首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7856篇
  免费   571篇
  国内免费   3篇
  8430篇
  2022年   36篇
  2021年   55篇
  2019年   58篇
  2018年   79篇
  2017年   56篇
  2016年   80篇
  2015年   134篇
  2014年   184篇
  2013年   570篇
  2012年   293篇
  2011年   290篇
  2010年   188篇
  2009年   183篇
  2008年   287篇
  2007年   349篇
  2006年   314篇
  2005年   330篇
  2004年   360篇
  2003年   290篇
  2002年   347篇
  2001年   295篇
  2000年   335篇
  1999年   280篇
  1998年   101篇
  1997年   82篇
  1996年   67篇
  1995年   96篇
  1994年   79篇
  1993年   73篇
  1992年   189篇
  1991年   234篇
  1990年   179篇
  1989年   177篇
  1988年   200篇
  1987年   172篇
  1986年   151篇
  1985年   144篇
  1984年   112篇
  1983年   90篇
  1982年   66篇
  1981年   82篇
  1980年   72篇
  1979年   84篇
  1978年   74篇
  1977年   56篇
  1976年   43篇
  1975年   45篇
  1974年   43篇
  1973年   48篇
  1972年   41篇
排序方式: 共有8430条查询结果,搜索用时 0 毫秒
71.
Unfolding rates of globular proteins determined by kinetics of proteolysis   总被引:2,自引:0,他引:2  
A convenient method for the determination of unfolding rates of small globular proteins under physiological conditions was developed using digestion with proteases. The apparent first-order rate constants for digestion of lysozyme with thermolysin and with Pronase at pH 8 and 50 degrees C were shown to be saturated with increases of concentrations of these proteases. The maximum rate constants extrapolated were identical in digestions with two different proteases, and were found to be equal to the unfolding rate constant of lysozyme. Similarly, the unfolding rate constant of RNase A at pH 8 and 50 degrees C, and those of lysozyme, RNase A and beta-lactoglobulin at pH 8 and 40 degrees C, were determined by the digestion method. Thus, it was shown that digestion by proteases proceeds mainly via the unfolded state of proteins.  相似文献   
72.
Root cultures of various solanaceous plants grow well in vitro and produce large amounts of tropane alkaloids. Enzyme activity that converts hyoscyamine to 6β-hydroxyhyoscyamine is present in cell-free extracts from cultured roots of Hyoscyamus niger L. The enzyme hyoscyamine 6β-hydroxylase was purified 3.3-fold and characterized. The hydroxylation reaction has absolute requirements for hyoscyamine, 2-oxoglutarate, Fe2+ ions and molecular oxygen, and ascorbate stimulates this reaction. Only the l-isomer of hyoscyamine serves as a substrate; d-hyoscyamine is nearly inactive. Comparisons were made with a number of root, shoot, and callus cultures of the Atropa, Datura, Duboisia, Hyoscyamus, and Nicotiana species for the presence of the hydroxylase activity. Decarboxylation of 2-oxoglutarate during the conversion reaction was studied using [1-14C]-2-oxoglutarate. A 1:1 stoichiometry was shown between the hyoscyamine-dependent formation of CO2 from 2-oxoglutarate and the hydroxylation of hyoscyamine. Therefore, the enzyme can be classified as a 2-oxoglutarate-dependent dioxygenase (EC 1.14.11.-). Both the supply of hyoscyamine and the hydroxylase activity determine the amounts of 6β-hydroxyhyoscyamine and scopolamine produced in alkaloid-producing cultures.  相似文献   
73.
Two major isotypes of parvalbumins (PA1 and PA2) have been isolated from the skeletal muscle of bullfrog, Rana catesbeiana. The Mr values were estimated to be 10,100 (PA1) and 11,800 (PA2) by SDS/polyacrylamide gel electrophoresis, and the isoelectric points were determined to be 4.78 (PA1) and 4.97 (PA2) by polyacrylamide gel isoelectric focusing. The amino acid compositions and isoelectric points indicate that PA1 corresponds to Rana esculenta pI 4.50 and Rana temporaria pI 4.75 parvalbumins and PA2 to Rana esculenta pI 4.88 and Rana temporaria pI 4.97 parvalbumins, showing that PA1 is genetically a beta-parvalbumin and PA2 an alpha-parvalbumin. However, in terms of the amino acid compositions, PA1 and PA2 are distinctly different from the corresponding parvalbumins of Rana esculenta or Rana temporaria. The ultraviolet spectra of PA1 and PA2 are consistent with their amino acid compositions. An ultraviolet difference spectrum of the Ca2+-loaded form vs. metal-free form indicates that a Tyr and some Phe residues in PA1 are affected by a conformational change associated with the binding of Ca2+. On electrophoresis in polyacrylamide gel in 14 mM Tris and 90 mM glycine, the Ca2+-loaded form of PA1 migrated twice as fast as the Mg2+-loaded form. Both PA1 and PA2 show increased mobility in the Ca2+-loaded forms, like troponin C but different from calmodulin.  相似文献   
74.
Type IV collagen is a major structural component in basement membranes. It is considerably different from the fibrillar collagens, types I-III. For example, unlike fibrillar collagens, the triple helical domain of type IV collagen is frequently interrupted by nonhelical regions. In this report, we demonstrate several overlapping genomic clones which cover most of the mouse alpha 1(IV) chain. Electron microscopic analysis of R-loops revealed that there were at least 28 exons within 35 kilobases of the gene segment. The sizes of six exons were determined by DNA sequence analysis to be 81, 178, 134, 73, 129, and 213 base pairs. These sizes do not appear to be related to the 54-base pair coding unit which is characteristic of fibrillar collagen exons, suggesting that the alpha 1 type IV collagen gene evolved differently from the fibrillar collagen genes.  相似文献   
75.
Using affinity chromatography with immobilized gelatin and native type I collagen, we have identified the major collagen-binding proteins in Nonidet P-40 extracts of chick embryo fibroblasts labeled with [35S] methionine. After washing the gelatin- or collagen-Sepharose beads with high ionic strength buffer, a 47,000-dalton protein was the only major protein besides fibronectin found to bind to these affinity beads. The isoelectric point of this protein was approximately 9.0, with a closely spaced minor spot. The total amount and the synthesis of this collagen-binding protein were both decreased in Rous sarcoma virus-transformed cells. This collagen-binding protein was found to be phosphorylated by incubating intact cells with [32P]orthophosphate. Phosphoamino acid analysis revealed that serine and threonine residues were phosphorylated, but tyrosine was not. Although quantities of the 47,000-dalton protein labeled with [35S]methionine were decreased by a factor 2.5 after transformation, the incorporation of [32P]orthophosphate/unit of protein was 5-7-fold higher in transformed cells. In temperature-sensitive mutant virus-infected cells, the amount of the 47,000-dalton protein was also decreased at the temperature permissive for transformation, and the incorporation of [32P]orthophosphate/protein was also increased. These studies establish that a major membrane-associated collagen-binding protein of fibroblasts is phosphorylated and that it is altered in both total quantity and degree of phosphorylation after malignant transformation.  相似文献   
76.
Summary We examined cellular protein content in four temperature-sensitive (ts) mutants of rat 3Y1 fibroblasts (3Y1tsD123, 3Y1tsF121, 3Y1tsG125, and 3Y1tsH203) under various conditions of culture that affect cell proliferation. When proliferation of the ts mutants was inhibited at a nonpermissive temperature (39.8°C) in the G1 phase, prominent accumulation of cellular protein occurred in three mutants (3Y1tsF121, 3Y1tsG125, and 3Y1tsH203) but not in 3Y1tsD123. The over-accumulation of protein at 39.8°C in the former three mutants was inhibited at high cell densities. At low cell densities there was an upper limit in the protein accumulation at 39.8°C. When the three mutants, proliferation-arrested at high cell densities at 33.8°C, were replated sparsely in fresh medium and shifted to 39.8°C, proliferation was completely inhibited whereas over-accumulation of protein occurred. These results indicating dissociation of protein accumulation and cell proliferation suggest that the two events are regulated by different mechanisms. This work was supported in part by a Grant-in-Aid for Encouragement of Young Scientists (1984) to K. Y. from the Ministry of Education, Science, and Culture, Japan.  相似文献   
77.
In the reaction of the intramolecular cross-linking between Lys-13 (epsilon-NH3+) and Leu-129 (alpha-COO-) in lysozyme using imidazole and 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride [Yamada, H., Kuroki, R., Hirata, M., & Imoto, T. (1983) Biochemistry 22, 4551-4556], it was found that two-thirds of the protein (both the recovered and cross-linked lysozymes) showed a lower affinity than the rest against chitin-coated Celite, an affinity adsorbent for lysozyme. The protein with the reduced affinity was separated on chitin-coated Celite affinity chromatography and found to be slightly different from native lysozyme in the elution position of the tryptic peptide of Ile-98-Arg-112 on reversed-phase high-performance liquid chromatography. In contrast with native lysozyme, the limited hydrolysis of this abnormal tryptic peptide of Ile-98-Arg-112 in 6 N HCl at 110 degrees C gave a considerable amount of beta-aspartylglycine. Therefore, it was concluded that two-thirds of the protein obtained from this reaction possessed the beta-aspartylglycyl sequence at Asp-101-Gly-102. As a result, we obtained four lysozymes from this reaction, the derivative with the beta-aspartyl sequence at Asp-101 (101-beta-lysozyme), the cross-linked derivative between Lys-13 and Leu-129 (CL-lysozyme), the CL-lysozyme derivative with the beta-aspartyl sequence at Asp-101 (101-beta-CL-lysozyme), and native lysozyme. In the ethyl esterification of Asp-52 in lysozyme with triethyloxonium fluoroborate [Parsons, S. M., Jao, L., Dahlquist, F. W., Borders, C. L., Jr., Groff, T., Racs, J., & Raftery, M. A. (1969) Biochemistry 8, 700-712; Parsons, S. M., & Raftery, M. A. (1969) Biochemistry 8, 4199-4205], the same bond rearrangement was detected in the same ratio.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
78.
We have isolated two clones containing 19 kilobases (kb) of the human gene coding for a pro-alpha 1 (II) collagen chain from human lambda genomic DNA libraries. A 3' clone, HC2A, was selected by cross-hybridization with a cDNA clone containing sequences coding for the carboxy propeptide of chick type II procollagen. A second clone, HC2B, was obtained by screening the library with the 5' part of HC2A. The sequence analysis of exon 3 corresponding to the C propeptide reveals the presence of stretches of conserved nucleotides between the human and the chick type II procollagen genes. On Northern blots, the human collagen clone hybridizes strongly to a 5.5-kb RNA for the rat type II procollagen chain. Finally, studies of genomic DNAs from normal individuals reveal the presence of a HindIII and a BamHI polymorphic site at the 3' end of the gene.  相似文献   
79.
Distributional and morphological features, especially characteristics of the ramification of serotonin-containing supraependymal fibers (SEF), were studied in the ventricular systems of mammals (mouse, rat, guinea pig, rabbit, cat, dog, monkey) by means of a modified peroxidase antiperoxidase technique, using antiserotonin antiserum prepared in our laboratory. SEF were present in all ventricular systems, except on the third ventricle floor and in the choroid plexus. The density of SEF was higher in the smaller species. In the rat, light- and scanning electron microscopical SEF were almost completely abolished 1 week after intraventricular administration of 5,6-dihydroxytryptamine. Ramification of SEF was complicated; the SEF formed a true network with frequent anastomosing. In the ventricular system of rats rendered hydrocephalic by kaolin administration, the mode of axonal branching in the supraependymal plexus could best be analyzed by the scanning electron microscope because the meshes of the plexus were spread out.  相似文献   
80.
Oxidation of acyclic terpenoids by Corynebacterium sp.   总被引:2,自引:2,他引:0       下载免费PDF全文
Y Yamada  C W Seo    H Okada 《Applied microbiology》1985,49(4):960-963
Squalene analogs such as lycopersene, geranylfarnesyl, digeranyl, and 2-hydroxy-2,3-dihydrosqualene and terpene alcohol derivatives such as farnesyl benzyl ether, farnesyl pivalate, geranylgeranyl pivalate, geranyl pivalate, and geranyl benzyl ether were oxidized by Corynebacterium sp. strain SY-79, which was isolated from soil by using squalene as a carbon source. Lycopersene and geranylfarnesyl gave no major product. Digeranyl, geranyl benzyl ether, and geranyl pivalate gave terminal oxidation products, and 2-hydroxy-2,3-dihydrosqualene, farnesyl benzyl ether, farnesyl pivalate, and geranylgeranyl pivalate were degraded to give lower molecular carboxylic acids. Strain SY-79 showed promising oxidative activities toward acyclic terpenes, although the metabolites obtained were variable, depending upon the structure of the substrate.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号