全文获取类型
收费全文 | 1585篇 |
免费 | 147篇 |
国内免费 | 51篇 |
出版年
2023年 | 10篇 |
2022年 | 13篇 |
2021年 | 13篇 |
2020年 | 21篇 |
2019年 | 42篇 |
2018年 | 48篇 |
2017年 | 41篇 |
2016年 | 31篇 |
2015年 | 51篇 |
2014年 | 74篇 |
2013年 | 93篇 |
2012年 | 56篇 |
2011年 | 91篇 |
2010年 | 61篇 |
2009年 | 63篇 |
2008年 | 63篇 |
2007年 | 86篇 |
2006年 | 65篇 |
2005年 | 88篇 |
2004年 | 76篇 |
2003年 | 51篇 |
2002年 | 54篇 |
2001年 | 43篇 |
2000年 | 35篇 |
1999年 | 39篇 |
1998年 | 42篇 |
1997年 | 43篇 |
1996年 | 29篇 |
1995年 | 33篇 |
1994年 | 23篇 |
1993年 | 29篇 |
1992年 | 20篇 |
1991年 | 16篇 |
1990年 | 24篇 |
1989年 | 18篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 19篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 9篇 |
1979年 | 17篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1974年 | 7篇 |
1973年 | 7篇 |
排序方式: 共有1783条查询结果,搜索用时 328 毫秒
971.
The freezing effect on peptide coupling in organic solutions of different polarity has been investigated and compared with the results obtained in liquid phase. The model reaction of DCC-activated coupling of Boc-Ala-Phe-OH with H-Ala-OBut has been carried out in dioxane, dimethylsulfoxide and formamide, as well as in mixtures (90%/10%, v/v) of dioxane with acetonitrile, dimethylformamide, dimethylsulfoxide and formamide. The reactions have been traced and evaluated by RP-HPLC analysis. Freezing the reaction mixture resulted in all cases in a significant suppression of the N-dipeptidylurea side-product formation together with a slight decrease of tripeptide epimerization. The coupling yields and the side effects depended on the solvent, with the dioxane and dioxane/acetonitrile mixture produced the best results. The role of freezing and solvent in the improved results is discussed. © 1998 European Peptide Society and John Wiley & Sons, Ltd. 相似文献
972.
Himadri B. Bohidar 《Biopolymers》1998,45(1):1-8
Aggregation behavior and hydrodynamic parameters of insulin have been determined from static and dynamic light scattering experiments and intrinsic viscosity measurements carried out at pH 4.0, 7.5, and 9.0 in the temperature range 20–40°C in aqueous solutions. The protein aggregated extensively at elevated temperatures in the acidic solutions. Intermolecular interactions were found to be attractive and to increase with temperature. The measured intrinsic viscosity [η], diffusion coefficient D0, molecular weight M, and radius of gyration Rg exhibited the universal behavior: M[η] = (2.4 ± 02) × 10−27 (Re,η/Re,D)3(D0η/T)−3 and (D0√n)−1 ≃ (√6 πη0ζβ/kBT) [1 + 0.201)(v/β3)√n], where n is the number of segments in the polypeptide. The effective hydrodynamic radii deduced from [η], (Re, η) and the same deduced from D0, (Re,D) showed a constant ratio, (Re,η/Re,D = 1.1 ± 0.1). Re,D/Rg = ξ was found to be (0.76 ± 0.07). From the known solvent viscosity η0, the segment length β was deduced to be (10 ± 1) Å. The excluded volume was deduced to be (5 Å)3 regardless of pH. The Flory-Huggins interaction parameter was found to be χ = 0.45 ± 0.04, independent of pH and temperature. © 1998 John Wiley & Sons, Inc. Biopoly 45: 1–8, 1998 相似文献
973.
The solution conformation of a number of small, linear alanine oligomers was investigated via ir (or vibrational) CD (VCD). We find that these oligopeptides assume distinct solution conformations that depend primarily on chain lengths, and to a lesser degree on temperature, ionic strength, and pH. As expected, the longer chain oligomers exhibit more distinct VCD features and, presumably, more stable solution structures. At the level of the hexamer, however, aggregation of the peptide occurs. The fast time scale of VCD allows solution structures to be detected that may not be observable using slower techniques such as various forms of nmr spectroscopy. The VCD results reported here confirm that it is generally possible to obtain conformational information for small, linear homo- and heterooligopeptides via VCD spectroscopy. In this respect, the sensitivity of VCD is similar to that of electronic CD. Furthermore, the temperature dependence of the VCD results indicate that at elevated temperatures, the increasing number of conformational states results in a loss of discernible conformers, and consequently, a broadening and weakening of the VCD features. © 1998 John Wiley & Sons, Inc. Biopoly 46: 455–463, 1998 相似文献
974.
Rafat A. Siddiqui Denis English 《Biochimica et Biophysica Acta (BBA)/Molecular and Cell Biology of Lipids》2000,1483(1):161-173
Phosphatidylinositol 3′-kinase (PI 3′-kinase) plays an important role in the migration of hepatocytes, endothelial cells and neoplastic cells to agonists which activate cellular tyrosine kinases. We examined the PI 3′-kinase-dependent chemotactic responses of neutrophilic leukocytes induced by phosphatidic acid (PA) in order to clarify mechanisms by which the enzyme potentially influences cellular migration. Western analysis of immunoprecipitates indicated that PA induced the tyrosine phosphorylation of three distinct proteins involved in functional activation which co-immunoprecipitated in PA-stimulated cells. These proteins were identified as lyn, syk and the 85 kDa regulatory subunit of PI 3′-kinase. Chemotactic responses to PA but not to several other neutrophil agonists were inhibited by the PI 3′-kinase inhibitors wortmannin and LY294002. Chemotactic inhibition resulted from upstream inhibition of calcium mobilization. Chelation of extracellular calcium by ethylene glycol-bis(β-aminoethyl ether) N,N,N′,N′-tetraacetic acid (EGTA) did not affect the PA-induced chemotaxis, whereas chelation of intracellular calcium by 1,2-bis(2-aminophenoxy)-ethane-N,N,N′,N′-tetraacetic acid (BAPTA) attenuated this response. Thus, changes in intracellular Ca2+ levels that can be effected by Ca2+ mobilized from intracellular stores in the absence of Ca2+ influx regulate PA-induced chemotaxis. Furthermore, PI 3′-kinase inhibition blunted the agonist-dependent generation of inositol 1,4,5-trisphosphate (IP3), suggesting that PI 3′-kinase exerted its effects on calcium mobilization from intracellular sources by mediating activation of phospholipase C (PLC) in PA-stimulated cells. Moreover, the PI 3′-kinase inhibitor LY294002 also inhibited phosphorylation of syk in PA-stimulated cells. We, therefore, propose that products of PI 3′-kinase confined to the inner leaflet of the plasma membrane play a role in activation of syk, calcium mobilization and induction of chemotactic migration. 相似文献
975.
Element Fluxes and Landscape Position in a Northern Hardwood Forest Watershed Ecosystem 总被引:8,自引:3,他引:5
Chemical changes along headwater streams at the Hubbard Brook Experimental Forest in New Hampshire suggest that important
differences exist in biogeochemical cycles along an altitudinal gradient within small watershed ecosystems. Using data collected
during the period 1982–92, we have constructed element budgets [Ca, Mg, K, Na, Si, Al, dissolved organic carbon (DOC), S,
and N] for three subcatchments within watershed 6, a forested watershed last logged around 1917–20. The biogeochemistry of
the high-elevation spruce-fir–white birch subcatchment was dominated by processes involving naturally occuring organic compounds.
Stream water and soil solutions in this zone had elevated concentrations of organic acidity, DOC, and organically bound monomeric
aluminum (Alo), relative to lower-elevation sites. The middle-elevation subcatchment, dominated by hardwood vegetation, had the greatest
net production of inorganic-monomeric aluminum (Ali), and exhibited net immobilization of DOC and Alo. The low-elevation subcatchment, also characterized by deciduous vegetation, had the highest rates of net production of base
cations (Ca2+, Mg2+, K+, Na+) among the subcatchments. Living biomass of trees declined slightly in the spruce-fir–white birch subcatchment during the
study period, remained constant in the middle-elevation zone, and increased by 5% in the low-elevation subcatchment. Coupling
the corresponding changes in biomass nutrient pools with the geochemical patterns, we observed up to 15-fold differences in
the net production of Ca, Mg, K, Na, and Si in soils of the three subcatchments within this 13.2-ha watershed. Release of
Ca, Na, and dissolved Si in the highest-elevation subcatchment could be explained by the congruent dissolution of 185 mol
ha−1 y−1 of plagioclase feldspar. The rate of plagioclase weathering, based on the net output of Na, increased downslope to 189 and
435 mol ha−1 y−1 in the middle-elevation and low-elevation subcatchments, respectively. However, the dissolution of feldspar in the hardwood
subcatchments could account for only 26%–37% of the observed net Ca output. The loss of Ca from soil exchange sites and organic
matter is the most likely source of the unexplained net export. Furthermore, this depletion appears to be occurring most rapidly
in the lower half of watershed 6. The small watersheds at the Hubbard Brook Experimental Forest occupy a soil catena in which
soil depth and soil-water contact time increase downslope. By influencing hydrologic flowpaths and acid neutralization processes,
these factors exert an important influence on biogeochemical fluxes within small watersheds, but their influence on forest
vigor is less clear. Our results illustrate the sensitivity of watershed-level studies to spatial scale. However, it appears
that much of the variation in element fluxes occurs in the first 10–20 ha of drainage area.
Received 13 August 1998; accepted 7 September 1999. 相似文献
976.
目的和方法:实验在游离灌流的颈动脉体窦神经标本上,观察氧分压降低和渗透压升高对化学感受性单位放电的影响。共记录72个有自发放电的化学感受性单位对二者的反应。结果:仅对PO2降低有反应的化学感受性单位有22例(占30.6%),对两种自然刺激均有反应的化学感受性单位有35例(占48.6%),仅高渗引起化学感受性单位放电增加的有15例(占20.8%)。结论:在颈动脉体中不但存在着对缺氧敏感的化学感受性单位,还存在着对渗透压敏感的化学感受性单位,它们对氧分压、渗透压的改变有不同的敏感性。 相似文献
977.
描述神经元相互作用的非线性动力学方程的解析解 总被引:3,自引:0,他引:3
神经系统是由大量神经元构成的非线性动力系统,动力学行为十分复杂FHN模型提供了在该系统中观察时间周期振荡这种非线性现象的实验证据.本文利用扰动法从理论上求出了FHN模型所给出的非线性动力学方程的解析解,为进一步深入研究神经系统的动力学行为提供了理论依据 相似文献
978.
一类具周期系数的单种群模型及其最优收获策略 总被引:11,自引:0,他引:11
文[1]用直接求解的方法,得到了具周期系数的广义Logistic单种群收获模型的最优收获策略.本文在参照并推广文[2]中一类具周期系数的单种群收获模型周期解的全局渐近稳定性结果的基础上,用变分方法得到了其最优收获策略.所得结果包括了许多常见的自治单种群模型所对应的具周期系数的收获模型,如Logistic型[1],Gilpin和Ayala型, Gompertz型[3],以及具类似于Ⅱ,Ⅲ类Holling型功能性反应的密度制约函数[4,5]的模型等. 相似文献
979.
Pinton Roberto Cesco Stefano Iacolettig Gabriele Astolfi Stefania Varanini Zeno 《Plant and Soil》1999,215(2):155-161
The effect of a water extractable humic substances fraction (WEHS) on nitrate uptake and plasma membrane (pm) H+-ATPase activity of maize roots was investigated. Four days old maize root seedlings were exposed for 4 to 24 h to a nutrient
solution containing 200 μ M nitrate in the absence or presence of 5 mg org. C { L
-1 WEHS. Plants exposed to nitrate developed a higher capacity to absorb the anion (induction): the net uptake rate progressively
increased up to 12 h of contact with the solution; thereafter, a decline was observed. When WEHS was present together with
nitrate in the nutrient solution, the induction of nitrate uptake was evident and maximal already 4 h after starting the treatment.
The rate of net nitrate uptake decreased only slightly during the remaining period (4-24 h). Stimulation of net nitrate uptake
rate was also observed when WEHS was added to a nitrogen- or nitrate-free nutrient solution or to a 5 mM CaSO4 solution. The activity of pmH+-ATPase raised upon exposure of the roots to nitrate with the same pattern observed for nitrate uptake. The contemporary presence
of nitrate and WEHS caused a further stimulation of the pmH+-ATPase activity after 4 h treatment. An increase in the enzyme activity was also observed when plants were treated for 4
h in the presence of WEHS in CaSO4, nitrogen- or nitrate-free solutions. However, when nitrate was present the enhancement was even greater. Results support
the idea that the plasma membrane proton pump might be one of the primary targets of the action of humic substances on plant
nutrient acquisition. A role of WEHS in the modulation of nitrate uptake via an interaction with the pm H+-ATPase is also discussed.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
980.
Mobilization of aluminium in the rhizosphere soil solution of growing tree roots in an acidic soil 总被引:2,自引:0,他引:2
Chemical conditions in the rhizosphere in many respects are different from the bulk soil. Especially in acid forest soils
aluminium chemistry at the soil root interface is of particular interest because of its importance for evaluating the risk
of rhizotoxicity. In the present study we have used micro suction cups to collect soil solution from the rhizosphere of oak
seedlings (Quercus robur L.) in high spatial resolution and capillary electrophoresis for the determination of major ions
and Al3+. While the concentrations of nutrient cations, especially Ca2+ and Mg2+, decreased in the vicinity of growing roots the concentrations of Al3+ significantly increased. Al3+-ions were probably released when root-exuded protons were buffered by the soil. Their occurrence indicates, that the oak
roots in our experiments had only limited capabilities to detoxify Al in their rhizosphere. The restriction of this effect
to the very small soil compartment close to the roots suggests, that common soil analysis which neglect rhizosphere processes
might greatly underestimate the in situ concentration of Al3+ near tree roots. Our experiments furthermore indicate, that also suberized roots have a significant influence on rhizosphere
soil solution chemistry.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献