首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2318篇
  免费   168篇
  国内免费   2篇
  2488篇
  2023年   11篇
  2022年   26篇
  2021年   48篇
  2020年   43篇
  2019年   48篇
  2018年   56篇
  2017年   44篇
  2016年   68篇
  2015年   100篇
  2014年   83篇
  2013年   162篇
  2012年   169篇
  2011年   200篇
  2010年   128篇
  2009年   112篇
  2008年   152篇
  2007年   160篇
  2006年   133篇
  2005年   136篇
  2004年   118篇
  2003年   93篇
  2002年   99篇
  2001年   15篇
  2000年   11篇
  1999年   12篇
  1998年   17篇
  1997年   18篇
  1996年   8篇
  1995年   7篇
  1994年   8篇
  1993年   16篇
  1992年   13篇
  1991年   8篇
  1990年   14篇
  1989年   11篇
  1988年   7篇
  1987年   11篇
  1986年   7篇
  1985年   8篇
  1984年   6篇
  1983年   6篇
  1982年   9篇
  1981年   8篇
  1980年   7篇
  1979年   11篇
  1978年   11篇
  1975年   11篇
  1974年   5篇
  1973年   8篇
  1972年   5篇
排序方式: 共有2488条查询结果,搜索用时 28 毫秒
41.
The recovery of radioactivity in the urine of guineapigs following a bolus intravenous dose of chromatographically pure 14C-Ntau-methylhistidine was measured in order to test whether the excretion of Ntau-methylhistidine (Ntau-MH) is a valid index of myofibrillar protein breakdown in these animals. Four male and four female guineapigs were dosed and after 7 days, 91.65+/-2.82% and 3.58+/-0.91% of injected radioactivity was recovered in the excreta and tissues, respectively. The average total recovery of 95.2+/-3.0% was not significantly different from 100%. Male guineapigs excreted the radioactivity more slowly than females (70% of the dose excreted within 74 h vs 39 h, respectively) but cumulative excretion at 7 days was the same for each sex. Chromatographic analysis of the urine showed almost all of the radioactivity to be associated with a single peak corresponding to Ntau-MH, indicating a lack of significant metabolism. These data show that although the clearance of 14C-Ntau-MH is slower than in rats or humans the urinary excretion of Ntau-MH is a valid index for myofibrillar protein degradation in the guineapig.  相似文献   
42.
43.
The sequences of the 3′‐terminal region of four Czech Potato virus M isolates VIRUBRA 4/007, VIRUBRA 4/009, VIRUBRA 4/016 and VIRUBRA 4/035 were determined and compared with sequences of PVM isolates available in GenBank. Among the Czech isolates, VIRUBRA 4/007 and 4/016 as well as VIRUBRA 4/016 and 4/035 showed the highest nucleotide identity (93%). Isolates VIRUBRA 4/007, 4/016 and 4/035 were most similar to the PV0273 isolate from Germany and to the wild isolate from Russia. Interestingly, isolate VIRUBRA 4/009 significantly differed from the other three Czech isolates and was the only European isolate that showed the highest nucleotide identity with American isolates. Moreover, the PVM isolates from the Czech Republic and Germany differed in their host range. Phylogenetic analysis based on ORF5 coding for coat protein showed that the Czech isolates could be classified in two of the three groupings of the phylogenetic tree obtained. This is the first report on molecular and biological analysis of the genome sequences of PVM isolates from the Czech Republic.  相似文献   
44.
Properties of the excited states in reaction center core (RCC) complexes of the green sulfur bacterium Prosthecochloris aestuarii were studied by means of femtosecond time-resolved isotropic and anisotropic absorption difference spectroscopy at 275 K. Selective excitation of the different transitions of the complex resulted in the rapid establishment of a thermal equilibrium. At about 1 ps after excitation, the energy was located at the lowest energy transition, BChl a 835. Time constants varying between 0.26 and 0.46 ps were observed for the energy transfer steps leading to this equilibrium. These transfer steps were also reflected in changes in polarization. Our measurements indicate that downhill energy transfer towards excited BChl a 835 occurs via the energetically higher spectral forms BChl a 809 and BChl a 820. Low values of the anisotropy of about 0.07 were found in the ‘two-color’ measurements at 820 and 835 nm upon excitation at 800 nm, whereas the ‘one-color’ kinetics showed much higher anisotropies. Charge separation occurred with a time constant varying between 20 and 30 ps. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
45.
In previous work, a strongly stabilized variant of the β1 domain of streptococcal protein G (Gβ1) was obtained by an in vitro selection method. This variant, termed Gβ1-M2, contains the four substitutions E15V, T16L, T18I, and N37L. Here we elucidated the molecular basis of the observed strong stabilizations. The contributions of these four residues were analyzed individually and in various combinations, additional selections with focused Gβ1 gene libraries were performed, and the crystal structure of Gβ1-M2 was determined. All single substitutions (E15V, T16L, T18I, and N37L) stabilize wild-type Gβ1 by contributions of between 1.6 and 6.0 kJ mol− 1 (at 70 °C). Hydrophobic residues at positions 16 and 37 provide the major contribution to stabilization by enlarging the hydrophobic core of Gβ1. They also increase the tendency to form dimers, as shown by dependence on the concentration of apparent molecular mass in analytical ultracentrifugation, by concentration-dependent stability, and by a strongly increased van't Hoff enthalpy of unfolding. The 0.88-Å crystal structure of Gβ1-M2 and NMR measurements in solution provide the explanation for the observed dimer formation. It involves a head-to-head arrangement of two Gβ1-M2 molecules via six intermolecular hydrogen bonds between the two β strands 2 and 2′ and an adjacent self-complementary hydrophobic surface area, which is created by the T16L and N37L substitutions and a large 120° rotation of the Tyr33 side chain. This removal of hydrophilic groups and the malleability of the created hydrophobic surface provide the basis for the dimer formation of stabilized Gβ1 variants.  相似文献   
46.
Past anthrax attacks in the United States have highlighted the need for improved measures against bioweapons. The virulence of anthrax stems from the shielding properties of the Bacillus anthracis poly-γ-d-glutamic acid capsule. In the presence of excess CapD, a B. anthracis γ-glutamyl transpeptidase, the protective capsule is degraded, and the immune system can successfully combat infection. Although CapD shows promise as a next generation protein therapeutic against anthrax, improvements in production, stability, and therapeutic formulation are needed. In this study, we addressed several of these problems through computational protein engineering techniques. We show that circular permutation of CapD improved production properties and dramatically increased kinetic thermostability. At 45 °C, CapD was completely inactive after 5 min, but circularly permuted CapD remained almost entirely active after 30 min. In addition, we identify an amino acid substitution that dramatically decreased transpeptidation activity but not hydrolysis. Subsequently, we show that this mutant had a diminished capsule degradation activity, suggesting that CapD catalyzes capsule degradation through a transpeptidation reaction with endogenous amino acids and peptides in serum rather than hydrolysis.  相似文献   
47.
48.
Dynamin is essential for clathrin-dependent coated vesicle formation. It is required for membrane budding at a late stage during the transition from a fully formed pit to a pinched-off vesicle. Dynamin may also fulfill other roles during earlier stages of vesicle formation. We have screened about 16,000 small molecules and have identified 1, named here dynasore, that interferes in vitro with the GTPase activity of dynamin1, dynamin2, and Drp1, the mitochondrial dynamin, but not of other small GTPases. Dynasore acts as a potent inhibitor of endocytic pathways known to depend on dynamin by rapidly blocking coated vesicle formation within seconds of dynasore addition. Two types of coated pit intermediates accumulate during dynasore treatment, U-shaped, half formed pits and O-shaped, fully formed pits, captured while pinching off. Thus, dynamin acts at two steps during clathrin coat formation; GTP hydrolysis is probably needed at both steps.  相似文献   
49.

Objectives

To investigate the associations of environmental MS risk factors with clinical and MRI measures of progression in high-risk clinically isolated syndromes (CIS) after the first demyelinating event.

Methods

We analyzed 211 CIS patients (age: 28.9±7.8 years) enrolled in the SET study, a multi-center study of high-risk CIS patients. Pre-treatment samples were analyzed for IgG antibodies against cytomegalovirus (anti-CMV), Epstein Barr virus (EBV) early nuclear antigen-1 (EBNA-1), viral capsid antigen (VCA), early antigen-diffuse (EA-D), 25 hydroxy-vitamin D3 and cotinine levels and HLA DRB1*1501 status. The inclusion criteria required evaluation within 4 months of the initial demyelinating event, 2 or more brain MRI lesions and the presence of two or more oligoclonal bands in cerebrospinal fluid. All patients were treated with interferon-beta. Clinical and MRI assessments were obtained at baseline, 6, 12, and 24 months.

Results

The time to first relapse decreased and the number of relapses increased with anti-CMV IgG positivity. Smoking was associated with increased number and volume of contrast-enhancing lesions (CEL) during the 2-year period. The cumulative number of CEL and T2 lesions during the 2-year period was greater for individuals in the highest quartile of anti-EBV VCA IgG antibodies. The percent loss of brain volume was increased for those in the highest quartile of with anti-EBV VCA IgG antibodies.

Conclusions

Relapses in CIS patients were associated with CMV positivity whereas anti-EBV VCA positivity was associated with progression on MRI measures, including accumulation of CEL and T2 lesions and development of brain atrophy.  相似文献   
50.
Unicellular, diazotrophic cyanobacteria temporally separate dinitrogen (N2) fixation and photosynthesis to prevent inactivation of the nitrogenase by oxygen. This temporal segregation is regulated by a circadian clock with oscillating activities of N2 fixation in the dark and photosynthesis in the light. On the population level, this separation is not always complete, since the two processes can overlap during transitions from dark to light. How do single cells avoid inactivation of nitrogenase during these periods? One possibility is that phenotypic heterogeneity in populations leads to segregation of the two processes. Here, we measured N2 fixation and photosynthesis of individual cells using nanometer-scale secondary ion mass spectrometry (nanoSIMS) to assess both processes in a culture of the unicellular, diazotrophic cyanobacterium Crocosphaera watsonii during a dark-light and a continuous light phase. We compared single-cell rates with bulk rates and gene expression profiles. During the regular dark and light phases, C. watsonii exhibited the temporal segregation of N2 fixation and photosynthesis commonly observed. However, N2 fixation and photosynthesis were concurrently measurable at the population level during the subjective dark phase in which cells were kept in the light rather than returned to the expected dark phase. At the single-cell level, though, cells discriminated against either one of the two processes. Cells that showed high levels of photosynthesis had low nitrogen fixing activities, and vice versa. These results suggest that, under ambiguous environmental signals, single cells discriminate against either photosynthesis or nitrogen fixation, and thereby might reduce costs associated with running incompatible processes in the same cell.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号