全文获取类型
收费全文 | 95107篇 |
免费 | 501篇 |
国内免费 | 880篇 |
专业分类
96488篇 |
出版年
2023年 | 13篇 |
2022年 | 24篇 |
2021年 | 50篇 |
2020年 | 23篇 |
2019年 | 48篇 |
2018年 | 11872篇 |
2017年 | 10707篇 |
2016年 | 7499篇 |
2015年 | 694篇 |
2014年 | 376篇 |
2013年 | 414篇 |
2012年 | 4377篇 |
2011年 | 12973篇 |
2010年 | 12081篇 |
2009年 | 8298篇 |
2008年 | 9893篇 |
2007年 | 11492篇 |
2006年 | 367篇 |
2005年 | 612篇 |
2004年 | 1068篇 |
2003年 | 1111篇 |
2002年 | 852篇 |
2001年 | 300篇 |
2000年 | 186篇 |
1999年 | 47篇 |
1998年 | 21篇 |
1997年 | 32篇 |
1996年 | 22篇 |
1995年 | 11篇 |
1994年 | 16篇 |
1993年 | 44篇 |
1992年 | 36篇 |
1991年 | 56篇 |
1990年 | 19篇 |
1989年 | 26篇 |
1988年 | 29篇 |
1987年 | 23篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 26篇 |
1979年 | 12篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1972年 | 257篇 |
1971年 | 276篇 |
1970年 | 13篇 |
1965年 | 14篇 |
1962年 | 24篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
With the help of quantum mechanical calculations, we have examined the series of central system X(ML)3
+(X = O, S, Se; M = Au, Ag, Cu). Using a scalar–relativistic density functional approach, we studied the geometry structures, Mulliken populations and charges of the systems. Structure parameters of the experimental systems are reproduced well with Xα method. The metallophilic interaction energy is analyzed and decomposed. For the systems with different central atoms and different metal atoms, the nature of the metallophilic attraction interaction is analyzed. 相似文献
992.
In this study, we performed a molecular docking and dynamics simulation for a benzoxazinone–human oxytocin receptor system to determine the possible hydrophobic and electrostatic interaction points in the dynamic complex. After the homology modeling, the ligand was docked into the putative active using AutoDock 3.05. After the application of energetic and structural filters, the complexes obtained were further refined with a simulated annealing protocol (AMBER8) to remove steric clashes. Three complexes were selected for subjection to the molecular dynamics simulation (5 ns), and the results on the occurrence of average anchor points showed a stable complex between the benzoxazinone derivative and the receptor. The complex could be used as a good starting point for further analysis with site-directed mutagenesis, or further computational research.
Figure The location of the ligands (complex B – blue; complex E – red; and complex F –
green) in the transmembrane regions (TM1 – red; TM2 – blue; TM3 – yellow; TM4
– purple; TM5 – orange; TM6 – cyan; TM7 – pink) of the hOTR. For clarity, the EC
and IC loops are not shown
Electronic Supplementary Material Supplementary material is available at 相似文献
993.
Understanding the mechanism of the protein stability change is one of the most challenging tasks. Recently, the prediction
of protein stability change affected by single point mutations has become an interesting topic in molecular biology. However,
it is desirable to further acquire knowledge from large databases to provide new insights into the nature of them. This paper
presents an interpretable prediction tree method (named iPTREE-2) that can accurately predict changes of protein stability
upon mutations from sequence based information and analyze sequence characteristics from the viewpoint of composition and
order. Therefore, iPTREE-2 based on a regression tree algorithm exhibits the ability of finding important factors and developing
rules for the purpose of data mining. On a dataset of 1859 different single point mutations from thermodynamic database, ProTherm,
iPTREE-2 yields a correlation coefficient of 0.70 between predicted and experimental values. In the task of data mining, detailed
analysis of sequences reveals the possibility of the compositional specificity of residues in different ranges of stability
change and implies the existence of certain patterns. As building rules, we found that the mutation residues in wild type
and in mutant protein play an important role. The present study demonstrates that iPTREE-2 can serve the purpose of predicting
protein stability change, especially when one requires more understandable knowledge. 相似文献
994.
Botulinum neurotoxins type A (BoNT/A) are highly potent toxins, but are also useful in the treatment of illnesses. We studied
the properties of BoNT/A at various temperatures and pH values in order to understand its toxicity and structure variations.
The pH values of the environment of BoNT/A are obtained by changing the protonation states of certain titratable residue groups.
Our results show that certain parts of the protein are active at acidic pH environments or at high temperatures. The protein
is more stable in neutral environments at normal human body temperature, whereas, at high temperature, the protein is more
stable in acidic environments. Also, the three domains of the protein tend to have relative motion rather than within individual
domains. 相似文献
995.
Klara K. Nordn Jaeike W. Faber Frane Babarovi Thomas L. Stubbs Tara Selly James D. Schiffbauer Petra Peharec tefani Gerald Mayr Fiann M. Smithwick Jakob Vinther 《Evolution; international journal of organic evolution》2019,73(1):15-27
Some of the most varied colors in the natural world are created by iridescent nanostructures in bird feathers, formed by layers of melanin‐containing melanosomes. The morphology of melanosomes in iridescent feathers is known to vary, but the extent of this diversity, and when it evolved, is unknown. We use scanning electron microscopy to quantify the diversity of melanosome morphology in iridescent feathers from 97 extant bird species, covering 11 orders. In addition, we assess melanosome morphology in two Eocene birds, which are the stem lineages of groups that respectively exhibit hollow and flat melanosomes today. We find that iridescent feathers contain the most varied melanosome morphologies of all types of bird coloration sampled to date. Using our extended dataset, we predict iridescence in an early Eocene trogon (cf. Primotrogon) but not in the early Eocene swift Scaniacypselus, and neither exhibit the derived melanosome morphologies seen in their modern relatives. Our findings confirm that iridescence is a labile trait that has evolved convergently in several lineages extending down to paravian theropods. The dataset provides a framework to detect iridescence with more confidence in fossil taxa based on melanosome morphology. 相似文献
996.
997.
Lindsey L. Thurman Allison K. Barner Tiffany S. Garcia Tara Chestnut 《Ecography》2019,42(10):1658-1670
Species interactions are dynamic processes that vary across environmental and ecological contexts, and operate across scale boundaries, making them difficult to quantify. Nevertheless, ecologists are increasingly interested in inferring species interactions from observational data using statistical analyses of their spatial co‐occurrence patterns. Trophic interactions present a particular challenge, as predators and prey may frequently or rarely co‐occur, depending on the spatial or temporal scale of observation. In this study, we investigate the accuracy of inferred interactions among species that both compete and trophically interact. We utilized a long‐term dataset of pond‐breeding amphibian co‐occurrences from Mt Rainier National Park (Washington, USA) and compiled a new dataset of their empirical interactions from the literature. We compared the accuracy of four statistical methods in inferring these known species interactions from spatial associations. We then used the best performing statistical method, the Markov network, to further investigate the sensitivity of interaction inference to spatial scale‐dependence and the presence of predators. We show that co‐occurrence methods are generally inaccurate when estimating trophic interactions. Further the strength and sign of inferred interactions were dependent upon the spatial scale of observation and predator presence influenced the detectability of competitive interactions among prey species. However, co‐occurrence analysis revealed new patterns of spatial association among pairs of species with known interactions. Overall, our study highlights a limiting frontier in co‐occurrence theory and the disconnect between widely implemented methodologies and their ability to accurately infer interactions in trophically‐structured communities. 相似文献
998.
Andrew J. Edwards Patricia J. Sweeney David G. Reid John M. Walker Nabil Elshourbagy Charles E. Egwuagu James F. Young Curtis L. Patton 《Chirality》1996,8(8):545-550
Calmidazolium {R24571, 1-[bis(4-chlorophenyl)methyl]-3-[2-(2,4-dichlorophenyl)-2-[(2,4-dichlorophenyl)methoxy]ethyl]-1H-imidazolium chloride} is a potent calmodulin inhibitor. This paper describes the synthesis and properties of the enantiomers of calmidazolium from the enantiomers of miconazole {1(N)-(2-(2,4-dichlorobenzyloxy)-2-(2,4-dichlorophenyl))-ethyl imidazole}, prepared from the racemate by chiral preparative scale high performance liquid chromatography. Overlap between ligand and protein resonances in the aromatic region of the 1H NMR spectrum of the calmidazolium-calmodulin complexes has been obviated by preparation of the protein with all of its nine phenylalanine rings deuterated (Phe-d5 calmodulin). This has been accomplished by the overexpression of calmodulin derived from Trypanosoma brucei rhodiesiense in E. coli in a medium supplemented with ring-deuterated phenylalanine. The kinetics of binding of each enantiomer are slow on the 1H NMR time scale as judged by the behaviour of the H2 resonance of Histidine-107, which is clearly visible under the sample conditions used. The aromatic spectral regions of the protein-bound (+) and (−) enantiomers contrast strikingly, reflecting differences in bound environment and/or conformation. Chirality 8:545–500, 1996. © 1997 Wiley-Liss, Inc. 相似文献
999.
W F Diven J Sweeney V Warty A Sanghvi 《Biochemical and biophysical research communications》1988,155(1):7-13
In preliminary experiments, we have shown that rat liver microsomes possess phosphatase activity which was inhibited in the presence of sodium fluoride. We have now separated six microsomal phosphatase fractions appearing to be isoenzymes. They all possess different kinetic constants and are not equally inhibited by tartrate and fluoride ions, inhibitors of phosphatase activity. One phosphatase fraction, in fact, is almost completely unaffected by fluoride ion. More pertinent to our interest, these isoenzymes exhibit differing abilities to modulate the activities of hydroxymethylglutaryl CoA reductase, acyl-CoA:cholesterol O-acetyltransferase, and cholesterol 7 alpha-hydroxylase. Interaction of four of the fractions with rat liver microsomes resulted in a decrease in cholesterol 7 alpha-hydroxylase activity; two were without effect. 相似文献
1000.
Mutsumi Nakamura Takafumi Nagamine Chisato Harada Kiyoshi Tajima Hiroki Matsui Yoshimi Benno 《Current microbiology》2003,47(1):0071-0074
The objective of this study was to ligate the xylanase gene A (xynA) isolated from Ruminococcus albus 7 into the promoter and signal-peptide region of the lichenase [β-(1,3-1,4)-glucanase] gene of Streptococcus bovis JB1. This fusion gene was inserted into the pSBE11 vector, and the resulting recombinant, plasmid pXA, was used to transform
S. bovis 12-U-1 cells. The transformant, S. bovis 12UXA, secreted the xylanase, which was stable against freeze-thaw treatment and long-time incubation at 37°C. The introduction
of pXA and production of xylanase did not affect cell growth, and the xylanase produced degraded xylan from oat-spelt and
birchwood.
Received: 24 June 2002 / Accepted: 7 October 2002 相似文献