全文获取类型
收费全文 | 4657篇 |
免费 | 505篇 |
国内免费 | 4篇 |
出版年
2021年 | 75篇 |
2019年 | 47篇 |
2018年 | 71篇 |
2017年 | 54篇 |
2016年 | 101篇 |
2015年 | 122篇 |
2014年 | 166篇 |
2013年 | 201篇 |
2012年 | 247篇 |
2011年 | 255篇 |
2010年 | 192篇 |
2009年 | 132篇 |
2008年 | 268篇 |
2007年 | 241篇 |
2006年 | 176篇 |
2005年 | 182篇 |
2004年 | 181篇 |
2003年 | 162篇 |
2002年 | 170篇 |
2001年 | 99篇 |
2000年 | 87篇 |
1999年 | 92篇 |
1998年 | 69篇 |
1997年 | 56篇 |
1996年 | 58篇 |
1995年 | 52篇 |
1994年 | 43篇 |
1993年 | 46篇 |
1992年 | 81篇 |
1991年 | 86篇 |
1990年 | 69篇 |
1989年 | 73篇 |
1988年 | 85篇 |
1987年 | 54篇 |
1986年 | 60篇 |
1985年 | 52篇 |
1984年 | 59篇 |
1983年 | 49篇 |
1982年 | 37篇 |
1981年 | 41篇 |
1980年 | 51篇 |
1979年 | 49篇 |
1978年 | 48篇 |
1976年 | 45篇 |
1975年 | 34篇 |
1974年 | 34篇 |
1973年 | 42篇 |
1972年 | 34篇 |
1970年 | 33篇 |
1969年 | 33篇 |
排序方式: 共有5166条查询结果,搜索用时 15 毫秒
971.
Synthesis, spectroscopic measurements and X-ray structure determination of a new 16-oxa-5β-14β-steroid is presented, and its use in a plausible approach to the synthesis of eurycomalactone is discussed. 相似文献
972.
Interstitial deletion of the long arm of chromosome 3 总被引:3,自引:0,他引:3
A patient with multiple congenital malformations and developmental delay is reported. Her karyotype is 46,XX,del(3)(q23q25). 相似文献
973.
974.
975.
Wenping Zhang Jishan Lin Jianguo Li Shaoquan Zheng Xingtan Zhang Shuai Chen Xiaokai Ma Fei Dong Haifeng Jia Xiuming Xu Ziqin Yang Panpan Ma Fang Deng Ban Deng Yongji Huang Zhanjie Li Xiaozhou Lv Yaying Ma Zhenyang Liao Zhicong Lin Jing Lin Shengcheng Zhang Tracie Matsumoto Rui Xia Jisen Zhang Ray Ming 《The Plant journal : for cell and molecular biology》2021,108(4):1037-1052
Rambutan is a popular tropical fruit known for its exotic appearance, has long flexible spines on shells, extraordinary aril growth, desirable nutrition, and a favorable taste. The genome of an elite rambutan cultivar Baoyan 7 was assembled into 328 Mb in 16 pseudo-chromosomes. Comparative genomics analysis between rambutan and lychee revealed that rambutan chromosomes 8 and 12 are collinear with lychee chromosome 1, which resulted in a chromosome fission event in rambutan (n = 16) or a fusion event in lychee (n = 15) after their divergence from a common ancestor 15.7 million years ago. Root development genes played a crucial role in spine development, such as endoplasmic reticulum pathway genes, jasmonic acid response genes, vascular bundle development genes, and K+ transport genes. Aril development was regulated by D-class genes (STK and SHP1), plant hormone and phenylpropanoid biosynthesis genes, and sugar metabolism genes. The lower rate of male sterility of hermaphroditic flowers appears to be regulated by MYB24. Population genomic analyses revealed genes in selective sweeps during domestication that are related to fruit morphology and environment stress response. These findings enhance our understanding of spine and aril development and provide genomic resources for rambutan improvement. 相似文献
976.
Lindsey Chambers Ying Yang Heather Littier Partha Ray Tong Zhang Amy Pruden Michael Strickland Katharine Knowlton 《PloS one》2015,10(8)
Although dairy manure is widely applied to land, it is relatively understudied compared to other livestock as a potential source of antibiotic resistance genes (ARGs) to the environment and ultimately to human pathogens. Ceftiofur, the most widely used antibiotic used in U.S. dairy cows, is a 3rd generation cephalosporin, a critically important class of antibiotics to human health. The objective of this study was to evaluate the effect of typical ceftiofur antibiotic treatment on the prevalence of ARGs in the fecal microbiome of dairy cows using a metagenomics approach. β-lactam ARGs were found to be elevated in feces from Holstein cows administered ceftiofur (n = 3) relative to control cows (n = 3). However, total numbers of ARGs across all classes were not measurably affected by ceftiofur treatment, likely because of dominance of unaffected tetracycline ARGs in the metagenomics libraries. Functional analysis via MG-RAST further revealed that ceftiofur treatment resulted in increases in gene sequences associated with “phages, prophages, transposable elements, and plasmids”, suggesting that this treatment also enriched the ability to horizontally transfer ARGs. Additional functional shifts were noted with ceftiofur treatment (e.g., increase in genes associated with stress, chemotaxis, and resistance to toxic compounds; decrease in genes associated with metabolism of aromatic compounds and cell division and cell cycle), along with measureable taxonomic shifts (increase in Bacterioidia and decrease in Actinobacteria). This study demonstrates that ceftiofur has a broad, measureable and immediate effect on the cow fecal metagenome. Given the importance of 3rd generation cephalospirins to human medicine, their continued use in dairy cattle should be carefully considered and waste treatment strategies to slow ARG dissemination from dairy cattle manure should be explored. 相似文献
977.
978.
Anjana Bagchi Partha Neogi Mahendra Sahai Anil B. Ray Yoshiteru Oshima Hiroshi Hikino 《Phytochemistry》1984,23(4):853-855
Novel withanolides, withaperuvin E and nicandrin B, isolated respectively, from Physalis peruviana and Nicandra physaloides, were fully characterized by chemical and spectroscopic means. 相似文献
979.
1H and 31P NMR pH titrations were conducted to monitor changes in the environment and protonation state of the histidine residues and phosphoserine group of rabbit muscle phosphoglucomutase on binding of metal ions at the activating site and of substrate (glucose phosphate) at the catalytic site. Imidazole C epsilon-H signals from 8 of the 10 histidines present in the free enzyme were observed in 1H NMR spectra obtained by a spin-echo pulse sequence at 470 MHz; their pH (uncorrected pH meter reading of a 2H2O solution measured with a glass electrode standardized with H2O buffer) titration properties (in 99% 2H2O) were determined. Three of these histidine residues, which have pKa values ranging from 6.5 to 7.9, exhibited an atypical pH-dependent perturbation of their chemical shifts with a pHmid of 5.8 and a Hill coefficient of about 2. Since none of the observed histidines has a pKa near 5.8, it appears that these three histidines interact with a cluster consisting of two or more groups which become protonated cooperatively at this pH. Binding of Cd2+ at the activating site of the enzyme abolishes the pH-dependent transition of these histidines; hence, the putative anion cluster may constitute the metal ion binding site, or part of it. Two separate 31P NMR peaks from phosphoserine-116 of the phosphoenzyme were observed between pH 6 and 9. Apparently, the metal-free enzyme exists as a pH-dependent mixture of conformers that provide two different environments, I and II, for the enzymic phosphate group; the transition of the phosphate group between these two environments is slow on the NMR time scale.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
980.
Metal binding at the activating site of rabbit muscle phosphoglucomutase has been studied by 31P, 7Li, and 113Cd NMR spectroscopy. A 7Li NMR signal of the binary Li+ complex of the phosphoenzyme was not observed probably because of rapid transverse relaxation of the bound ion due to chemical exchange with free Li+. The phosphoenzyme-Li+-glucose 6-phosphate ternary complex is more stable, kinetically, and yields a well-resolved peak from bound Li+ at -0.24 ppm from LiCl with a line width of 5 Hz and a T1 relaxation time of 0.51 +/- 0.07 s at 78 MHz. When glucose 1-phosphate was bound, instead, the chemical shift of bound 7Li+ was -0.13 ppm; and in the Li+ complex of the dephosphoenzyme and glucose bisphosphate a partially broadened 7Li+ peak appeared at -0.08 ppm. Thus, the bound metal ion has a somewhat different environment in each of these three ternary complexes. The 113Cd NMR signal of the binary Cd2+ complex of the phosphoenzyme appears at 22 ppm relative to Cd(ClO4)2 with a line width of 20 Hz at 44.4 MHz. Binding of substrate and formation of the Cd2+ complex of the dephosphoenzyme and glucose bisphosphate broaden the 113Cd NMR signal to 70 Hz and shift it to 75 ppm. The 53 ppm downfield shift upon the addition of substrate along with 1H NMR data suggests that one oxygen ligand to Cd2+ in the binary complex is replaced by a nitrogen ligand at some intermediate point in the enzymic reaction.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献