全文获取类型
收费全文 | 3879篇 |
免费 | 276篇 |
国内免费 | 150篇 |
出版年
2023年 | 44篇 |
2022年 | 25篇 |
2021年 | 57篇 |
2020年 | 72篇 |
2019年 | 119篇 |
2018年 | 117篇 |
2017年 | 88篇 |
2016年 | 107篇 |
2015年 | 93篇 |
2014年 | 199篇 |
2013年 | 291篇 |
2012年 | 160篇 |
2011年 | 221篇 |
2010年 | 157篇 |
2009年 | 173篇 |
2008年 | 194篇 |
2007年 | 214篇 |
2006年 | 178篇 |
2005年 | 147篇 |
2004年 | 137篇 |
2003年 | 135篇 |
2002年 | 121篇 |
2001年 | 84篇 |
2000年 | 68篇 |
1999年 | 74篇 |
1998年 | 47篇 |
1997年 | 47篇 |
1996年 | 48篇 |
1995年 | 33篇 |
1994年 | 41篇 |
1993年 | 30篇 |
1992年 | 43篇 |
1991年 | 21篇 |
1990年 | 26篇 |
1989年 | 12篇 |
1988年 | 20篇 |
1987年 | 17篇 |
1985年 | 46篇 |
1984年 | 69篇 |
1983年 | 39篇 |
1982年 | 45篇 |
1981年 | 68篇 |
1980年 | 49篇 |
1979年 | 53篇 |
1978年 | 43篇 |
1977年 | 43篇 |
1976年 | 38篇 |
1975年 | 44篇 |
1974年 | 41篇 |
1973年 | 41篇 |
排序方式: 共有4305条查询结果,搜索用时 15 毫秒
101.
Summary We have extended earlier experiments on the radiolysis of DL-tryptophan using32P-radiation to longer reaction times, observing complete destruction of the tryptophan by secondary, non-radiolytic processes. We have also undertaken the irradiation of DL-leucine with32P's at -196°, achieving radiolyses to the extents of ca. 20–30%, but observing no concomittant asymmetric bias. The implications of these observations are discussed with regard to the Vester-Ulbricht mechanism for the origin of optical activity. 相似文献
102.
The crystal structure of the cyclic peptide disulfide has been determined by X-ray diffraction. The peptide crystallizes in the space group P212121, with a = 8.646(1), b = 18.462(2), c = 19.678(3)Å and Z = 4. The molecules adopt a highly folded compact conformation, stabilized by two intramolecular 4→ 1 hydrogen bonds between the Cys (1) and Pro (2) CO groups and the Cys (4) and methylamide NH groups, respectively. The backbone conformational angles for the peptide lie very close to those expected for a 310 helix. The S-S bridge adopts a right handed twist with a dihedral angle of 82°. The structure illustrates the role of stereochemically constrained residues, in generating novel peptide conformations. 相似文献
103.
Using steady-state relaxation spectrophotometric technique a P700 component () has been detected which was sensitized by low concentration (10?7M) DCMU in isolated broken chloroplasts of pea. The relative quantum yield of electron flux through DCMU-sensitized P700 was similar to that with methyl viologen or NADP as terminal electron acceptor and water as electron donor. Kinetic analysis showed that a small fraction (10%) of the total P700 reaction centers was sensitized by low DCMU. 相似文献
104.
Gonadally undifferentiated Tilapia nilotica fry were fed with methyl testosterone at the dosage of 50 mg/kg of dry diet. The fry used in the investigation belonged to
two age groups with a mean total length of 10 mm and 13.5 mm. Treatment lasted for a minimum of four weeks and a maximum of
six weeks. 100% males were obtained in all the various groups under androgen treatment. 相似文献
105.
D Rosson H B Otwell R B Dunlap 《Biochemical and biophysical research communications》1980,97(2):500-505
Sulfhydryl-blocked thymidylate synthetase (EC 2.1.1.4.5) is rapidly inactivated by low concentrations of tetranitromethane. This reagent first nitrates two non-essential tyrosines per dimeric enzyme molecule followeed by two essential tyrosines with no oxidation of sulfhydryl groups. dUMP affords significant protection against inactivation. These results suggest that essential tyrosyl residues are present in the active sites of the enzyme. 相似文献
106.
The cytotoxic and mutagenic properties of antitumor drugs such as adriamycin, acridines, diacridine, actinomycin D and Pt compounds are related to their interaction with nucleic acids and inhibition of protein synthesis. We have examined their interaction with human erythrocyte ghost membranes and murine mastocytoma cells using spin labeling techniques. These drugs induce changes in electron spin resonance of the spin labeled ghost membranes and in the mastocytoma cells. These alterations suggest that these drugs induce changes in protein conformation of the membranes. The membrane binding properties of these drugs may be important in their mechanism of action. 相似文献
107.
Spermatozoa of the American oyster Crassostrea virginica were frozen to ?196 °C in concentrated Hanks' salt solution (2.6×) containing 8% dimethyl sulfoxide. About 0.2 ml of spermatozoa that were stored in liquid nitrogen for 68 days fertilized 91% of 65,600 eggs compared to fresh spermatozoa, which fertilized 92% of approximately the same number of eggs from the same females. Larvae resulting from fertilization of fresh eggs with 39-day-old cryopreserved spermatozoa appeared normal after 11 days. 相似文献
108.
S L Marcus S W Smith C I Jarowski M J Modak 《Biochemical and biophysical research communications》1976,70(1):37-44
Guanosine, rather than its methylated derivative, was found to be the inverted nucleoside present in the 5′ terminal capping structure of insect oocyte messenger RNA. Since methylation of the terminal guanosine at the 7 position is necessary for the initiation of protein synthesis in eukaryotes, this evidence suggests that the translational inactivity of the mRNA prior to fertilization may be associated with the absence of methylation. 相似文献
109.
Thomas A. Steitz Robert J. Flatterick Wayne F. Anderson Charles M. Anderson 《Journal of molecular biology》1976,104(1):197-222
The structure determination of yeast hexokinase has been extended to 3.5 Å resolution for the dimer and to 2.7 Å resolution for the monomer using multiple isomorphous replacement. The electron density maps of both the monomer and dimer crystal forms have been substantially improved by an averaging procedure. From these maps the course of the polypeptide backbone and some aspects of the dimer interaction have been established.The hexokinase subunit arrangement is contrary to a major tenet of the Monod et al. (1965) theory of allosteric proteins which postulated that only symmetric or isologous interactions of subunits would occur in oligomeric proteins. One subunit of the dimer is related to the other by a 156 ° rotation about and a 13.8 Å translation along a molecular screw axis. In the hexokinase dimer the set of residues in one subunit that is interacting with the other subunit is different from the set of residues in the second subunit that is interacting with the first subunit. This heterologous or non-symmetric interaction of subunits is associated with some small differences in the structure of the two subunits, particularly at the subunit interface, and accounts for some of this enzyme's non-symmetric interactions with substrates and activators. Indeed, the non-symmetric subunit association may play an important role in the control of this enzyme's activity.The overall structure of hexokinase is considerably different than the known structures of the other enzymes in the glycolytic pathway. Although there is a striking similarity between the domain of hexokinase that binds AMP and the domain of lactate dehydrogenase that binds NAD, the former structure contains both antiparallel and parallel β-pleated strands, while the latter contains only parallel β-structure. In an attempt to assess the significance of this structural similarity, the structure of the nucleotide binding domains of hexokinase and lactate dehydrogenase are compared to a portion of carboxypeptidase A. The observed similarities among these structures suggests that a central β-pleated sheet flanked by α-helices is a common supersecondary structure that probably arose by convergent as well as divergent evolution. Thus, there appears to be no compelling evidence at this time to support the hypothesis that a part of hexokinase has evolved from the same gene as the dinucleotide binding domain of lactate dehydrogenase. 相似文献
110.
Cain G. D. and Bassow F. 1976. Porphyrins in the perienteric fluid of Ascaris lumbricoides. International Journal for Parasitology6: 79–82. Porphyrins in the perienteric fluid of adult female A. lumbricoides were esterified in methanolic H2SO4, extracted in chloroform, separated by thin-layer chromatography, and identified spectrophotometrically before and after conversion to their zinc and copper chelates. Protoporphyrin IX was the major component, comprising 95·4% of the total; the remaining 4·6 % was coproporphyrin III. Uroporphyrin was not detected; no porphyrins were recovered from other worm tissues. Fluid from worms with light and dark colored guts varied in protoporphyrin content from 0·58 to 4·08 nmoles/ml, respectively, but fluid from both groups contained similar molar ratios of protoporphyrin, coproporphyrin and heme. 相似文献