全文获取类型
收费全文 | 7401篇 |
免费 | 637篇 |
国内免费 | 2篇 |
出版年
2023年 | 23篇 |
2021年 | 90篇 |
2020年 | 70篇 |
2019年 | 72篇 |
2018年 | 108篇 |
2017年 | 95篇 |
2016年 | 140篇 |
2015年 | 231篇 |
2014年 | 305篇 |
2013年 | 416篇 |
2012年 | 438篇 |
2011年 | 470篇 |
2010年 | 327篇 |
2009年 | 307篇 |
2008年 | 412篇 |
2007年 | 431篇 |
2006年 | 413篇 |
2005年 | 412篇 |
2004年 | 358篇 |
2003年 | 344篇 |
2002年 | 349篇 |
2001年 | 118篇 |
2000年 | 99篇 |
1999年 | 129篇 |
1998年 | 127篇 |
1997年 | 115篇 |
1996年 | 96篇 |
1995年 | 84篇 |
1994年 | 79篇 |
1993年 | 86篇 |
1992年 | 80篇 |
1991年 | 89篇 |
1990年 | 72篇 |
1989年 | 86篇 |
1988年 | 71篇 |
1987年 | 51篇 |
1986年 | 59篇 |
1985年 | 63篇 |
1984年 | 51篇 |
1983年 | 45篇 |
1982年 | 69篇 |
1981年 | 50篇 |
1980年 | 52篇 |
1979年 | 49篇 |
1978年 | 31篇 |
1977年 | 40篇 |
1976年 | 28篇 |
1975年 | 32篇 |
1974年 | 42篇 |
1972年 | 23篇 |
排序方式: 共有8040条查询结果,搜索用时 15 毫秒
21.
22.
Roland von Bothmer Maija Kotimäki Ib Linde-Laursen 《Plant Systematics and Evolution》1987,156(3-4):183-188
Hybrids between the Chinese endemic speciesPsathyrostachys huashanica
Keng and the SW. Asian speciesP. fragilis (Boiss.)Nevski (all 2n = 14) developed normally but were completely sterile. Meiotic analyses revealed a high chiasma frequency indicating that the two species as well asP. juncea (Fisch.)Nevski share the same basic genome (called N). The hybrid nature of the plants was established through karyotype analysis and Giemsa C-banding. 相似文献
23.
T Tamura L L Kaiser J E Watson C H Halsted L S Hurley E L Stokstad 《Archives of biochemistry and biophysics》1987,256(1):311-316
To study the effect of zinc deficiency on folate metabolism, three groups of male Sprague-Dawley rats (zinc deficient (ZD), restricted-fed (RF + Zn), and ad libitum-fed control (control] were given a semipurified 25% egg white protein diet. The ZD group received less than 10.3 nmol zinc/g of diet, while the RF + Zn and control groups were given 1620 nmol zinc/g of diet. After 6-7 weeks of feeding, severe zinc deficiency developed in ZD rats. Hepatic methionine synthetase activity was increased in the ZD group compared to both the RF + Zn and control groups, but hepatic 5,10-CH2-H4folate reductase activity was similar in all groups. This increased methionine synthetase activity found in zinc-deficient rats might induce secondary alterations in folate metabolism. These changes include significantly lowered plasma folate levels, decreased 5-CH3-H4folate in liver, and increased rates of histidine and formate oxidation. The latter two findings suggest that the available non-5-CH3-H4folate is increased in zinc deficiency. 相似文献
24.
Conformation of Leu-Arg-Arg-Ala-Ser-Leu-Gly bound in the active site of adenosine cyclic 3',5'-phosphate dependent protein kinase 总被引:1,自引:0,他引:1
H N Bramson N E Thomas W T Miller D C Fry A S Mildvan E T Kaiser 《Biochemistry》1987,26(14):4666-4670
Studies utilizing NMR spectroscopy have shown that adenosine cyclic 3',5'-phosphate dependent protein kinase (A-kinase) probably binds Leu-Arg-Arg-Ala-Ser-Leu-Gly (peptide 1) in one of two extended coil conformations (A or B). The relative reactivities of a series of N-methylated peptides based on the structure of peptide 1 might, therefore, be related to how well each can assume the A or B conformation. From estimates of the magnitude of steric interactions that would be induced by N-methylation of an amide in peptide 1 that is locked in either conformation, the ability of each peptide to form that conformation was predicted. The ability of A-kinase to catalyze phosphorylation of the N-methylated peptides correlated well with the ability of each peptide to form conformation A, but not conformation B. In accord with these findings, the reactivity of an unreactive N-methylated peptide was partially restored by a second change, which allowed the peptide to assume conformation A. These results suggest that, when bound in the enzymatic active site, peptide 1 has a conformation that resembles structure A much more closely than structure B. 相似文献
25.
Oligodeoxyribonucleotides containing 1,3-propanediol as nucleoside substitute. 总被引:10,自引:8,他引:2 下载免费PDF全文
1,3-Propanediol was protected with one dimethoxytrityl residue and converted into the methoxy- and cyanoethoxyphosphoramidites 2a and 2b, respectively. Solid-phase oligonucleotide synthesis, employing the phosphoramidite 2a resulted in the dodecamers d(CGCGAATTCGCG) (6-9), in which dA or dT residues were replaced by 1,3-propanediol. These oligomers showed a high tendency to form hairpins. Their phosphodiester bonds between the 3'-position of a nucleoside and the propanediol moiety was not cleaved by snake venom phosphodiesterase. 相似文献
26.
Francis Fabre Nieve Magana-Schwencke Roland Chanet 《Molecular & general genetics : MGG》1989,215(3):425-430
Summary The RAD18 gene of Saccharomyces cerevisiae is involved in mutagenic DNA repair. We describe its isolation from a yeast library introduced into the centromeric YCp50 vector, a low copy number plasmid. The insert was sublconed into YCp50 and into the multicopy YRp7 plasmid. RAD18 is not toxic when present in multiple copies but the UV survival response indicates an heterogeneity in the cell population, a fraction of it being more sensitive. A DNA segment, close to RAD18, is toxic on the multicopy plasmid and may correspond to the tRAN sup61 known to be tightly linked to RAD18. Chromosomal deletions of RAD18 were constructed. The gene is not essential and the deleted strains have the properties of single site mutants. Thus, RAD18 appears to be essentially involved in DNA repair metabolism. 相似文献
27.
Gerd Reuter Roland Schauer Claudia Szeiki Johannis P Kamerling Johannes FG Vliegenthart 《Glycoconjugate journal》1989,6(1):35-44
Periodate oxidation of terminalN-acetyl- andN-glycoloylneuraminic acid residues in the mucins from edible bird nest substance and pig submandibular gland, respectively, can be carried out under conditions which exclusively give rise to the formation of the C-7 analogues of these sialic acids. In contrast, the C-8 compounds can be obtained in a maximum yield of about 40%. Under identical conditions,N-glycoloylneuraminic acid is oxidized about 1.5 times faster than theN-acetylated derivative. After release of the sialic acids by acid hydrolysis, the characterization of the oxidation products was carried out by TLC, by GLC and GLC-MS of the corresponding pertrimethylsilyl derivatives, and by 500-MHz1H-NMR spectroscopy. In addition, molar response factors for GLC analysis and extinction coefficients in the orcinol/Fe3+/HCl assay were determined. 相似文献
28.
I. M. Birk R. Dierstein I. Kaiser U. Matern W. A. König R. Krebber J. Weckesser 《Archives of microbiology》1989,151(5):411-415
Toxic and nontoxic peptides were isolated from the cyanobacterium Microcystis aeruginosa PCC 7806 by a procedure including extraction of cells with water-saturated 1-butanol, chromatography of the extract on silica
gel plates and high performance liquid chromatography (HPLC) on Partisil-5. The toxin was shown to be only a minor constituent,
being negatively charged and thus separable by electrophoresis, within the HPLC-purified fraction. It contained erythro-β-methyl-D-Asp, D-Glu, D-Ala, L-Leu, and L-Arg known to be part of the Microcystis peptide-toxin with Mr 994. The major part of the HPLC-purified fraction was assigned, however, to a nontoxic peptide with a Mr of 956. Partial hydrolysis studies of the nontoxic peptide(s) revealed amino acid sequences composed of D-Glu, N-methyl-Phe, and 3,4-dehydro-Pro, aside from the common L-amino acids. Cyclic linkage in the nontoxic peptide(s) appears likely. 相似文献
29.
CMP-Neu5Gc has been shown to be transported into mouse liver Golgi vesicles by a specific carrier the characteristics of which were investigated in detail. In the system employed, CMP-Neu5Gc enters the Golgi vesicles within 2 min; transport was saturable with high concentrations of the sugar-nucleotide and was dependent on temperature. A kinetic analysis gave an apparent Km of 1.3 μM and a maximal transport velocity of 335 pmol/mg protein per min. Almost identical values were obtained with CMP-Neu5Ac, under the same incubation conditions. Furthermore, the uptake of CMP-Neu5Gc was inhibited by CMP-Neu5Ac, a substrate analogue. Conversely, the uptake of CMP-Neu5Ac was inhibited by CMP-Neu5Gc to the same extent, leading to the conclusion that the transport of CMP-Neu5Ac and CMP-Neu5Gc is mediated by the same carrier molecule. This transport system for CMP-Neu5Gc involves both CMP and CMP-Neu5Gc since intravesicular CMP induced the entry of external CMP-Neu5Gc. 相似文献
30.
B W Lennon I I Kaiser 《Comparative biochemistry and physiology. B, Comparative biochemistry》1990,97(4):695-699
1. A crotoxin-like protein was isolated from the venom of a South American rattlesnake Crotalus durissus collilineatus. 2. Many of its properties are similar to those of crotoxin, including its non-covalent heterodimeric structure, electrophoretic mobility on SDS-PAGE, isoelectric focusing properties, toxicity in mice, immunological reactivity, multiple isoforms, phospholipase activity, peptide map, and instability on an anion-exchange column. 3. Results indicate that "collilineatus toxin" is strongly homologous with crotoxin, found in the venom of Crotalus durissus terrificus, and all other characterized rattlesnake neurotoxins. 相似文献