全文获取类型
收费全文 | 92488篇 |
免费 | 6735篇 |
国内免费 | 6400篇 |
专业分类
105623篇 |
出版年
2024年 | 201篇 |
2023年 | 1254篇 |
2022年 | 2940篇 |
2021年 | 4868篇 |
2020年 | 3193篇 |
2019年 | 4016篇 |
2018年 | 3957篇 |
2017年 | 2866篇 |
2016年 | 4052篇 |
2015年 | 5842篇 |
2014年 | 6887篇 |
2013年 | 7247篇 |
2012年 | 8492篇 |
2011年 | 7736篇 |
2010年 | 4482篇 |
2009年 | 4186篇 |
2008年 | 4775篇 |
2007年 | 4146篇 |
2006年 | 3532篇 |
2005年 | 2820篇 |
2004年 | 2312篇 |
2003年 | 2105篇 |
2002年 | 1698篇 |
2001年 | 1471篇 |
2000年 | 1342篇 |
1999年 | 1407篇 |
1998年 | 820篇 |
1997年 | 892篇 |
1996年 | 813篇 |
1995年 | 775篇 |
1994年 | 673篇 |
1993年 | 570篇 |
1992年 | 682篇 |
1991年 | 535篇 |
1990年 | 455篇 |
1989年 | 331篇 |
1988年 | 278篇 |
1987年 | 219篇 |
1986年 | 185篇 |
1985年 | 210篇 |
1984年 | 124篇 |
1983年 | 118篇 |
1982年 | 54篇 |
1981年 | 23篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1976年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
足月分娩的新鲜胎盘组织制成匀浆后,经高速离心、超速离心,谷胱甘肽(GSH)Sepharose 6B亲合层析,Amicon pM-10膜超过滤及高效液相层析,最终经SDS-PAGE鉴定,结果呈现单一亚基区带,其亚基分子量为25000。 根据我们现有高效液相设备条件,用ODS柱代替RadulovicL等报道的特异阴离子柱,用磷酸盐洗脱液代替含谷胱甘肽、二硫苏糖醇及氯化钾的梯度洗脱液,从人胎盘组织成功地制备了谷胱甘肽硫转移酶(GST)纯酶,全过程在15min内完成,保留时间及主峰面积的重复性均较理想,7次实验结果的变异系数为0.2%,最终纯化578.9倍。本研究为各种形式GST的纯化制备提供了一个新的、重复性好、分辨率高及回收理想的简易方法。 相似文献
62.
本文介绍用二相分配法制备蚕豆叶片原生质膜上的Ca~(2+)·Mg~(2+)-ATPase,用以研究镧系,稀土离子对此酶活性的影响。初步证实Pr~(3+)、Nd~(3+)对依赖于CaM的以及不依赖于CaM的蚕豆叶片原生质膜上Ca~(2+)·Mg~(2+)-ATPase活性的抑制不是CaM专一的。 相似文献
63.
64.
Dissociative inhibition of dimeric enzymes. Kinetic characterization of the inhibition of HIV-1 protease by its COOH-terminal tetrapeptide 总被引:7,自引:0,他引:7
Z Y Zhang R A Poorman L L Maggiora R L Heinrikson F J Kézdy 《The Journal of biological chemistry》1991,266(24):15591-15594
Human immunodeficiency virus 1 (HIV-1) protease is an aspartyl protease composed of two identical protomers linked by a four-stranded antiparallel beta-sheet consisting of the NH2- and COOH-terminal segments (Weber, I.T. (1990) J. Biol. Chem. 265, 10492-10496). Kinetic analysis of the HIV-1 protease-catalyzed hydrolysis of a fluorogenic substrate demonstrates that the enzyme is an obligatory dimer. At pH = 5.0, 0.1 M sodium acetate, 1 M NaCl, 1 mM EDTA buffer, 37 degrees C, the equilibrium dissociation constant, Kd = 3.6 +/- 1.9 nM. We found that the tetrapeptide Ac-Thr-Leu-Asn-Phe-COOH, corresponding to the COOH-terminal segment of the enzyme, is an excellent inhibitor of the enzyme. Kinetic analysis shows that the inhibitor binds to the inactive protomers and prevents their association into the active dimer (dissociative inhibition). The dissociative nature of this inhibition is consistent with the results obtained from sedimentation equilibrium experiments in which the apparent molecular weight of the enzyme was observed to be 20,800 +/- 1,500 and 12,100 +/- 300, in the absence and presence of the COOH-terminal tetrapeptide, respectively. The dissociation constant of the protomer-inhibitor complex is Ki = 45.1 +/- 1.8 microM. This is the first kinetic analysis and direct experimental demonstration of noncovalent dissociative inhibition. 相似文献
65.
M Jourdan X G Zhang M Portier J M Boiron R Bataille B Klein 《Journal of immunology (Baltimore, Md. : 1950)》1991,147(12):4402-4407
IL-6 is a major tumor growth factor in human multiple myeloma. Myeloma cell lines, which have the same phenotypic characteristics and Ig gene rearrangements as the original fresh myeloma cells and whose growth is strictly dependent on exogenous IL-6 similar to fresh myeloma cells, have been reproducibly established. We show here that IFN-alpha stimulated the growth of five of six of these human myeloma cell lines by inducing an autocrine production of IL-6 in myeloma cells. Indeed, IFN-alpha induced IL-6 mRNA accumulation and IL-6 production in myeloma cells and the IFN-alpha-induced growth of these cells was inhibited by anti-IL-6 mAb. Moreover, IFN-alpha made possible the rapid emergence of autonomously growing myeloma cell sublines, which produced IL-6 as an autocrine growth factor. As IFN-alpha has a potential therapeutical interest for multiple myeloma, the present study opens up new directions for studying its effects on the myeloma clone in vivo. 相似文献
66.
Hydra are characterized by having their body wall organized as an epithelial bilayer with an intervening acellular layer termed the mesoglea. As an extension of the previous study which indicated that mesoglea is a primitive basement membrane which has retained some characteristics of interstitial extracellular matrix, the present study was undertaken to analyze the role of mesoglea components during head regeneration in Hydra vulgaris. Studies were conducted that utilized drugs that affect collagen processing or secondary collagen structure (beta-aminoproprionitrile; 2,2'-dipydridyl; and cis-4-hydroxy-L-proline) and a drug that inhibits addition of glycosaminoglycan chains to proteoglycan core proteins (p-nitrophenyl-beta-D-xylopyranoside). These studies indicated that alterations in the structure of collagens or proteoglycans caused blockage of head regeneration in Hydra as monitored over a 48-hr period. Blockage of head regeneration was reversible once the drugs were removed, indicating that the drugs were not having a general toxic effect on the organism. Radiotracer studies also indicated that blockage of head regeneration was not simply due to a general depression of protein synthesis by the drugs. Various controls indicated that each drug was affecting mesoglea components under the conditions utilized in these studies. These observations indicate that preservation of normal mesoglea structure is required for Hydra head regeneration to proceed. 相似文献
67.
Trigonal crystals of porin from Escherichia coli 总被引:3,自引:0,他引:3
R A Pauptit H Zhang G Rummel T Schirmer J N Jansonius J P Rosenbusch 《Journal of molecular biology》1991,218(3):505-507
Trigonal crystals of the integral membrane protein porin from Escherichia coli have been grown and characterized. They belong to space group P321 with unit cell constants a = b = LL8.4, c = 52.7 A, alpha = beta = 90 degrees, gamma = 120 degrees. The crystals grow as well-defined hexagonal prisms to a size of 0.25 mm in all dimensions, and diffract to 2.7 A. The molecular symmetry coincides with 3-fold crystallographic symmetry, giving two trimers per unit cell (1 monomer/asymmetric unit). This corresponds to VM = 2.9 A3/Da. Native X-ray data to 3.0 A resolution have been collected on a FAST area detector and a search for heavy atom derivatives is underway. 相似文献
68.
我们分子鉴别了一个缺失型中国(A_γδβ)°-地贫家系。先证者为这一缺失的纯合子,具有中度贫血症状。家系的另五个成员均为这一缺失的杂合子,其胎儿血红蛋白(HbF)为16—21%,接近或达到HPFH杂合子的HbF水平,并且几乎不表现贫血症状。限制性内切酶图谱分析证明了β-珠蛋白基因簇内的DNA顺序缺失,缺失的5′端点位于Aγ基因IVSⅡ内,3′端点在β-珠蛋白基因下游区远端,距HPFH-2的3′缺失端点上游区约11kb。缺失的总长度约为80kb。本文讨论了这一缺失导致胎儿血红蛋白在成人中持续活跃表达的可能机制。 相似文献
69.
70.
Investigation of reduced serum and serum-free media for the cultivation of insect cells (Bm5) and the production of baculovirus (BmNPV) 总被引:1,自引:0,他引:1
An experimental study has been carried out to investigate the effectivenes of several reduced serum and serum-free media for the cultivation of an ovarian cell line, Bm5, of the lepidopteran insect Bombyx mori. Bm5 cell were successfully adapted to grow in a medium containing 5% serum and a serum-free medium (EX-CELL 400). On the other hand, this cell line could not be adapted to grow in several other media suggested in the literature, including IPL-41 + 2% fetal bovine serum (FBS), SF-900, and a serum-free medium (ISFM). Furthermore, a comparative study was conducted to determine the production levels of B. mori nuclear polyhedrosis virus (BmNPV) in Bm5 cells cultured in three different medium formulations. The production levels of BmNPV in adapted Bm5 cells grown in a 5% serum-supplemented medium and a serum-free medium (EX-CELL 400) were comparable to those obtained in Bm5 cells grown 10% serum-supplemented medium. (c) 1992 John Wiley & Sons, Inc. 相似文献