全文获取类型
收费全文 | 490326篇 |
免费 | 54750篇 |
国内免费 | 237篇 |
出版年
2018年 | 4390篇 |
2017年 | 4443篇 |
2016年 | 7194篇 |
2015年 | 12208篇 |
2014年 | 12867篇 |
2013年 | 15989篇 |
2012年 | 16501篇 |
2011年 | 14289篇 |
2010年 | 10285篇 |
2009年 | 9707篇 |
2008年 | 11539篇 |
2007年 | 11837篇 |
2006年 | 11082篇 |
2005年 | 16428篇 |
2004年 | 14882篇 |
2003年 | 13163篇 |
2002年 | 10497篇 |
2001年 | 20589篇 |
2000年 | 20183篇 |
1999年 | 16891篇 |
1998年 | 5749篇 |
1997年 | 6177篇 |
1996年 | 5829篇 |
1995年 | 5491篇 |
1994年 | 5389篇 |
1993年 | 5476篇 |
1992年 | 14971篇 |
1991年 | 14844篇 |
1990年 | 14271篇 |
1989年 | 14040篇 |
1988年 | 12939篇 |
1987年 | 12449篇 |
1986年 | 11602篇 |
1985年 | 11555篇 |
1984年 | 9431篇 |
1983年 | 8039篇 |
1982年 | 6013篇 |
1981年 | 5383篇 |
1980年 | 5159篇 |
1979年 | 8926篇 |
1978年 | 7061篇 |
1977年 | 6339篇 |
1976年 | 6034篇 |
1975年 | 6759篇 |
1974年 | 7075篇 |
1973年 | 7006篇 |
1972年 | 6389篇 |
1971年 | 5939篇 |
1970年 | 4973篇 |
1969年 | 4835篇 |
排序方式: 共有10000条查询结果,搜索用时 34 毫秒
991.
992.
The quality of milk products is threatened by the formation of biofilms of thermophilicstreptococci on the internal surfaces of plate heat exchangers used in milk processing. Althoughattachment to stainless steel surfaces is one of the first stages in the development of a biofilm, themechanisms involved in attachment have not been reported. The cell surface properties of 12strains of thermophilic streptococci were examined to determine their importance in attachment tostainless steel surfaces. Hydrophobicity, extracellular polysaccharide production and cell surfacecharge varied between the different strains but could not be related to numbers attaching. Treatingthe cells with sodium metaperiodate, lysozyme or trichloroacetic acid to disrupt cell surfacepolysaccharide had no effect on attachment. Treatment with trypsin or sodium dodecyl sulphate toremove cell surface proteins resulted in a 100-fold reduction in the number of bacteria attaching.This result suggests that the surface proteins of the thermophilic streptococci are important intheir attachment to stainless steel. 相似文献
993.
994.
Horseradish peroxidase-catalyzed oxidation of p-phenetidine in the presence of either glutathione (GSH), cysteine, or N-acetylcysteine led to the production of the appropriate thioyl radical which could be observed using EPR spectroscopy in conjunction with the spin trap 5,5-dimethyl-1-pyrroline-N-oxide. This confirms earlier work using acetaminophen (Ross, D., Albano, E., Nilsson, U., and Moldéus, P. (1984) Biochem. Biophys. Res. Commun. 125, 109-115). The further reactions of glutathionyl radicals (GS.), generated during horseradish peroxidase-catalyzed oxidation of p-phenetidine and acetaminophen in the presence of GSH, were investigated by following kinetics of oxygen uptake and oxidized glutathione (GSSG) formation. Oxygen uptake and GSSG generation were dependent on the concentration of GSH but above that which was required for maximal interaction with the primary amine or phenoxy radical generated during peroxidatic oxidation of p-phenetidine or acetaminophen, suggesting that a secondary GSH-dependent process was responsible for oxygen uptake and GSSG production. GSSG was the only product of thiol oxidation detected during peroxidatic oxidation of p-phenetidine or acetaminophen in the presence of GSH, but under nitrogen saturation conditions its production was reduced to 8 and 33% of the corresponding amounts obtained under aerobic conditions in the cases of p-phenetidine and acetaminophen, respectively. Nitrogen saturation conditions did not affect horseradish peroxidase-catalyzed metabolism. This shows that the main route of GSSG generation in such reactions is not by dimerization of GS. but via mechanism(s) involving oxygen consumption such as via GSSG-. or via GSOOH. 相似文献
995.
A D Noble 《BMJ (Clinical research ed.)》1985,290(6462):123-125
A reduction has been recorded in National Health Service gynaecological bed occupancy in Winchester and Wessex. At least part of this change may be explained by an increase in private hospital practice. NHS managers should plan for similar changes elsewhere in the United Kingdom. 相似文献
996.
D Mitchell H P Laburn K E Cooper R F Hellon W I Cranston Y Townsend 《The Yale journal of biology and medicine》1986,59(2):159-168
We have reviewed the evidence in favor of a prostaglandin mediator of the thermal responses in fever and found that PGE injected into the hypothalamus does not always cause fever, that cerebrospinal fluid concentrations of PGE are not reliable reflections of hypothalamic events, and that antipyretic drugs may act in ways other than inhibiting PGE synthesis. Fever is not blocked by prostaglandin antagonists, nor by ablation of PGE-sensitive areas of the brain. There is poor correlation between the effects of pyrogens and of PGE on cerebral neurons. There is evidence that at least one prostanoid other than prostaglandin is a mediator of fever, but the prostanoid has not been identified yet. We conclude that PGE may contribute to the neural responses in fever but is not essential. 相似文献
997.
Deoxyadenosine/deoxycytidine kinase from Bacillus subtilis. Purification, characterization, and physiological function 总被引:3,自引:0,他引:3
H M?llgaard 《The Journal of biological chemistry》1980,255(17):8216-8220
Three different deoxyribonucleoside kinases with specificities toward thymidine, deoxyguanosine, and deoxyadenosine/deoxycytidine, respectively, are identified in Bacillus subtilis. The deoxyadenosin/deoxycytidine kinase is purified 950-fold employing blue Sepharose CL-6B column chromatography. The two deoxyribonucleoside kinase activities copurify and are present in the same band after polyacrylamide gel electrophoresis. The molecular weight is determined by gel filtration to be 47,000. Cytidine, adenosine, arabinosylcytosine, and arabinosyladenine are substrates for the enzyme. The activities toward these substrates are less than 20% of the activities obtained with deoxyadenosin and deoxycytidine. The deoxycytidine and deoxyadenosine saturation curves are hyperbolic with Km values for both nucleosides around 5 microM. The maximal velocities for the two deoxyribonucleosides are nearly identical with GTP as phosphate donor. GTP is the best donor showing hyperbolic saturation curves and Km values around 150 microM depending on the deoxyribonucleoside concentration. dATP and dCTP are inhibitors when GTP is the phosphate donor. They may both act as phosphate donors themselves. A divalent metal ion is required, Mg2+ giving the highest activity. A spontaneous mutant, selected as resistant to 5-fluorodeoxycytidine, lacks both deoxycytidine and deoxyadenosine kinase activity, while it retains normal activities toward deoxyguanosine, deoxyuridine, and thymidine. 相似文献
998.
Don Trinh Nguyen Jens Christian G?pfert Nobuhiro Ikezawa Gillian MacNevin Meena Kathiresan Jürgen Conrad Otmar Spring Dae-Kyun Ro 《The Journal of biological chemistry》2010,285(22):16588-16598
Sesquiterpene lactones are characteristic natural products in Asteraceae, which constitutes ∼8% of all plant species. Despite their physiological and pharmaceutical importance, the biochemistry and evolution of sesquiterpene lactones remain unexplored. Here we show that germacrene A oxidase (GAO), evolutionarily conserved in all major subfamilies of Asteraceae, catalyzes three consecutive oxidations of germacrene A to yield germacrene A acid. Furthermore, it is also capable of oxidizing non-natural substrate amorphadiene. Co-expression of lettuce GAO with germacrene synthase in engineered yeast synthesized aberrant products, costic acids and ilicic acid, in an acidic condition. However, cultivation in a neutral condition allowed the de novo synthesis of a single novel compound that was identified as germacrene A acid by gas and liquid chromatography and NMR analyses. To trace the evolutionary lineage of GAO in Asteraceae, homologous genes were further isolated from the representative species of three major subfamilies of Asteraceae (sunflower, chicory, and costus from Asteroideae, Cichorioideae, and Carduoideae, respectively) and also from the phylogenetically basal species, Barnadesia spinosa, from Barnadesioideae. The recombinant GAOs from these genes clearly showed germacrene A oxidase activities, suggesting that GAO activity is widely conserved in Asteraceae including the basal lineage. All GAOs could catalyze the three-step oxidation of non-natural substrate amorphadiene to artemisinic acid, whereas amorphadiene oxidase diverged from GAO displayed negligible activity for germacrene A oxidation. The observed amorphadiene oxidase activity in GAOs suggests that the catalytic plasticity is embedded in ancestral GAO enzymes that may contribute to the chemical and catalytic diversity in nature. 相似文献
999.
S. E. Franklin L. Young D. Watson A. Cigan T. Meyer L. A. Bulla Jr. 《Molecular genetics and genomics : MGG》1997,256(5):517-524
Various subspecies of the gram-positive bacterium Bacillus thuringiensis are known to produce a wide array of insecticidal crystal proteins (ICPs) upon sporulation. These ICPs act primarily on the brush border of midgut epithelial cells of susceptible larvae. Recently, a protein of 210?kDa, isolated from the midgut of Manduca sexta, has been demonstrated to bind the Cry1Ab toxin produced by B. thuringiensis subsp. berliner and is therefore postulated to be involved in mediating the toxicity of Cry1Ab. The cDNA encoding the 210?kDa protein, termed BT-R1 (Bacillus thuringiensis receptor-1), was recently cloned, and shows limited homology to the cadherin superfamily of proteins. Quite naturally, there is a great deal of interest in the characterization of BT-R 1 , the gene encoding the 210?kDa Cry1Ab binding protein. The studies presented here involve the use of various restriction fragments prepared from the cDNA encoding BT-R1 as probes of Southern blots bearing M. sexta genomic DNA cleaved with a variety of restriction endonucleases. These Southern blot data reveal that there are two discrete regions within the M. sexta genome which encode sequences homologous to BT-R1. On the basis of the signal intensities seen on Southern blots, it appears that only one of these genes encodes BT-R1, whereas the other is a closely related homologue. 相似文献
1000.