全文获取类型
收费全文 | 742篇 |
免费 | 130篇 |
专业分类
872篇 |
出版年
2021年 | 6篇 |
2020年 | 8篇 |
2018年 | 6篇 |
2017年 | 11篇 |
2016年 | 9篇 |
2015年 | 16篇 |
2014年 | 10篇 |
2013年 | 26篇 |
2012年 | 24篇 |
2011年 | 25篇 |
2010年 | 14篇 |
2009年 | 21篇 |
2008年 | 29篇 |
2007年 | 27篇 |
2006年 | 42篇 |
2005年 | 27篇 |
2004年 | 37篇 |
2003年 | 28篇 |
2002年 | 26篇 |
2001年 | 35篇 |
2000年 | 25篇 |
1999年 | 30篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 13篇 |
1993年 | 13篇 |
1992年 | 18篇 |
1991年 | 20篇 |
1990年 | 31篇 |
1989年 | 17篇 |
1988年 | 18篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 14篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1972年 | 6篇 |
1971年 | 7篇 |
1966年 | 5篇 |
排序方式: 共有872条查询结果,搜索用时 15 毫秒
151.
Some rRNA operons in E. coli have tRNA genes at their distal ends. 总被引:25,自引:0,他引:25
We have previously isolated seven rRNA operons on plasmids or lambda transducing phages and identified various tRNAs encoded by these operons. Each of the seven operons has one of two different spacer tRNA gene arrangements between the genes for 16S and 23S rRNA: either tRNAGlu2 or both tRNAIle1 and tRNAAla1B genes. In addition, various tRNA genes are located at or near the distal ends of rRNA operons. In particular, genes for tRNATrp and tRNAAsp1 are located at the distal end of rrnC at 83 min on the E. coli chromosome. Experiments with various hybrid plasmids, some of which lack the rRNA promoter, have now demonstrated that this promoter is necessary for expression of the distal tRNA genes. Rifampicin run-out experiments have also provided evidence that the tRNATrp gene is located farther from its promoter than the spacer tRNA gene or the 5S RNA gene. These results confirm the localization of genes for tRNATrp and tRNAAsp1 at the distal end of rrnC and strongly suggest that they are co-transcribed with the genes for 16S, tRNAGlu2, 23S and 5S RNA. Other such distal tRNAs have been identified, and it is suggested that they too are part of rRNA operons. 相似文献
152.
K99 Fimbriae from enterotoxigenicEscherichia coli (ETEC) were found to bind specifically to sialic acid, as measured in a haemagglutination inhibition assay using the intact bacteria and human erythrocytes. The affinity forN-glycolylneuraminic acid was about twice that ofN-acetylneuraminic acid (NeuAc), and other monosaccharides were found to be at least ten-fold less effective as inhibitors. The specificity was found to depend on electrostatic interaction where the carboxyl group and its orientation plays an important role. 2--Benzyl-NeuAc was a better inhibitor than 2--methyl-NeuAc suggesting a hydrophobic patch near the binding site on the protein. Axially oriented hydroxyl groups as in 4-epi-NeuAc and 3-hydroxy-NeuAc seemed to participate in binding since these derivatives were better inhibitors thanN-acetylneuraminic acid. K99 was found to have a higher affinity for 4-O-acetyl-NeuAc and lower affinity forN-acetylneuraminic acid withO-substituents at C7-C9 as compared toN-acetylneuraminic acid. Hence, the degree ofO-acetylation of sialic acid in the mucosa of the small intestine may influence colonization and determine susceptibility to infection. 相似文献
153.
Samuel Murail Rebecca J. Howard Torben Broemstrup Edward J. Bertaccini R. Adron Harris James R. Trudell Erik Lindahl 《PLoS computational biology》2012,8(10)
Cys-loop receptors constitute a superfamily of pentameric ligand-gated ion channels (pLGICs), including receptors for acetylcholine, serotonin, glycine and γ-aminobutyric acid. Several bacterial homologues have been identified that are excellent models for understanding allosteric binding of alcohols and anesthetics in human Cys-loop receptors. Recently, we showed that a single point mutation on a prokaryotic homologue (GLIC) could transform it from a channel weakly potentiated by ethanol into a highly ethanol-sensitive channel. Here, we have employed molecular simulations to study ethanol binding to GLIC, and to elucidate the role of the ethanol-enhancing mutation in GLIC modulation. By performing 1-µs simulations with and without ethanol on wild-type and mutated GLIC, we observed spontaneous binding in both intra-subunit and inter-subunit transmembrane cavities. In contrast to the glycine receptor GlyR, in which we previously observed ethanol binding primarily in an inter-subunit cavity, ethanol primarily occupied an intra-subunit cavity in wild-type GLIC. However, the highly ethanol-sensitive GLIC mutation significantly enhanced ethanol binding in the inter-subunit cavity. These results demonstrate dramatic effects of the F(14′)A mutation on the distribution of ligands, and are consistent with a two-site model of pLGIC inhibition and potentiation. 相似文献
154.
Simen Rød Sandve Heidi Rudi Guro Dørum Paul Ragnar Berg Odd Arne Rognli 《Molecular breeding : new strategies in plant improvement》2010,26(4):711-718
Novel high-throughput genotyping technologies have facilitated rapid genotyping of single nucleotide polymorphisms in non-model
organisms. Most plant species have complex genomes with a large proportion of their genes having one or more paralogous copies
due to single gene duplications and ancient or recent polyploidization events. These paralogous gene copies are potential
sources of genotyping errors, and hence genotyping of plant genomes is inherently difficult. Here we present a case study
that exemplifies paralog-related problems in high-throughput genotyping of plant genomes. We used the MassARRAY genotyping
platform to genotype the LpIRI locus in L. perenne populations; this gene is thought to be involved in low-temperature stress tolerance. The dissection of the molecular genetics
underlying the genotyping results provides a good example of how unknown paralogs can mask the true genotype of the locus,
instructive to the non-specialist plant researcher and breeder. 相似文献
155.
Changes of the proteinase binding properties and conformation of bovine alpha 2-macroglobulin on cleavage of the thio ester bonds by methylamine 总被引:1,自引:0,他引:1
Cleavage of the thio ester bonds of human alpha2-macroglobulin (alpha 2M) by methylamine leads to an extensive conformational change and to inactivation of the inhibitor. In contrast, cleavage of these bonds in bovine alpha 2M only minimally perturbs the hydrodynamic volume of the protein [Dangott, L. J., & Cunningham, L. W. (1982) Biochem. Biophys. Res. Commun. 107, 1243-1251], as well as its spectroscopic properties, as analyzed by ultraviolet difference spectroscopy, circular dichroism, and fluorescence in this work. A conformational change analogous to that undergone by human alpha 2M thus does not occur in the bovine inhibitor. However, changes of several functional properties of bovine alpha 2M are induced by the amine. The apparent stoichiometry of inhibition of trypsin thus is reduced from about 1.2 to about 0.7 mol of enzyme/mol of inhibitor. In spite of this decrease, the interaction with the proteinase induces similar conformational changes in methylamine-treated alpha 2M as in intact alpha 2M, as revealed by spectroscopic analyses, indicating that the mode of binding of the proteinase to the inhibitor is essentially unperturbed by thio ester bond cleavage. The reaction with methylamine also greatly increases the sensitivity of bovine alpha 2M to proteolysis by trypsin at sites other than the "bait" region. Moreover, the second-order rate constant for the reaction with thrombin is reduced by about 10-fold. These results indicate that the thio ester bonds of bovine alpha 2M, although not required per se for the binding of proteinases, nevertheless are responsible for maintaining certain structural features of the inhibitor that are of importance for full activity. 相似文献
156.
M?ssbauer, EPR, and magnetization studies of the Azotobacter vinelandii Fe protein. Evidence for a [4Fe-4S]1+ cluster with spin S = 3/2 总被引:1,自引:0,他引:1
P A Lindahl E P Day T A Kent W H Orme-Johnson E Münck 《The Journal of biological chemistry》1985,260(20):11160-11173
We have studied the Fe protein (Av2) of the Azotobacter vinelandii nitrogenase system with M?ssbauer and EPR spectroscopies and magnetic susceptometry. In the oxidized state the protein exhibits M?ssbauer spectra typical of diamagnetic [4Fe-4S]2+ clusters. Addition of Mg.ATP or Mg.ADP causes a pronounced decline in the quadrupole splitting of the M?ssbauer spectra of the oxidized protein. Our studies show that reduced Av2 in the native state is heterogeneous. Approximately half of the molecules contain a [4Fe-4S]1+ cluster with electronic spin S = 1/2 and half contain a [4Fe-4S]1+ cluster with spin S = 3/2. The former yields the characteristic g = 1.94 EPR signal whereas the latter exhibits signals around g = 5. The magnetization of reduced Av2 is dominated by the spin S = 3/2 form of its [4Fe-4S]1+ clusters. These results explain a long standing puzzle, namely why the integrated spin intensity of the g = 1.94 EPR signal is substantially less than 1 spin/4 Fe atoms. In 50% ethylene glycol, 90% of the clusters are in the spin S = 1/2 form whereas, in 0.4 M urea, 85% are in the S = 3/2 form. In 0.4 M urea, the EPR spectrum of reduced Av2 exhibits well defined resonances at g = 5.8 and 5.15, which we assign to the S = 3/2 system. The EPR and M?ssbauer studies yield a zero-field splitting of 2D approximately equal to -5 cm-1 for this S = 3/2 state. 相似文献
157.
158.
We have studied with M?ssbauer spectroscopy the metal clusters of CO dehydrogenase from Clostridium thermoaceticum. At potentials greater than -200 mV, all of the approximately 12 irons reside in diamagnetic environments and contribute a quadrupole doublet characteristic of [Fe4S4]2+ clusters. At lower potentials a variety of components are observed. About 40% of the Fe appears to belong to one [Fe4S4]1+ cluster. We have also observed the M?ssbauer spectrum (approximately 18% of Fe) of the complex which yields EPR with g = 2.01, 1.81, and 1.65. Also present is a doublet (9% of Fe) with delta EQ = 2.90 mm/s and delta = 0.70 mm/s, values typical of a ferrous FeS4 complex. This component seems to interact with a nickel site to form an EPR-silent complex with half-integral electronic spin. We have also characterized the iron environments of the S = 1/2 NiFeC complex. This complex contributes approximately 20% of the total M?ssbauer absorption when the EPR signal has approximately 0.35 spins/12 Fe. From isomer shift comparisons in the oxidized and CO-reacted states of this center, we speculate that the NiFeC complex may consist of a nickel site exchange-coupled to a [Fe4S4]2+ cluster. Finally, the M?ssbauer and EPR data, taken together, force us to conclude that current preparations, while homogeneous according to purifications standards, are spectroscopically heterogeneous, thus rendering the development of a model of the cluster types and compositions in this enzyme premature. 相似文献
159.
160.