全文获取类型
收费全文 | 3684篇 |
免费 | 336篇 |
出版年
2021年 | 40篇 |
2020年 | 31篇 |
2019年 | 34篇 |
2018年 | 34篇 |
2017年 | 39篇 |
2016年 | 58篇 |
2015年 | 98篇 |
2014年 | 131篇 |
2013年 | 126篇 |
2012年 | 143篇 |
2011年 | 181篇 |
2010年 | 80篇 |
2009年 | 111篇 |
2008年 | 133篇 |
2007年 | 126篇 |
2006年 | 136篇 |
2005年 | 127篇 |
2004年 | 105篇 |
2003年 | 114篇 |
2002年 | 91篇 |
2001年 | 100篇 |
2000年 | 116篇 |
1999年 | 97篇 |
1998年 | 40篇 |
1997年 | 57篇 |
1996年 | 50篇 |
1994年 | 30篇 |
1993年 | 38篇 |
1992年 | 78篇 |
1991年 | 90篇 |
1990年 | 78篇 |
1989年 | 70篇 |
1988年 | 68篇 |
1987年 | 52篇 |
1986年 | 56篇 |
1985年 | 77篇 |
1984年 | 81篇 |
1983年 | 68篇 |
1982年 | 55篇 |
1981年 | 46篇 |
1980年 | 46篇 |
1979年 | 61篇 |
1978年 | 53篇 |
1977年 | 46篇 |
1976年 | 36篇 |
1975年 | 41篇 |
1974年 | 45篇 |
1973年 | 26篇 |
1972年 | 36篇 |
1969年 | 26篇 |
排序方式: 共有4020条查询结果,搜索用时 31 毫秒
941.
942.
D. Glenn Kennedy Finbarr P. M. O'Harte W. John Blanchflower Desmond A. Rice 《Biological trace element research》1991,28(3):233-241
The changes in propionate metabolism that accompany cobalt deficiency in sheep are described. Two groups of sheep, fed either a cobalt sufficient or deficient diet, were given an iv propionate load at intervals during a 14 w experiment. There was a tendency towards increased propionate half-life as the animals became cobalt deficient. However, significant changes in the area under the plasma methylmalonic acid-time curve occurred very early, indicating significant impairment of propionate metabolism. Despite this, the area under the plasma glucose-time curve was unaffected by cobalt deficiency, suggesting that the impairment of propionate metabolism, although significant, is not extensive. 相似文献
943.
944.
11B n.m.r. spectroscopy studies of solutions of sugars in the presence of borax have shown the existence of boron-containing complexes. Chemical-shift values indicate the size of the ring in which boron is involved. Use of this method and p.m.r. spectroscopy showed that D-glucose forms a 1,2-furanoid and 1,2-pyranoid complex in the presence of borax and benzeneboronic acid. 相似文献
945.
946.
Hazinski T. A.; France M.; Kennedy K. A.; Hansen T. N. 《Journal of applied physiology》1989,67(6):2586-2592
We previously reported that pretreatment with endotoxin significantly reduced acute pulmonary O2 toxicity in lambs (J. Appl. Physiol. 65: 1579-1585, 1988). One of endotoxin's many effects is to inhibit cytochrome P-450 mono-oxygenation reactions, which are believed to produce toxic O2 species. Therefore, one possible explanation for endotoxin's beneficial effect is that it inhibited P-450-mediated O2 radical production during hyperoxia. To test this hypothesis, we administered a single dose of cimetidine, a noncompetitive inhibitor of P-450 activity, to nine lambs before continuous exposure to greater than 95% O2. Compared with six control O2-exposed lambs, the cimetidine-treated O2-exposed lambs maintained normal gas exchange for a longer period of time (P less than 0.01), accumulated lung water at a slower rate (P less than 0.01), and had normal microvascular permeability after 72 h of O2 exposure. Postmortem levels of antioxidant enzymes in blood-free lung homogenate were not increased in cimetidine-treated lambs. However, the levels of oxidized glutathione were significantly lower in cimetidine-treated lambs, and the ratio of reduced to oxidized glutathione concentrations (GSH/GSSG ratio) was sevenfold higher than the ratio measured in control O2-exposed lambs (P less than 0.001). In four lambs, pretreatment with ranitidine (a drug chemically related to cimetidine but without P-450 inhibitory activity) had no effect either on the time course of O2 injury or on postmortem antioxidants. Microsomes were isolated from blood-free lung of all study animals and P-450 activity of the form 2 isozyme was measured.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
947.
Summary A large increase in the incidence of bacteriophage mutants is found after photoreactivation of UV-irradiated phage S13. The
increase was seen only when the irradiated phage were stored before they were photoreactivated; the maximum mutation frequency
was achieved after storage for 2 h at 4° C or 30 min at 37° C. The mutations can be attributed entirely to deamination of
cytosine in cyclobutane dimers. Naked S13 DNA was stored for 2 h at 37° C after being irradiated with wavelengths ≥290 nm
in the presence of 0.2% acetophenone, which sensitizes the formation of thymine-thymine but not cytosine-containing dimers;
the specific mutation frequency was 7.2-fold lower compared to the frequency produced by irradiation in the absence of the
photosensitizer, confirming that cytosine dimers are a major source of mutations. These results undermine the basis for the
two-step model of UV mutagenesis in which a distinctly separate misincorporation step is supposed to precede the lesion bypass
step; instead the results support a different two-step model, in which a deamination step precedes the bypass. The S13 capsid
appears to completely inhibit the putative deamination reaction at about 75% of the dimer sites. 相似文献
948.
949.
13C-nmr spectra of lysozyme obtained at 50.3 MHz using both static and magic-angle-spinning-cross-polarization methods are reported at several water contents. The line widths and consequent resolution in the hydrated material is substantially improved over that in the lyophilized protein. The line narrowing is not commensurate with loss of a proton-carbon dipole-dipole coupling or dramatic changes in the relaxation parameters characterizing magnetization transfer from protons to carbon in the Hartmann-Hahn cross-polarization experiment. We interpret these data in terms of the water inducing a decrease in the distribution of local conformations sampled by the protein, although the magnitude of the conformational reorientations required to account for the data are not necessarily large nor do they imply a major unfolding of the protein on dehydration. 相似文献
950.