全文获取类型
收费全文 | 3201篇 |
免费 | 302篇 |
出版年
2022年 | 22篇 |
2021年 | 46篇 |
2020年 | 25篇 |
2019年 | 25篇 |
2018年 | 36篇 |
2017年 | 35篇 |
2016年 | 53篇 |
2015年 | 110篇 |
2014年 | 135篇 |
2013年 | 146篇 |
2012年 | 209篇 |
2011年 | 197篇 |
2010年 | 129篇 |
2009年 | 108篇 |
2008年 | 189篇 |
2007年 | 159篇 |
2006年 | 158篇 |
2005年 | 140篇 |
2004年 | 142篇 |
2003年 | 146篇 |
2002年 | 131篇 |
2001年 | 51篇 |
2000年 | 41篇 |
1999年 | 43篇 |
1998年 | 23篇 |
1997年 | 36篇 |
1996年 | 27篇 |
1994年 | 21篇 |
1993年 | 33篇 |
1992年 | 28篇 |
1991年 | 26篇 |
1990年 | 28篇 |
1989年 | 23篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 26篇 |
1985年 | 34篇 |
1984年 | 38篇 |
1983年 | 29篇 |
1982年 | 38篇 |
1981年 | 32篇 |
1980年 | 37篇 |
1979年 | 21篇 |
1978年 | 32篇 |
1977年 | 34篇 |
1976年 | 27篇 |
1973年 | 24篇 |
1972年 | 19篇 |
1971年 | 16篇 |
1970年 | 16篇 |
排序方式: 共有3503条查询结果,搜索用时 62 毫秒
91.
The stoichiometry of calcium-ion chelation to alginate chains has been investigated by circular dichroism (c.d.), and by equilibrium dialysis in the presence of various concentrations of sodium chloride. C.d. intensity in the carboxylate π → π * spectral region increases linearly with calcium-ion concentration up to a level equivalent to half the total poly-L-guluronate stoichiometric requirement, and thereafter shows little further change. Similarly, the level of bound calcium resistant to displacement by swamping concentrations of sodium ions is equivalent to half the stoichiometric requirement of poly-L-guluronate chain-sequences alone. In terms of the previously developed “egg-box” model of co-operative junction-zone formation in alginate gelation, these results are interpreted as showing that the primary mechanism of interchain association is by dimerisation of poly-L-guluronate chain-segments in a regular, buckled, two-fold conformation related to that characterized for the free acid in the solid state, with tight interchain chelation of calcium to the carboxylate groups on the interior faces of the dimer (i.e., half the carboxylate residues of the participating chain-sequences). This interpretation is entirely consistent with previous evidence from electron microscopy, and offers a simple rationalisation of experimental results from competitive-ion binding studies. 相似文献
92.
93.
94.
When Lemna minor was cultured in the presence of 0.25 mM l-lysine, the concentration of free methionine and formyl and methyl tetrahydrofolate (THFA) were decreased. l-lysine, l-homoserine, l-threonine and l-methionine at concentrations up to 8 mM did not affect N10-formyl THFA synthetase (E.C. 6.3.4.3) and N5,N10-methylene THFA reductase (E.C. 1.1.1.68). In contrast, serine hydroxymethyltransferase (E.C. 2.1.2.1) activity was inhibited by lysine. This inhibition gave a sigmoidal curve when plotted for a range of l-lysine or THFA concentrations. Exogenous lysine also reduced the incorporation of glycine [14C] and serine [3-14C] into free and protein methionine. Lysine, which is known to control synthesis of homocysteine in L. minor, may also regulate production of C-1 units for methionine synthesis by inhibition of serine hydroxymethyltransferase. 相似文献
95.
Potato slices treated with spirovetiva-1(10),11-diene-2-one yield lubimin and rishitin within 24 hr. A vetispirane which has not been detected in fungally infected potatoes was also isolated. This compound, isolubimin, appears to be an intermediate in the conversion of the above spirovetivone to lubimin. 相似文献
96.
Summary The host-parasite relationship of HeLa M cells artificially infected with a bovine species of Mycoplasma was studied by light microscopy, transmission electron microscopy and scanning electron microscopy. The use of morphometry to quantitate some of the findings was explored. The parasites were seen in locations extracellular to the cell surface. The detection of small numbers of organisms by light microscopy was well demonstrated by use of the fluorescent antibody technique. Scanning electron microscopy proved to be an excellent method for revealing the surface details of cell-parasite morphology. Ultra-thin sections showed that the parasites are aligned mostly parallel to the plasma membrane of the host cell but separated by a gap of 10 nm. Morphometry indicated an average of 69 organisms per cell surface occupying 1.7% of the surface area. An increase of 26% in diameter of the HeLa cells, possibly as a result of infection, was observed.The authors wish to thank Christiana Ulness and Andrea Erickson for expert technical assistance and Arnold Schmidt for the operation of the scanning electron microscope. This work was supported by grants from the U.S.P.H.S.: AI 09586, AI 10743, and AI 06720 相似文献
97.
Edwin M. Westbrook Paul B. Sigler Helen Berman Jenny P. Glusker Gerard Bunick Ann Benson Paul Talalay 《Journal of molecular biology》1976,103(3):665-667
A monoclinic crystal form (, β= 130.1 °) of Δ5-3-ketosteroid isomerase from Pseudomonas testosteroni (EC 5.3.3.1), grown at pH 7.0, has been characterized. Crystal-density measurements show that the asymmetric unit contains 12 protomers (Mr = 13,394). 相似文献
98.
99.
100.