全文获取类型
收费全文 | 2909篇 |
免费 | 219篇 |
国内免费 | 3篇 |
出版年
2021年 | 36篇 |
2020年 | 17篇 |
2019年 | 31篇 |
2018年 | 33篇 |
2017年 | 30篇 |
2016年 | 58篇 |
2015年 | 102篇 |
2014年 | 95篇 |
2013年 | 150篇 |
2012年 | 220篇 |
2011年 | 207篇 |
2010年 | 126篇 |
2009年 | 107篇 |
2008年 | 154篇 |
2007年 | 194篇 |
2006年 | 160篇 |
2005年 | 145篇 |
2004年 | 161篇 |
2003年 | 173篇 |
2002年 | 146篇 |
2001年 | 32篇 |
2000年 | 16篇 |
1999年 | 34篇 |
1998年 | 32篇 |
1997年 | 34篇 |
1996年 | 25篇 |
1995年 | 30篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 29篇 |
1991年 | 24篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 25篇 |
1985年 | 27篇 |
1984年 | 28篇 |
1983年 | 27篇 |
1982年 | 26篇 |
1981年 | 26篇 |
1980年 | 20篇 |
1979年 | 17篇 |
1978年 | 19篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1969年 | 14篇 |
1968年 | 15篇 |
排序方式: 共有3131条查询结果,搜索用时 537 毫秒
81.
Rajendra M. Srivastava Norman Hudson Fred R. Seymour Bernard Weissmann 《Carbohydrate research》1978,60(2)
The earlier preparation of cyclohexylammonium (phenyl α-l-idopyranosid)-uronate has been improved, and (4-methylumbelliferyl α-l-idopyranosid)uronic acid (14), a more sensitive substrate for α-l-iduronidase, has been synthesized by an analogous route. Zinc chloride-catalyzed condensation of 4-methylumbelliferone with 1,2,3,4,6-penta-O-acetyl-α-l-idopyranose (4) in 1,2-ethanediol diacetate gave crystalline 4-methylumbelliferyl 2,3,4,6-tetra-O-acetyl-α-l-idopyranoside (7). O-Deacetylation and catalytic oxidation gave 14, characterized as a cyclohexylammonium salt. The starting material 4 was prepared, in 21 % yield from l-glucose, by conversion of the intermediate 1,2,3,4,6-penta-O-acetyl-β-l-glucopyranose to 2,3,4,6-tetra-O-acetyl-β-l-glucopyranosyl chloride and acetoxonium ion rearrangement, as described for the D-series. 相似文献
82.
Mutants of Bacillus subtilis constitutive for L-leucine dehydrogenase synthesis were selected. Using these mutants we could determine two functional roles for the L-leucine dehydrogenase. This enzyme liberates ammonium ions from branched chain amino acids when supplied as the sole nitrogen source. Another function is to synthesize from L-isoleucine, L-leucine, and L-valine the branched chain -keto acids which are precursors of branched chain fatty acid biosynthesis. These results together with the inducibility of the enzyme suggest that the L-leucine dehydrogenase has primarily a catabolic rather than an anabolic function in the metabolism of Bacillus subtilis. 相似文献
83.
Influence of divalent cations upon complement-mediated enzyme release from human polymorphonuclear leukocytes. 总被引:9,自引:0,他引:9
I M Goldstein S T Hoffstein G Weissmann 《Journal of immunology (Baltimore, Md. : 1950)》1975,115(3):665-670
The complement component, C5a provokes the selective release of granule-associated enzymes from the intact, viable cytochalasin B-treated human polymorphonuclear leukocytes (PMN) in the absence of phagocytosis or cellular adherence to surfaces. Consquently, in this experimental system the influence of divalent cations on these two processes can be disregarded and their effects on enzymes secretion can be studied directly. Cytochalasin B-treated PMN exposed to C5a in calcium and magnesium-free media consistently secreted significant amounts of the granule-associated enzymes, beta-glucuronidase and lysozyme. The basal secretory response was not diminished if cells were preincubated with 5.0 mM EDTA, nor was it influenced if 1.0 mm or 2.0 mM EDTA were present in the reaction mixtures. The addition of calcium (up to 1.5 to 2.0 mM) produced a concentration-dependent enhancement of beta-glucuronidase release, whereas increasing amounts of calcium (above 2.0 mM) inhibited secretion of this enzyme. Lysozyme release was similarly enhanced by the addition of calcium, but inhibition with high concentrations was not observed. Calcium per se, in the absence of C5a, provoked only the release of lysozyme from these cells. The effects of calcium upon enzyme release were not associated with alterations in the state of assembly of cytoplasmic microtubules. These findings provide another example of the role of calcium in "stimulus-secretion coupling" and provide evidence that exocytosis of various granules in human PMN is regulated by independent mechanisms involving calcium. 相似文献
84.
Anne Durand Fabien Chauveau Tae-Hee Cho Radu Bolbos Jean-Baptiste Langlois Laure Hermitte Marlène Wiart Yves Berthezène Norbert Nighoghossian 《PloS one》2012,7(11)
Injection of thrombin into the middle cerebral artery (MCA) of mice has been proposed as a new model of thromboembolic stroke. The present study used sequential multiparametric Magnetic Resonance Imaging (MRI), including Magnetic Resonance Angiography (MRA), Diffusion-Weighted Imaging (DWI) and Perfusion-Weighted Imaging (PWI), to document MCA occlusion, PWI-DWI mismatch, and lesion development. In the first experiment, complete MCA occlusion and reproducible hypoperfusion were obtained in 85% of animals during the first hour after stroke onset. In the second experiment, 80% of animals showed partial to complete reperfusion during a three-hour follow-up. Spontaneous reperfusion thus contributed to the variability in ischemic volume in this model. The study confirmed the value of the model for evaluating new thrombolytic treatments, but calls for extended MRI follow-up at the acute stage in therapeutic studies. 相似文献
85.
86.
Steven Collins Njonte Wouamba Gervais Mouth Happi Michel Nguiam Pouofo Joseph Tchamgoue Jean‐Bosco Jouda Frida Longo Bruno Ndjakou Lenta Norbert Sewald Simeon Fogue Kouam 《化学与生物多样性》2020,17(9)
An extensive phytochemical study of the aerial parts of Vernonia guineensis Benth. (Asteraceae) led to the isolation of a new flavone, vernoguinoflavone and a naturally isolated glycerol ester, eicosanoic acid 2‐hydroxy‐1,3‐propanediyl ester, together with eighteen known secondary metabolites including quercetin, luteolin, vernopicrin, vernomelitensin, β‐amyrin, oleanolic acid, ursolic acid, lupeol, betulinic acid, β‐carotene, a mixture of stigmasterol and β‐sitosterol, β‐sitosterol‐3‐O‐β‐D‐glucoside, 2,3‐dihydroxypropyl heptacosanoate, pentacosanoic acid, docosan‐1‐ol, tritriacontan‐1‐ol, and heptatriacontan‐1‐ol. Eleven compounds are reported herein for the first time from this species. The structures of these compounds were elucidated on the basis of extensive spectroscopic analyses, particularly 1D and 2D NMR, and HR‐ESI‐MS and by comparison of their data with those reported in the literature. The crude extract, fractions and some isolated compounds were evaluated for their antibacterial activity against Gram‐negative bacteria: Escherichia coli (ATCC 25922), Shigella flexineri (NR 518), Salmonella muenchen, Salmonella typhimurium and Salmonella typhi (ATCC 19430). All the tested compounds demonstrated inhibitory activities against the tested enteric bacteria with MIC values ranging from 3.12 to 100 μg/ml. Three flavonoids isolated from the most active fraction demonstrated the best bioactivities against Escherichia coli, Salmonella muenchen and Salmonella typhimurium with MIC values ranging from 3.12 to 25 μg/mL. 相似文献
87.
Kalocayova Barbora Snurikova Denisa Vlkovicova Jana Navarova-Stara Veronika Michalikova Dominika Ujhazy Eduard Gasparova Zdenka Vrbjar Norbert 《Molecular and cellular biochemistry》2021,476(12):4323-4330
Molecular and Cellular Biochemistry - Previously it was shown that for reduction of anxiety and stress of experimental animals, preventive handling seems to be one of the most effective methods.... 相似文献
88.
Francová Kateřina Šumberová Kateřina Kučerová Andrea Šorf Michal Grill Stanislav Exler Norbert Vrba Jaroslav 《Wetlands Ecology and Management》2021,29(1):93-110
Wetlands Ecology and Management - Plants play an important role in fishpond littorals, but little is known about factors influencing their presence and growth patterns. We surveyed vegetation of... 相似文献
89.
Elyeza Bakaze Beloved Mensah Dzomeku Jens‐Norbert Wünsche 《The Annals of applied biology》2021,178(1):98-108
Each year 25–75% of banana and plantain yields are lost because of rhizome damages caused by banana weevil (Cosmopolites sordidus) in growing regions of sub‐Saharan Africa. However, the specific plant defence response of the rhizome tissue in relation to the C. sordidus attack is unknown. Consequently, in this study, we evaluated whether plant defence substances in the rhizome are correlated with the degree of larval damage and whether applications of methyl jasmonate (MJ) elicit a greater induction of the plant defence potential against C. sordidus. Moreover, we attempted to reveal cellular modifications in response to the root feeding herbivore through histochemical staining. The banana cultivars “Km5” and “Mbwazirume” with tolerance and susceptibility to C. sordidus, respectively, were used in a pot experiment to evaluate percent rhizome damage, leaf chlorophyll content, total phenolic content (TPC), antioxidant capacity and cell morphology in response to C. sordidus attack and/or MJ applications compared to untreated control plants. We found that C. sordidus‐induced rhizome damage was 30% in the susceptible cultivar but less than 5% in the tolerant cultivar. The percent rhizome damage was not related to leaf chlorophyll content but showed a significant negative linear relationship to both TPC and antioxidant capacity. Larvae feeding induced a considerably greater increase of polyphenolic defence compounds in Km5 than in Mbwazirume; however, this response was opposite in the MJ treatment, suggesting that the phytohormone induced the susceptible plant to invest more into the synthesis of defence chemicals that in turn lead to reduced C. sordidus damage. Tissue staining demonstrated a greater deposition of lignin and suberin in C. sordidus challenged rhizome, presumably to seal off healthy tissue with a physical barrier from continued pest attack. It is concluded that MJ induces polyphenolics in susceptible Mbwazirume banana that reduced C. sordidus damage. 相似文献
90.
Timothy A. Swearengin Eugene E. Fibuch Norbert W. Seidler 《Journal of enzyme inhibition and medicinal chemistry》2013,28(5):575-579
The mechanism of inhalation anesthesia remains to be fully elucidated. While certain neuronal membrane proteins are considered sites of action, cytosolic proteins may also be targets. We hypothesize that inhaled anesthetics may act via glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which has recently been shown to participate in neuronal inhibition. We examined the effects of sevoflurane, a halogenated ether anesthetic, on the catalytic and fluorescence properties of GAPDH. Initial rates of oxidoreductase activity decreased approximately 30% at saturating levels of sevoflurane. NADH-stimulated oxidoreductase activity (25 μM NADH; 0.8 mM NAD+) increased with sevoflurane. Sevoflurane quenched tryptophan fluorescence emission and increased polarization. Additionally, sevoflurane increased the susceptibility of GAPDH to thermal denaturation suggesting an effect on conformation. Our findings warrant further research on sevoflurane's effect on GAPDH and indicate that this approach may lead to delineation of a novel contribution to the mechanism of anesthesia. 相似文献