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   6725篇
  免费   592篇
  国内免费   5篇
  2022年   90篇
  2021年   169篇
  2020年   101篇
  2019年   108篇
  2018年   142篇
  2017年   135篇
  2016年   196篇
  2015年   279篇
  2014年   310篇
  2013年   394篇
  2012年   363篇
  2011年   363篇
  2010年   268篇
  2009年   201篇
  2008年   291篇
  2007年   251篇
  2006年   225篇
  2005年   219篇
  2004年   222篇
  2003年   194篇
  2002年   210篇
  2001年   184篇
  2000年   152篇
  1999年   134篇
  1998年   70篇
  1997年   59篇
  1996年   62篇
  1995年   59篇
  1994年   51篇
  1993年   49篇
  1992年   86篇
  1991年   96篇
  1990年   88篇
  1989年   77篇
  1988年   73篇
  1987年   72篇
  1986年   62篇
  1985年   62篇
  1984年   58篇
  1983年   53篇
  1981年   48篇
  1980年   45篇
  1979年   49篇
  1978年   48篇
  1976年   41篇
  1975年   56篇
  1974年   65篇
  1973年   40篇
  1971年   55篇
  1970年   42篇
排序方式: 共有7322条查询结果,搜索用时 0 毫秒
101.
102.
103.
104.
105.
106.
Acylphosphonic acids, R-CO-PO(OH)2, have been synthesized by the steps [formula: see text] of which the last is new and provides a mild method for de-esterifying acylphosphonic acids. Their reductive amination gives a simple way of making 1-aminoalkylphosphonic acids. Acetylphosphonic acid inhibited NAD+ reduction by pyruvate with the pyruvate dehydrogenases from Escherichia coli and Bacillus stearothermophilus. The inhibition was competitive with pyruvate, with Ki of 6 microM for the E. coli enzyme (pyruvate Km 0.5 mM) and one of 0.4 mM of the B. stearothermophilus enzyme (pyruvate Km 0.1 mM). Acetylphosphonate and its monomethyl ester are substates for pig heart lactate dehydrogenase, with Km values of 15 mM and 10 mM respectively (pyruvate Km 0.05 mM) and specificity constants one thousandth that for pyruvate.  相似文献   
107.
108.
Structural studies of turnip crinkle virus have been extended to include the identification of high-affinity coat protein binding sites on the RNA genome. Virus was dissociated at elevated pH and ionic strength, and a ribonucleoprotein complex (rp-complex) was isolated by chromatography on Sephacryl S-200. Genomic RNA fragments in the rp-complex, resistant to RNase A and RNase T1 digestion and associated with tightly bound coat protein subunits, were isolated using coat-protein-specific antibodies. The identity of the protected fragments was determined by direct RNA sequencing. These approaches allowed us to study the specific RNA-protein interactions in the rp-complex obtained from dissociated virus particles. The location of one protected fragment downstream from the amber terminator codon in the first and largest of the three viral open reading frames suggests that the coat protein may play a role in the regulation of the expression of the polymerase gene. We have also identified an additional cluster of T1-protected fragments in the region of the coat protein gene that may represent further high-affinity sites involved in assembly recognition.  相似文献   
109.
Methandrostenolone dose (amount and duration) and methods of isolation from urine can influence the identification and quantitation of methandrostenolone metabolites. Long-term use of methandrostenolone at high dosages led to the appearance of unmetabolized drug in the urine and contributed to the identification of a previously unreported metabolite, 3 beta, 6 section, 17 beta-trihydroxy-17 alpha-methyl-5 section-1-androstene. Exposure of methandrostenolone in vitro to acid conditions induced a retropinacol rearrangement in the D-ring of the methandrostenolone molecule, causing the formation of 18-nor-17,17-dimethyl-1,4,13(14)-androstatrien-3-one in large amounts. The same acidic conditions led to the addition of a hydroxyl at the 6 position of the B-ring of either the retropinacol rearrangement products or native methandrostenolone resulting in the formation of 6 beta-hydroxy-18-nor-17,17-dimethyl-1,4,13(14)-androstatrien-3-one, 6 alpha- hydroxy-18-nor-17,17-dimethyl-1,4,13(14)-androstatrien, 6 beta-17 alpha-methyl-1,4-androstadien-3-one and 6 alpha,17 beta-dihydroxy-17 alpha-methyl-1,4-androstadien-3-one. Hydroxylation of native methandrostenolone at the 6 position also occurs endogenously. However, no evidence of an endogenous retropinacol rearrangement was found. Silylating agents alone can induce the formation of small amounts of 6 beta-17 beta-dihydroxy-17 alpha-methyl-1,4-androstadien-3-one. Discrepancies between previously published reports on methandrostenolone metabolism in man are discussed and compared with an animal model.  相似文献   
110.
A stratified binomial marker model for bone-marrow repopulation experiments   总被引:1,自引:0,他引:1  
The paper considers bone-marrow repopulation experiments with injected mixtures of two types, A and B, of genetically marked donor cells. The covariance of the proportions of type A erythrocytes and lymphocytes is analysed as the sum of two components, under a stratified binomial model allowing the proportions of type A cells to vary in postulated strata of the mixture and with the assumption that the genetic marker does not influence cell development. The ratio of the two components is not experimentally estimable, but each of them has an interesting "demographic" interpretation. Possible inferences about certain "two-cell probabilities" are derived, and the experimental findings that necessitated the stratified model are illustrated.  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