全文获取类型
收费全文 | 90074篇 |
免费 | 7023篇 |
国内免费 | 6040篇 |
出版年
2024年 | 35篇 |
2023年 | 1100篇 |
2022年 | 1325篇 |
2021年 | 4360篇 |
2020年 | 3191篇 |
2019年 | 4016篇 |
2018年 | 3956篇 |
2017年 | 2866篇 |
2016年 | 4051篇 |
2015年 | 5842篇 |
2014年 | 6886篇 |
2013年 | 7248篇 |
2012年 | 8492篇 |
2011年 | 7737篇 |
2010年 | 4484篇 |
2009年 | 4187篇 |
2008年 | 4780篇 |
2007年 | 4146篇 |
2006年 | 3534篇 |
2005年 | 2820篇 |
2004年 | 2313篇 |
2003年 | 2105篇 |
2002年 | 1697篇 |
2001年 | 1471篇 |
2000年 | 1338篇 |
1999年 | 1403篇 |
1998年 | 802篇 |
1997年 | 877篇 |
1996年 | 801篇 |
1995年 | 775篇 |
1994年 | 675篇 |
1993年 | 571篇 |
1992年 | 682篇 |
1991年 | 535篇 |
1990年 | 455篇 |
1989年 | 331篇 |
1988年 | 278篇 |
1987年 | 219篇 |
1986年 | 185篇 |
1985年 | 210篇 |
1984年 | 124篇 |
1983年 | 118篇 |
1982年 | 54篇 |
1981年 | 23篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1976年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 84 毫秒
101.
Shu-Qun Liu Yan Tao Zhao-Hui Meng Yun-Xin Fu Ke-Qin Zhang 《Journal of molecular modeling》2011,17(2):289-300
The native serine protease proteinase K binds two calcium cations. It has been reported that Ca2+ removal decreased the enzyme’s thermal stability and to some extent the substrate affinity, but has discrepant effects on
catalytic activity of the enzyme. Molecular dynamics simulations were performed on the Ca2+-bound and Ca2+-free proteases to investigate the mechanism by which the calciums affect the structural stability, molecular motions, and
catalytic activity of proteinase K. Very similar structural properties were observed between these two forms of proteinase
K during simulations; and several long-lived hydrogen bonds and salt bridges common to both forms of proteinase K were found
to be crucial in maintaining the local conformations around these two Ca2+ sites. Although Ca2+ removal enhanced the overall flexibility of proteinase K, the flexibility in a limited number of segments surrounding the
substrate-binding pockets decreased. The largest differences in the equilibrium structures of the two simulations indicate
that, upon the removal of Ca2+, the large concerted motion originating from the Ca1 site can transmit to the substrate-binding regions but not to the catalytic
triad residues. In conjunction with the large overlap of the essential subspaces between the two simulations, these results
not only provide insight into the dynamics of the underlying molecular mechanism responsible for the unchanged enzymatic activity
as well as the decreased thermal stability and substrate affinity of proteinase K upon Ca2+ removal, but also complement the experimentally determined structural and biochemical data. 相似文献
102.
Li Zhang Shan Lei Hui Xie Qianqian Li Shuo Liu Qiang Liu Weijin Huang Xinyue Xiao Youchun Wang 《中国病毒学》2020,35(2):235-239
正Dear Editor,Marburg virus(MARV) belongs to the Filoviridae family,along with the related Ebola virus(EBOV). Although MARV is less renowned than EBOV, it causes an equally devastating disease, with clinical symptoms similar to EBOV infection and a remarkably high mortality rate. To 相似文献
103.
Fertilized eggs of Sebastes schlegelii, Sebastes pachycephalus and Sebastes hubbsi are morphologically similar under stereomicroscope. However, under the scanning electron microscope, significant differences in the ultrastructures of the egg surface among the three species were observed, and herein, a taxonomic key was proposed for future applications. A clustering analysis based on the ultrastructures of the egg surface and the diameter of the oil globule suggested that among the three species, S. hubbsi and S. pachycephalus were more genetically similar, while S. schlegelii had diverged earlier. The results agree with the conclusions drawn from morphological and molecular analyses on adult samples of the same species. 相似文献
104.
Bioprocess and Biosystems Engineering - To overcome the contamination in open pond, microalgal strain selection should focus on species with tolerability to extreme environments. In this study, a... 相似文献
105.
While host proteins incorporated into virions during viral budding from infected cell are known to play essential roles in multiple process of the life cycle of progeny virus, these characteristics have been largely neglected in studies on rabies virus(RABV). Here, we purified the RABV virions with good purity and integrity, and analyzed their proteome by nano LC–MS/MS, followed by the confirmation with immunoblot and immuno-electronic microscopy. In addition to the 5 viral proteins, 49 cellular proteins were reproducibly identified to be incorporated into matured RABV virions. Function annotation suggested that 24 of them were likely involved in virus replication. Furthermore, cryo-EM was employed to observe the purified RABV virions, generating high-resolution pictures of the bullet-shaped virion structure of RABV. This study has provided new insights into the host proteins composition in RABV virion and shed the light for further investigation on molecular mechanisms of RABV infection, as well as the discovery of new anti-RABV therapeutics. 相似文献
106.
107.
Hari G. Garg Hicham Mrabat Craig Freeman Fuming Zhang Charles A. Hales 《Carbohydrate research》2010,345(9):1084-1087
Heparin (HP) inhibits the proliferation of bovine pulmonary artery smooth muscle cells (BPASMC’s), among other cell types in vitro. In order to develop a potential therapeutic agent to reverse vascular remodeling, we are involved in deciphering the relationship between the native HP structure and its antiproliferative potency. We have previously reported the influence of the molecular size and the effects of various O-sulfo and N-acetyl groups of HP on growth-inhibitory activity. In this study, to understand the influence of carboxyl groups in the HP structure required for endogenous activity, a chemically modified derivative of native HP was prepared by converting the carboxyl groups of hexuronic acid residues in HP to primary hydroxyl groups. This modification procedure involves the treatment of HP with N-(3-dimethylaminopropyl)-N-ethylcarbodiimide followed by reduction with NaBH4 to yield carboxyl-reduced heparin (CR-HP). When compared to the antiproliferative potency of native HP on cultured BPASMC’s at three dose levels (1, 10, and 100 μg/mL), the CR-HP showed significantly less potency at all the doses. These results suggest that hexuronic acid residues in both major and variable sequences in HP are essential for the antiproliferative properties of native HP. 相似文献
108.
109.
Twenty ewes of mixed breeds were randomly assigned in equal numbers to one of four groups in a 2 x 2 factorial design. The factors were x-irradiation to destroy ovarian follicles or sham irradiation and the administration of estradiol-containing or empty (placebo) implants. Surgery for irradiation was performed on Day 8 of the cycle. Blood samples were withdrawn from jugular catheters at 1.5-h intervals from Day 10 to Day 17. Luteolysis was not observed by Day 17 in 4 of 5 placebo-treated ewes after destruction of ovarian follicles. Luteolysis was observed in 4 of 5 ewes of the sham-irradiated, placebo-treated group and in all ewes that received estradiol whether or not ovarian follicles had been destroyed. The longest (p less than 0.07) interval between peaks of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) was observed in the x-irradiated, placebo-treated group, whereas the administration of estradiol reduced (p less than 0.01) the interval between PGFM peaks. These findings indicate that a short interpulse interval in the secretion of prostaglandin F2 alpha (PGF2 alpha) is associated with luteolysis. It is possible that the reduced interpulse interval was either an effect of estradiol that caused luteolysis or a secondary event resulting from luteolysis. The administration of estradiol decreased (p less than 0.05) the number of episodes of oxytocin secretion during luteolysis and increased (p less than 0.01) the interval between episodes.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
110.