murine model bearing a transplantable T-cell lymphoma of spontaneous origin designated as Dalton's lymphoma (DL). Since tumor-infiltrating dendritic cells (DC) also play an important role in the host's antitumor response and are as such in an immunocompromised state in a tumor-bearing host, in the present investigation we studied if thy1 is able to influence the differentiation of tumor-associated macrophages (TAM) into DC with granulocyte macrophage colony stimulating factor (GM-CSF), interleukin (IL)-4 and tumor necrosis factor (TNF) and whether these TAM-derived DC show enhanced antitumor activity. It was observed that DC generated from thy1-administered tumor-bearing mice showed augmented antitumor activity in vitro. Adoptive immunotherapy using TAM-derived DC showed a significant delay in the tumor growth and a prolongation of the survival time in tumor-bearing mice. DC obtained from TAM of thy1-administered mice also produced an enhanced amount of cytokines like IL-1 and TNF-. This is the first study of its kind regarding the effect of thy1 on the differentiation of DC from TAM and the role of TAM-derived DC in tumor progression. 相似文献
992.
Botulinum neurotoxins (BoNTs) are highly potent toxins that inhibit neurotransmitter release from peripheral cholinergic synapses and associate with infant botulism. BoNT is a approximately 150kDa protein, consisting of a binding/translocating heavy chain (HC; 100kDa) and a toxifying light chain (LC; 50kDa) linked through a disulfide bond. C-terminal half of the heavy chain is binding domain, and N-terminal half of the heavy chain is translocation domain that includes transmembrane domain. A functional botulinum neurotoxin type B heavy chain transmembrane and binding domain (Ile 624-Glu 1291) has been cloned into a bacterial expression vector pET 15b and produced as an N-terminally six-histidine-tagged fusion protein (BoNT/B HC TBD). (His(6))-BoNT/B HC TBD was highly expressed in Escherichia coli BL21-CodonPlus (DE3)-RIL and isolated from the E. coli inclusion bodies. After solubilizing the purified inclusion bodies with 6M guanidine-HCl in the presence of 10mM beta-mercaptoethanol, the protein was purified and refolded in a single step on Ni(2+) affinity column by removing beta-mercaptoethanol first, followed by the removal of urea. The purified protein was determined to be 98% pure as assessed by SDS-polyacrylamide gel. (His(6))-BoNT/B HC TBD retained binding to synaptotagmin II, the receptor of BoNT/B, which was confirmed by immunological dot blot assay, also to ganglioside, which was investigated using enzyme-linked immunosorbent assay. 相似文献
993.
Pteridine reductase 1 (PTR1) is part of a novel metabolic pathway in Leishmania associated with folate metabolism. Its main function is to salvage pterins but a second one is to reduce folates. The novelty and possible uniqueness of the pathway in which PTR1 is involved opens the possibility of developing specific inhibitors, which in combination with dihydrofolate reductase inhibitors could be highly effective against Leishmania. In order to increase our understanding of this putative important chemotherapeutic target, we present here the cloning, overexpression and purification of this enzyme from a clinical isolate of Leishmania donovani causing kala azar in India. This recombinant enzyme will set the basis for inhibition studies as well as for structure-function relationships. 相似文献
994.
The evolutionary relationship of peculiar and poorly known Ganges River dolphin with extinct and extant cetaceans has been in the state of confusion for more than a century. The close resemblance of platanistidae with some of the extinct taxon viz., Dalpiaziniidae and Waipatiidae and their sister group relationship with many of the extant lineages of cetaceans has been reported but none of the alternative hypotheses provide an unambiguous placement for this species. The present study provides insights into the molecular relationships of Platanista with other cetaceans based on comprehensive analyses of the mitochondrial cytochrome b and nuclear interphotoreceptor retinoid-binding protein gene sequences, obtained from 15 specimens of Ganges dolphin from India and Bangladesh. The mean substitution distance analysis of phylogenetically informative characters in the cytochrome b sequences suggested that Platanista gangetica is significantly closer (P<0.001) to Mysticeti than to any other group of toothed whales. However, the conventional methods of phylogenetic reconstruction supported this finding with low to moderate (41-69%) bootstrap values. 相似文献
995.
996.
Inactivation of Cyanobacterial Nitrogenase After Exposure to Ultraviolet-B Radiation 总被引:2,自引:0,他引:2
Exposure of the N2-fixing cyanobacterium Anabaena BT2 to ultraviolet-B radiation (2.5 W m−2) for 30 min resulted in complete loss of nitrogenase activity but 100% cell killing occurred only after a 90-min exposure.
Inactivation of nitrogenase activity was not specific to Anabaena BT2; other species also showed a similar effect. The time required for 100% killing and inactivation of nitrogenase activity
differed in various species, and this difference may be ascribed to the presence of different levels of UV-B protection mechanisms
in individual species. Inhibition of nitrogenase activity was immediate, since exposure of cultures to UV-B for as little
as 5 min elicited some inhibition of activity. The activity of UV-B-inhibited nitrogenase did not recover upon transfer of
exposed cells to fluorescent light, suggesting that the inhibition may be due to specific inactivation of the enzyme. By employment
of inhibitors of protein synthesis and PS-II activity, it was demonstrated that restoration of nitrogenase activity in a UV-B-treated
culture occurred by fresh synthesis of nitrogenase polypeptide. Our findings suggest that estimation of nitrogenase activity
in diazotrophic species may be used as a marker enzyme for assessing the impact of UV-B radiation.
Received: 13 June 2002 / Accepted: 22 July 2002 相似文献
997.
Ca(2+) enhanced the transformation frequency of Thermoactinomyces vulgaris (stock no. 1278) of an auxotrophic strain by the chromosomal DNA isolated from a prototrophic strain (stock no. 1227). The number of transformants showed a marked increase with increasing concentration of CaCl(2) upto 0.05 mM; and above this concentration, the transformation frequency decreased significantly. Antipsychotic drugs that are potent calmodulin inhibitors, like trifluoperazine and chlorpromazine, when applied in the concentration range of 0.01-0.04 mM along with optimal CaCl(2) concentration to the cultures of the recipient cells, resulted in a significant inhibition in the frequency of Ca(2+)-stimulated transformation. The results of present investigation suggest the involvement of a Ca(2+)-dependent protein activator in the development of Ca(2+)-mediated competence, which could have played an important role in the enhancement of genetic transformation in this aerobic spore forming thermophilic actinomycete. 相似文献
998.
Suneetha LM Singh SS Vani M Vardhini D Scollard D Archelos JJ Srinivasulu M Suneetha S 《Neurochemical research》2003,28(9):1393-1399
We have previously shown that a major phosphorylated 25-kDa glycoprotein of the human peripheral nerve binds to Mycobacterium leprae. In the present study, we confirm that the 25-kDa glycoprotein of the human peripheral nerve is myelin P zero (P0) by immunoprecipitation and Western blot experiments using monoclonal antibodies to myelin P0. Immunohistochemical studies on human nerve using these antibodies to myelin P0 exhibited a strong immunoreactivity to the myelin and Schwann cells. Myelin P0 is a peripheral nerve specific protein; therefore it could likely be one of the key target molecules for M. leprae binding/internalization or even contact-dependent demyelination. This finding of M. leprae binding to myelin P0 adds to the present understanding on neural predilection of M. leprae. 相似文献
999.
Singh SB Zink DL Bills GF Teran A Silverman KC Lingham RB Felock P Hazuda DJ 《Bioorganic & medicinal chemistry letters》2003,13(4):713-717
Integration of viral DNA into host cell DNA is an essential step in retroviral (HIV-1) replication and is catalyzed by HIV-1 integrase. HIV-1 integrase is a novel therapeutic target and is the focus of efforts to identify effective inhibitors that will prevent/or cure HIV infections. Four novel naphtho-gamma-pyrones, belonging to the chaetochromin and ustilaginoidin family, were discovered as inhibitors of HIV-1 integrase from the screening of fungal extracts using a recombinant in vitro assay. These compounds inhibit both the coupled and strand transfer activity of HIV-1 integrase with IC(50) values of 1-3 and 4-12 microM, respectively. The discovery, structure elucidation, chemical modification and the structure-activity relationship of these compounds are described. 相似文献
1000.
Singh SB Zink DL Dombrowski AW Polishook JD Ondeyka JG Hirshfield J Felock P Hazuda DJ 《Bioorganic & medicinal chemistry》2003,11(7):1577-1582
HIV-1 integrase is a critical enzyme in the replication of HIV-1. It is absent in the host cells and therefore is a good target for treatment of HIV-1 infections. Integracides are members of the tetracyclic triterpenoids family that were isolated from the fermentation broth of a Fusarium sp. Integracide A, a sulfated ester, exhibited significant inhibitory activity against strand transfer reaction of HIV-1 integrase. The discovery, structure elucidation including single crystal X-ray structure and HIV-1 inhibitory activity of these compounds are described. 相似文献