首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   57850篇
  免费   4594篇
  国内免费   4506篇
  66950篇
  2024年   143篇
  2023年   789篇
  2022年   1851篇
  2021年   3045篇
  2020年   2080篇
  2019年   2501篇
  2018年   2344篇
  2017年   1805篇
  2016年   2544篇
  2015年   3627篇
  2014年   4380篇
  2013年   4436篇
  2012年   5286篇
  2011年   4762篇
  2010年   2882篇
  2009年   2598篇
  2008年   2937篇
  2007年   2634篇
  2006年   2262篇
  2005年   1887篇
  2004年   1509篇
  2003年   1420篇
  2002年   1072篇
  2001年   910篇
  2000年   889篇
  1999年   810篇
  1998年   499篇
  1997年   454篇
  1996年   477篇
  1995年   422篇
  1994年   413篇
  1993年   325篇
  1992年   446篇
  1991年   324篇
  1990年   284篇
  1989年   260篇
  1988年   210篇
  1987年   194篇
  1986年   176篇
  1985年   154篇
  1984年   115篇
  1983年   122篇
  1982年   81篇
  1981年   45篇
  1980年   51篇
  1979年   63篇
  1976年   46篇
  1974年   54篇
  1973年   45篇
  1972年   53篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
31.
32.
The conformation of poly(L-ornithine) (PLO) and poly(L-lysine) (PLL) in solutions of sodium alkyl sulfates, CH3(CH2)nSO4Na with n = 7, 9, 11, 13 and 15 was studied by circular dichroism. PLO adopts a helical conformation in all 5 homologs and PLL a β-form in only 4 of the homologs. With octyl sulfate PLL has a helical conformation instead. These conformations were observed in solution of surfactants both below and above the critical micelle concentration.  相似文献   
33.
A computerized calibration of the circular dichrometer   总被引:5,自引:0,他引:5  
J Y Cassim  J T Yang 《Biochemistry》1969,8(5):1947-1951
  相似文献   
34.
35.
36.
A method for the quantitative determination of cycasin from cycad flour by gas-liquid chromatography is described. The flour is extracted with 70% ethanol and the residue from the dried extract is directly trimethylsilylated. Androsterone was found to be an excellent internal standard. The average content of cycasin from ten separate analyses of one lot of flour was 0.429 gm100 gm. The method is rapid, sensitive, and not hindered by contaminating compounds.  相似文献   
37.
Long-chain unsaturated fatty acids (UFA) having up to six double bonds are derivatized to 2-substituted 4,4-dimethyloxazolines (DMOX) and then analyzed by combined in-beam electron impact (IBEI)-B/E-linked scan mass spectrometry. This technique provides highly characteristic mass spectra and may serve as an auxiliary means for direct structure determination of individual UFA in mixtures.  相似文献   
38.
The activity of the rat class I alcohol dehydrogenase (ADH) is enriched in certain tissues including the liver, intestine and testis. The tissue-specific expression of the gene encoding ADH in the rat was studied and found to closely correlate with tissue isozymic activity. A factor designated enhancer-site downstream binding protein (EDBP) was recently identified in the rat liver and found to interact with the proximal promoter of the class I ADH gene. The distribution of EDBP in nuclear extracts obtained from various tissues was examined based on its sequence-specific DNA binding property and found to correlate with tissue ADH expression. These findings suggest that EDBP is potentially a positive regulatory factor which is involved in controlling the tissue-specific expression of the ADH gene.  相似文献   
39.
An effort to identify the major general esterases of rat liver cytosol that are insensitive to the serine esterase inhibitor paraoxon (diethyl 4-nitrophenyl phosphate) has led to the isolation of a dozen enzymes. Four of these are electrophoretically homogeneous. Although purified on the basis of their hydrolytic activity toward 4-nitrophenyl acetate, each of the enzymes has a very broad and overlapping substrate specificity for aromatic esters. Thiol esters serve as substrates but, within the limits of the methods used, amides are not hydrolyzed.  相似文献   
40.
We propose two approximate methods (one based on parsimony and one on pairwise sequence comparison) for estimating the pattern of nucleotide substitution and a parsimony-based method for estimating the gamma parameter for variable substitution rates among sites. The matrix of substitution rates that represents the substitution pattern can be recovered through its relationship with the observable matrix of site pattern frequences in pairwise sequence comparisons. In the parsimony approach, the ancestral sequences reconstructed by the parsimony algorithm were used, and the two sequences compared are those at the ends of a branch in the phylogenetic tree. The method for estimating the gamma parameter was based on a reinterpretation of the numbers of changes at sites inferred by parsimony. Three data sets were analyzed to examine the utility of the approximate methods compared with the more reliable likelihood methods. The new methods for estimating the substitution pattern were found to produce estimates quite similar to those obtained from the likelihood analyses. The new method for estimating the gamma parameter was effective in reducing the bias in conventional parsimony estimates, although it also overestimated the parameter. The approximate methods are computationally very fast and appear useful for analyzing large data sets, for which use of the likelihood method requires excessive computation.   相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号