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991.
Aldo Franco Frank Marí 《International journal of peptide research and therapeutics》1999,6(4):199-207
Summary α-conotoxin EI is an 18-residue peptide (RDOCCYHPTCNMSNPQIC; 4–10, 5–18) isolated from the venom ofConus ermineus, the only fish-hunting cone snail of the Atlantic Ocean. This peptide targets specifically the nicotinic acetylcholine receptor
(nAChR) found in mammalian skeletal muscle and the electric organTorpedo, showing a novel selectivity profile when compared to other α-conotoxins. The 3D structure of EI has been determined by 2D-NMR
methods in combination with dynamical simulated annealing protocols. A total of 133 NOE-derived distances were used to produce
13 structures with minimum energy that complied with the NOE restraints. The structure of EI is characterized by a helical
loop between THr9 and Met12 that is stabilized by the Cys4-Cys10 disulfide bond and turns involving Cys4-Cys5 and Asn14-Pro15. Other regions of the peptide appear to be flexible. The overall fold of EI is similar to that of other α4/7-conotoxins (PnIA/B,
MII, EpI). However, unlike these other α4/7-conotoxins, EI targets the muscular type nAChR. The differences in selectivity
can be attributed to differences in the surface charge distribution among these α4/7-conotoxins. The implications for binding
of EI to the muscular nAChR are discussed with respect to the current NMR structure of EI.
Supplementary material available:1H resonance assignments of α-conotoxin EI. 相似文献
992.
氟微量元素矿化液对葡糖基转移酶及乳酸脱氢酶的影响 总被引:4,自引:1,他引:3
目的 采用氟与微量元素联合制成以钙、磷为基础,含有氟和微量元素的矿化液,探讨该矿化液对致龋毒力因子--变形链球菌和远缘链球菌的胞外葡糖基转移酶(GTF)和乳酸脱氢酶(LDH)的影响。方法 选用国际标准菌株ATCC S.mutans 25175和S.Sobrinus6715为实验菌珠,采用厌氧菌的连续培养技术培养细菌,用微量酶化学分析方法检测矿化液对GTF和LDH酶活性的影响,用菌液的吸光度(ABS)来描述。结果 无氟矿化液对GTF酶活性无明显的抑制作用(P〉0.05),而含氟矿化液和氟微量元素矿化液对GTF酶活性有明显抑制作用(P〈0.01),两两比较,万2以氟微量元素矿化液的作用最强;氟微量元素矿化液对LDH酶活性具有明显的抑制作用(P〈0.01),而无氟矿化液和含氟矿化液对其无明显的统计学意义(P〉0.05 相似文献
993.
结合运用Liapunov泛函数,研究二维Lotka-Volterra捕食系统周期正解的存在唯一性。 相似文献
994.
中立型时滞模型周期正解的一个充分条件 总被引:3,自引:0,他引:3
研究了一类中立型时滞模型:N’(t)=N(t)「α(t)-β(t)N(t)-b(t)N(t-τ(t))-c(t)N’(t-τ(t))」周期正解的存在性,其中α(t),β(t),b(t),c(t),τ(t)是周期T的非负连续函数。一个新的充分条件被给出,这一结果肯定的回答了文「1」的分开问题9.2。 相似文献
995.
冬小麦等4种作物对铵,硝态氮的吸收能力 总被引:5,自引:1,他引:4
采用水培试验探讨了冬小麦、大豆、油菜和莴笋4种作物对硝、铵态氮的相对吸收能力以及这两种氮源对它们生长发育的影响。试验表明:(1)不同氮源对供试作物的生长发育影响极大。供给硝态氮,这些作物生长发育良好,供给等量的NO^-3和NH^-4(1:1)时,蔬菜作物莴笋生长量下降幅度最大;供给铵态氨,莴笋和大豆极为敏感,供给NO^-3时莴笋吸氮量显著高于供给等氮量NO^-3和NH^+4,莴上麦供给等量NO^- 相似文献
996.
从无钼、无氨而含铬的固氮培养基中生长的棕色固氮菌(Azotobacter vinelandii Lipmann)突变种UW3中纯化得到了部分纯的CrFe蛋白.在试图培养CrFe蛋白大晶体时发现,棕色晶体和砖红色晶体可同时或单独出现.SDS-PAGE和厌氧天然PAGE皆表明,棕色晶体主要由与固氮酶钼铁蛋白(Av1)类似大小的亚基(~60 kD)组成,而砖红色晶体则由~20kD亚基组成.免疫分析表明只有~60kD的亚基可与固氮酶钼铁蛋白的抗体反应,而~20kD亚基则无这种反应.在部分纯的CrFe蛋白溶液中,~20 kD的总蛋白含量远低于~60 kD蛋白的含量,表明由这种小亚基组成的蛋白只是CrFe蛋白溶液中的一种污染蛋白.用3,5-二氨基苯甲酸染色的天然电泳表明,形成砖红色和棕色晶体的蛋白是迁移率不同的两种含铁蛋白.质谱分析表明砖红色晶体蛋白为棕色固氮菌的细菌铁蛋白.分辨率为2.34 A的X射线衍射结果也表明,砖红色晶体属于H3空间群,晶胞参数为a=124.965A,b=124.965A和c=287.406 A.即将发表的三维结构解析表明,此砖红色晶体确为24聚体的细菌铁蛋白. 相似文献
997.
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
998.
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
999.
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
1000.
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