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   711533篇
  免费   75850篇
  国内免费   546篇
  2018年   6593篇
  2016年   8866篇
  2015年   12956篇
  2014年   14386篇
  2013年   20733篇
  2012年   23350篇
  2011年   23209篇
  2010年   15278篇
  2009年   13543篇
  2008年   20496篇
  2007年   20935篇
  2006年   19801篇
  2005年   19019篇
  2004年   18645篇
  2003年   17721篇
  2002年   17218篇
  2001年   29014篇
  2000年   29467篇
  1999年   23511篇
  1998年   8319篇
  1997年   8644篇
  1996年   8142篇
  1995年   7582篇
  1994年   7489篇
  1993年   7473篇
  1992年   19206篇
  1991年   18686篇
  1990年   18094篇
  1989年   17655篇
  1988年   16531篇
  1987年   15807篇
  1986年   14805篇
  1985年   15042篇
  1984年   12515篇
  1983年   10730篇
  1982年   8521篇
  1981年   7906篇
  1980年   7272篇
  1979年   12167篇
  1978年   9684篇
  1977年   9046篇
  1976年   8505篇
  1975年   9431篇
  1974年   10438篇
  1973年   10316篇
  1972年   9349篇
  1971年   8644篇
  1970年   7627篇
  1969年   7454篇
  1968年   6834篇
排序方式: 共有10000条查询结果,搜索用时 704 毫秒
131.
132.
133.
134.
Reactivity of histidine residues in equine growth hormone to ethoxyformic anhydride was studied. The existence of two kinetically different sets was demonstrated: one of them including only the slow reacting histidine 169 (k = 0.164 min-1) and the other containing fast reacting histidines 19 and 21 (k = 0.892 min-1). A correlation between the decrease in the capacity to compete with 125I-labeled hormone for rat liver binding sites and the degree of ethoxyformylation of the fast group was found. Circular dichroism studies indicated no significant conformational changes in the protein with all three residues modified. These results fully agree with those obtained for bovine growth hormone which is further evidence supporting the vinculation of histidines 19 and/or 21 with the binding site of these hormones to their specific receptors.  相似文献   
135.
136.
Neurospora grows vegetatively as a syncytium in which multiple nuclei exist within a connected cytoplasm. Because of the ability of separate and distinct mycelia to fuse, the possibility exists of generating heterocaryotic cultures in which the nuclei and cytoplasms of two different strains are comingled into the same syncytium. We have used such heterocaryons, in which the component parts differed with respect to their circadian clock phase, to examine whether or not clock-dominant phases exist in the circadian cycle. To this end, the phase subsequent to the formation of heterocaryons by pairs of mycelial discs that are initially at different circadian phases was examined in Neurospora crassa. The resulting phase was an average of the parent phases in many cases, but was sometimes observed to correspond more closely to just one of the original parental phases. In these cases, we did not observe any dominant phases in the circadian cycle; the phase of a particular parent disc was more dominant in the heterocaryon when the proportion of the nuclei from that parent was greater in the heterocaryon. In some instances, which occurred mostly when the difference in phase of the parental discs was large, the resultant phase could not be related in a simple way to the parental phases. An interpretation based on a limit cycle model of the circadian oscillation is possible.  相似文献   
137.
Chromatophores from Rhodopseudomonas capsulata cells grown semiaerobically in the dark oxidize NADH, succinate, and dichlorophenolindophenol. In the presence of N3? these activities are inhibited, but light induces oxidation of dichlorophenolindophenol with O2 as a terminal electron acceptor. Cyanide also inhibits electron transport but much higher concentrations are required to inhibit the photooxidation than the dark oxidation. The photooxidation was studied in a mutant strain of Rhodopseudomonas capsulata (YIV) which cannot grow anaerobically in the light, but similarly to the wild type, grows in the presence of oxygen. Chromatophores from YIV mutant catalyze photophosphorylation and dark oxidation activities with the same properties as those of the wild type. However, the rate of photooxidation in the mutant is only one-third that of the wild type. Based on the differential inhibitor sensitivity and on the mutation it is suggested that the photooxidase is different from the two respiratory oxidases and that this photooxidation activity might be essential for growth of the cells under anaerobic conditions in the light.  相似文献   
138.
139.
140.
HbA1c is the most prevalent of the minor human hemoglobins. It is formed by the nonenzymatic addition of glucose to the alpha-amino group of the beta chain by an initial condensation reaction and a subsequent intermolecular Amadori rearrangement. We have developed a method of analysis which utilizes high performance liquid chromatography to follow the formation of HbA1c and greatly simplifies the determination of the kinetic parameters associated with this reaction. This has allowed us to study the effects of several Hb ligands, including the hydrogen ion, on the kinetics of this glycosylation reaction. Both the initial condensation reaction and the subsequent rearrangement are shown to exhibit acid catalysis, but the rate of the condensation step is limited by the extent of protonation of the alpha-amino group. The variation in kinetic parameters as a function of hydrogen ion concentration has allowed us to determine the probable reaction mechanism of HbA1c formation by comparison to previously reported model systems of Schiff base formation and Amadori rearrangement. The formation of pre-HbA1c from deoxy-Hb shows an increased forward rate when compared to oxy-Hb. The presence of physiologic concentrations of CO2 causes a proportional decrease in both k1 and k-1. 2,3-Diphosphoglycerate causes a significant increase in the keq of the formation reaction. The effects of CO and the substitution of L-glucose for D-glucose are not significant.  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