全文获取类型
收费全文 | 1642篇 |
免费 | 206篇 |
专业分类
1848篇 |
出版年
2021年 | 21篇 |
2020年 | 17篇 |
2019年 | 23篇 |
2018年 | 16篇 |
2017年 | 22篇 |
2016年 | 42篇 |
2015年 | 53篇 |
2014年 | 67篇 |
2013年 | 73篇 |
2012年 | 103篇 |
2011年 | 114篇 |
2010年 | 76篇 |
2009年 | 48篇 |
2008年 | 74篇 |
2007年 | 50篇 |
2006年 | 48篇 |
2005年 | 46篇 |
2004年 | 52篇 |
2003年 | 55篇 |
2002年 | 55篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 40篇 |
1998年 | 25篇 |
1997年 | 21篇 |
1996年 | 34篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 21篇 |
1992年 | 28篇 |
1991年 | 30篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 22篇 |
1987年 | 20篇 |
1986年 | 23篇 |
1984年 | 21篇 |
1983年 | 20篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 14篇 |
1979年 | 16篇 |
1976年 | 15篇 |
1974年 | 18篇 |
1973年 | 14篇 |
1972年 | 14篇 |
1971年 | 16篇 |
1969年 | 21篇 |
1967年 | 12篇 |
1966年 | 14篇 |
排序方式: 共有1848条查询结果,搜索用时 0 毫秒
41.
Infrared dichroic studies and deuterium exchange measurements of somatostatin and some of its analogs incorporated in uniaxially oriented polyoxyethylene are described. Band positions and dichroic ratios in the N-H stretching and amide I and II regions are similar to those of flexible and nonordered peptides like valinomycin and poly[Glu(ONa)]. This information, together with fast deuterium exchange rates, suggests that somatostatin exists in a flexible nonordered conformation in polyoxyethylene. One analog, di-S3,14-acetamidomethyl dihydrosomatostatin, was found to exist in both nonordered and β-like conformations. 相似文献
42.
Proline specific endo- and exopeptidases 总被引:12,自引:0,他引:12
Roderich Walter William H. Simmons Tadashi Yoshimoto 《Molecular and cellular biochemistry》1980,30(2):111-127
Summary Peptidases which are specific for proline residues have been described and include endopeptidases (post-proline cleaving enzyme and proline specific endopeptidase), N-terminal exopeptidases (post-proline dipeptidyl aminopeptidase, proline iminopeptidase, aminopeptidase P), C-terminal exopeptidases (prolylcarboxypeptidase, and carboxypeptidase P) and dipeptidases (prolyl dipeptidase and proline dipeptidase). The properties, distinguishing characteristics, and possible significance of these proline specific endo- and exopeptidases are discussed. In addition, reference is made to a series of enzymes which can hydrolyze proline containing peptide bonds, but which are not specific for proline. 相似文献
43.
D W Green R Aschaffenburg A Camerman J C Coppola P Dunnill R M Simmons E S Komorowski L Sawyer E M Turner K F Woods 《Journal of molecular biology》1979,131(2):375-397
Bovine β-lactoglobulin is a dimer with a molecular weight of 2 × 18,400. In solution it undergoes a pH-dependent transition at pH 7.0 between two alternative structures, named N and R. The structures of four different crystal forms have been determined by multiple isomorphous replacement with heavy-atoms. Two of them, lattices K and X, were crystallised at pH 6.5, corresponding to the N state in solution; and the other two, lattices Y and Z, were crystallised at pH 7.5, corresponding to the R state in solution. The figures of merit of the phase angles determined for these lattices were 0.76, 0.77, 0.80 and 0.80, respectively. The four structures that emerged are similar and show certain features suggestive of α-helices and pleated sheets, but the resolution is insufficient to trace the entire course of the polypeptide chain. No clear distinction can yet be made between the structures above or below pH 7.0, nor between the native molecule and the molecule from which the C-terminal leucine and histidine residues have been cleaved. Analyses at higher resolution are in progress. 相似文献
44.
45.
46.
Analysis of adenosine triphosphate (ATP) from surficial sediment layers in two antarctic lakes and two temperate lakes showed a high degree of similarity in spite of differences between trophic state, mictic state, or geographic location. Adenosine triphosphate was found at all levels sampled in temperate lake sediment cores but occasionally was present only in surficial layers of antarctic cores. Surficial sediment layers from antarctic lakes contained high chlorophylla (Chla) levels due to the extensive benthic algal mats which occur there. In some antarctic cores, Chla was detectable in deep, old mat layers, whereas Chla was not found in any of the temperate lake cores. Antarctic lake sediments appear to be unique environments where Chla molecules can remain intact for long periods of time due to low light, temperature, and microbial activity. As such, these lakes are important natural laboratories where a long history of microbial interactions can be studied without metazoan perturbation effects. Although there was much variability in concentration of Chla and ATP between samples, there appears to be no relationship between Chla or ATP levels to mictic or trophic states of the lakes. These data suggest that sediment microbial communities may be independent of environmental and biological properties of the overlying water masses. 相似文献
47.
S N Vinogradov P K Sharma A N Qabar J S Wall J A Westrick J H Simmons S J Gill 《The Journal of biological chemistry》1991,266(20):13091-13096
Repeated dissociation of the approximately 3600-kDa hexagonal bilayer extracellular hemoglobin of Lumbricus terrestris in 4 M urea followed by gel filtration at neutral pH produces a subunit that retains the oxygen affinity of the native molecule (approximately 12 torr), but only two-thirds of the cooperativity (nmax = 2.1 +/- 0.2 versus 3.3 +/- 0.3). The mass of this subunit was estimated to be 202 +/- 15 kDa by gel filtration and 202 +/- 26 kDa from mass measurements of unstained freeze-dried specimens by scanning transmission electron microscopy. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this subunit showed that it consists predominantly of the heme-containing subunits M (chain I, 17 kDa) and T (disulfide-bonded chains II-IV, 50 kDa). Mixing of subunits M and T isolated concurrently with the 200-kDa subunit resulted in partial association into particles that had a mass of 191 +/- 13 kDa determined by gel filtration and 200 +/- 38 kDa determined by scanning transmission electron microscopy and whose oxygen affinity and cooperativity were the same as those of the 200-kDa subunit. The results imply that the 200-kDa subunit is a dodecamer of globin chains, consisting of three copies each of subunits M and T (3 x chains (I + II + III + IV], in good agreement with the mass of 209 kDa calculated from the amino acid sequences of the four chains, and represents the largest functional subunit of Lumbricus hemoglobin. Twelve copies of this subunit would account for two-thirds of the total mass of the molecule, as suggested earlier (Vinogradov, S. N., Lugo, S. L., Mainwaring, M. G., Kapp, O. H., and Crewe, A. V. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 8034-8038). The retention of only partial cooperativity by the 200-kDa subunit implies that full cooperativity is dependent on the presence of a complete hexagonal bilayer structure, wherein 12 200-kDa subunits are linked together by approximately 30-kDa heme-deficient chains. 相似文献
48.
J R Lancaster J M Langrehr H A Bergonia N Murase R L Simmons R A Hoffman 《The Journal of biological chemistry》1992,267(16):10994-10998
The paramagnetic molecule nitric oxide (NO), produced from L-arginine by a specific enzyme (NO synthase), has been shown to be involved in a surprising variety of mammalian cellular responses, including the regulation of T cell immunity to alloantigens in vitro. In cytotoxic activated macrophages, NO production results in a characteristic pattern of alteration of iron-containing enzyme function that is mimicked by exposure to NO. Electron paramagnetic resonance (EPR) studies have shown the formation of iron-nitrosyl species during macrophage activation and also during sepsis, indicating that alteration of iron-containing protein function may be the result of the well-documented tendency of NO to bind to metal ions. We have recently shown that the NO synthesis induced during alloantigenic activation of rat splenocytes inhibits lymphocyte proliferation and cytotoxic T-lymphocyte generation. This report demonstrates that iron-nitrosyl EPR signals similar to those observed in macrophages and during sepsis are present in the blood and in the grafted tissue of rats during the rejection of allogeneic (but not syngeneic) heart grafts. These signals are found in the blood and at the site of allograft rejection, but are not found in other tissues (such as spleen and lung), and are obliterated by administration of the immunosuppressant FK506. These results directly demonstrate the formation of iron-nitrosyl complexes during vascularized allograft rejection and suggest that consequent destruction of iron-containing protein function plays an important role in the rejection response. 相似文献
49.
Nitric oxide mediates iron release from ferritin 总被引:16,自引:0,他引:16
Nitric oxide (NO) synthesis by cytotoxic activated macrophages has been postulated to result in a progressive loss of iron from tumor target cells as well as inhibition of mitochondrial respiration and DNA synthesis. In the present study, the addition of an NO-generating agent, sodium nitroprusside, to the iron storage protein ferritin resulted in the release of iron from ferritin and the released iron-catalyzed lipid peroxidation. Hemoglobin, which binds NO, and superoxide anion, which reacts with NO, inhibited nitroprusside-dependent iron release from ferritin, thereby providing evidence that NO can mobilize iron from ferritin. These results suggest that NO generation in vivo could lead to the mobilization of iron from ferritin disrupting intracellular iron homeostasis and increasing the level of reactive oxygen species. 相似文献
50.
N. L. Simmons 《The Journal of membrane biology》1981,59(2):105-114
Summary Cultured monolayers of MDCK cells grown upon filter supports display many features ofin vivo epithelia. Previously reported values of transmonolayer resistance of 100 cm–2 (Misfeldt, Hamamoto & Pitelka, 1976; Cereijido, Robbins, Dolan, Rotunno & Sabatini, 1978) indicate a leaky epithelium. This paper describes the properties of a strain of MDCK cells which displays entirely different electrophysiological properties. The results show that (i) the mean transmonolayer resistance is 4.16 k cm–2, (ii) transmonolayer ion transport is of small magnitude since the mean spontaneous open circuit PD is only 2.17 mV basal surface positive and isotopic Na and Cl flux measurements fail to demonstrate a significant net flux, (iii) the action of ouabain, amiloride and ion substitutions are consistent with transmonolayer net Na movement being largely responsible for the spontaneous PD, and (iv) asymmetry in the localization of the Na-K ATPase is evident on the basis of3H-ouabain binding to cell monolayer. 相似文献