全文获取类型
收费全文 | 4186篇 |
免费 | 333篇 |
国内免费 | 182篇 |
出版年
2024年 | 11篇 |
2023年 | 86篇 |
2022年 | 86篇 |
2021年 | 129篇 |
2020年 | 140篇 |
2019年 | 204篇 |
2018年 | 162篇 |
2017年 | 163篇 |
2016年 | 139篇 |
2015年 | 146篇 |
2014年 | 246篇 |
2013年 | 413篇 |
2012年 | 202篇 |
2011年 | 170篇 |
2010年 | 158篇 |
2009年 | 195篇 |
2008年 | 189篇 |
2007年 | 190篇 |
2006年 | 177篇 |
2005年 | 159篇 |
2004年 | 133篇 |
2003年 | 107篇 |
2002年 | 121篇 |
2001年 | 86篇 |
2000年 | 76篇 |
1999年 | 82篇 |
1998年 | 66篇 |
1997年 | 66篇 |
1996年 | 61篇 |
1995年 | 55篇 |
1994年 | 37篇 |
1993年 | 56篇 |
1992年 | 35篇 |
1991年 | 36篇 |
1990年 | 30篇 |
1989年 | 24篇 |
1988年 | 20篇 |
1987年 | 29篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 20篇 |
1982年 | 29篇 |
1981年 | 16篇 |
1980年 | 21篇 |
1979年 | 18篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1975年 | 6篇 |
排序方式: 共有4701条查询结果,搜索用时 227 毫秒
1.
Filippo Genovese Stefania Ferrari Giambattista Guaitoli Monica Caselli M. Paola Costi Glauco Ponterini 《Protein science : a publication of the Protein Society》2010,19(5):1023-1030
An ad hoc bioconjugation/fluorescence resonance energy transfer (FRET) assay has been designed to spectroscopically monitor the quaternary state of human thymidylate synthase dimeric protein. The approach enables the chemoselective engineering of allosteric residues while preserving the native protein functions through reversible masking of residues within the catalytic site, and is therefore suitable for activity/oligomerization dual assay screenings. It is applied to tag the two subunits of human thymidylate synthase at cysteines 43 and 43′ with an excitation energy donor/acceptor pair. The dimer–monomer equilibrium of the enzyme is then characterized through steady‐state fluorescence determination of the intersubunit resonance energy transfer efficiency. 相似文献
2.
Malav A. Kapadia 《Inorganica chimica acta》2009,362(9):3292-3298
Coordination polymers of HEAP-ED with La(III), Pr(III), Nd(III), Sm(III), Gd(III), Tb(III) and Dy(III) metal ions have been synthesized and characterized by elemental analyses, electronic spectra, magnetic susceptibilities, FTIR, NMR, scanning electronic microscopy (SEM) and thermogravimetric analyses. Catalytic activity of selected coordination polymers was examined for pharmaceutical important organic synthesis. Antimicrobial activity of isolated Ln(III) coordination polymers against Escherichia coli, Bacillus subtilis, Staphylococcus aureus (bacteria) and Saccharomyces cerevisiae (yeast) were measured. It was observed from the study that the Ln(III) coordination polymers acted as an efficient and effective catalysts and antimicrobial agents. 相似文献
3.
《Bioorganic & medicinal chemistry》2016,24(8):1778-1785
As part of a selection strategy for artificial nucleic acids (XNA) (to be considered as potential new information systems in vivo), we have carried out a modelling study on cyclohexanyl nucleic acids (CNA) duplexes and hairpins. CNA may form a duplex as well as hairpin structures, having the carbocyclic nucleosides in the 4C1 conformation (with equatorial basis). The geometry of ds CNA is close to that of a HNA:RNA duplex. We demonstrated that CNA triphosphates function as a substrate for polymerases. Modelling experiments indicate that the monomers are probably presented to the polymerase in the 1C4 conformation. 相似文献
4.
《Anthrozo?s》2013,26(2):150-165
AbstractThis paper describes a field study on the effects of attachment styles on the subjective well-being of institutionalized older persons after participating in a pet visitation program. We compared how attachment style (i.e., secure, anxious ambivalent, dismissive avoidant, and fearful avoidant) moderated the impact of a dog visitation program in 52 elderly residents of an assisted-living facility. Controlling for pre-visitation mood, the results revealed that the secure style related to increases in positive mood after interacting with the dogs. The anxious ambivalent style related to increases in positive mood and decreases in ratings of depression. Most importantly, the fearful avoidant style related to increases in depression after interacting with the dogs. The results highlight the notion that not all participants benefit from taking part in pet visitation programs. Theoretically, this study presents preliminary evidence for the relevance of attachment styles in understanding human–pet interactions. 相似文献
5.
Aim Species capable of vigorous growth under a wide range of environmental conditions should have a higher chance of becoming invasive after introduction into new regions. High performance across environments can be achieved either by constitutively expressed traits that allow for high resource uptake under different environmental conditions or by adaptive plasticity of traits. Here we test whether invasive and non‐invasive species differ in presumably adaptive plasticity. Location Europe (for native species); the rest of the world and North America in particular (for alien species). Methods We selected 14 congeneric pairs of European herbaceous species that have all been introduced elsewhere. One species of each pair is highly invasive elsewhere in the world, particularly so in North America, whereas the other species has not become invasive or has spread only to a limited degree. We grew native plant material of the 28 species under shaded and non‐shaded conditions in a common garden experiment, and measured biomass production and morphological traits that are frequently related to shade tolerance and avoidance. Results Invasive species had higher shoot–root ratios, tended to have longer leaf‐blades, and produced more biomass than congeneric non‐invasive species both under shaded and non‐shaded conditions. Plants responded to shading by increasing shoot–root ratios and specific leaf area. Surprisingly, these shade‐induced responses, which are widely considered to be adaptive, did not differ between invasive and non‐invasive species. Main conclusions We conclude that high biomass production across different light environments pre‐adapts species to become invasive, and that this is not mediated by plasticities of the morphological traits that we measured. 相似文献
6.
The interaction of jatrophone with sRNA from Escherichia coli has been investigated through UV, CD, and 1H NMR measurements. The results obtained show that the interaction with jatrophone increases the stability of the polynucleotide. It appears that the optical properties of jatrophone depend upon the jatrophone/nucleotide ratio. The observed behaviour can only be explained by the existence of different types of interaction between jatrophone and sRNA. Even for a jatrophone/nucleotide ratio as low as 0.17 the 1H NMR spectra show a multiplicity of resonances that can only be explained by the simultaneous existence of two different binding modes involving the jatrophone molecules. 相似文献
7.
The effects of repeated annual application of methiocarb-based slug pellets, broadcast on the soil surface and drilled into the seed bed, on carabid beetle activity were investigated over a four year period on a winter-sown cereal field using pitfall traps in barriered plots. Following applications in late autumn all winter-active carabid populations were severely depressed; total carabid activity falling to less than 5% and 10–15% following broadcast and drilled applications, respectively, compared with untreated plots. Spring and summer-active species, not active at the time of application, were largely unaffected by applications and were responsible for a gradual recovery of total activity from early spring onwards. Activity of all affected winter species remained demonstrably depressed on treated areas for the remainder of their seasonal incidence. However, all except one species, Bembidion obtusum, recovered to normal activity levels in the following season prior to reapplication. Recovery patterns are discussed in terms of the known biology of the species involved. Evidence that a minority of summer-active species were also affected by treatments, sometimes positively and sometimes negatively, were attributed to indirect effects possibly involving prey availability and foraging behaviour. The long-term ecological and short-term agronomic implications of methiocarb effects on carabid populations in winter-sown cereals are discussed. 相似文献
8.
P. Usobiaga J. J. Calvete J. L. Saíz M. T. Eirín J. González-Rodríguez 《European biophysics journal : EBJ》1987,14(4):211-218
Sedimentation equilibrium and low-angle laser-light scattering were used to determine the molar mass of the glycoprotein moieties in the complexes of sodium dodecyl sulphate with the human platelet membrane glycoproteins IIb (GPIIb), IIIa (GPIIIa), and the (GPIIb) and (GPIIb) subunits of GPIIb. The values obtained by both procedures, except those for GPIIb, agree within experimental error with those calculated from their chemical composition: GPIIb (114,000 g mol-1), GPIIb (22,200 g mol-1), and GPIIIa (91,500 g mol-1). The molar mass of GPIIb determined by light scattering (142,000 g mol-1) and sedimentation equilibrium at different solvent densities (134,000 g mol-1) also agree, within experimental error, with the values calculated either from its chemical composition (136,500 g mol-1) or from the sum of the molar masses of its subunits. However the molar mass determined by sedimentation equilibrium at constant solvent density, is consistently underestimated (116,000 g mol-1).High-performance size-exclusion chromatography in sodium dodecyl sulphate solutions overestimates the molar mass of these glycoproteins and their Stokes radii, and therefore the maximal frictional ratios derived from them.Abbreviations GPIIb
glycoprotein IIb
- GPIIIa
glycoprotein IIIa
- GPIIb and GPIIb
and subunits of GPIIb, respectively
- CM-GPIIb
CM-GPIIb, and CM-GPIIIa, totally reduced and carboxymethylated forms of GPIIb, GPIIb, and GPIIIa, respectively
- SDS
sodium dodecyl sulphate
- eosin-ITC
eosin-5-isothiocyanate 相似文献
9.
Summary The effect of artificial acid rain on the reproduction and survival of the aphid Euceraphis betulae on silver birch was studied in Turku, southern Finland. Eight bioassays were done during 1984–1986. In four of the bioassays the aphids produced 40% to over 100% more progeny on birches watered with dilute sulphuric acid (pH 3.5) than on control trees. In four other cases the performance of aphids did not differ between the treatments. An index of aphid reproduction pooled over the whole study was significantly higher on acid-treated than on control birches. The reproduction of aphids on acid-treated birches was enhanced when precipitation was below long term average, suggesting an interaction between the stress caused by acid treatment and dry periods. 相似文献
10.
G. R. Findenegg 《Plant and Soil》1987,103(2):239-243
Seedlings of 14 species were grown for 14–28 days on nutrient solution with 6 mmol.l−1 NH4 as the sole nitrogen source. Solutions acidity was were kept constant at pH 4.0, 5.0, 6.0 and 7.0 by continuous titration
with diluted KOH. The following species were used: barley, maize, oats sorghum, yellow and white lupin, pea, soybean, carrot,
flax, castor-oil, spinach, sugarbeet and sunflower. Most plant species grew optimally at pH 6.0 with slight reductions at
pH 5.0. Growth of many species was severely inhibited at pH 4.0, but this inhibition was not observed with the legume and
cereal species. Yield depressions at pH 4.0 relative to pH 6.0 were well correlated with the respective relative decreases
of the K concentration in their roots (P<0.002). In the roots of two species (sunflower and flax) total N concentrations were also strongly reduced at pH 4.0. apparently,
the interactions between uptake of K, NH4 and H ions become the prevalent problem at suboptimal pH. At pH 7.0, yields were also considerably decreased, with the exception
of the lupines. At this pH, the roots of the growth inhibited plants were characterized by increased levels of total N and
free NH4. It is thought that the binding capacity of the roots for NH4 is an important factor in preventing NH4/NH3 toxicity at supraoptimal pH. 相似文献