全文获取类型
收费全文 | 23158篇 |
免费 | 2710篇 |
国内免费 | 5篇 |
出版年
2021年 | 291篇 |
2020年 | 224篇 |
2019年 | 287篇 |
2018年 | 308篇 |
2017年 | 305篇 |
2016年 | 510篇 |
2015年 | 835篇 |
2014年 | 844篇 |
2013年 | 1043篇 |
2012年 | 1407篇 |
2011年 | 1380篇 |
2010年 | 780篇 |
2009年 | 750篇 |
2008年 | 1113篇 |
2007年 | 1098篇 |
2006年 | 1077篇 |
2005年 | 1025篇 |
2004年 | 990篇 |
2003年 | 909篇 |
2002年 | 866篇 |
2001年 | 515篇 |
2000年 | 490篇 |
1999年 | 468篇 |
1998年 | 266篇 |
1997年 | 282篇 |
1996年 | 254篇 |
1995年 | 216篇 |
1994年 | 245篇 |
1993年 | 191篇 |
1992年 | 401篇 |
1991年 | 354篇 |
1990年 | 335篇 |
1989年 | 296篇 |
1988年 | 307篇 |
1987年 | 305篇 |
1986年 | 308篇 |
1985年 | 309篇 |
1984年 | 280篇 |
1983年 | 223篇 |
1982年 | 212篇 |
1981年 | 160篇 |
1980年 | 169篇 |
1979年 | 218篇 |
1978年 | 225篇 |
1977年 | 167篇 |
1976年 | 179篇 |
1975年 | 157篇 |
1974年 | 181篇 |
1973年 | 191篇 |
1972年 | 146篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
1.
Igor Loncaric Gabrielle L. Stalder Kemal Mehinagic Renate Rosengarten Franz Hoelzl Felix Knauer Chris Walzer 《PloS one》2013,8(12)
In order to test whether rooks (Corvus frugilegus) represent good indicators for the potential circulation of antibiotics in their native habitat, two populations with different migratory behavior were tested for the presence of beta-lactamase producing Enterobacteriaceae and methicillin-resistant Staphylococcus aureus (MRSA). In all, 54 and 102 samples of fresh feces of a migratory and a resident population were investigated. A total of 24 and 3 cefotaxime-resistant enterobacterial isolates were obtained from the migratory and resident population, respectively. In these isolates CTX-M-1 (n = 15), CTX-M-3 (n = 3), and CTX-M-15 (n = 3) genes were detected. TEM-1 and OXA-1 were associated with CTX-M in 3 and 2 isolates, respectively. In two E. coli isolates CMY-2 could be detected, where from one isolate displayed an overexpression of chromosomal AmpC as well. Among E. coli isolates the most common phylogenetic group was A (n = 11) and ST1683 (n = 5). In one E. coli of B2-ST131 the rfbO25b locus was detected. Three Enterobacter isolates were stably derepressed AmpC-producers. In five samples of the migratory population, PVL positive MRSA could be isolated. Two isolates were typed SCCmec IVa, spa type t127, and ST1. Three isolates carried a SCCmec type IVc, with spa type t852 and ST22. The highly significant difference of the occurrence of antibiotic resistance between the migratory population from eastern Europe compared to resident population in our study indicates that rooks may be good indicator species for the evaluation of environmental contamination with antibiotic resistant bacteria, especially due to their ecology, foraging behavior and differing migratory behavior. 相似文献
2.
Protein metal-binding sites. 总被引:2,自引:0,他引:2
Metal ions have a role in a variety of important functions in proteins including protein folding, assembly, stability, conformational change, and catalysis. The presence or absence of a given metal ion is crucial to the conformation or activity of over one third of all proteins. Recent developments have been made in the understanding and design of metal-binding sites in proteins, an important and rapidly advancing area of protein engineering. 相似文献
3.
4.
D Roberts 《Acta anatomica》1978,101(2):160-169
Retention of a retromandibular space has necessitated developing a system of mandibular protrusion in hominid species. It is possible that mandibular protrusion can be effected by a single muscle-the lateral pterygoid-and the motion controlled by the excentrically placed mandibular suspensory ligaments. The elasticity of the ligaments produces an integrity maintenance force between the articular condyle and eminence which is normally of minimal size. Excessive craniofacial flexion, or the retention of a juvenile configuration of the mandible, could result in increasing this integrity maintenance force and cause crepitation and clicking. Ajustment of the ligaments could reduce these pathological manifestations. 相似文献
5.
The RAINFOR database: monitoring forest biomass and dynamics 总被引:1,自引:0,他引:1
Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage. 相似文献
6.
C F Roberts 《Biochimica et biophysica acta》1970,201(2):267-283
7.
8.
Paul D. Bonnitcha Simon R. Bayly Mark B.M. Theobald Helen M. Betts Jason S. Lewis Jonathan R. Dilworth 《Journal of inorganic biochemistry》2010,104(2):126-9888
Combination agents comprising two different pharmacophores with the same biological target have the potential to show additive or synergistic activity. Bis(thiosemicarbazonato)copper(II) complexes (e.g. 64Cu-ATSM) and nitroimidazoles (e.g. 18F-MISO) are classes of tracer used for the delineation of tumor hypoxia by positron emission tomography (PET). Three nitroimidazole-bis(thiosemicarbazonato)copper(II) conjugates were produced in order to investigate their potential as combination hypoxia imaging agents. Two were derived from the known bifunctional bis(thiosemicarbazone) H2ATSM/A and the third from the new precursor diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-ethylamino-3-thiosemicarbazone) - H2ATSM/en. Oxygen-dependent uptake studies were performed using the 64Cu radiolabelled complexes in EMT6 carcinoma cells. All the complexes displayed appreciable hypoxia selectivity, with the nitroimidazole conjugates displaying greater selectivity than a simple propyl derivative used as a control. Participation of the nitroimidazole group in the trapping mechanism is indicated by the increased hypoxic uptake of the 2- vs. the 4-substituted 64Cu-ATSM/A derivatives. The 2-nitroimidazole derivative of 64Cu-ATSM/en demonstrated superior hypoxia selectivity to 64Cu-ATSM over the range of oxygen concentrations tested. Biodistribution of the radiolabelled 2-nitroimidazole conjugates was carried out in EMT6 tumor-bearing mice. The complexes showed significantly different uptake trends in comparison to each other and previously studied Cu-ATSM derivatives. Uptake of the Cu-ATSM/en conjugate in non-target organs was considerably lower than for derivatives based on Cu-ATSM/A. 相似文献
9.
10.