全文获取类型
收费全文 | 809957篇 |
免费 | 97824篇 |
国内免费 | 433篇 |
专业分类
908214篇 |
出版年
2018年 | 6314篇 |
2016年 | 9263篇 |
2015年 | 14193篇 |
2014年 | 16319篇 |
2013年 | 22552篇 |
2012年 | 25814篇 |
2011年 | 25694篇 |
2010年 | 17414篇 |
2009年 | 16465篇 |
2008年 | 23350篇 |
2007年 | 23945篇 |
2006年 | 22444篇 |
2005年 | 21559篇 |
2004年 | 21365篇 |
2003年 | 20558篇 |
2002年 | 19978篇 |
2001年 | 34699篇 |
2000年 | 35077篇 |
1999年 | 28395篇 |
1998年 | 11039篇 |
1997年 | 11618篇 |
1996年 | 11137篇 |
1995年 | 10566篇 |
1994年 | 10385篇 |
1993年 | 10217篇 |
1992年 | 23838篇 |
1991年 | 23057篇 |
1990年 | 22676篇 |
1989年 | 22029篇 |
1988年 | 20297篇 |
1987年 | 19937篇 |
1986年 | 18257篇 |
1985年 | 18380篇 |
1984年 | 15306篇 |
1983年 | 13373篇 |
1982年 | 10722篇 |
1981年 | 9558篇 |
1980年 | 9097篇 |
1979年 | 14675篇 |
1978年 | 11809篇 |
1977年 | 10677篇 |
1976年 | 10080篇 |
1975年 | 10909篇 |
1974年 | 11719篇 |
1973年 | 11512篇 |
1972年 | 10314篇 |
1971年 | 9558篇 |
1970年 | 8194篇 |
1969年 | 7738篇 |
1968年 | 6901篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
141.
G J Moore 《International journal of peptide and protein research》1985,26(5):469-481
The possible existence of intramolecular interactions involving the tyrosine and histidine residues in angiotensin II has been investigated by measuring the reactivities of the functional groups in the molecule. Angiotensin II catalyzed the hydrolysis of p-nitrophenylacetate in the pH range 6.6-8.2 at higher rates than were consistent with the reactivities of the free constituent functional groups, and had 2-4% of the activity of chymotrypsin between pH 6.6 and 7.5. Treatment of angiotensin II with acetic anhydride demonstrated that the tyrosine hydroxyl and the imidazole side-chain in angiotensin II acetylated and deacetylated at markedly higher rates than for the free amino acids, indicating increased nucleophilicities and the presence of intrinsic deacetylation mechanisms for these residues in angiotensin II. These findings are consistent with the presence of tyrosine hydroxyl-histidine-carboxylate charge relay system in ANG II in aqueous environments, and suggest that ANG II may act at membrane receptors by a mechanism which is analogous to that operating in serine proteases. 相似文献
142.
M. A. Riddin G. J. Venter K. Labuschagne M. H. Villet 《Medical and veterinary entomology》2019,33(4):467-475
An upsurge in African horse sickness (AHS) in the Eastern Cape, South Africa, from 2006 led to an epidemiological reassessment of the disease there. Light trapping surveys carried out near horses, donkeys and zebras in 2014–2016 collected 39 species of Culicoides midge (Diptera: Ceratopogonidae) that are potential vectors of AHS. To establish if these midges fed on equids, DNA sequences were obtained from the gut contents of 52 female midges (35 freshly blood‐fed, 13 gravid and four parous), representing 11 species collected across 11 sites. Culicoides leucostictus fed on all three equids. Culicoides bolitinos, Culicoides imicola and Culicoides magnus fed on both horses and donkeys. Culicoides onderstepoortensis fed on donkeys, and Culicoides similis and Culicoides pycnostictus fed on zebras. Bloodmeals from cows, pigs, warthogs, impalas and a domestic dog were also identified in various species, but none of the midges tested had fed on birds. These results contribute to knowledge of the vectorial capacity of several species of Culicoides with regard to AHS in the Eastern Cape and point to potential reservoir hosts, of which donkeys, zebras and domestic dogs have previously been found to harbour AHS. Blood‐fed midges were also obtained throughout winter, indicating the potential for endemic AHS in the province. 相似文献
143.
Patterns of Sleeping Site and Sleeping Tree Selection by Black-and-Gold Howler Monkeys (Alouatta caraya) in Northern Argentina 总被引:1,自引:0,他引:1
Brividoro Melina V. Kowalewski Martin M. Scarry Clara J. Oklander Luciana I. 《International journal of primatology》2019,40(3):374-392
International Journal of Primatology - The selection of sleeping sites and sleeping trees in nonhuman primates is related to social and ecological factors. We investigate the role of body... 相似文献
144.
145.
Defective transducing phages carrying aroG, the structural gene for phenylalanine (phe)-inhibitable phospho-2-keto-heptonate aldolase (EC 4.1.2.15; previously known as 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase[phe]), have been isolated, and DNA from two of these phages has been used to construct a restriction map of the region from att lambda to aroG. A 7.6-kb PstI-HindIII fragment from one of these phages was cloned into pBR322 and shown to contain aroG. The location of aroG within the 7.6 kb was established by subcloning and Tn3 transpositional mutagenesis. A fragment carrying the aroG promoter and operator has been cloned into a high copy number promoter-cloning vector (pMC489), and the resulting aroGpo-LacZ' (alpha) fusion subcloned in a low copy number vector. Strains with this fusion on the low copy number vector exhibit negative regulation of beta-galactosidase expression by both phenylalanine and tryptophan and positive regulation by tyrosine in a tyrR+ background. 相似文献
146.
A simple new procedure was described for producing a sequential series of overlapping clones for use in DNA sequencing. The technique used single-stranded M13 DNA and complementary DNA oligomers to form specific cleavage and ligation substrates. It was, therefore, independent of the sequence of the DNA cloned into the vector. Deletions of varying sizes were generated from one end of the insert through the 3' to 5' exonuclease activity of T4 DNA polymerase. The approximate size of the deletion and therefore the starting point for DNA sequencing could be estimated by electrophoresis of the subcloned phage DNA on a agarose gel. This greatly reduced the number of templates that must be sequenced to obtain a complete sequence. The entire procedure could be carried out in one tube in less than a day. The procedure was used to subclone and sequence the maize mitochondrial 18 S rDNA and 5' flanking region (2622 bases) in less than a week. Other applications of oligomers and single-stranded DNA in the construction of insertions, deletions, and cDNAs are discussed. 相似文献
147.
148.
Monoclonal antibody as a probe for structure and function of an Escherichia coli outer membrane protein 总被引:16,自引:0,他引:16
Eight independently derived monoclonal antibodies directed against the LamB protein were produced and characterized. By using these antibodies as probes, we identified four distinct topological and functional regions in the LamB molecule. Four monoclonal antibodies recognize antigenic determinants of the protein exposed on the outer side of the membrane. Two of these have their binding sites located in a region involved in maltose transport. One monoclonal antibody presumably binds to a determinant which is normally hidden in the membrane and three monoclonal antibodies recognize determinants facing the periplasmic space. 相似文献
149.
State of aggregation of the (Ca2+ + Mg2+)-ATPase studied using saturation-transfer electron spin resonance 总被引:1,自引:0,他引:1
The state of aggregation of the (Ca2+ + Mg2+)-ATPase in the membrane of sarcoplasmic reticulum and in reconstituted membrane systems has been studied using saturation-transfer electron spin resonance (ST-ESR). Saturation-transfer ESR spectra show that in the sarcoplasmic reticulum, the ATPase is relatively free to rotate, with an effective rotational correlation time of approx. 33 microseconds at 4 degrees C, consistent with a monomeric or dimeric structure. The rate of rotation is observed to decrease with decreasing molar ratio of lipid to protein. In reconstituted systems, rotational motion of the ATPase on the millisecond time scale ceases when the lipids are in the gel phase. Addition of decavanadate, which causes the formation of crystalline arrays in negatively stained electron micrographs, results in only a small reduction in rotation rate for the ATPase in the membrane. The experiments are interpreted in terms of a short-lived (on the millisecond time scale) protein-protein interaction, with the formation of crystalline clusters of ATPase molecules which form and melt rapidly. 相似文献
150.