全文获取类型
收费全文 | 78853篇 |
免费 | 6532篇 |
国内免费 | 4881篇 |
专业分类
90266篇 |
出版年
2024年 | 138篇 |
2023年 | 901篇 |
2022年 | 2075篇 |
2021年 | 3602篇 |
2020年 | 2327篇 |
2019年 | 2839篇 |
2018年 | 2869篇 |
2017年 | 2029篇 |
2016年 | 2874篇 |
2015年 | 4585篇 |
2014年 | 5294篇 |
2013年 | 5963篇 |
2012年 | 6897篇 |
2011年 | 6355篇 |
2010年 | 3818篇 |
2009年 | 3372篇 |
2008年 | 4112篇 |
2007年 | 3652篇 |
2006年 | 3173篇 |
2005年 | 2679篇 |
2004年 | 2276篇 |
2003年 | 1972篇 |
2002年 | 1731篇 |
2001年 | 1559篇 |
2000年 | 1565篇 |
1999年 | 1447篇 |
1998年 | 846篇 |
1997年 | 797篇 |
1996年 | 808篇 |
1995年 | 736篇 |
1994年 | 687篇 |
1993年 | 530篇 |
1992年 | 818篇 |
1991年 | 657篇 |
1990年 | 601篇 |
1989年 | 531篇 |
1988年 | 421篇 |
1987年 | 362篇 |
1986年 | 336篇 |
1985年 | 299篇 |
1984年 | 221篇 |
1983年 | 199篇 |
1982年 | 112篇 |
1981年 | 118篇 |
1980年 | 86篇 |
1979年 | 147篇 |
1978年 | 84篇 |
1977年 | 95篇 |
1975年 | 111篇 |
1974年 | 116篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Affinity labeling studies of NADP(+)-glutamate dehydrogenase from Salmonella typhimurium have shown that the peptide Leu-282-Lys-286 is located near the coenzyme site [Haeffner-Gormley et al. (1991) J. Biol. Chem. 266, 5388-5394]. The present study was undertaken to evaluate the role of lysine-286. The mutant enzymes K286R, K286Q, and K286E were prepared by site-directed mutagenesis, expressed in Escherichia coli, and purified. The Vmax values (micromoles of NADPH per minute per milligram of protein) were similar for WT (270), K286R (529), K296Q (409), and K286E (382) enzymes. As measured at pH 7.9, the Km value for NADPH was much greater for K286E (280 microM) than for WT (9.8 microM), K286R (30 microM), or K286Q (66 microM) enzymes. The efficiencies (kcat/Km) of the WT and K286R mutant were similar (1.2 x 10(3) min-1 microM-1 and 1.0 x 10(3) min-1 microM-1, respectively) while those of K286Q (0.30 x 10(3) min-1 microM-1) and K286E (0.07 x 10(3) min-1 microM-1) were greatly reduced. The decreased efficiency of the K286E mutant results from the increase in Km-NADPH, consistent with a role for a basic residue at position 286 which enhances the binding of NADPH. Plots of Vmax vs pH showed the pH optima to be 8.1-8.3 for all enzymes at saturating NADPH concentrations. A 40-fold increase in Km-NADPH for K286E was observed as the pH increased from 5.98 to 8.08, from which a unique pKe of 6.5 was calculated.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
32.
S D Kroll J Chen M De Vivo D J Carty A Buku R T Premont R Iyengar 《The Journal of biological chemistry》1992,267(32):23183-23188
33.
L L Chen J J Rosa S Turner R B Pepinsky 《The Journal of biological chemistry》1991,266(27):18237-18243
We have developed a three-step cross-linking procedure that is specifically targeted at the carbohydrate on a protein and applied it to CD4 as a model system for studying the role of multivalent interactions in function. In the first step CD4 was oxidized with periodate, creating aldehydes that served as targets for the subsequent chemistry. Next the aldehydes were modified with cystamine, converting the reactive group into a thiol. Finally cross-linking through the thiol moiety was generated with the homobifunctional cross-linker bismaleimidohexane. With this procedure, approximately 60% of the CD4 was converted into higher molecular weight complexes that were soluble and retained function as assessed by glycoprotein gp120 binding activity. CD4 dimers and tetramers by mass were 4 and 15 times as active as CD4 monomer in blocking virus infection with HTLV-IIIB in an in vitro cellular assay. The cross-linking chemistry provides an efficient method for producing homomultimers of a glycoprotein. 相似文献
34.
35.
A strategy for high cell density culture of heterotrophic microalgae with inhibitory substrates 总被引:5,自引:0,他引:5
Substrate inhibition is one of the major problems preventing high cell densities of microalgae in heterotrophic culture, so
the possibility of overcoming the problem by various culture techniques was examined. It was found that perfusion culture
may be the most appropriate technique for high cell densities in heterotrophic culture using inhibitory substrates. An experimental
example in which a hollow fibre cell recycle system (HFCRS) was employed to achieve high cell densities of Chlamydomonas reinhardtii on acetate under heterotrophic conditions of growth was demonstrated. The cell density in the HFCRS was much higher than
that reported in the literature for this species. 相似文献
36.
James M. Chen Rosalyn Grad Regina Monaco Matthew R. Pincus 《Journal of Protein Chemistry》1996,15(1):11-16
rap-1A, an anti-oncogene-encoded protein, is aras-p21-like protein whose sequence is over 80% homologous to p21 and which interacts with the same intracellular target proteins and is activated by the same mechanisms as p21, e.g., by binding GTP in place of GDP. Both interact with effector proteins in the same region, involving residues 32–47. However, activated rap-1A blocks the mitogenic signal transducing effects of p21. Optimal sequence alignment of p21 and rap-1A shows two insertions of rap-1A atras positions 120 and 138. We have constructed the three-dimensional structure of rap-1A bound to GTP by using the energy-minimized three-dimensional structure ofras-p21 as the basis for the modeling using a stepwise procedure in which identical and homologous amino acid residues in rap-1A are assumed to adopt the same conformation as the corresponding residues in p21. Side-chain conformations for homologous and nonhomologous residues are generated in conformations that are as close as possible to those of the corresponding side chains in p21. The entire structure has been subjected to a nested series of energy minimizations. The final predicted structure has an overall backbone deviation of 0.7 å from that ofras-p21. The effector binding domains from residues 32–47 are identical in both proteins (except for different side chains of different residues at position 45). A major difference occurs in the insertion region at residue 120. This region is in the middle of another effector loop of the p21 protein involving residues 115–126. Differences in sequence and structure in this region may contribute to the differences in cellular functions of these two proteins. 相似文献
37.
Oat leaf base: tissue with an efficient regeneration capacity 总被引:1,自引:1,他引:0
Summary An efficient short term regeneration system using seedling derived oat (Avena sativa) leaf tissue has been developed. Callus derived from the leaf base showed a higher response of plant regeneration than callus initiated from mesocotyls and more mature parts of the leaves. A correlation between the nuclear DNA content of the donor material, as analysed with flow cytometry, and its ability to form callus was observed. Somatic embryogenesis was histologically recognised from callus derived from tissue close to the apical meristem. Plant regeneration media with various concentrations of auxin were tested. Callus from three different cultivars had a similar regeneration potential with an optimal regeneration frequency of 60%. About 2 months after inoculation regenerated plantlets could be moved to a greenhouse for cultivation.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- DAPI
6-diamidino-2-phenylindole
- IAA
indole-3-acetic acid
- KT
kinetin
- MS
Murashige and Skoog's medium
- NAA
naphthalene acetic acid 相似文献
38.
39.
RNA polymerase I was purified from chromatin isolated from auxin-treated soybean hypocotyl. Purification was achieved by using Agarose A-1.5m gel filtration, DEAE-cellulose, CM-sephadex, and phosphocellulose chromatography, and sucrose density gradient centrifugation. With denatured calf thymus DNA as template, the enzyme has a high specific activity (200-300 nmol/mg/30 min at 28 degrees C) which is comparable to other RNA polymerase I enzymes purified from animals and yeast. While the gel profiles indicate that purification to homogeneity (greater than 90%) may not have been achieved, the enzyme appears to be composed of possibly 7 subunits, several of which are similar to the subunits of yeast RNA polymerase I. The putative subunits and molar ratios are 183 000 (1), 136 000 (1), 50 000 (0.5), 46 000 (0.5), 40 000 (0.5), 33 000 (0.2), and 28 000 (2). The purified enzyme strongly prefers a completely denatured template such as poly(dC). 相似文献
40.
Xue-Yuan Dong Yun-Yan Li Xiao-Ang Yang Wei-Feng Chen 《Biochimie et biologie cellulaire》2004,82(5):577-582
In an effort to identify novel Cancer-Testis genes, we analyzed the sequence in the q26-28 region of human X chromosome by several on-line tools. The candidate sequences were then confirmed by experiments. We have obtained a novel Cancer-Testis gene, BJ-HCC-20. In vivo, it was found to have two isoforms. In samples of liver, colon, gastric and lung cancer tested, the expression frequency of BJ-HCC-20 is 25%, 17%, 21% and 15%, respectively. Full-length cDNAs of both BJ-HCC-20 isoforms were isolated and their gene structures and promoter regions were characterized. BJ-HCC-20 might have implications in theoretical and practical tumor biology. 相似文献