全文获取类型
收费全文 | 2862篇 |
免费 | 294篇 |
专业分类
3156篇 |
出版年
2021年 | 29篇 |
2020年 | 26篇 |
2019年 | 25篇 |
2018年 | 38篇 |
2017年 | 26篇 |
2016年 | 55篇 |
2015年 | 94篇 |
2014年 | 98篇 |
2013年 | 108篇 |
2012年 | 123篇 |
2011年 | 155篇 |
2010年 | 84篇 |
2009年 | 77篇 |
2008年 | 113篇 |
2007年 | 106篇 |
2006年 | 103篇 |
2005年 | 113篇 |
2004年 | 113篇 |
2003年 | 116篇 |
2002年 | 92篇 |
2001年 | 84篇 |
2000年 | 78篇 |
1999年 | 78篇 |
1998年 | 50篇 |
1997年 | 31篇 |
1996年 | 30篇 |
1995年 | 33篇 |
1994年 | 27篇 |
1993年 | 26篇 |
1992年 | 70篇 |
1991年 | 60篇 |
1990年 | 58篇 |
1989年 | 50篇 |
1988年 | 44篇 |
1987年 | 49篇 |
1986年 | 50篇 |
1985年 | 50篇 |
1984年 | 38篇 |
1983年 | 37篇 |
1982年 | 27篇 |
1980年 | 28篇 |
1979年 | 31篇 |
1978年 | 29篇 |
1977年 | 39篇 |
1976年 | 20篇 |
1974年 | 29篇 |
1973年 | 33篇 |
1972年 | 26篇 |
1971年 | 22篇 |
1969年 | 22篇 |
排序方式: 共有3156条查询结果,搜索用时 15 毫秒
31.
32.
C L Brooks 《Journal of molecular biology》1992,227(2):375-380
We have used molecular dynamics simulation methods to study the structure and fluctuations of "native" apomyoglobin in aqueous solution for a period of greater than 0.5 nanosecond. This work was motivated by the recent attempts of Hughson et al. to characterize the structure and motion of both this molecule and the less compact, acid stabilized I stage, using methods of pulsed H/2H exchange. The study of these systems provides new insights into protein folding intermediates and our simulation has yielded a detailed model for structure and fluctuations in apomyoglobin which complements the experimental studies. We find that local (short-time) fluctuations agree well with fluctuations observed for the holoprotein in aqueous solution, as well as results from the crystallographic B-factors. In addition, the structural features we observe for native apomyoglobin are very similar to the holoprotein, in basic agreement with the findings of Hughson et al. By examining larger-scale motions, developing only over timescales in excess of a 100 picoseconds, we are able to identify conformationally "labile" and "non-labile" regions within native apomyoglobin. These regions correspond extremely well to those identified in the nuclear magnetic resonance experiments as unstable and stable "folding subdomains" in the I state of apomyoglobin. Overall we find that helices A, B, E, G and H show the least amount of motion and helices C, D and F move substantially over the timescales examined. The major motions, and the primary difference between the holo and apo structures as we have observed them, are due to the shifting motion of helices C, D and F into the vacant heme cavity. We also find that motions at the interface of helical segments can be large, with one important exception being the chain segment connecting helices G and H. This segment of chain interacts with the conformationally "non-labile" helix A to form a relatively rigid subdomain composed of helices A, G and H. We believe that these findings provide direct support for the suggestion of Hughson et al. that helices A, G and H constitute a compact subdomain that remains in a native-like conformation as the protein begins to unfold in environments of decreasing pH. 相似文献
33.
Twenty-three livetrapped and two trapper-caught river otters (Lutra canadensis) from northeastern Pennsylvania (USA) were examined for ectoparasites immediately after their captures during 1981 to 1985. Ectoparasites were collected from both trapper-caught otters, but from only one livetrapped otter. One species of tick (Ixodes cookei) and one flea (Oropsylla arctomys) were collected. 相似文献
34.
The finer control of mating activity by testosterone in male sheep was investigated using the castrated ram (wether) as an experimental model. Adult wethers that had been castrated before puberty were injected with graded doses of testosterone propionate (TP) and mating behavior was assessed in standardized libido trials at various times during treatment. Doses of 1 to 2 mg TP/day elicited mounting behavior in wethers but did not result in intromission or ejaculation. On the other hand, TP doses of 4 mg/day or greater stimulated the complete mating response which included intromission and the ejaculatory reflex. The threshold dose of TP required for complete mating activity (4 mg/day) produced plasma testosterone levels which were lower than those normally observed in intact rams. The results of this study indicate that the behavioral aspects (arousal mechanisms) of mating in rams have a lower testosterone threshold than intromission and ejaculation (consummatory mechanisms). Also, since complete mating activity was stimulated in wethers having relatively low plasma testosterone levels, this may explain why there is no apparent relationship between plasma testosterone and mating drive in intact rams. 相似文献
35.
36.
A passive hemagglutination assay was developed to measure Trypanosoma musculi-specific antibody in mice. Indicator-erythrocyte donor mice received 550 rad 60Co 24 hr before intraperitoneal injection of 3 × 104T. musculi. T. musculi antigen-coated erythrocytes were obtained from these mice on Day 9 postinfection. T. musculi antigen-coated erythrocytes obtained in this manner were used as indicator erythrocytes in a passive hemagglutination procedure. Serum from hyperimmunized mice (three consecutive infections at 21-day intervals) gave titers as high as 1:1024. Titers of 1:256 and 1:512 were obtained from singly infected mice on Days 18 and 28 postinfection, respectively. In marked contrast, nude mice infected with T. musculi did not produce a detectable agglutinating antibody response. Erythrocytes obtained from either irradiated (550 rad 60Co) uninfected mice, nonirradiated infected mice, or normal mice did not agglutinate when combined with any of the sera tested. These data suggest the usefulness of this passive hemagglutination assay for the measurement of antibody to T. musculi in the serum of infected mice. 相似文献
37.
The kidney exerts both prohypertensive and antihypertensive functions. Part of the anti-hypertensive function of the kidney is mediated by the renomedullary interstitial cells (RIC), as an endocrine-type function. Six experimental models of hypertension and their relation to the antihypertensive function of the RIC are discussed. It is proposed that the anti-hypertensive function of the RIC may be deficient by the three mechanisms: 1) absence of the cells (as in the renoprival state); 2) severe damage to the cells (as in partial nephrectomy-salt hypertension of the rat and late malignant hypertension of the rabbit); and 3) constraint of the function of these cells (as in angiotensin-salt hypertension due to a lower salt intake). The constraint may result from excessive angiotensin, either by a direct effect or via a hemodynamic mechanism. The converting enzyme inhibitors (CEI) fail to exert their antihypertensive function when the RIC are absent or damaged. Conversely, the CEI are effective in those models associated with intact RIC. CEI appear to exert their antihypertensive action partly through an effect on RIC. 相似文献
38.
This study describes and quantifies the social organization and agonistic behaviour of adult male collared lemmings (Dicrostonyx groenlandicus) in an indoor enclosure. Five groups of four lemmings were observed during separate 8-day tests. Experimental design permitted an increase in group size while density remained constant. When in groups of two, males established stable dominant-subordinate relationships and, when in groups of four, they formed stable non-linear dominance hierarchies. For either group size, frequency of agonistic behaviour declined over time. Habituation, formation of dominance relationships and spatio-temporal partitioning of space are suggested as possible explanations of this decline. Percentage of initial body weight lost during the experiments varied inversely with social rank. Possible causes of this weight loss are discussed. Social relationships in this study are discussed in light of field observations of male lemming home range distribution. 相似文献
39.
The tolerance of a cell line (IMC-HZ-1) from a moth, Heliothis zea, for the monovalent cations Na+ and K+ were defined. Cells shifted to media containing more than 70 mM of K+ showed decreased growth rates. No evidence was obtained for Na+ toxicity. The osmotic pressure tolerances were influenced by the K+ concentration of the medium. The richer the medium was in K+, the narrower was the spectrum of osmotic pressure tolerance. Once the limit of K+ tolerance was exceeded, the rate of decline of growth was linear with respect to further increases in K+. This rate of decline was independent of osmotic pressure. The initial responses of cells during one subculture (2 to 4 population doublings) in media differing from the standard medium (used to maintain the cell line) were not reliable indicators of the growth potential of the cells. Continued subculture in such media resulted in an upward trend in population growth rates in most cases. 相似文献
40.
P M Brooks J J Walker M A Bell W W Buchanan A R Rhymer 《BMJ (Clinical research ed.)》1975,3(5975):69-11
Plasma profiles of indomethacin after a 50-mg oral dose were constructed in six healthy volunteers before and after a week of aspirin treatment. Aspirin did not interfere with indomethacin plasma levels. To examine the clinical effect of concurrent indomethacin and aspirin treatment 20 patients with seropositive rheumatoid arthritis were given indomethacin 100 mg/day, aspirin soluble 4 g/day, and the two drugs taken together in random order. Analysis of the clinical indices of inflammation--articular index and mean pain score--and of the efficacy of each treatment showed no significant differences between the three treatment groups. With the proliferation in the number of anti-rheumatic drugs available, the case for giving two or more nonsteroidal anti-inflammatory drugs concurrently remains unproved. 相似文献