全文获取类型
收费全文 | 48104篇 |
免费 | 4052篇 |
国内免费 | 1458篇 |
专业分类
53614篇 |
出版年
2023年 | 708篇 |
2022年 | 1037篇 |
2021年 | 1286篇 |
2020年 | 1650篇 |
2019年 | 2161篇 |
2018年 | 1952篇 |
2017年 | 1377篇 |
2016年 | 1346篇 |
2015年 | 1517篇 |
2014年 | 2907篇 |
2013年 | 3395篇 |
2012年 | 2186篇 |
2011年 | 2838篇 |
2010年 | 2122篇 |
2009年 | 2357篇 |
2008年 | 2550篇 |
2007年 | 2466篇 |
2006年 | 2071篇 |
2005年 | 1902篇 |
2004年 | 1701篇 |
2003年 | 1440篇 |
2002年 | 1259篇 |
2001年 | 869篇 |
2000年 | 677篇 |
1999年 | 731篇 |
1998年 | 669篇 |
1997年 | 584篇 |
1996年 | 544篇 |
1995年 | 542篇 |
1994年 | 536篇 |
1993年 | 428篇 |
1992年 | 394篇 |
1991年 | 357篇 |
1990年 | 276篇 |
1989年 | 255篇 |
1988年 | 215篇 |
1987年 | 227篇 |
1986年 | 176篇 |
1985年 | 325篇 |
1984年 | 458篇 |
1983年 | 405篇 |
1982年 | 420篇 |
1981年 | 344篇 |
1980年 | 350篇 |
1979年 | 274篇 |
1978年 | 218篇 |
1977年 | 210篇 |
1976年 | 210篇 |
1975年 | 179篇 |
1974年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Isolation and identification of granule-associated proteins relevant for poly(3-hydroxyalkanoic acid) biosynthesis in Chromatium vinosum D 总被引:1,自引:0,他引:1
Matthias Liebergesell Bernhard Schmidt Alexander Steinbüchel 《FEMS microbiology letters》1992,99(2-3):227-232
Abstract Poly(3-hydroxybutyric acid) granules, which harbored only four major granule-associated proteins as revealed by SDS polyacrylamide gel electrophoresis, were isolated from crude cellular extracts of Chromatium vinosum D by centrifugation in a linear sucrose gradient. N-Terminal amino acid sequence determination identified two proteins of M r 41 000 and M r 40 000 as the phaE Cv and phaC Cv translational products, respectively, of C. vinosum D. In a previous study it was shown that both proteins are required for the expression opf poly(3-hydroxyalkanoic acid) synthase activity. The N-terminus of the third protein ( M r 17 000) exhibited no homology to other proteins. Lysozyme, which was during purification of the granules, exhibited a strong affinity to PHB granules and was identified as the fourth protein enriched with the granules. 相似文献
82.
Alessandro Negro Irene Martini Emilio Bigon Flavia Cazzola Cristina Minozzi Stephen D. Skaper Lanfranco Callegaro 《Gene》1992,110(2)
The gene(NGFB) encoding the β subunit of mature human nerve growth factor (hNGFB) was subcloned into the pJLA503 expression vector under the control of bacteriophage promoters pR and pL, and expressed in Escherichia coli. The recombinant protein represented approximately 3% of the total cellular protein. Biologically active hNGFB was solubilized (0.2% total NGFB) and purified by cation-exchange chromatography and it yielded two bands on polyacrylamide-gel electrophoresis under nonreducing conditions, corresponding to the monomeric (14 kDa) and homodimeric (26.5 kDa) forms of the molecule. Both hNGFB forms were immunopositive on Western blots with rabbit anti-NGFB antibodies; however, following additional purification, only the species corresponding to the hNGFB homodimer was biologically active on cultured chicken dorsal root ganglion neurons. These results demonstrate the feasibility of synthesizing the biologically active form of hNGFB in E. coli. 相似文献
83.
84.
The packing of peptide helices in crystals of the leucine-rich decapeptide Boc-Aib-Leu-Aib-Aib-Leu-Leu-Leu-Aib-Leu-Aib-OMe provides an example of ladder-like leucylleucyl interactions between neighboring molecules. The peptide molecule forms a helix with five 5----1 hydrogen bonds and two 4----1 hydrogen bonds near the C terminus. Three head-to-tail NH ... O = C hydrogen bonds between helices form continuous columns of helices in the crystal. The helicial columns associate in an antiparallel fashion, except for the association of Leu ... Leu side chains, which occurs along the diagonal of the cell where the peptide helices are parallel. The peptide, with formula C56H102N10O13, crystallizes in space group P2(1)2(1)2(1) with Z = 4 and cell parameters a = 16.774(3) A, b = 20.032(3) A and c = 20.117(3) A; overall agreement factor R = 10.7% for 2014 data with magnitude of F(obs) greater than 3 sigma (F); resolution 1.0 A. 相似文献
85.
A directed conformational search algorithm using the program CONGEN (ref. 3), which samples backbone conformers, is described. The search technique uses information from the partially built structures to direct the search process and is tested on the problem of generating a full set of backbone Cartesian coordinates given only alpha-carbon coordinates. The method has been tested on six proteins of known structure, varying in size and classification, and was able to generate the original backbone coordinates with RMSs ranging from 0.30-0.87A for the alpha-carbons and 0.5-0.99A RMSs for the backbone atoms. Cis peptide linkages were also correctly identified. The procedure was also applied to two proteins available with only alpha-carbon coordinates in the Brookhaven Protein Data Bank; thioredoxin (SRX) and triacylglycerol acylhydrolase (TGL). All-atom models are proposed for the backbone of both these proteins. In addition, the technique was applied to randomized coordinates of flavodoxin to assess the effects of irregularities in the data on the final RMS. This study represents the first time a deterministic conformational search was used on such a large scale. 相似文献
86.
Phototropic stimulation does not induce unequal distribution of indole-3-acetic acid in maize coleoptiles 总被引:3,自引:0,他引:3
Distribution of endogenous diffusible auxin into agar blocks from phototropically stimulated maize coleoptile tips was studied using a bioassay and a physicochemical assay, to clarify whether phototropism in maize coleoptiles involves a lateral gradient in the amount of auxin. At 50 min after the onset of phototropic stimulation, when the phototropic response was still developing, direct assay of the blocks with the Avena curvature test showed that the auxin activity in the blocks from the shaded half-tips was twice that of the lighted side, at both the first and second positive phototropic curvatures. However, physicochemical determination following purification showed that the amount of indole-3-acetic acid (IAA) was evenly distributed in the blocks from lighted and shaded coleoptile half-tips at both the first and second positive phototropic curvatures. The even distribution of the IAA was also confirmed with the Avena curvature test following purification by HPLC. These results indicate that phototropism in maize coleoptiles is not caused by a lateral gradient of IAA itself and thus cannot be described by the Cholodny-Went theory. Furthermore, the lower auxin activity in the blocks from the lighted half-tips suggests the presence of inhibitor(s) interfering with the action of auxin and their significant diffusion from unilaterally illuminated coleoptile tips. 相似文献
87.
Viqar Uddin Ahmad Mohammad Ghani Shah Faryal Vali Mohammad Nargis Ismail Mushtaq Noorwala 《Phytochemistry》1991,30(12)
A new flavone glucoside macrophylloside has been isolated from the whole plant of Primula macrophylla and its structure was determined by spectroscopic methods as 2′-hydroxy-7-O-β-
-glucopyranosyloxyflavone. Sitosterol glucoside was also isolated for the first time from this plant. 相似文献
88.
S. Hinnisdaels L. Bariller A. Mouras V. Sidorov J. Del-Favero J. Veuskens I. Negrutiu M. Jacobs 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1991,82(5):609-614
Summary Extremely asymmetric nuclear hybrids have been obtained via protoplast fusion in an intergeneric combination. Irradiated (cobalt60,100 krad) kanamycinresistant Petunia hybrida mesophyll protoplasts were chemically fused with wild-type mesophyll protoplasts of Nicotiana plumbaginifolia. Eighty-six hybrid colonies were selected on kanamycin-containing medium, and twenty-four of these could be induced to regenerate numerous shoots. Cytological analysis of the regenerants showed the presence of a few chromosome fragments in some lines, and even a metacentric chromosome in yet another line. Besides additional chromosome fragments some lines only possessed typical Nicotiana chromosomes, and this at the diploid (2n = 2X = 20) as well as the tetraploid (2n = 2X = 40) level. Biochemical analysis showed that all regenerants had neomycin phosphotransferase activity (NPTII), which suggests that intergenomic recombination and or translocation events took place at least in those lines where no additional chromosome fragments could be detected. The presence of the NPTII gene was shown by Southern hybridization. All regenerants tested were fertile, and the segregation ratios for the kanamycin gene (for self and backcross pollinations to the recipient partner) for some of the regenerants correspond with Mendelian rules for a monogenic dominant marker. Most of the regenerants showed abnormal segregation ratios; in this case, no correlation could be made between segregation ratio and chromosome composition.Our results demonstrate the existence of intergenomic recombination and translocations evens in nuclear somatic hybrid plants obtained via gamma-fusion. 相似文献
89.
90.
R. J. Collins P. J. Boyle A. E. Clague A. E. Barr S. C. Latham 《Biological trace element research》1991,30(3):233-244
Patients with phenylketonuria (PKU) are frequently deficient in the essential trace element selenium (Se), because of their very low protein diet. Using two approaches to investigate T-cell response to proliferative signaling, viz, mitogenesis caused by the monoclonal antibody OKT3 and the plant lectin phytohaemagglutinin (PHA), we demonstrated significantly reduced responses to optimal concentrations of OKT3 in a group of PKU patients with reduced serum Se compared with a normal group (p = 0.0005) and with a group of PKU patients whose serum Se was normal (p = 0.0023). The response of the Se-deficient group to optimal levels of PHA did not differ from that of the normal controls or from that of Se-normal PKU patients. A dose-dependent relationship between serum Se levels and mitogenic response was evident for OKT3 (r = 0.34, p = 0.0154), but not for PHA (r = -0.02, p = 0.9086). We suggest that the reduced response to OKT3 mitogenesis in Se-deficient PKU patients is possibly the consequence of impaired Se-dependent metabolic activity, which affects mitogenic signaling via the T cell antigen receptor (TCR/CD3) complex. 相似文献