全文获取类型
收费全文 | 24812篇 |
免费 | 2358篇 |
国内免费 | 2469篇 |
出版年
2023年 | 403篇 |
2022年 | 424篇 |
2021年 | 707篇 |
2020年 | 740篇 |
2019年 | 874篇 |
2018年 | 790篇 |
2017年 | 773篇 |
2016年 | 788篇 |
2015年 | 899篇 |
2014年 | 1017篇 |
2013年 | 1456篇 |
2012年 | 910篇 |
2011年 | 1188篇 |
2010年 | 1052篇 |
2009年 | 1502篇 |
2008年 | 1459篇 |
2007年 | 1497篇 |
2006年 | 1452篇 |
2005年 | 1452篇 |
2004年 | 1321篇 |
2003年 | 1120篇 |
2002年 | 985篇 |
2001年 | 630篇 |
2000年 | 581篇 |
1999年 | 611篇 |
1998年 | 591篇 |
1997年 | 484篇 |
1996年 | 349篇 |
1995年 | 447篇 |
1994年 | 374篇 |
1993年 | 312篇 |
1992年 | 272篇 |
1991年 | 225篇 |
1990年 | 213篇 |
1989年 | 197篇 |
1988年 | 164篇 |
1987年 | 152篇 |
1986年 | 109篇 |
1985年 | 157篇 |
1984年 | 170篇 |
1983年 | 118篇 |
1982年 | 137篇 |
1981年 | 79篇 |
1980年 | 96篇 |
1979年 | 59篇 |
1978年 | 47篇 |
1977年 | 38篇 |
1976年 | 37篇 |
1973年 | 44篇 |
1972年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
961.
Abstract The process of primary growth in 2-year-old seedlings of 11 populations of Pinus halepensis Mill. is described. At the end of the first growing season one type of apical structure was observed: type-1, a tuft of primary needles placed close together, surrounding and protecting a meristematic apex. At the end of the 2nd growing season, three types of apical structure were observed: type-1; type-2, a terminal winter bud; and type-3; a «bud» with characteristics of both type-1 and type-2. Morphological observation along with an anatomical examination of the winter bud led to the conclusion that the definitive growth pattern in juvenile P. halepensis is monocyclic with a variable number of summer shoots. This growth pattern is reached by some P. halepensis populations in 3–4 years, by contrast, in other pine species two years are usually needed. The populations studied differed both in growth potential (differences in number of cycles, ratio of first cycle to total growth, growth rates) and in the developmental stages of the apical meristem. Four groups could be identified: (i) Morocco and Spain, (a limited growth, few cycles, a high ratio of 1st cycle to total growth, and growth in 2nd season almost entirely due to free growth); (ii) Algeria and Greece, (moderate to low growth, a large number of cycles, a low ratio of 1st cycle to total growth, and very early formation of apical structure with preformed primordia); (iii) Israel and Central Italy (a high growth, a large number of cycles, a medium ratio of 1st cycle to total growth, and early formation of apical structure with preformed primordia); (iiii) Greece, France and Italy, which was intermediate between group (i) and the other 2 groups. 相似文献
962.
David J Craik 《Journal of peptide science》2013,19(7):393-407
Cyclotides are plant‐derived peptides of approximately 30 amino acids that have the characteristic structural features of a head‐to‐tail cyclized backbone and a cystine knot arrangement of their three conserved disulfide bonds. This article gives a personal account of the discovery of cyclotides, their characterization and their applications, based on work carried out in my laboratory over the last 20 years. It describes some of the background to their discovery and focuses on how their unique structural features lead to exceptional stability. This stability and their amenability to chemical synthesis have made it possible to use cyclotides as templates in protein engineering and drug design applications. These applications complement the interest in cyclotides deriving from their unique structures and natural function as host defense molecules. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd. 相似文献
963.
G. A. Perry J. D. Jackson T. L. McDonald D. A. Crouse J. G. Sharp 《Preparative biochemistry & biotechnology》2013,43(5):431-447
Recent increases in the ability to detect low levels of immunofluorescence have shown the need for highly purified primary immunoreagents. There are now reports of purification of monoclonal antibodies using HPLC with reverse phase columns. In this study we have utilized standard size exclusion HPLC to purify both biotinylated and non-biotinylated monoclonal antibodies from hybridoma culture supernatants. Results indicated that both biotinylated and non-biotinylated monoclonal antibodies retained their antigen binding capacity after purification, and were not different in this capacity from commercially available, affinity purified reagents. These findings indicate that size exclusion HPLC may be used in the purification of biologically active monoclonal antibodies, and suggest that this technique may be used in the large scale production of antibodies and their fragments, in antibody purification from ascites fluid, and in antisera quality control. 相似文献
964.
965.
Abstract We quantified the effects of anthropogenic disturbances on the structure and biodiversity of boreal forests on acidic soils and created a statistically supported rational set of indicators to monitor the stand “naturalness”. For that, we surveyed various traits of tree layer, understory, herb layer, forest floor and several widely accepted biodiversity epiphytic indicators in 252 old‐aged boreal stands in Estonia, mostly dominated by Scots pine or Norway spruce. Multifactorial general linear model analyses showed that many forest characteristics and potential indicators were confounded by the gradient of soil productivity (reflected by the forest site type), local biogeographic gradients and also by stand age. Considering confounding effects, boreal forests in a near‐natural state have more large‐diameter trees (diameter at breast height >40 cm) and larger variety of diameter classes, higher proportion of spruce or deciduous trees, a larger amount of coarse woody debris in various stages, a more closed tree canopy and denser understory than managed mature forests. By increasing light availability above the field layer, forest management indirectly increases the coverage of herbs and lichens on the forest floor but reduces the alpha‐ and beta‐diversity of herbs and the proportion of graminoids. Human disturbances reduce the relative incidence of many commonly accepted biodiversity indicators such as indicator lichens, woodpeckers, wood‐dwelling insects or fungi on trees. The test for the predictive power of characteristics reacting on disturbance revealed that only a fraction of them appeared to be included in a diagnostic easy‐to‐apply set of indicators to assess the nature quality of boreal forest: the amount of dead wood, the proportion of deciduous trees, the presence of specially shaped trees and woodpeckers and, as an indicator of disturbances, the forest herb Melampyrum pratensis. Many of these indicators have already been implemented in practice. 相似文献
966.
Nobuo N Noda Yuko Fujioka Takao Hanada Yoshinori Ohsumi Fuyuhiko Inagaki 《EMBO reports》2013,14(2):206-211
Atg12 is conjugated to Atg5 through enzymatic reactions similar to ubiquitination. The Atg12–Atg5 conjugate functions as an E3‐like enzyme to promote lipidation of Atg8, whereas lipidated Atg8 has essential roles in both autophagosome formation and selective cargo recognition during autophagy. However, the molecular role of Atg12 modification in these processes has remained elusive. Here, we report the crystal structure of the Atg12–Atg5 conjugate. In addition to the isopeptide linkage, Atg12 forms hydrophobic and hydrophilic interactions with Atg5, thereby fixing its position on Atg5. Structural comparison with unmodified Atg5 and mutational analyses showed that Atg12 modification neither induces a conformational change in Atg5 nor creates a functionally important architecture. Rather, Atg12 functions as a binding module for Atg3, the E2 enzyme for Atg8, thus endowing Atg5 with the ability to interact with Atg3 to facilitate Atg8 lipidation. 相似文献
967.
968.
Zahid H. Chohan 《Journal of enzyme inhibition and medicinal chemistry》2013,28(1):120-130
Synthesis, characterization and biological studies of Schiff base-derived sulfonamides and their Co (II), Cu (II), Ni (II) and Zn (II) complexes have been reported and screened for in-vitro antibacterial activity against six Gram-negative; E. coli, K. pneumoniae, P. aeruginosa, P. mirabilis, S. typhi and S. dysenteriae and four Gram-positive; B. cereus, C. diphtheriae, S. aureus and S. pyogenes bacterial strains and for in-vitro antifungal activity against T. longifusus, C. albicans, A. flavus, M. canis, F. solani, and C. glaberata. All compounds showed moderate to significant antibacterial activity, however, the zinc (II) complexes were found to be more active. Some of the compounds also showed significant antifungal activity against various fungal strains. Only compounds (6) and (10) displayed potent cytotoxic activity with LD50 = 4.644 × 10? 4 and 4.106 × 10? 4 moles/mL respectively, against Artemia salina. The X-ray structure of 4-[(2-hydroxybenzylidene)amino]benzenesulfonamide is also reported. 相似文献
969.
《Journal of enzyme inhibition and medicinal chemistry》2013,28(6):1199-1204
A series of 5,7-dihydroxyflavanone derivatives were synthesized and identified as reversible and competitive protein tyrosine phosphatase (PTP) 1B inhibitors with IC50 values in the micromolar range. Compound 4k had the most potent in vitro inhibition activity against PTP1B (IC50 = 2.37?±?0.37 μM) and the greatest selectivity (3.7-fold) for PTP1B relative to T-cell protein tyrosine phosphatase. Cell-based studies revealed that 4k was membrane-permeable and enhanced insulin receptor tyrosine phosphorylation in CHO/hIR cells. 相似文献
970.