全文获取类型
收费全文 | 55249篇 |
免费 | 3973篇 |
国内免费 | 3194篇 |
专业分类
62416篇 |
出版年
2024年 | 77篇 |
2023年 | 676篇 |
2022年 | 1208篇 |
2021年 | 1354篇 |
2020年 | 1241篇 |
2019年 | 1619篇 |
2018年 | 1623篇 |
2017年 | 1161篇 |
2016年 | 1331篇 |
2015年 | 1913篇 |
2014年 | 2803篇 |
2013年 | 3812篇 |
2012年 | 2050篇 |
2011年 | 2851篇 |
2010年 | 2277篇 |
2009年 | 2884篇 |
2008年 | 3094篇 |
2007年 | 3155篇 |
2006年 | 2877篇 |
2005年 | 2835篇 |
2004年 | 2493篇 |
2003年 | 2238篇 |
2002年 | 2066篇 |
2001年 | 1367篇 |
2000年 | 1167篇 |
1999年 | 1256篇 |
1998年 | 1260篇 |
1997年 | 1070篇 |
1996年 | 852篇 |
1995年 | 945篇 |
1994年 | 880篇 |
1993年 | 781篇 |
1992年 | 700篇 |
1991年 | 501篇 |
1990年 | 407篇 |
1989年 | 376篇 |
1988年 | 401篇 |
1987年 | 354篇 |
1986年 | 291篇 |
1985年 | 347篇 |
1984年 | 453篇 |
1983年 | 303篇 |
1982年 | 299篇 |
1981年 | 186篇 |
1980年 | 176篇 |
1979年 | 147篇 |
1978年 | 85篇 |
1977年 | 48篇 |
1976年 | 43篇 |
1975年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
141.
Two strains ofAspergillus niger were cultured in solid-state fermentation system on carob pods ground from 1.25 to 8 mm diam. A particle size of 2.5 mm gave the highest protein content of the final product (20%, w/w) and 52% of the total soluble carbohydrates were utilized. The total tannin concentration of the carob pods decreased by 83% in 4 days of fermentation.T. Smail and O. Salhi are with the Laboratory of Microbiology, U.R.B.A.F., Institute of Biology, Tizi-Ouzou University, Algeria. J.S. Knapp is with the Department of Microbiology, The University of Leeds, Leeds LS2 9JT, UK; 相似文献
142.
M. B. Swindells 《Protein science : a publication of the Protein Society》1995,4(1):103-112
A procedure is described for detecting domains in proteins of known structure. The method is based on the intuitively simple idea that each domain should contain an identifiable hydrophobic core. By applying the algorithm described in the companion paper (Swindells MB, 1995, Protein Sci 4:93-102) to identify distinct cores in multi-domain proteins, one can use this information to determine both the number and the location of the constituent domains. Tests have shown the procedure to be effective on a number of examples, even when the domains are discontinuous along the sequence. However, deficiencies also occur when hydrophobic cores from different domains continue through the interface region and join one another. 相似文献
143.
Ethanol utilization regulatory protein: profile alignments give no evidence of origin through aldehyde and alcohol dehydrogenase gene fusion. 下载免费PDF全文
H. B. Nicholas Jr B. Persson H. Jrnvall J. Hempel 《Protein science : a publication of the Protein Society》1995,4(12):2621-2624
The suggestion that the ethanol regulatory protein from Aspergillus has its evolutionary origin in a gene fusion between aldehyde and alcohol dehydrogenase genes (Hawkins AR, Lamb HK, Radford A, Moore JD, 1994, Gene 146:145-158) has been tested by profile analysis with aldehyde and alcohol dehydrogenase family profiles. We show that the degree and kind of similarity observed between these profiles and the ethanol regulatory protein sequence is that expected from random sequences of the same composition. This level of similarity fails to support the suggested gene fusion. 相似文献
144.
The SEA module: a new extracellular domain associated with O-glycosylation. 总被引:4,自引:1,他引:3 下载免费PDF全文
Using a variety of homology search methods and multiple alignments, a new extracellular module was identified in (1) agrin, (2) enterokinase, (3) a 63-kDa sea urchin sperm protein, (4) perlecan, (5) the breast cancer marker MUCI (episialin), (6) the cell surface antigen 114/A10, and (7/8) two functionally uncharacterized, probably extracellular, Caenorhabditis elegans proteins. Despite the functional diversity of these adhesive proteins, a common denominator seems to be their existence in heavily glycosylated environments. In addition, the better characterized proteins mentioned above contain all O-glycosidic-linked carbohydrates such as heparan sulfate that contribute considerably to their molecular masses. The common module might regulate or assist binding to neighboring carbohydrate moieties. 相似文献
145.
R. S. DeWitte S. W. Michnick E. I. Shakhnovich 《Protein science : a publication of the Protein Society》1995,4(9):1780-1791
We present an efficient new algorithm that enumerates all possible conformations of a protein that satisfy a given set of distance restraints. Rapid growth of all possible self-avoiding conformations on the diamond lattice provides construction of alpha-carbon representations of a protein fold. We investigated the dependence of the number of conformations on pairwise distance restraints for the proteins crambin, pancreatic trypsin inhibitor, and ubiquitin. Knowledge of between one and two contacts per monomer is shown to be sufficient to restrict the number of candidate structures to approximately 1,000 conformations. Pairwise RMS deviations of atomic position comparisons between pairs of these 1,000 structures revealed that these conformations can be grouped into about 25 families of structures. These results suggest a new approach to assessing alternative protein folds given a very limited number of distance restraints. Such restraints are available from several experimental techniques such as NMR, NOESY, energy transfer fluorescence spectroscopy, and crosslinking experiments. This work focuses on exhaustive enumeration of protein structures with emphasis on the possible use of NOESY-determined distance restraints. 相似文献
146.
Hints on the evolutionary design of a dimeric RNase with special bioactions. 总被引:2,自引:2,他引:0 下载免费PDF全文
A. Di Donato V. Cafaro I. Romeo G. D'Alessio 《Protein science : a publication of the Protein Society》1995,4(8):1470-1477
Residues P19, L28, C31, and C32 have been implicated (Di Donato A, Cafaro V, D'Alessio G, 1994, J Biol Chem 269:17394-17396; Mazzarella L, Vitagliano L, Zagari A, 1995, Proc Natl Acad Sci USA: forthcoming) with key roles in determining the dimeric structure and the N-terminal domain swapping of seminal RNase. In an attempt to have a clearer understanding of the structural and functional significance of these residues in seminal RNase, a series of mutants of pancreatic RNase A was constructed in which one or more of the four residues were introduced into RNase A. The RNase mutants were examined for: (1) the ability to form dimers; (2) the capacity to exchange their N-terminal domains; (3) resistance to selective cleavage by subtilisin; and (4) antitumor activity. The experiments demonstrated that: (1) the presence of intersubunit disulfides is both necessary and sufficient for engendering a stably dimeric RNase; (2) all four residues play a role in determining the exchange of N-terminal domains; (3) the exchange is the molecular basis for the RNase antitumor action; and (4) this exchange is not a prerequisite in an evolutionary mechanism for the generation of dimeric RNases. 相似文献
147.
Solution structure of the potassium channel inhibitor agitoxin 2: caliper for probing channel geometry. 总被引:5,自引:1,他引:4 下载免费PDF全文
A. M. Krezel C. Kasibhatla P. Hidalgo R. MacKinnon G. Wagner 《Protein science : a publication of the Protein Society》1995,4(8):1478-1489
The structure of the potassium channel blocker agitoxin 2 was solved by solution NMR methods. The structure consists of a triple-stranded antiparallel beta-sheet and a single helix covering one face of the beta-sheet. The cysteine side chains connecting the beta-sheet and the helix form the core of the molecule. One edge of the beta-sheet and the adjacent face of the helix form the interface with the Shaker K+ channel. The fold of agitoxin is homologous to the previously determined folds of scorpion venom toxins. However, agitoxin 2 differs significantly from the other channel blockers in the specificity of its interactions. This study was thus focused on a precise characterization of the surface residues at the face of the protein interacting with the Shaker K+ channel. The rigid toxin molecule can be used to estimate dimensions of the potassium channel. Surface-exposed residues, Arg24, Lys27, and Arg31 of the beta-sheet, have been identified from mutagenesis studies as functionally important for blocking the Shaker K+ channel. The sequential and spatial locations of Arg24 and Arg31 are not conserved among the homologous toxins. Knowledge on the details of the channel-binding sites of agitoxin 2 formed a basis for site-directed mutagenesis studies of the toxin and the K+ channel sequences. Observed interactions between mutated toxin and channel are being used to elucidate the channel structure and mechanisms of channel-toxin interactions. 相似文献
148.
S.N. Khrapunov A.I. Dragan A.F. Protas G.D. Berdyshev 《International journal of biological macromolecules》1984,6(1):31-34
It was shown that the histone tetramer (H3-H4)2 fluorescence spectra were shifted by about 2 nm towards the long-wave region and had a larger halfwidth than the free tyrosine fluorescence spectra. Denaturation with 8 m urea resulted in a shift towards the short-wave region and a decrease in the halfwidth of the histone tetramer (H3-H4)2 tyrosine fluorescence spectra. Fluorescence quenching of the histone tetramer (H3-H4)2 by iodine ions was analysed by the Stern-Volmer equation. It was estimated that at 0.1 m NaCl and 0.3–0.8 m NaCl, 45% and 60% tyrosyl fluorescence, respectively, was quenched by I? ions. The results obtained suggests that histone tetramer (H3-H4)2 may have several structural forms distinguished by the amount of ‘exposed’ and ‘buried’ tyrosyls depending upon the conditions of the medium. 相似文献
149.
Salt-tolerance in plants. I. Ions, compatible organic solutes and the stability of plant ribosomes 总被引:7,自引:4,他引:3
C. J. BRADY T. S. GIBSON† E. W. R. BARLOW‡ J. SPEIRS R. G. WYN JONES§ 《Plant, cell & environment》1984,7(8):571-578
Abstract Polysomes and ribosomes recovered from a number of plant species were tested for stability when incubated at 25°C in salt solutions in the absence of ATP and initiation factors. Stability was assessed by sucrose density gradient analysis. The stability was inversely proportional to salt concentrations above 125 mol m−3 KCl. Polysomes were less stable in the presence of Na+ than K+ salts, and were much less stable in Cl− than in acetate salts. Polysomes from Triticum aestivum. Hordeum vulgare, Capsicum annuum, Helianthus annuus. Pisum sativum, Atriplex nummularia, Beta vulgaris, Cladophora sp., Enteromorpha sp. and Corallina cuvieri were similarly sensitive to KCl. Polysomes from Ulva lactuca were more sensitive than the other species. Cytoplasmic and plastid polysomes from T. aestivum were similarly unstable in 500 mol m−3 KCl. Unprogrammed ribosomal subunit couples from T. aestivum, B. vulgaris and U. lactuca showed Mg2+-dependent conformational instability and dissociation in KCl. Slight differences in ribosomal stability were observed between species, but these were unrelated to the salt tolerances of the plants. The ‘compatible’ organic solutes, glycinebetaine and proline, failed to reduce ion-induced instability. Ribosome yield and polysome profiles were similar in leaves of B. vulgaris containing significantly different levels of both Na+ and Cl− after growth in media containing 50 or 200 mol m−3 NaCl. The results are consistent with the hypothesis that plants maintain a cytoplasmic solute environment that is compatible with ribosomal stability. 相似文献
150.
Paul S. Moss Dennis H. Spector Charles A. Glass Richard C. Strohman 《In vitro cellular & developmental biology. Plant》1984,20(6):473-478
Summary As part of an effort to optimize conditions required for the complete maturation of muscle cells in vitro, we have investigated
the effects of the antibiotics penicillin, streptomycin, and Fungizone (amphotericin B) on the development of cultured chick
embryo skeletal muscle. It is shown that even low dosages of streptomycin, but not penicillin or Fungizone, retard protein
synthesis and accumulation in these cultures. Myosin accumulation was also reduced and the appearance of striations in fused
cells was delayed in myotubes formed in medium containing streptomycin. Additional data suggest that this overall retardation
of myogenesis is due to the influence of streptomycin on maturing myotubes rather than early proliferation and cell fusion.
These results are discussed with regard to recent efforts to promote the full maturation of muscle cells grown in culture.
This research was supported by National Institutes of Health Grant NS 155882 and a Task Force on Drug Development Research
Contract from The Muscular Dystrophy Association. 相似文献