全文获取类型
收费全文 | 13749篇 |
免费 | 956篇 |
国内免费 | 4篇 |
出版年
2024年 | 5篇 |
2023年 | 87篇 |
2022年 | 122篇 |
2021年 | 417篇 |
2020年 | 256篇 |
2019年 | 303篇 |
2018年 | 449篇 |
2017年 | 379篇 |
2016年 | 540篇 |
2015年 | 809篇 |
2014年 | 861篇 |
2013年 | 1078篇 |
2012年 | 1241篇 |
2011年 | 1229篇 |
2010年 | 728篇 |
2009年 | 647篇 |
2008年 | 838篇 |
2007年 | 773篇 |
2006年 | 769篇 |
2005年 | 644篇 |
2004年 | 537篇 |
2003年 | 510篇 |
2002年 | 501篇 |
2001年 | 101篇 |
2000年 | 80篇 |
1999年 | 90篇 |
1998年 | 136篇 |
1997年 | 79篇 |
1996年 | 60篇 |
1995年 | 43篇 |
1994年 | 52篇 |
1993年 | 34篇 |
1992年 | 42篇 |
1991年 | 35篇 |
1990年 | 23篇 |
1989年 | 25篇 |
1988年 | 22篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 17篇 |
1981年 | 14篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1971年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Lucimar G. Milagres Maria Cristina C. Brandileone Claudio T. Sacchi Vera S.D. Vieira Rosemeire C. Zanella Carl E. Frasch 《FEMS immunology and medical microbiology》1996,13(1):9-17
Abstract Since 1988, N. meningitidis , B:4:P1.15, ET-5 complex, has been responsible for an epidemic of meningococcal disease in Greater São Paulo, Brazil. Despite current trials to develop an effective vaccine against group B meningococci, children less than 2 years old have not been protected. It has been suggested that iron-regulated proteins (IRPs) should be considered as potential antigens for meningococcal vaccines. The vaccines under study consisted of outer-membrane vesicles depleted of lipooligosaccharide from three serogroup B strains and one serogroup C strain, IRPs, meningococcal group C polysaccharide and aluminum hydroxide. Four different protein and C polysaccharide concentrations were studied. The ELISA and bactericidal results showed a higher antibody response when 2 injections of 2.0 μg doses were administered. Despite higher IgG reactivity against antigen preparations containing IRPs seen in ELISA, the bactericidal activity was not increased if the target strain was grown in iron-restricted medium. The influence of addition of alkaline-detoxified lipooligosaccharide (dLOS) on immunogenicity of the vaccine was also investigated, and the dLOS provided for a more functionally specific antibody response. 相似文献
92.
Stefan Mihailescu John M. Matsoukas Francis L. Abel Valeriu Nestianu Florin Romanescu Cristina Alecu-Parvu 《Letters in Peptide Science》1996,3(4):181-184
Summary The angiotensin II (ANG II) receptor blocker properties of sarmesin and its influence on the homotropic cooperativity of ANG II receptors were studied in two experimental models: isolated rabbit aorta and isolated rabbit atria. The results show that: (i) sarmesin is a specific competitive antagonist of ANG II receptors, with high affinity (pA2=8.93 in the isolated aorta and 8.66 in the isolated atria); and (ii) the slope of the concentration-response curves to ANG II and the Hill coefficient increased in the presence of sarmesin, the latter suggesting an enhancement of the positive homotropic cooperativity of ANG II receptors. These results may be explained overall by the reciprocal negative modulation of receptor affinity between sarmesin and ANG II, due, possibly, to opposite effects on the binding of G-proteins to ANG II receptors. 相似文献
93.
Paula Veríssimo Cristina Esteves Carlos Faro Euclides Pires 《Biotechnology letters》1995,17(6):621-626
The flowers of cardoon (genus Cynara) are traditionally used in Portugal for cheese making. In this work the vegetable rennet of the species Cynara cardunculus L. was characterized in terms of enzymic composition and proteolytic specificity of its proteinases (cardosin A and cardosin B). Cardosin A was found to cleave insulin B chain at the bonds Leu15-Tyr16, Leu17-Val18 and Phe25-Tyr26. In addition to the bonds mentioned cardosin B cleaves also Glu13-Ala14, Ala14-Leu15 and Phe24-Phe25 indicating that it has a broader specificity. The kinetic parameters for the hydrolysis of the synthetic peptide Leu-Ser-Phe(NO2)-Nle-Ala-Leu-oMe were also determined and compared to those of chymosin and pepsin. The results obtained indicate that in terms of specificity and kinetic parameters cardosin A is similar to chymosin whereas cardosin B is similar to pepsin. It appears therefore that the enzyme composition of cardoon rennet closely resembles that of calf rennet. 相似文献
94.
Abstract: We studied the action of H2 O2 on the exocytosis of glutamate by cerebrocortical synaptosomes. The treatment of synaptosomes with H2 O2 (50–150 µ M ) for a few minutes results in a long-lasting depression of the Ca2+ -dependent exocytosis of glutamate, induced by KCl or by the K+ -channel inhibitor 4-aminopyridine. The energy state of synaptosomes, as judged by the level of phosphocreatine and the ATP/ADP ratio, was not affected by H2 O2 , although a transient decrease was observed after the treatment. H2 O2 did not promote peroxidation, as judged by the formation of malondialdehyde. In indo-1-loaded synaptosomes, the treatment with H2 O2 did not modify significantly the KCl-induced increase of [Ca2+ ]i . H2 O2 inhibited exocytosis also when the latter was induced by increasing [Ca2+ ]i with the Ca2+ ionophore ionomycin. The effects of H2 O2 were unchanged in the presence of superoxide dismutase and the presence of the Fe3+ chelator deferoxamine. These results appear to indicate that H2 O2 , apparently without damaging the synaptosomes, induces a long-lasting inhibition of the exocytosis of glutamate by acting directly on the exocytotic process. 相似文献
95.
Factors controlling somatic embryogenesis 总被引:5,自引:0,他引:5
Histological and ultrastructural, molecular and elemental distribution changes were investigated during the induction of direct somatic embryogenesis using theCamellia japonica leaf culture system. In this culture system, direct somatic embryogenesis is induced in a controlled way in a specific leaf region (leaf blade) within a leaf. Embryogenic and non-embryogenic leaf regions have characteristic energy-dispersive X-ray spectra already before induction. According to these results electron probe X-ray microanalysis (EPMA) can be a tool for early diagnosis of embryogenic competence. Histological studies showed that severe fluctuations in the number of calcium oxalate crystals and in starch accumulation occur after induction but only in induced tissues. Changes in the cell wall composition of competent cells occur shortly after the induction treatment. The induction of morphogenesis is linked to the appearance of callose covering the surface cells of induced leaves and calluses. A 2nd deposition of material (cutin) is necessary for normal somatic embryogenesis to occur. The involvement of lipid transfer proteins in the appearance of cutin in the embryogenic regions of the explant is suggested. 相似文献
96.
Influence of external factors on secondary embryogenesis and germination in somatic embryos from leaves of Quercus suber 总被引:2,自引:0,他引:2
Bárbara Fernández-Guijarro Cristina Celestino Mariano Toribio 《Plant Cell, Tissue and Organ Culture》1995,41(2):99-106
Somatic embryogenesis was obtained in cultures of leaves from young seedlings of Quercus suber L. A two-stage process, in which benzyladenine and naphthaleneacetic acid were added first at high and then at low concentrations, was required to initiate the process. Somatic embryos arose when the explants were subsequently placed on medium lacking plant growth regulators. The embryogenic lines remained productive, by means of secondary embryogenesis, on medium without growth regulators. However, this repetitive induction was influenced by the macronutrient composition of the culture medium. Both low total nitrogen content and high reduced nitrogen concentration decreased the percentage of somatic embryos that showed secondary embryogenesis. Our results suggest that alternate culture on medium that increases embryo proliferation and a low salt medium prohibiting embryo formation will partially synchronize embryo development. Chilling slightly reduced secondary embryogenesis but gave a modest increase in germination. Maturation under light followed by storage at 4 °C for at least 30 days gave the best results in switching embryos from an embryogenic pathway to a germinative one. Under these conditions 15% of embryos showed coordinated root and shoot growth and 35% formed either shoots or mostly roots. These percentages were higher than those of embryos matured in darkness. This result indicates that a specific treatment is required after maturation and before chilling to activate the switch from secondary embryo formation to germination.Abbreviations BA
benzyladenine
- NAA
naphthaleneacetic acid
- 2,4-D
2,4-dichlorophenoxyacetic acid
- BA
indolebutyric acid
- MS
Murashige & Skoog (1962) medium
- SH
Schenk & Hildebrandt (1972) medium
- G
Gamborg (1966, PRL-4-C) medium (macronutrients in mg l–1: NaH2PO4·H2O, 90; Na2HPO4, 30; KCl, 300; (NH4)2SO4, 200; MgSO4·7H2O, 250; KNO3, 1000, CaCl2·2H2O, 150)
- PGR
plant growth regulator 相似文献
97.
An automatic system was installed for continuous analyses of ozone, sulphur dioxide, nitrogen monoxide and nitrogen dioxide in an experimental orchard with a canopy ofPrunus cerasifera plants in summer 1993. Air samples from three elevations (0.8 m, 1.6 m and 3 m above ground) were sequentially analyzed. Ozone concentrations above the canopy were usually higher than within the canopy; their relationships with stomatal resistance have been investigated. Sulphur dioxide levels were negligible. Nitrogen oxides showed a complex profile, with no particular trend, likely due to a reciprocal exchange between the atmosphere and the ground surface. 相似文献
98.
Alessandro Sisto Fabio Bonelli Felice Centini Christopher I. Fincham Edoardo Potier Edith Monteagudo Paolo Lombardi Federico Arcamone Cristina Goso Stefano Manzini Alessandro Giolitti Carlo A. Maggi Mariano Venanzi Basilio Pispisa 《Biopolymers》1995,36(4):511-524
In the course of a program aimed at synthesizing novel, potent NK-1 tachykinin receptor antagonists, we developed upon a bioactive model by comparing the low energy structures of a series of peptide and nonpeptide Substance P antagonists. The comparison was based on the super imposition of the aromatic rings, assuming that the rest of the molecule behaves predominantly as a template to arrange the key aromatic groups in the right spatial position. A series of 2-aminocyclohexane carboxylic acid analogues were then selected as the best templates for reproducing the postulated bioactive structure, leading to several pseudo-peptides with interesting biological activity. According to the molecular modeling, these compounds exhibit a neat parallel facing of the indolyl and naphthyl groups at about 3 Å distance. Ultraviolet absorption and steady state fluorescence measurements support this conclusion, showing a linear correlation between the spectral properties and the binding affinity of these analogues. Stacking of the indole ring with naphthalene gives rise to a complex characterized by a well-defined molar extinction coefficient. Consistently, steady state and lifetime fluorescence measurements suggest that the quenching process is ascribable to ground-state interactions between the chromophores. Implications of the π stacking propensity of aromatic groups in the biological activity of the compounds examined are briefly discussed. © 1995 John Wiley & Sons, Inc. 相似文献
99.
Keith Ashman Tony Houthaeve Jonathan Clayton Matthias Wilm Alexandre Podtelejnikov Ole N. Jensen Matthias Mann 《Letters in Peptide Science》1997,4(2):57-65
The rapid accumulation of sequence data generated by the various genome sequencingprojects and the generation of expressed sequence tag databases has resulted in the need forthe development of fast and sensitive methods for the identification and characterisation oflarge numbers of gel electrophoretically separated proteins to translate the sequence data intobiological function. To achieve this goal it has been necessary to devise new approaches toprotein analysis: matrix-assisted laser desorption and electrospray mass spectrometry havebecome important protein analytical tools which are both fast and sensitive. When combinedwith a robotic system for the in-gel digestion of electrophoretically separated proteins, itbecomes possible to rapidly identify many proteins by searching databases with MS data. Thepower of this combination of techniques is demonstrated by an analysis of the proteins presentin the myofibrillar lattice of the indirect flight muscle of Drosophila melanogaster. Theproteins were separated by SDS-PAGE and in-gel proteolysis was performed bothautomatically and manually. All 16 major proteins could quickly be identified by massspectrometry. Although most of the protein components were known to be present in theflight muscle, two new components were also identified. The combination of methodsdescribed offers a means for the rapid identification of large numbers of gel separatedproteins. 相似文献
100.
Mart M. Cristina Bocaz Gastn Asp Estrella Roeckel Marlene 《Biotechnology Techniques》1997,11(3):163-166
The simultaneous determination by capillary ion electrophoresis of nitrate and nitrite in wastewaters with chloride concentrations of 15 to 23g/l is described. Chloride concentrations over 200mg/l hampered the determination; thus, lead, mercuric and silver acetate were used to precipitate chloride. Silver acetate added in 1.5 times the stoichiometric amount for AgCl formation gave the best results in terms of nitrate and nitrite peak resolution. 相似文献