全文获取类型
收费全文 | 5745篇 |
免费 | 265篇 |
国内免费 | 14篇 |
出版年
2024年 | 14篇 |
2023年 | 55篇 |
2022年 | 159篇 |
2021年 | 323篇 |
2020年 | 149篇 |
2019年 | 198篇 |
2018年 | 217篇 |
2017年 | 175篇 |
2016年 | 237篇 |
2015年 | 291篇 |
2014年 | 332篇 |
2013年 | 457篇 |
2012年 | 413篇 |
2011年 | 426篇 |
2010年 | 234篇 |
2009年 | 204篇 |
2008年 | 221篇 |
2007年 | 216篇 |
2006年 | 199篇 |
2005年 | 166篇 |
2004年 | 134篇 |
2003年 | 100篇 |
2002年 | 106篇 |
2001年 | 74篇 |
2000年 | 73篇 |
1999年 | 58篇 |
1998年 | 30篇 |
1997年 | 19篇 |
1996年 | 20篇 |
1995年 | 21篇 |
1993年 | 18篇 |
1992年 | 39篇 |
1991年 | 53篇 |
1990年 | 50篇 |
1989年 | 36篇 |
1988年 | 40篇 |
1987年 | 48篇 |
1986年 | 34篇 |
1985年 | 42篇 |
1984年 | 34篇 |
1983年 | 23篇 |
1982年 | 19篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 26篇 |
1978年 | 28篇 |
1977年 | 14篇 |
1975年 | 14篇 |
1974年 | 19篇 |
1973年 | 21篇 |
排序方式: 共有6024条查询结果,搜索用时 31 毫秒
961.
Seyed Jafar Mousavy Gholam Hossein Riazi Mahmood Kamarei Hadi Aliakbarian Naghmeh Sattarahmady Ahmad Sharifizadeh Shahrokh Safarian Faizan Ahmad Ali Akbar Moosavi–Movahedi 《International journal of biological macromolecules》2009,44(3):278-285
Widespread use of mobile phones has increased the human exposure to electromagnetic fields (EMFs). It is required to investigate the effect of EMFs on the biological systems. In this paper the effect of mobile phone RF (910 MHz and 940 MHz) on structure and function of HbA was investigated. Oxygen affinity was measured by sodium dithionite with UV–vis spectrophotometer. Structural changes were studied by circular dichroism and fluorescence spectroscopy. The results indicated that mobile phone EMFs altered oxygen affinity and tertiary structure of HbA. Furthermore, the decrease of oxygen affinity of HbA corresponded to the EMFs intensity and time of exposure. 相似文献
962.
Khalid Mahmood Zia Mehdi Barikani Ijaz Ahmad Bhatti Mohammad Zuber Mohammad Barmar 《Carbohydrate polymers》2009,77(1):54-58
Chitin based polyurethane (PU) elastomers constituted on 4,4´-diphenylmethane diisocyanate (MDI), poly(ε-caprolactone) (PCL) and extended with blends of chitin/1,4-butane diol were synthesized via two step polymerization technique. The synthesized samples were irradiated for 50, 100 and 200 h in an UV exposure chamber as such the spectral distribution of the light is good match for terrestrial solar radiation. The crystalline behavior of the irradiated PU samples were investigated by X-ray diffraction (XRD), differential scanning calorimetery (DSC) and dynamic mechanical thermal analysis (DMTA) techniques. The effect of irradiation time and chitin contents on crystallinity were studied and investigated. The maximum decrease in the crystalline behavior of samples after irradiation observed by XRD, DSC and tan δ peaks were found for the PU samples extended with lower contents of chitin (chitin/BDO; 0/100). In comparison with irradiation times the 200 h irradiation showed maximum change in the crystalline behavior. 相似文献
963.
Friederike Khn Ahmad Reza Sharifi Henner Simianer 《Applied animal behaviour science》2009,120(1-2):68-75
Goettingen minipigs are laboratory animals with an increasing demand over the last few years. At the moment, Goettingen minipigs are not selected for a low reactivity to humans and this trait is not included in the breeding programme. However, it is obvious that there is a need for genetically non-responding minipigs during handling to facilitate the treatment and restraint of the animals which is often needed in biomedical experiments. A first testing scheme was developed to evaluate the reactivity of Goettingen minipigs to humans and to analyse whether the trait reactivity to humans can be considered in the breeding programme. In this study temperament scores of this testing scheme for nine different traits from 10,033 animals collected from 2005 to 2008 were analysed. Temperament was subjectively scored on a scale from 1 to 5 while the pig is caught (C), held on the arms (A), standing in a box for weighing (W), standing on a table (T) and walking on the ground (G). The traits were a combination of these situations evaluated at three different ages (2, 4 and 6 months). Genetic parameters were estimated using bivariate models and different possible selection strategies were examined. Heritabilities were low to moderate with a range from 0.09 to 0.22 and phenotypic and genetic correlations between the nine traits were moderate to high with phenotypic correlations between 0.12 (W2 and G4) and 0.64 (W2 and A2) and genetic correlations between 0.44 (A4 and C6) and 1.00 (e.g. W2 and A4). It was shown that the highest genetic progress per year can be obtained when all nine traits are considered in the selection index. Under an economical point of view the selection on the basis of the two arm traits plus the trait W2 should be preferred.Based on a critical discussion of the explanatory power of the used scoring system a new evaluation scheme was developed. In this scheme the minipigs can be divided into responding and non-responding animals whereas the latter are desired for selection. The suggested scoring system offers better possibilities for statistical analyses. It is planned to include the selection for non-responding Goettingen minipigs in the routine breeding programme. 相似文献
964.
Plant cells are constantly exposed to environmental agents and endogenous processes that inflict damage to DNA and cause genotoxic stress, which can reduce plant genome stability, growth and productivity. Plants are most affected by solar UV-B radiation, which damage the DNA by inducing the formation of two main UV photoproducts such as cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs). Reactive oxygen species (ROS) are also generated extra- or intra-cellularly, which constitute yet another source of genotoxic stress. As a result of this stress, the cellular DNA-damage responses (DDR) are activated, which transiently arrest the cell cycle and allow cells to repair DNA before proceeding into mitosis. DDR requires the activation of Ataxia telangiectasia-mutated (ATM) and Rad3-related (ATR) genes, which regulate the cell cycle and transmit the damage signals to downstream effectors of cell-cycle progression. Since genomic protection and stability are fundamental to ensure and sustain plant diversity and productivity, therefore, repair of DNA damages is essential. In plants the bulky DNA lesions, CPDs and 6-4PPs, are repaired by a simple and error-free mechanism: photoreactivation, which is a light-dependent mechanism and requires CPD or 6-4PP specific photolyases. In addition to this direct repair process, the plants also have sophisticated light-independent general repair mechanisms, such as the nucleotide excision repair (NER) and base excision repair (BER). The completed plant genome sequences reveal that most of the genes involved in NER and BER are present in higher plants, which suggests that the network of in-built DNA-damage repair mechanisms is conserved. This article describes the insight underlying the DNA damage and repair pathways in plants. The comet assay to measure the DNA damage and the role of DNA repair helicases such as XPD and XPB are also covered. 相似文献
965.
Moradi Ahmad B. Conesa Héctor M. Robinson Brett Lehmann Eberhard Kuehne Guido Kaestner Anders Oswald Sascha Schulin Rainer 《Plant and Soil》2009,320(1-2):243-254
Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and 13C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched 13C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests. 相似文献
966.
Divsalar A Saboury AA Mansoori-Torshizi H Moghaddam MI Ahmad F Hakimelahi GH 《Journal of biomolecular structure & dynamics》2009,26(5):587-597
Abstract An new water-soluble Pd(II) complex, 2,2'-bipyridin n-butyl dithiocarbamato Pd(II) nitrate has been synthesized. The Pd(II) complex has been characterized by elemental analysis and conductivity measurements as well as spectroscopic methods such as infrared, 1H NMR, and ultraviolet-visible. The interaction between this new design Pd(II)-complex, an anti-tumor component, with carrier proteins of β-lactoglobulin-A and -B (BLG-A and -B) were studied at different temperatures of 27, 37, 42, and 47 °C by fluorescence spectroscopy and far-UV circular dichroism (CD) spectrophotometric techniques. A strong fluorescence quenching interaction of Pd(II) complex with BLG-A and -B was observed at different temperatures. The binding parameters were evaluated by fluorescence quenching method. The thermodynamic parameters, including ΔH°, ΔS°, and ΔG° were calculated by fluorescence quenching method indicated that the electrostatic and hydrophobic forces might play a major role in the interactions of Pd(II) complex with BLG-A and -B, respectively. The distances between donors (Trps of the BLG-A and -B) and acceptor (Pd(II) complex) were obtained according to the fluorescence resonance energy transfer (FRET). Far-UV CD studies showed that the Pd(II) complex did not represent any significant changes in the secondary structures of BLG- A and -B. The difference in the interaction properties observed for BLG-A and -B with Pd(II) complex is related to the difference in the amino acid sequences between these two variants. 相似文献
967.
Nagendra Singh Ramasamy Prem Kumar Sanjit Kumar Sujata Sharma Rafia Mir Punit Kaur Alagiri Srinivasan Tej P. Singh 《Journal of molecular recognition : JMR》2009,22(6):437-445
A novel ligand‐binding site with functional implications has been identified in phospholipase A2 (PLA2). The binding of non‐steroidal anti‐inflammatory agent indomethacin at this site blocks both catalytic and anti‐coagulant actions of PLA2. A group IIA PLA2 has been isolated from Daboia russelli pulchella (Russell's viper) which is enzymatically active as well as induces a strong anti‐coagulant action. The binding studies have shown that indomethacin reduces the effects of both anti‐coagulant and pro‐inflammatory actions of PLA2. A group IIA PLA2 was co‐crystallized with indomethacin and the structure of the complex has been determined at 1.4 Å resolution. The structure determination has revealed the presence of an indomethacin molecule in the structure of PLA2 at a site which is distinct from the conventional substrate‐binding site. One of the carboxylic group oxygen atoms of indomethacin interacts with Asp 49 and His 48 through the catalytically important water molecule OW 18 while the second carboxylic oxygen atom forms an ionic interaction with the side chain of Lys 69. It is well known that the residues, His 48 and Asp 49 are essential for catalysis while Lys 69 is a part of the anti‐coagulant loop (residues, 54–77). Indomethacin binds in such a manner that it blocks the access to both, it works as a dual inhibitor for catalytic and anti‐coagulant actions of PLA2. This new binding site in PLA2 has been observed for the first time and indomethacin is the first compound that has been shown to bind at this novel site resulting in the prevention of anti‐coagulation and inflammation. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
968.
969.
This study investigated the adsorption potential of oil palm shell-based activated carbon to remove 2,4,6-trichlorophenol from aqueous solution using fixed-bed adsorption column. The effects of 2,4,6-trichlorophenol inlet concentration, feed flow rate and activated carbon bed height on the breakthrough characteristics of the adsorption system were determined. The regeneration efficiency of the oil palm shell-based activated carbon was evaluated using ethanol desorption technique. Through ethanol desorption, 96.25% of the adsorption sites could be recovered from the regenerated activated carbon. 相似文献
970.
Ginny D. Ho John Anthes Ana Bercovici John P. Caldwell Kuo-Chi Cheng Xiaoming Cui Ahmad Fawzi Xiomara Fernandez William J. Greenlee John Hey Walter Korfmacher Sherry X. Lu Robbie L. McLeod Fay Ng April Smith Torhan Zheng Tan Deen Tulshian Geoffrey B. Varty Xiaoying Xu Hongtao Zhang 《Bioorganic & medicinal chemistry letters》2009,19(9):2519-2523
The discovery of 1 as a high-affinity ligand for the nociceptin receptor has led to the synthesis of a series of tropane (8-methyl-8-azabicyclo[3.2.1]octane) derivatives as optimized ligands. These compounds exhibit high affinity for the nociceptin receptor, moderate to excellent selectivity over the opioid μ receptor, and behave as full agonists. In this Letter, we present the synthesis and highlight the structure–activity relationship of tropane derivatives culminating in the identification of 24 and 32 as potent and orally active antitussive and anxiolytic agents. The in vitro and in vivo activities, pharmacokinetic profile, and the hPXR activity, which predicts the potential 3A4 induction in human, are disclosed. 相似文献