全文获取类型
收费全文 | 698篇 |
免费 | 73篇 |
出版年
2021年 | 7篇 |
2016年 | 9篇 |
2015年 | 14篇 |
2014年 | 27篇 |
2013年 | 16篇 |
2012年 | 50篇 |
2011年 | 24篇 |
2010年 | 18篇 |
2009年 | 16篇 |
2008年 | 32篇 |
2007年 | 27篇 |
2006年 | 19篇 |
2005年 | 29篇 |
2004年 | 27篇 |
2003年 | 20篇 |
2002年 | 28篇 |
2001年 | 25篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 14篇 |
1997年 | 7篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 16篇 |
1991年 | 8篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1972年 | 5篇 |
1970年 | 8篇 |
1969年 | 6篇 |
1966年 | 4篇 |
1962年 | 4篇 |
1941年 | 4篇 |
1935年 | 5篇 |
排序方式: 共有771条查询结果,搜索用时 15 毫秒
1.
2.
3.
The dry weight and the crest length of the upper and lower beak, the length of the radula ribbon, the average width of the base of the six proximal and distal rachidian teeth as well as the total number of these teeth have all been related to the live body weight of octopuses between 1.1 and 4440 g. From any one of these parameters it is possible to estimate the size and approximate age of the animal. 相似文献
4.
5.
Pyruvate decarboxylase (PDC, EC 4.1.1.1) is a thiamin diphosphate-dependent enzyme about which there is a large body of structural and functional information. The active site contains several absolutely conserved ionizable groups and all of these appear to be important, as judged by the fact that mutation diminishes or abolishes catalytic activity. Previously we have shown [Schenk, G., Leeper, F.J., England, R., Nixon, P.F. & Duggleby, R.G. (1997) Eur. J. Biochem. 248, 63-71] that the activity is pH-dependent due to changes in kcat/Km while kcat itself is unaffected by pH. The effect on kcat/Km is determined by a group with a pKa of 6.45; the identity of this group has not been determined, although H113 is a possible candidate. Here we mutate five crucial residues in the active site with ionizable side-chains (D27, E50, H113, H114 and E473) in turn, to residues that are nonionizable or should have a substantially altered pKa. Each protein was purified and characterized kinetically. Unexpectedly, the pH-dependence of kcat/Km is largely unaffected in all mutants, ruling out the possibility that any of these five residues is responsible for the observed pKa of 6.45. We conjecture that the kcat/Km profile reflects the protonation of an alcoholate anion intermediate of the catalytic cycle. 相似文献
6.
Human erythrocyte transketolase could be resolved from thiamin diphosphate (TDP) by acidification of the ammonium sulfate precipitate to pH 3.5, but not by other tested procedures. Resolution was 98% by chemical measurement of residual thiamin and 95% by residual enzyme activity. Reconstitution of the resolved preparation by incubation with TDP was dependent upon TDP concentration, duration, temperature, and the presence of dithiothreitol. At low TDP concentrations, 1 h was required for maximum activation; kinetic analysis then yielded an apparent Km value for TDP of 65 nM (SD 14 nM) from 100 erythrocyte lysates and similar values for reconstituted resolved preparations previously purified 400-fold and 10,000-fold. Velocity data obtained by transketolase assays in which the TDP was added to resolved preparations simultaneously with substrates yielded an apparent Km value for TDP of 2.3 microM (SD 1.6 microM) from 114 erythrocyte lysates and similar values for purified preparations. The recovery of activity following resolution and reconstitution ranged from 21 to 60% from lysates and 38 to 70% from purified preparations. Residual ammonium sulfate up to 4.9 mM decreased the apparent Km value for TDP, while a concentration of 11.3 mM increased the value in a manner competitive with TDP and with an apparent Ki value of 2.3 mM. The spectrophotometric assay of transketolase activity was greatly affected by storage of frozen solutions of the substrate ribose 5-phosphate. 相似文献
7.
AN AMPLIFICATION OF THE PHYLOGENETIC SPECIES CONCEPT 总被引:9,自引:0,他引:9
Kevin C. Nixon Quentin D. Wheeler 《Cladistics : the international journal of the Willi Hennig Society》1990,6(3):211-223
Abstract— The goal of a phylogenetic species concept is to reveal the smallest units that are analysable by cladistic methods and interpretable as the result of phylogenctic history. We define species as the smallest aggregation of populations (sexual) or lineagcs (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts). A character state is an inherited attribute distributed among all comparable individuals (semaphoronts) of the same historical population, clade, or terminal lineage. This definition of species is character-based and pattern oriented. Evolutionary explanations of phylogenetic species are consistent with contemporary explanations of processes of speciation, but require only the assumption of nested hierarchical pattern. We discuss the compatibility of the phylogenetic species concept with various biological needs for species and justify its use at the exclusion of alternative species concepts. 相似文献
8.
9.
R A Nixon 《Current opinion in cell biology》1992,4(1):8-14
New studies provide further evidence that the neuronal cytoskeleton is the product of a dynamic interplay between axonal transport processes and locally regulated assembly mechanisms. These data confirm that the axonal cytoskeleton in mammalian systems is largely stationary and is maintained by a smaller pool of moving subunits or polymers. Slow axonal transport in certain lower species, however, may exhibit quite different features. 相似文献
10.
A rapid and sensitive spot test amenable to visual or spectrophotometric quantitation has been developed for a wide variety of biochemical reagents by utilizing the transition metal salt cupric chloride and its large number of related colored compounds. This assay is potentially a widely applicable multipurpose test for rapidly detecting the presence of unknown substances. Combination of the test sample with the working reagent results in the immediate formation of a distinctive colored product that may be precipitable. Some compounds require the further addition of sodium hydroxide in order to generate the distinctively colored product. Distinctive reactions occur with the following reagents, and their limit of visual detection is indicated in parentheses: ammonium bicarbonate (12.5 mM), ammonium acetate (25 mM), ammonium hydroxide (0.1%), ammonium sulfate (2%), ammonium persulfate (0.02 mM), L-(+)-cysteine (0.07 mM), dithiothreitol (DTT) (1.25 mM), EDTA (0.6 mM), ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (5 mM), D-glucose (6 mM), glycerol (0.3%), imidazol (12.5 mM), DL-methionine (100 mM), mercaptoethanol (0.05%), sodium azide (19 mM, 0.1%), sodium dithionite (0.25%), sodium metabisulfite (25 mM), sodium nitrite (6.2 mM), sodium periodate (3.1 mM), sodium sulfite (12.5 mM), sodium thiosulfite (12.5 mM), sucrose (6 mM), and N,N,N',N'-tetramethylethylenediamine (0.05%). A distinctive exothermic reaction occurs with hydrogen peroxide, but without color change. Compounds reacting insignificantly include 50 mM Tris buffer, urea, N,N'-methylene bisacrylamide, sodium dodecyl sulfate, isopropyl alcohol, sodium fluoride, trichloroacetic acid, phenol, mannose, K2HPO4, guanidine HCl, chloramine-T, magnesium chloride, and boric acid, where the solids were tested at approximately 10 mg/ml. Spectrophotometric standard curves were developed for DTT and sodium azide utilizing the clear supernatants resulting from these reactions. Combinations of at least four reagents could be discriminated, as demonstrated with mixtures of glucose, sodium azide, EDTA, and DTT. In addition ammonium sulfate could be detected to a limit of 4% in the presence of protein, DTT, and EDTA in a 50 mM Tris buffer. Spot tests were developed which utilized reagent-impregnated filter paper and gave distinctive colored products on addition of 5 microliter of test sample. 相似文献