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   135902篇
  免费   4879篇
  国内免费   823篇
  2022年   305篇
  2021年   587篇
  2020年   398篇
  2019年   476篇
  2018年   12279篇
  2017年   11088篇
  2016年   8317篇
  2015年   2379篇
  2014年   2043篇
  2013年   2599篇
  2012年   6855篇
  2011年   15051篇
  2010年   13432篇
  2009年   9594篇
  2008年   11656篇
  2007年   13199篇
  2006年   2116篇
  2005年   2254篇
  2004年   2610篇
  2003年   2467篇
  2002年   2215篇
  2001年   1505篇
  2000年   1419篇
  1999年   1168篇
  1998年   656篇
  1997年   612篇
  1996年   591篇
  1995年   578篇
  1994年   514篇
  1993年   568篇
  1992年   1040篇
  1991年   803篇
  1990年   797篇
  1989年   771篇
  1988年   683篇
  1987年   621篇
  1986年   628篇
  1985年   721篇
  1984年   555篇
  1983年   446篇
  1982年   362篇
  1981年   322篇
  1980年   257篇
  1979年   388篇
  1978年   356篇
  1977年   260篇
  1974年   300篇
  1973年   255篇
  1972年   438篇
  1971年   422篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms.  相似文献   
12.
13.
14.

Background  

Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation.  相似文献   
15.
Prediction of sequential antigenic regions in proteins   总被引:30,自引:0,他引:30  
Prediction of antigenic regions in a protein will be helpful for a rational approach to the synthesis of peptides which may elicit antibodies reactive with the intact protein. Earlier methods are based on the assumption that antigenic regions are primarily hydrophilic regions at the surface of the protein molecule. The method presented here is based on the amino acid composition of known antigenic regions in 20 proteins which is compared with that of 314 proteins [(1978) Atlas of Protein Sequence and Structure, vol. 5, suppl. 3, 363-373]. Antigenicity values were derived from the differences between the two data sets. The method was applied to bovine ribonuclease, the B-subunit of cholera toxin and herpes simplex virus type 1 glycoprotein D. There was a good correlation between the predicted regions and previously determined antigenic regions.  相似文献   
16.
2-(N-Benzyloxycarbonyl)aminoethyl 7-O-acetyl-6-O-allyl-2-O-benzoyl-4-O-benzyl-3-O-chloroacetyl-l-glycero-α-d-manno-heptopyranosyl-(1→3)-[2,3,4,6-tetra-O-benzoyl-β-d-glucopyranosyl-(1→4)]-6,7-di-O-acetyl-2-O-benzyl-l-glycero-α-d-manno-heptopyranoside, a spacer-equipped protected derivative of the common 3,4-branched diheptoside trisaccharide structure of the lipopolysaccharide core of Neisseria meningitidis and Haemophilus influenzae has been synthesized. The protecting group pattern installed allows regioselective introduction of phosphoethanolamine residues in the 3- and 6-position of the second heptose unit in accordance with native structures. From this intermediate the 3-and 6-monophosphoethanolamine as well as the non-phosphorylated deprotected trisaccharides have been synthesized to be used in evaluation of antibody binding specificity and in investigation of the substrate specificity of glycosyl transferases involved in the biosynthesis of LPS core structures.  相似文献   
17.
18.
19.
20.

Background  

Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes.  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