全文获取类型
收费全文 | 1977篇 |
免费 | 82篇 |
国内免费 | 37篇 |
专业分类
2096篇 |
出版年
2024年 | 6篇 |
2023年 | 34篇 |
2022年 | 26篇 |
2021年 | 44篇 |
2020年 | 51篇 |
2019年 | 86篇 |
2018年 | 68篇 |
2017年 | 76篇 |
2016年 | 77篇 |
2015年 | 43篇 |
2014年 | 90篇 |
2013年 | 153篇 |
2012年 | 47篇 |
2011年 | 116篇 |
2010年 | 53篇 |
2009年 | 98篇 |
2008年 | 99篇 |
2007年 | 114篇 |
2006年 | 88篇 |
2005年 | 78篇 |
2004年 | 69篇 |
2003年 | 47篇 |
2002年 | 42篇 |
2001年 | 44篇 |
2000年 | 30篇 |
1999年 | 31篇 |
1998年 | 41篇 |
1997年 | 28篇 |
1996年 | 24篇 |
1995年 | 33篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 18篇 |
1990年 | 27篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1974年 | 3篇 |
1971年 | 1篇 |
排序方式: 共有2096条查询结果,搜索用时 15 毫秒
1.
J.G. Roddick 《Phytochemistry》1974,13(1):9-25
The literature relating to chemical, biochemical and biological aspects of the steroidal glycoalkaloid, α-tomatine, is reviewed. The alkaloid, which can be used as a starting compound for the synthesis of steroidal hormones, is toxic to a wide range of living organisms. The significance of tomatine to plants which elaborate it is discussed and some possible uses of the compound are mentioned. 相似文献
2.
Haems are the cofactors of cytochromes and important catalysts of biological electron transfer. They are composed of a planar porphyrin structure with iron coordinated at the centre. It is known from spectroscopy that ferric low-spin haem has one unpaired electron at the iron, and that this spin is paired as the haem receives an electron upon reduction (I. Bertini, C. Luchinat, NMR of Paramagnetic Molecules in Biological Systems, Benjamin/Cummins Publ. Co., Menlo Park, CA, 1986, pp. 165-170; H.M. Goff, in: A.B.P. Lever, H.B. Gray (Eds.), Iron Porphyrins, Part I, Addison-Wesley Publ. Co., Reading, MA, 1983, pp. 237-281; G. Palmer, in: A.B.P. Lever, H.B. Gray (Eds.), Iron Porphyrins, Part II, Addison-Wesley Publ. Co., Reading, MA, 1983, pp. 43-88). Here we show by quantum chemical calculations on a haem a model that upon reduction the spin pairing at the iron is accompanied by effective delocalisation of electrons from the iron towards the periphery of the porphyrin ring, including its substituents. The change of charge of the iron atom is only approx. 0.1 electrons, despite the unit difference in formal oxidation state. Extensive charge delocalisation on reduction is important in order for the haem to be accommodated in the low dielectric of a protein, and may have impact on the distance dependence of the rates of electron transfer. The lost individuality of the electron added to the haem on reduction is another example of the importance of quantum mechanical effects in biological systems. 相似文献
3.
《Bioorganic & medicinal chemistry》2016,24(6):1209-1215
Metal-free click chemistry has become an important tool for pretargeted approaches in the molecular imaging field. The application of bioorthogonal click chemistry between a pretargeted trans-cyclooctene (TCO) derivatized monoclonal antibody (mAb) and a 99mTc-modified 1,2,4,5-tetrazine for tumor imaging was examined in vitro and in vivo. The HYNIC tetrazine compound was synthesized and structurally characterized, confirming its identity. Radiolabeling studies demonstrated that the HYNIC tetrazine was labeled with 99mTc at an efficiency of >95% and was radiochemically stable. 99mTc–HYNIC tetrazine reacted with the TCO–CC49 mAb in vitro demonstrating its selective reactivity. In vivo biodistribution studies revealed non-specific liver and GI uptake due to the hydrophobic property of the compound, however pretargeted SPECT imaging studies demonstrated tumor visualization confirming the success of the cycloaddition reaction in vivo. These results demonstrated the potential of 99mTc–HYNIC–tetrazine for tumor imaging with pretargeted mAbs. 相似文献
4.
Big Moose Basin: simulation of response to acidic deposition 总被引:2,自引:1,他引:1
George F. Davis John J. Whipple Steven A. Gherini Carl W. Chen Robert A. Goldstein Arland H. Johannes Peter W.H Chan Ronald K. Munson 《Biogeochemistry》1987,3(1-3):141-161
The ILWAS model has been enhanced for application to multiple-lake hydrologic basins. This version of the model has been applied to the Big Moose basin, which includes Big Moose Lake and its tributary streams, lakes, and watersheds. The basin, as defined, includes an area of 96 km2, with over 20 lakes and ponds, and 70 km of streams. Hydrologic and chemical calibrations have been made using data from seven sampling stations. When total atmospheric sulfur loading to the basin is halved, the model predicts, after four years of simulation, a decreasing sulfate concentration and to a lesser extent a rising alkalinity at Big Moose Lake outlet. At the end of four years, the results show an increase in pH of 0.1 to 0.5 pH units depending upon season. 相似文献
5.
Arthur L. Weber 《Journal of molecular evolution》1987,25(1):7-11
Summary Reaction ofDl-serine and adenosine-5-phosphorimidazolide in the presence of adenosine-5-(O-methylphosphate) and imidazole resulted in the stereoselective synthesis of the aminoacyl nucleotide ester 2(3)-O-seryl-adenosine-5-(O-methylphosphate). The enantiomeric excess ofd-serine incorporated into 2(3)-O-seryl-adenosine-5-(O-methylphosphate) was about 9%. Adenylyl-(5N)-serine and an unknown product also incorporated an excess ofd-serine; however, serylserine showed an excess ofl-serine. The relationship of these results to the origin of the biological pairing ofl-amino acids and nucleotides containingd-ribose is discussed. 相似文献
6.
Hematological and serum chemistry values, as well as rectal temperatures, were obtained from greater galagos (Galago garnettii and G. crassicaudatus), in order to establish normative values. No species or sex differences were found for four hematological parameters and 15 serum chemistry parameters. Species differences were seen in phosphate, magnesium, cholesterol, alkaline phosphate, G-glutamyl transferase, mean corpuscular volume and leucocyte, neutrophil, and lymphocyte number. Significant sex differences were observed in glucose, hemoglobin, and hematocrit values. Species and sex differences were seen in chloride and erythrocyte number. 相似文献
7.
Dissolved nutrient inputs in bulk precipitation and outputs in streamwater were measured during 3 years of contrasting hydrological conditions in a 6.3-ha, grazed heathland watershed on schists in the Montseny mountains (NE Spain), drained by an intermittent stream. On average, 39% of the precipitation became streamflow. Bulk precipitation delivered positive net alkalinity (mean 0.22 keq/ha/yr), sulphate input was moderate (9.0 kg SO4-S/ha/yr), and the mean input of inorganic N was not exceptionally high (6.6 kg/ha/yr). Ion concentrations were relatively low in streamwater; SO4
2- was the dominant anion. Most concentrations in streamwater varied seasonally, with maxima in late summer or early autumn and minima in spring. This pattern probably resulted from increased availability of ions for leaching due to decomposition of organic matter and chemical weathering during the warm period. Nitrate concentrations were relatively high in winter and dropped sharply in early spring, probably because of biological uptake. Annual element outputs in streamwater varied between years and seemed to be controlled by both the amount of annual streamflow and its seasonal distribution. Annual inputs exceeded outputs for dissolved inorganic N. The watershed accumulated H+ and Ca2+, had net losses of Na+ and Mg2+, and was close to steady state for K+, SO4
2-, Cl- and alkalinity. The chloride budgets gave no evidence of substantial dry deposition in this system. The cationic denudation rate was negative (-0.14 keq/ha/yr) because Ca2+ retention was higher than net exports of Na+ and Mg2+ from silicate weathering. Low nutrient export and little production of alkalinity suggest that this watershed has a low buffering capacity. 相似文献
8.
Joseph A. N. Zasadzinski 《生物化学与生物物理学报:生物膜》1988,946(2)
Mixtures of sn-1 (
) and sn-3 (
) enantiomers of fully hydrated dipalmitoylphosphatidylcholine (DPPC) were studied with differential scanning calorimetry and freeze-fracture microscopy. The pretransition temperature of racemic mixtures of DPPC was 1.8 C° below that of either pure sn-1 or sn-3 enantiomers, which had similar pretransition temperatures. The main transition temperature of racemic mixtures was also depressed, but to a lesser extent, 0.8 C°. Freeze-fracture images of liposomes of sn-1, sn-3, and racemic mixtures of DPPC frozen from the Pβ′ phase showed well-defined ripples of wavelength 13 nm. Lipid stereoconfiguration had no effect on ripple wavelength, configuration or amplitude, or on the number and nature of surface defects. 相似文献
9.
A survey of oligotrophic Lake Rara, the biggest lake in Nepal, was carried out from 1982 till 1984. Mean depth is 100 m, and
maximum depth is 167 m. The surface area covers 9.8 km2, and the lake contains 0.98 km3 volume of water.
Transparency was about 16 m, photoquantum yield decreased exponentially with depth below 5 m, and the extinction coefficient
was 8.3 × 10−2. The concentration of Chl.-a was in the range of 0.06–0.46 mg m−3, and total nitrogen was 18–30 μg 1−1. The whole water column was well oxygenated. Primary productivity was extremely low. It has more than 30 inflowing brooks
and one outlet. The water quality of the brooks changes drastically with their location. The pH, electrical conductivity,
and EDTA hardness in the waters from a landslide area were high. In the waters from a rich pine forest they were extremely
low.
The zooplankton consisted of two species of protozoa, five species of rotifers, two species of Cladocera, and two species
of Copepoda. The zooplankton density range was 6200–16200 individuals m−3. The minimum was on November 11th, 1983 and the maximum on August 19th, 1983. 相似文献
10.
M. A. Khan 《Hydrobiologia》1986,135(3):233-242
L. Naranbagh (alt. 1587 m) is a polymictic, shallow marl lake in the flood-plain valley of Kashmir, India. Macrofloral affinities resemble Potamogeton Type of Forsberg (1965) with alkaline waters, not rich in phosphorus. CaCO3 precipitation coupled with decline in Ca2+ and alkalinity values are characteristic of the lake. Fluctuations in Mg2+, Na+, K+, and Cl– were relatively conservative. The levels of PO
inf4
sup3–
-P and NO
inf3
sup–
-N indicate moderate fertility of the lake water.Persistence of a summer-autumn planktonic algal pulse is related to favourable irradiance, high water temperatures, and increased photosynthetic efficiency values. The most striking seasonality in photosynthetic rates (m–2 h–1) between winter minimum (3 mg Cassim) and summer maximum (75.4 mg Cassim) is determined by mainly climatic changes. Energy flow gave annual phytoplankton production of 51.95 × 102 KJ m–2 for the ecosystem.The nutrient levels and productivity rates suggest mesotrophic status of L. Naranbagh in classic oligoeutrophic classification of lake types. 相似文献