全文获取类型
收费全文 | 3626篇 |
免费 | 260篇 |
国内免费 | 1篇 |
专业分类
3887篇 |
出版年
2022年 | 19篇 |
2021年 | 34篇 |
2019年 | 32篇 |
2018年 | 40篇 |
2017年 | 39篇 |
2016年 | 79篇 |
2015年 | 99篇 |
2014年 | 143篇 |
2013年 | 145篇 |
2012年 | 238篇 |
2011年 | 257篇 |
2010年 | 189篇 |
2009年 | 144篇 |
2008年 | 197篇 |
2007年 | 198篇 |
2006年 | 172篇 |
2005年 | 178篇 |
2004年 | 160篇 |
2003年 | 179篇 |
2002年 | 165篇 |
2001年 | 44篇 |
2000年 | 30篇 |
1999年 | 66篇 |
1998年 | 61篇 |
1997年 | 46篇 |
1996年 | 62篇 |
1995年 | 41篇 |
1994年 | 44篇 |
1993年 | 33篇 |
1992年 | 59篇 |
1991年 | 38篇 |
1990年 | 46篇 |
1989年 | 36篇 |
1988年 | 22篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 24篇 |
1984年 | 29篇 |
1983年 | 25篇 |
1982年 | 40篇 |
1981年 | 34篇 |
1980年 | 25篇 |
1979年 | 35篇 |
1978年 | 19篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1973年 | 16篇 |
1971年 | 16篇 |
1970年 | 18篇 |
1967年 | 15篇 |
排序方式: 共有3887条查询结果,搜索用时 15 毫秒
61.
The covalent hydration of β-halopyruvic acids and β-halopyruvamides has been investigated in the uv region by stopped-flow techniques. In aqueous solutions of β-bromopyruvate and related compounds, the geminal diol is the predominant form at all pH values. Kinetic investigations have also been carried out on these hydrations. General bases enhance the rate of approaching the equilibrium. Specific acid catalysis was not detected. In the water-catalyzed reaction of pyruvamides, the relationship between the equilibrium constants K and the rate constants of the forward reaction
shows that the transition state is more productlike. 相似文献
62.
Chromatin-bound, DNA-dependent RNA polymerase (EC 2.7.7.6) activity and chromatin template availability, as measured with saturating amounts of E. coli RNA polymerase, changes rhythmically during the formation, dormancy, and sprouting of potato tubers. Active growth processes coincide with the highest RNA polymerase activity as well as the greatest template accessibility, during tuberization and sprouting. Consequently, chromatin-associated RNA and protein content is highest in young developing tubers and in old tubers at the onset of sprouting. Ribosomal RNA content, in turn, is maximal in small tubers, remains constant during dormancy, and decreases when sprouting begins, probably due to the translocation of rRNA into the sprouts. The nucleolus changes its shape and size concomitantly with the process of tuberization. 相似文献
63.
Wayss Klaus Reyes-Mayes Denis Volm Manfred 《Virchows Archiv. B, Cell pathology including molecular pathology》1981,38(1):13-21
Virchows Archiv B Cell Pathology - The long-term (34 weeks) topical administration of 7,12-dimethylbenz(a)anthracene (DMBA) to the skin of male and femaleMastomys induced a broad spectrum of benign... 相似文献
64.
The effect of placebo and ACTH-1-17 (Synchrodyn®, Hoechst) upon urinary free cortisol was examined at 5 different circadian stages on 10 men with Steinbrocker Stage II–III rheumatoid arthritis. A mean cosinor analysis of urinary cortisol data from the subjects prior to treatment with either ACTH or placebo revealed a statistically highly-significant rhythm. A circadian variation in a response of urinary free cortisol to a placebo was also seen. Moreover, the response of the midline-estimating statistic of rhythm (rhythm-adjusted circadian average) of urinary free cortisol to ACTH-1-17 by patients with rheumatoid arthritis is circadian rhythmic. This reactivity rhythm is out of phase with the spontaneous rhythm in urinary cortisol acrophases—in the tests limited thus far to midsummer. The further assessment of the circadian component in the context of broader interactions by rhythms with other frequencies in various conditions in health and disease is warranted by the demonstration of rhythms here presented for men with rheumatoid arthritis. 相似文献
65.
Summary In the American cockroach, the distribution and connections of neuronal elements of the terminal ganglion-proctodeal nerve-hindgut system were investigated by means of immunohistochemical methods and axonal CoCl2 iontophoresis. Proctolinlike immunoreactivity was localized within neurons of the terminal ganglion projecting into the proctodeal nerve on the one hand, and in nerve cells without a direct connection to this system on the other. Immunohistochemically, in whole mount preparations fibres of the proctodeal nerve and terminal structures in the hindgut musculature exhibit strong proctolinlike immunoreactivity. At the light- and electron-microscopic levels the pathways of about 30 somata of the proctodeal neural system were characterized by cobalt chloride iontophoresis. The relationships of cobalt filled and immunoreactive neuronal structures are discussed.For the preparation of tritiated proctolin we thank Dr. S. Reißmann, WB Biochemie, Sektion Biologie, FSU JenaThe authors wish to thank G. Schörlitz, Film- und Bildstelle, FSU Jena, for photographs of whole mount preparations and Ms. A. Zinßer and Mrs. B. Cosack for excellent technical assistance. 相似文献
66.
Manfred Horter 《Ethology : formerly Zeitschrift fur Tierpsychologie》1977,45(3):256-287
The possibility of learning time intervals was examined. 7 male BDE-rats (70 days old at the beginning of the experiments) were trained (shock avoidance learning) in a modified test chamber to learn bar-pressing only in distinguished time periods (LD 12: 12). In a second part of the experiment it was examined with 5 rats what effect an extension of the light phase has. The results show that there is a possibility of learning time intervals and of maintaining this behaviour under these specific conditions of training. 相似文献
67.
S-Alkylated 2-thiocytidine can be substituted at C-2 by nucleophilic agents. This reaction has been investigated with model compounds as well as with tRNA using the amino acids cysteine and lysine in order to develop a new affinity label linking covalently tRNA and a protein. Reaction with N-protected cysteine gives 2-S-alkyl-pyrimidines, while unprotected cysteine yields an N-alkyl-pyrimidine, after intramolecular substitution. With the -amino group of lysine a fast replacement at C-2 is observed, leading to an unstable 2-N-alkyl-pyrimidine. All products have been characterized both chemically and spectroscopically. 相似文献
68.
Anthony A. Ribeiro Robert P. Saltman Murray Goodman Manfred Mutter 《Biopolymers》1982,21(11):2225-2239
The use of 1H-nmr spectroscopy is demonstrated to be a useful analytical method to characterize the structure of synthetic peptides attached to soluble, macromolecular polyoxyethylene (POE) supports in the liquid-phase method (LPM) of peptide synthesis. We report an extensive 360-MHz 1H-nmr study of POE-bound homo-oligo-L -methionine peptides. A combination of high field and selective saturation or Redfield pulse methods allows resolution of individual backbone NH and α-CH resonances of dilute peptides in the presence of strong resonances from macromolecular POE and/or protonated solvents. The nmr spectra for the POE-bound peptides in CDCl3 are qualitatively similar to those of the low-molecular-weight Boc-L -Metn-OMe peptide esters. This corroborates other observations that POE has little effect on peptide stucture. The backbone α-CH region of peptides is overlapped by signals from the terminal oxyethylene group of POE, but the peptide side-chain and low-field backbone NH resonances are well resolved. In trifluoroethanol the Boc-(L -Met)n-NH-POE heptamer and octamer adopt the right-handed α-helical structure, and the present nmr studies provide evidence for two strong intramolecular hydrogen bonds to stabilize the helices. In water, the N-deblocked derivatives, (L -Met)n-NH-POE oligomers adopt β-sheet structure and manifest well-resolved nonequivalent NH resonances with 6–7 Hz 3JNH-CH coupling constants. 相似文献
69.
70.
Manfred Braun Jong Min Kim Rolf D. Schmid 《Applied microbiology and biotechnology》1992,37(5):594-598
Summary The soil isolate Cellulomonas cellulans AM8 produces an extracellular l-amino acid oxidase (L-AAO) with broad substrate specificity. The strain produced up to 0.35 unit (U)/ml of the extracellular L-AAO in a simple medium containing glycerol and yeast extract. The enzyme was easily purified up to 30 U/mg protein using Phenyl-Sepharose fast flow. The purified enzyme migrated as single band on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) with a molecular mass of 55 kDa. On native PAGE the molecular mass was approx. 300 000 kDa, which may be due to aggregation. With the exception of glycine, proline, and threonine, all the amino acids normally constituting proteins were oxidized. The V
max values from 0.7 to 35.2 U/mg for aspartic acid and lysine, respectively, and the K
m values from 0.007 to 7.1 mm for cysteine and valine, respectively, were obtained at 25° C and pH 7.0 in oxygen-saturated solutions. The L-AAO had a pH optimum of 6.5–7.5. It was stable for several months at — 30° C and for some days at 35° C. Ferricyanide served as an electron acceptor with a V
max of 50 U/mg and K
m for 0.3 mm with phenylalanine as the substrate.
Correspondence to: R. D. Schmid 相似文献