全文获取类型
收费全文 | 24854篇 |
免费 | 1191篇 |
国内免费 | 838篇 |
出版年
2023年 | 468篇 |
2022年 | 840篇 |
2021年 | 1031篇 |
2020年 | 743篇 |
2019年 | 746篇 |
2018年 | 758篇 |
2017年 | 492篇 |
2016年 | 491篇 |
2015年 | 627篇 |
2014年 | 1305篇 |
2013年 | 1804篇 |
2012年 | 1026篇 |
2011年 | 1466篇 |
2010年 | 1019篇 |
2009年 | 1048篇 |
2008年 | 1156篇 |
2007年 | 1156篇 |
2006年 | 977篇 |
2005年 | 923篇 |
2004年 | 802篇 |
2003年 | 728篇 |
2002年 | 612篇 |
2001年 | 421篇 |
2000年 | 360篇 |
1999年 | 423篇 |
1998年 | 349篇 |
1997年 | 344篇 |
1996年 | 279篇 |
1995年 | 340篇 |
1994年 | 246篇 |
1993年 | 237篇 |
1992年 | 211篇 |
1991年 | 214篇 |
1990年 | 161篇 |
1989年 | 142篇 |
1988年 | 152篇 |
1987年 | 107篇 |
1986年 | 104篇 |
1985年 | 232篇 |
1984年 | 323篇 |
1983年 | 205篇 |
1982年 | 276篇 |
1981年 | 241篇 |
1980年 | 208篇 |
1979年 | 189篇 |
1978年 | 180篇 |
1977年 | 145篇 |
1976年 | 130篇 |
1975年 | 123篇 |
1974年 | 122篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
《Journal of molecular biology》2021,433(15):167097
DNA glycosylases remove damaged or modified nucleobases by cleaving the N-glycosyl bond and the correct nucleotide is restored through subsequent base excision repair. In addition to excising threatening lesions, DNA glycosylases contribute to epigenetic regulation by mediating DNA demethylation and perform other important functions. However, the catalytic mechanism remains poorly defined for many glycosylases, including MBD4 (methyl-CpG binding domain IV), a member of the helix-hairpin-helix (HhH) superfamily. MBD4 excises thymine from G·T mispairs, suppressing mutations caused by deamination of 5-methylcytosine, and it removes uracil and modified uracils (e.g., 5-hydroxymethyluracil) mispaired with guanine. To investigate the mechanism of MBD4 we solved high-resolution structures of enzyme-DNA complexes at three stages of catalysis. Using a non-cleavable substrate analog, 2′-deoxy-pseudouridine, we determined the first structure of an enzyme-substrate complex for wild-type MBD4, which confirms interactions that mediate lesion recognition and suggests that a catalytic Asp, highly conserved in HhH enzymes, binds the putative nucleophilic water molecule and stabilizes the transition state. Observation that mutating the Asp (to Gly) reduces activity by 2700-fold indicates an important role in catalysis, but probably not one as the nucleophile in a double-displacement reaction, as previously suggested. Consistent with direct-displacement hydrolysis, a structure of the enzyme-product complex indicates a reaction leading to inversion of configuration. A structure with DNA containing 1-azadeoxyribose models a potential oxacarbenium-ion intermediate and suggests the Asp could facilitate migration of the electrophile towards the nucleophilic water. Finally, the structures provide detailed snapshots of the HhH motif, informing how these ubiquitous metal-binding elements mediate DNA binding. 相似文献
2.
Gad Avigad Priscilla J. Bohrer 《Biochimica et Biophysica Acta (BBA)/General Subjects》1984,798(3):317-324
6-Phosphofructo-2-kinase (ATP: D-fructose-6-phosphate-2-phosphotransferase) and D-fructose-2,6-bisphosphatase activities have been found in extracts prepared from etiolated mung bean seedlings. The activity of 6-phosphofructo-2-kinase exhibits a sigmoidal shape in response to changes in concentrations of both substrates, D-fructose 6-phosphate and ATP (S0.5 values of 1.8 and 1.2 mM, respectively). Inorganic orthophosphate (Pi) has a strong stimulating effect on the 2-kinase activity (A0.5 at about 2 mM), moderately increasing the Vmax and modifying the response into hyperbolic curves with Km values of 0.4 and 0.2 mM for fructose 6-phosphate and ATP, respectively. 3-Phosphoglycerate (I0.5 about 0.15 mM) partially inhibited the kinase activity by counteracting the Pi activation. In contrast, the activity of D-fructose-2,6-bisphosphatase (Km 0.38 mM) is strongly inhibited by Pi (I0.5 0.8 mM) lowering its affinity to fructose-2,6-P2 (Km 1.4 mM). 3-Phosphoglycerate activites the enzyme (A0.5 at about 0.3 mM) without causing a significant change in its Km for fructose-2,6-P2. The activities of both of these enzymes in relationship to the metabolic role of D-fructose 2,6-bisphosphate in the germinating seed is discussed. 相似文献
3.
4.
The biologically important pyridoxinato(1−) ligand (anionic vitamin B6) shows the rare phenolate-hydroxymethyl chelation plus bridging mode through the pyridine-nitrogen atom towards zinc(II) to give the one-dimensional (1D) coordination polymer {(acetato-κO)-aqua-μ-[2-methyl-3-oxy-4,5-di(hydroxymethyl)pyridine-κN:O,O′]zinc(II)}·monohydrate with polar packing of adjacent chains along the polar c axis (in space group Pc) through strong inter-chain hydrogen bonding. 相似文献
5.
《Biocatalysis and Biotransformation》2013,31(4):307-316
Lipases are enzymes that usually hydrolyze acylglycerols, but will hydrolyze the carboxylic esters in many other compounds. They also catalyze esteriftcations and transesterifications. In addition to specificity for carboxylic esters, the lipases are selective for lipid classes and show selectivity for primary vs. secondary alcohols (positional or regio-), fatty acids, enantiomers (chirality of either the acid or alcohol residue) and combinations of these. Uses of the enzymes have depended to some extent on regio- and fatty acid selectivities. Newer applications, such as ester synthesis and asymmetric hydrolysis, may not be based on selectivities. Factors affecting selectivities are discussed and some areas for research are mentioned. 相似文献
6.
From three Fouquieria sp. 12 iridoid glucosides were isolated and identified. Eight of these were structurally related to galioside (monotropein methylester), while four were hydroxy substitution products of deoxyloganin. In three cases the glucoside occurred together with the corresponding 10-O-acetate. 相似文献
7.
Cynthia L. Deitrick Richard E. Katholi David J. Huddleston Kathy Hardiek Lucienne Burrus 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2001,751(2)
Vitamin B6, measured as pyridoxal 5′-phosphate (PLP), is a co-enzyme in the transsulfuration pathway of homocysteine metabolism. Since depletion of PLP has been suggested as an independent risk factor for coronary artery disease, PLP is frequently measured to guide patient care. By a change and utilization of an Aquasil C18 column and the addition of an acetonitrile clean-up gradient to the potassium phosphate, with sodium perchlorate and bisulfite buffer between samples we report the modification of a previously described method for analysis of PLP. The result is a more practical, efficient, reliable and robust method for daily clinical use. We also determined and report that it is critical to protect freshly prepared standard PLP samples from light exposure during assay preparation. 相似文献
8.
《Developmental cell》2022,57(8):995-1008.e5
- Download : Download high-res image (144KB)
- Download : Download full-size image
9.
《Molecular & cellular proteomics : MCP》2020,19(7):1161-1178
- Download : Download high-res image (250KB)
- Download : Download full-size image
- •XL-MS reveals new PPIs in yeast mitochondria under glycerol and glucose condition.
- •Significant but limited results from quantitative XL-MS experiments.
- •Ndi1 participates in a CIII2CIV2 respiratory supercomplex.
- •Min8 promotes assembly of Cox12 into an intermediate complex IV.
10.
Results from a large-scale study, comprising 75 different breeding herds, are reported on predicting the halothane ( Hal ) genotypes of individual pigs by making use of the known close linkage between Hal and three C blood marker loci ( Phi, Po2, Pgd ). The parents haplotypes (involving Hal and marker loci) were determined from the HAL phenotypes (halothane test results) and marker loci phenotypes of their offspring in the first one or two litters studied. In subsequent litters of the Hal -marker loci haplotyped parents, the offspring's expected Hal genotypes could be predicted on the basis of the marker loci haplotypes inherited by them. By comparing the expected and observed HAL phenotypes of offspring in subsequent litters, the predicted Hal genotype was found to be correct in 90–95 % of the 4000 offspring (from Nn × Nn and Nn × nn matings) of Swedish Landrace and Yorkshire breeds studied.
The order of the three marker loci was confirmed as Phi-Po2-Pgd but the position of Hal with regards to Phi could not be resolved. The recombination frequencies between the most distant loci in this region, viz. Hal-Pgd and Phi-Pgd , were estimated to be 3–4.5 % and 4–6 % , respectively. The easy and rapid electrophoretic techniques described in the study to phenotype PHI, PO2, PGD, also allowed phe-notyping of six other polymorphic protein systems on the same gels. Thus Hal genotyping and effective parentage control can be conducted simultaneously. 相似文献
The order of the three marker loci was confirmed as Phi-Po2-Pgd but the position of Hal with regards to Phi could not be resolved. The recombination frequencies between the most distant loci in this region, viz. Hal-Pgd and Phi-Pgd , were estimated to be 3–4.5 % and 4–6 % , respectively. The easy and rapid electrophoretic techniques described in the study to phenotype PHI, PO2, PGD, also allowed phe-notyping of six other polymorphic protein systems on the same gels. Thus Hal genotyping and effective parentage control can be conducted simultaneously. 相似文献