全文获取类型
收费全文 | 6961篇 |
免费 | 731篇 |
专业分类
7692篇 |
出版年
2023年 | 58篇 |
2022年 | 92篇 |
2021年 | 235篇 |
2020年 | 105篇 |
2019年 | 133篇 |
2018年 | 166篇 |
2017年 | 144篇 |
2016年 | 240篇 |
2015年 | 417篇 |
2014年 | 441篇 |
2013年 | 469篇 |
2012年 | 631篇 |
2011年 | 580篇 |
2010年 | 330篇 |
2009年 | 247篇 |
2008年 | 373篇 |
2007年 | 332篇 |
2006年 | 297篇 |
2005年 | 271篇 |
2004年 | 211篇 |
2003年 | 231篇 |
2002年 | 208篇 |
2001年 | 104篇 |
2000年 | 92篇 |
1999年 | 86篇 |
1998年 | 55篇 |
1997年 | 49篇 |
1996年 | 48篇 |
1995年 | 45篇 |
1994年 | 29篇 |
1993年 | 39篇 |
1992年 | 51篇 |
1991年 | 46篇 |
1990年 | 58篇 |
1989年 | 43篇 |
1988年 | 39篇 |
1987年 | 56篇 |
1986年 | 37篇 |
1985年 | 32篇 |
1984年 | 39篇 |
1983年 | 34篇 |
1982年 | 32篇 |
1981年 | 29篇 |
1980年 | 25篇 |
1979年 | 35篇 |
1978年 | 29篇 |
1977年 | 22篇 |
1973年 | 25篇 |
1972年 | 33篇 |
1967年 | 22篇 |
排序方式: 共有7692条查询结果,搜索用时 0 毫秒
1.
Antoine Wystrach Sebastian Schwarz Patrick Schultheiss Guy Beugnon Ken Cheng 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2011,197(2):167-179
The Australian desert ant Melophorus bagoti often follows stereotypical routes through a cluttered landscape containing both distant panoramic views and obstacles (plants)
to navigate around. We created an artificial obstacle course for the ants between a feeder and their nest. Landmarks comprised
natural objects in the landscape such as logs, branches, and tussocks. Many ants travelled stereotypical routes home through
the obstacle course in training, threading repeatedly the same gaps in the landmarks. Manipulations altering the relations
between the landmarks and the surrounding panorama, however, affected the routes in two major ways. Both interchanging the
positions of landmarks (transpositions) and displacing the entire landmark set along with the starting position of the ants
(translations) (1) reduced the stereotypicality of the route, and (2) increased turns and meanders during travel. The ants
might have used the entire panorama in view-based travel, or the distal panorama might prime the identification and use of
landmarks en route. Despite the large data set, both options (not mutually exclusive) remain viable. 相似文献
2.
The effect of substrate modification on binding of porcine pancreatic alpha amylase: hydrolysis of modified amylose containing D-allose residues 总被引:1,自引:0,他引:1
A modified amylose containing 10% of tritiated D-allose residues has been hydrolyzed by porcine pancreatic alpha amylase (PPA). This reaction produced a number of radioactive oligosaccharides of low molecular weight, including modified mono-, di-, and tri-saccharides, as well as larger products. Analysis of these products by chemical and enzymic methods identified D-allose, two isomers of modified maltose, and isomers of modified maltotriose. These results may be interpreted in terms of current PPA models to indicate that D-allose residues may be productively bound at all five subsites of the active site of the enzyme. The distribution of modified residues in these products, however, further suggests that productive binding of D-allose at the subsite where catalytic attack occurs (subsite 3) is less favorable than binding of D-glucose. These results are compared with results of a series of PPA substrates having modifications at C-3 and at other positions. Trends observed in enzyme hydrolysis of these modified substrates reflect factors that contribute to PPA catalysis, with respect to steric, electronic, and hydrogen-bonding interactions between enzyme and substrate. 相似文献
3.
Carbonic anhydrase subunits of the mitochondrial NADH dehydrogenase complex (complex I) in plants 总被引:2,自引:0,他引:2
The mitochondrial nicotinamide adenine dinucleotide, reduced (NADH) dehydrogenase complex (complex I) of plants has a molecular mass of about 1000 kDa and is composed of more than 40 distinct protein subunits. About three quarter of these subunits are homologous to complex I subunits of heterotrophic eukaryotes, whereas the remaining subunits are unique to plants. Among them are three to five structurally related proteins that resemble an archaebacterial γ-type carbonic anhydrase (γCA). The γCA subunits are attached to the membrane arm of complex I on the matrix-exposed side and form an extra spherical domain. At the same time, they span the inner mitochondrial membrane and are essential for assembly of the protein complex. Expression of the genes encoding γCA subunits is reduced if plants are cultivated in the presence of elevated CO2 concentration. The functional role of these subunits within plant mitochondria is currently unknown but might be related to photorespiration. We propose that the complex I–integrated γCAs are involved in mitochondrial HCO3 − formation to allow efficient recycling of inorganic carbon for CO2 fixation in chloroplasts under high light conditions. 相似文献
4.
Monoclonal antibodies to the multienzyme enniatin synthetase. Production and use in structural studies 总被引:1,自引:0,他引:1
A Billich R Zocher H Kleinkauf D G Braun D Lavanchy H K Hochkeppel 《Biological chemistry Hoppe-Seyler》1987,368(5):521-529
Monoclonal antibodies have been prepared against the multifunctional enzyme enniatin synthetase, which catalyses the biosynthesis of the cyclodepsipeptide antibiotic enniatin. Five different antibodies (designated 1.56, 21.1, 25.91, 28.7 and 28.34) were characterized. 1.56, 21.1 and 25.91 were of IgG1 and 28.7 and 28.34 of IgM subclass. Binding studies showed that 21.1 and 25.91 are obviously directed against determinants based on the primary structure of the enzyme, whereas 28.7, 28.34 and 1.56 bind to the native enzyme. All antibodies inhibited enniatin formation. Based on their ability to inhibit different partial reactions of the multienzyme the antibodies could be divided into three groups: 21.1 and 25.91 inhibit valyl thioester formation, 1.56 additionally inhibits D-2-hydroxyisovaleric acid thioesterification, and 28.7 and 28.34 block both thioester sites as well as the N-methylation step. None of the antibodies affected the formation of L-valyl or D-hydroxyisovaleryl adenylate by the enzyme. The results indicate that there must be distinct thioester activation sites for valine and D-hydroxyisovalerate close to each other and in the neighbourhood of the methyltransferase site. The adenylation sites for D-hydroxy-isovalerate and L-valine are obviously located at some distance. 相似文献
5.
6.
7.
N-Terminal Sequence of Pig Brain Choline Acetyltransferase Purified by a Rapid Procedure 总被引:6,自引:5,他引:1
Axel Braun Yves-Alain Barde Friedrich Lottspeich Werner Mewes Hans Thoenen 《Journal of neurochemistry》1987,48(1):16-21
A procedure is reported that allows the purification and amino terminal sequencing of pig brain choline acetyltransferase. The enzyme (present in extremely low amounts in this tissue) is eluted together with its antibody from an affinity column by a mild pH shift and the resulting enzyme-antibody complex separated by gel electrophoresis. The band corresponding to the enzyme is electroeluted from the gel using volatile solutions allowing the direct determination of the amino acid composition and partial sequence. The first 11 residues are: Pro-Ile-Leu-Glu-Lys-Thr-Pro-Pro-Lys-Met-Ala. 相似文献
8.
Local deformation studies of chain molecules: differential conditions for changes of dihedral angles
W Braun 《Biopolymers》1987,26(10):1691-1704
New first and second-order differential equations for changes of dihedral angles characterizing local deformations of chain molecules with fixed bond lengths and bond angles are derived. Two methods for integrating the differential relations are given. The proposed method is used to generate a path of locally deformed conformations around a β-turn region of a small protein, bovine pancreatic trypsin inhibitor. The variable regions change their conformations by more than 3 Å root-mean-square distance value whereas the fixed regions stay within 0.02 Å. Possible applications of this method are in the field of computer graphics, Monte Carlo simulations, and energy minimization calculations of chain molecules. 相似文献
9.
Timothy J. Strabala Sebastian Y. Bednarek Gregory Bertoni Richard M. Amasino 《Molecular & general genetics : MGG》1989,216(2-3):388-394
Summary A 1.9 kb clone of the T-DNA region of the Agrobacterium tumefaciens Ti plasmid Bo542 which exhibited homology to the isopentenyl transferase (ipt) locus of pTiA6 was identified by low stringency DNA hybridization. Introduction of this segment of pTiBo542 DNA into cells of Nicotiana tabacum or N. glauca caused tumor formation in vivo, and allowed hormone independent growth in vitro. Furthermore, this DNA segment complemented ipt mutant strains of A. tumefaciens, restoring their ability to cause tumors on Kalanchöe leaves and tomato stems. The complete DNA sequence of this segment has been determined, revealing an open reading frame homologous to other known Agrobacterium ipt genes. 相似文献
10.
The immobilization of penicillin G acylase on chitosan 总被引:3,自引:0,他引:3