全文获取类型
收费全文 | 23668篇 |
免费 | 2383篇 |
国内免费 | 16篇 |
出版年
2021年 | 234篇 |
2020年 | 180篇 |
2019年 | 215篇 |
2018年 | 257篇 |
2017年 | 262篇 |
2016年 | 391篇 |
2015年 | 664篇 |
2014年 | 702篇 |
2013年 | 987篇 |
2012年 | 1095篇 |
2011年 | 1126篇 |
2010年 | 862篇 |
2009年 | 679篇 |
2008年 | 854篇 |
2007年 | 889篇 |
2006年 | 843篇 |
2005年 | 841篇 |
2004年 | 945篇 |
2003年 | 875篇 |
2002年 | 824篇 |
2001年 | 696篇 |
2000年 | 687篇 |
1999年 | 639篇 |
1998年 | 333篇 |
1997年 | 342篇 |
1996年 | 331篇 |
1995年 | 311篇 |
1994年 | 251篇 |
1993年 | 282篇 |
1992年 | 594篇 |
1991年 | 491篇 |
1990年 | 538篇 |
1989年 | 527篇 |
1988年 | 432篇 |
1987年 | 431篇 |
1986年 | 378篇 |
1985年 | 387篇 |
1984年 | 406篇 |
1983年 | 315篇 |
1982年 | 288篇 |
1981年 | 274篇 |
1980年 | 244篇 |
1979年 | 329篇 |
1978年 | 267篇 |
1977年 | 289篇 |
1976年 | 219篇 |
1975年 | 238篇 |
1974年 | 242篇 |
1973年 | 211篇 |
1971年 | 183篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
In order to prevent enzymatic degradation of beta-casomorphin-5 (1) and morphiceptin, reduced peptide bonds were incorporated at the 2-3 and 3-4 bonds, respectively. The analogues were synthesized by a combination of solid phase methodology and reductive alkylation of resin-bound peptide amines with Boc-amino acid aldehydes (Boc: tert-butyloxycarbonyl) in the presence of NaBH3CN. During reversed phase high pressure liquid chromatography purification, peak shape distortions could be observed. Epimerization was excluded, based on gas chromatography/mass spectroscopy analysis, which indicated acceptable levels of racemization (less than 3%) in the crude product. Instead, the phenomena could be attributed to slow cis/trans isomerizations originating from the Xxx-Pro bonds in the sequence. The presence of different conformational isomers was also established by 1H-nmr spectroscopy in DMSO-d6. All analogues showed high stability in blood plasma, enhanced binding affinity for the mu receptor, and very low binding to the delta receptor. While the Phe 3 psi(CH2-N)Pro4 analogues (3) and (5) displayed agonist activity, the Pro 2 psi(CH2-NH)Phe3 modified analogue (2) showed antagonist activity comparable to D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2. 相似文献
42.
C. M. Van Der Loos J. J. Van Den Oord P. K. Das H. J. Houthoff 《The Histochemical journal》1988,20(8):409-413
Summary An immunoenzyme double-staining method for the simultaneous detection of two cellular epitopes, using commercially available mouse monoclonal antibodies, is described. The method employs a combination of the suppression of endogenous biotin and two successive indirect techniques with a blocking step in between. The first indirect method involves an unlabelled monoclonal antibody followed by an alkaline phosphatase-conjugated goat anti-mouse immunoglobulin. After a blocking step with normal mouse serum, the second indirect method is applied using a biotinylated monoclonal antibody followed by the visualization of this antibody by avidin-biotinylated peroxidase complex (ABC) or rabbit anti-biotin and peroxidase-conjugated swine anti-rabbit immunoglobulin in successive steps. Using these methods in combination with the introduction of dioctyl sodium sulphosuccinate and tetramethylbenzidine as chromogens for peroxidase activity, two cellular epitopes could be distinguished clearly in tissue sections by the green-and violet-stained peroxidase and alkaline phosphatase activities, respectively. The expression of two epitopes on the same cellular constituent is outlined by the coappearance of both enzyme activities as a bluish-purple colour. This method allows for the simultaneous identification, localization and enumeration of two cellular epitopes. These can serve as parameters for a number of pathological processes. 相似文献
43.
44.
Van Eldik G; Ruiter R; Van Herpen M; Schrawen J; Wullems G 《Journal of experimental botany》1998,49(325):1453-1454
The reproductive function of the pistil requires the production of
compounds essential for pollen tube growth. A cold-plaque screening of a
pollinated pistil cDNA library of Solanum tuberosum
resulted in the isolation of cDNA clone cp67. Northern blot analyses
revealed that cp67 is specifically expressed in potato pistils and in a
limited number of plant species. The deduced CP67 protein displays
similarity to long-chain zinc-containing alcohol dehydrogenases (ADHs),
although at a similarity level much lower than between other plant
ADHs. 相似文献
45.
46.
Effects of ethanol and other alkanols on passive proton influx in the yeast Saccharomyces cerevisiae
Ethanol, isopropanol, propanol and butanol enhanced the passive influx of protons into deenergized cells of Saccharomyces cerevisiae. The influx followed first-order kinetics with a rate constant that increased exponentially with the alkanol concentration. The exponential enhancement constants increased with the lipid solubility of the alkanols, which indicated hydrophobic membrane regions as the target sites. While the enhancement constants were independent of pH over the range tested (3.3–5.0), the rate constants decreased linearly with increasing extracellular proton concentration, indicating the presence of an additional surface barrier against proton penetration, the effectiveness of which increased with protonation. The alkanols affected the acidification curves of energized yeast suspensions in such a way that the final pH values were linear functions of the alkanol concentrations. These results were consistent with a balance between active and passive proton movements at the final pH, the exponential enhancement constants calculated from the slopes being nearly identical with those obtained with deenergized cells. It was concluded that passive proton influx contributes to the kinetics of acidification in S. cerevisiae and that uncoupling contributes to the overall kinetics of alkanol-inhibited secondary active transport across the yeast plasma membrane. 相似文献
47.
48.
A statistical mechanical theory of the helix-coil transition in sequential polypeptides is developed assuming that the statistical weights of the Zimm-Bragg parameters of a given residue depend on the type of adjacent residues. In the case of a sequential polypeptide consisting of two kinds of residues, the theory describes the helix- coil transition of the polypeptide in terms of the Zimm-Bragg parameters associated with the corresponding residues. The theory is then used to determine this parameter, as a function of temperature, from experimental data for transition temperature as a function of solvent composition, for a series of sequential polypeptides consisting of Glu(OBzl) and Lys(Chz) residues in mixtures of dichloroacetic acid and 1,2-dichlorethane. This parameter is then combined with the Zimm-Bragg parameters for the parent homopolypeptides, and the theory used to predict helix coil transition curves which are in good agreement with the experimental ones for the sequential polypeptides studied. 相似文献
49.
50.