全文获取类型
收费全文 | 3683篇 |
免费 | 289篇 |
出版年
2023年 | 13篇 |
2022年 | 40篇 |
2021年 | 75篇 |
2020年 | 48篇 |
2019年 | 61篇 |
2018年 | 86篇 |
2017年 | 72篇 |
2016年 | 124篇 |
2015年 | 150篇 |
2014年 | 194篇 |
2013年 | 232篇 |
2012年 | 258篇 |
2011年 | 251篇 |
2010年 | 193篇 |
2009年 | 172篇 |
2008年 | 232篇 |
2007年 | 197篇 |
2006年 | 191篇 |
2005年 | 212篇 |
2004年 | 176篇 |
2003年 | 156篇 |
2002年 | 141篇 |
2001年 | 71篇 |
2000年 | 47篇 |
1999年 | 64篇 |
1998年 | 35篇 |
1997年 | 39篇 |
1996年 | 31篇 |
1995年 | 22篇 |
1994年 | 35篇 |
1993年 | 26篇 |
1992年 | 30篇 |
1991年 | 31篇 |
1990年 | 23篇 |
1989年 | 27篇 |
1988年 | 26篇 |
1987年 | 20篇 |
1986年 | 15篇 |
1985年 | 26篇 |
1984年 | 13篇 |
1983年 | 18篇 |
1982年 | 13篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有3972条查询结果,搜索用时 15 毫秒
991.
The 0.93A crystal structure of sphericase: a calcium-loaded serine protease from Bacillus sphaericus
Almog O González A Klein D Greenblatt HM Braun S Shoham G 《Journal of molecular biology》2003,332(5):1071-1082
We have previously isolated sphericase (Sph), an extracellular mesophilic serine protease produced by Bacillus sphaericus. The Sph amino acid sequence is highly homologous to two cold-adapted subtilisins from Antarctic bacilli S39 and S41 (76% and 74% identity, respectively). Sph is calcium-dependent, 310 amino acid residues long and has optimal activity at pH 10.0. S41 and S39 have not as yet been structurally analysed.In the present work, we determined the crystal structure of Sph by the Eu/multiwavelength anomalous diffraction method. The structure was extended to 0.93A resolution and refined to a crystallographic R-factor of 9.7%. The final model included all 310 amino acid residues, one disulfide bond, 679 water molecules and five calcium ions. Although Sph is a mesophilic subtilisin, its amino acid sequence is similar to that of the psychrophilic subtilisins, which suggests that the crystal structure of these subtilisins is very similar.The presence of five calcium ions bound to a subtilisin molecule, as found here for Sph, has not been reported for the subtilisin superfamily. None of these calcium-binding sites correlates with the well-known high-affinity calcium-binding site (site I or site A), and only one site has been described previously. This calcium-binding pattern suggests that a reduction in the flexibility of the surface loops of Sph by calcium binding may be responsible for its adaptation to mesophilic organisms. 相似文献
992.
Thomas-Oates J Bereszczak J Edwards E Gill A Noreen S Zhou JC Chen MZ Miao LH Xie FL Yang JK Zhou Q Yang SS Li XH Wang L Spaink HP Schlaman HR Harteveld M Díaz CL van Brussel AA Camacho M Rodríguez-Navarro DN Santamaría C Temprano F Acebes JM Bellogín RA Buendía-Clavería AM Cubo MT Espuny MR Gil AM Gutiérrez R Hidalgo A López-Baena FJ Madinabeitia N Medina C Ollero FJ Vinardell JM Ruiz-Sainz JE 《Systematic and applied microbiology》2003,26(3):453-465
We have analysed 198 fast-growing soybean-nodulating rhizobial strains from four different regions of China for the following characteristics: generation time; number of plasmids; lipopolysaccharide (LPS), nodulation factors (LCOs) and PCR profiles; acidification of growth medium; capacity to grow at acid, neutral, and alkaline pH; growth on LC medium; growth at 28 and 37 degrees C; melanin production capacity; Congo red absorption and symbiotic characteristics. These unbiased analyses of a total subset of strains isolated from specific soybean-cropping areas (an approach which could be called "strainomics") can be used to answer various biological questions. We illustrate this by a comparison of the molecular characteristics of five strains with interesting symbiotic properties. From this comparison we conclude, for instance, that differences in the efficiency of nitrogen fixation or competitiveness for nodulation of these strains are not apparently related to differences in Nod factor structure. 相似文献
993.
San Miguel-Chávez R Soto-Hernández M Ramos-Valdivia AC Kite G Martínez-Vázquez M del Rosario García M M Terrazas S T 《Biotechnology letters》2003,25(13):1055-1059
The production of erythroidines and other alkaloids was studied in cotyledons, callus and cell suspension cultures of Erythrina americana Miller. The cell suspension cultures, grown in Murashige & Skoog medium with naphthaleneacetic acid (3 mg l–1) and kinetin (2 mg l–1), produced 89 and 17 g - and -erythroidines respectively per g dry wt. 相似文献
994.
Hövel K Shallom D Niefind K Belakhov V Shoham G Baasov T Shoham Y Schomburg D 《The EMBO journal》2003,22(19):4922-4932
High-resolution crystal structures of alpha-L-arabinofuranosidase from Geobacillus stearothermophilus T-6, a family 51 glycosidase, are described. The enzyme is a hexamer, and each monomer is organized into two domains: a (beta/alpha)8-barrel and a 12-stranded beta sandwich with jelly-roll topology. The structures of the Michaelis complexes with natural and synthetic substrates, and of the transient covalent arabinofuranosyl-enzyme intermediate represent two stable states in the double displacement mechanism, and allow thorough examination of the catalytic mechanism. The arabinofuranose sugar is tightly bound and distorted by an extensive network of hydrogen bonds. The two catalytic residues are 4.7 A apart, and together with other conserved residues contribute to the stabilization of the oxocarbenium ion-like transition state via charge delocalization and specific protein-substrate interactions. The enzyme is an anti-protonator, and a 1.7 A electrophilic migration of the anomeric carbon takes place during the hydrolysis. 相似文献
995.
S100B inhibits myogenic differentiation and myotube formation in a RAGE-independent manner
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Sorci G Riuzzi F Agneletti AL Marchetti C Donato R 《Molecular and cellular biology》2003,23(14):4870-4881
S100B is a Ca(2+)-modulated protein of the EF-hand type with both intracellular and extracellular roles. S100B, which is most abundant in the brain, has been shown to exert trophic and toxic effects on neurons depending on the concentration attained in the extracellular space. S100B is also found in normal serum, and its serum concentration increases in several nervous and nonnervous pathological conditions, suggesting that S100B-expressing cells outside the brain might release the protein and S100B might exert effects on nonnervous cells. We show here that at picomolar to nanomolar levels, S100B inhibits myogenic differentiation of rat L6 myoblasts via inactivation of p38 kinase with resulting decrease in the expression of the myogenic differentiation markers, myogenin, muscle creatine kinase, and myosin heavy chain, and reduction of myotube formation. Although myoblasts express the multiligand receptor RAGE, which has been shown to transduce S100B effects on neurons, S100B produces identical effects on myoblasts overexpressing either full-length RAGE or RAGE lacking the transducing domain. This suggests that S100B affects myoblasts by interacting with another receptor and that RAGE is not the only receptor for S100B. Our data suggest that S100B might participate in the regulation of muscle development and regeneration by inhibiting crucial steps of the myogenic program in a RAGE-independent manner. 相似文献
996.
In this study, we investigated the involvement of reactive oxygen species (ROS) and calcium in staurosporine (STS)-induced apoptosis in cultured retinal neurons, under conditions of maintained membrane integrity. The antioxidants idebenone (IDB), glutathione-ethylester (GSH/EE), trolox, and Mn(III)tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP) significantly reduced STS-induced caspase-3-like activity and intracellular ROS generation. Endogenous sources of ROS production were investigated by testing the effect of the following inhibitors: 7-nitroindazole (7-NI), a specific inhibitor of the neuronal isoform of nitric oxide synthase (nNOS); arachidonyl trifluoromethyl ketone (AACOCF(3)), a phospholipase A(2) (PLA(2)) inhibitor; allopurinol, a xanthine oxidase inhibitor; and the mitochondrial inhibitors rotenone and oligomycin. All these compounds decreased caspase-3-like activity and ROS generation, showing that both mitochondrial and cytosolic sources of ROS are implicated in this mechanism. STS induced a significant increase in intracellular calcium concentration ([Ca(2+)](i)), which was partially prevented in the presence of IDB and GSH/EE, indicating its dependence on ROS generation. These two antioxidants and the inhibitors allopurinol and 7-NI also reduced the number of TdT-mediated dUTP nick-end labeling-positive cells. Thus, endogenous ROS generation and the rise in intracellular calcium are important inter-players in STS-triggered apoptosis. Furthermore, the antioxidants may help to prolong retinal cell survival upon apoptotic cell death. 相似文献
997.
Hint protein domains appear in inteins and in the C-terminal region of Hedgehog and Hedgehog-like animal developmental proteins. Intein Hint domains are responsible and sufficient for protein-splicing of their host-protein flanks. In Hedgehog proteins the Hint domain autocatalyses its cleavage from the N-terminal domain of the Hedgehog protein by attaching a cholesterol molecule to it. We identified two new types of Hint domains. Both types have active site sequence features of Hint domains but also possess distinguishing sequence features. The new domains appear in more than 50 different proteins from diverse bacteria, including pathogenic species of humans and plants, such as Neisseria meningitidis and Pseudomonas syringae. These new domains are termed bacterial intein-like (BIL) domains. Bacterial intein-like domains are present in variable protein regions and are typically flanked by domains that also appear in secreted proteins such as filamentous haemagglutinin and calcium binding RTX repeats. Phylogenetic and genomic analysis of BIL sequences suggests that they were positively selected for in different lineages. We cloned two BIL domains of different types and showed them to be active. One of the domains efficiently cleaved itself from its C-terminal flank and could also protein-splice its two flanks, in E. coli and in a cell free system. We discuss several possible biological roles for BIL domains including microevolution and post translational modification for generating protein variability. 相似文献
998.
999.
In the present study, we evaluated the effects of extended exposure procedures to contextual fear cues as a way of reducing intertrial responses (ITRs), a characteristic stereotyped behavior displayed in 2-way Shuttle Box avoidance overtraining. We used rat strains psychogenetically selected for efficient (Roman high-avoidance; RHA) versus poor 2-way Shuttle Box performance (Roman low-avoidance; RLA), the former being fearless/impulsive relative to the latter. Additionally, Sprague-Dawley (SD) rats were compared with RHAs to have a measure of the ITRs exhibited by another, non-selected group of animals. The main findings were that after extended exposure RHAs diminished their ITRs without affecting avoidance performance, although they emitted a pattern of ITRs somewhat excessive as compared with SDs. We concluded that three factors could influence the expression of ITRs in RHA rats: (1) the repetitive and stereotyped responding induced by 2-way Shuttle Box overtraining; (2) contextual fear conditioning; and (3) a genetic background associated with high impulsiveness. 相似文献
1000.
Vitrification of porcine embryos at various developmental stages using different ultra-rapid cooling procedures 总被引:6,自引:0,他引:6
Cuello C Gil MA Parrilla I Tornel J Vázquez JM Roca J Berthelot F Martinat-Botté F Martínez EA 《Theriogenology》2004,62(1-2):353-361
In this study, three different vitrification systems (open pulled straw: OPS; superfine open pulled straw: SOPS; and Vit-Master technology using SOPS: Vit-Master-SOPS) were compared in order to investigate the influence of cooling rate on in vitro development of vitrified/warmed porcine morulae, early blastocysts, or expanded blastocysts. Embryos were obtained surgically on Day 6 of the estrous cycle (D0 = onset of estrus) from weaned crossbred sows, classified and pooled according their developmental stage. A subset of embryos from each developmental stage was cultured to evaluate the in vitro development of fresh embryos; the remaining embryos were randomly allocated to each vitrification system. After vitrification and warming, embryos were cultured in vitro for 96 h in TCM199 with 10% fetal calf serum at 39 degrees C, in 5% CO(2) in humidified air. During the culture period, embryos were morphologically evaluated for their developmental progression. The developmental stage of embryos at collection affected the survival and hatching rates of vitrified/warmed embryos (P < 0.001). The vitrification system or the interaction of vitrification system and developmental stage had no effect on these parameters (P > 0.05). Vitrified expanded blastocysts showed the best development in vitro (P < 0.001), with survival and hatching rates similar to those of fresh expanded blastocysts. The hatching rate of fresh morula or early blastocyst stage embryos was higher than their vitrified counterparts. In conclusion, under our experimental conditions, cooling rates greater than 20,000 degrees C/min, as occurs when SOPS or Vit-Master-SOPS systems are used, do not enhance the efficiency of in vitro development of vitrified porcine embryos. 相似文献