全文获取类型
收费全文 | 2668篇 |
免费 | 261篇 |
出版年
2023年 | 16篇 |
2022年 | 26篇 |
2021年 | 58篇 |
2020年 | 38篇 |
2019年 | 37篇 |
2018年 | 71篇 |
2017年 | 53篇 |
2016年 | 73篇 |
2015年 | 110篇 |
2014年 | 127篇 |
2013年 | 135篇 |
2012年 | 183篇 |
2011年 | 199篇 |
2010年 | 110篇 |
2009年 | 96篇 |
2008年 | 150篇 |
2007年 | 146篇 |
2006年 | 146篇 |
2005年 | 110篇 |
2004年 | 103篇 |
2003年 | 104篇 |
2002年 | 99篇 |
2001年 | 63篇 |
2000年 | 64篇 |
1999年 | 47篇 |
1998年 | 34篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 26篇 |
1994年 | 23篇 |
1993年 | 26篇 |
1992年 | 42篇 |
1991年 | 22篇 |
1990年 | 32篇 |
1989年 | 26篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 19篇 |
1985年 | 32篇 |
1984年 | 15篇 |
1982年 | 14篇 |
1981年 | 9篇 |
1980年 | 17篇 |
1979年 | 19篇 |
1978年 | 14篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1972年 | 10篇 |
1971年 | 10篇 |
1970年 | 9篇 |
排序方式: 共有2929条查询结果,搜索用时 15 毫秒
61.
Oligosaccharides at individual glycosylation sites in glycoprotein 71 of Friend murine leukemia virus 总被引:4,自引:0,他引:4
R Geyer J Dabrowski U Dabrowski D Linder M Schlüter H H Schott S Stirm 《European journal of biochemistry》1990,187(1):95-110
Glycoprotein 71 from Friend murine leukemia virus was digested with proteases and the glycopeptides obtained were isolated and assigned, by amino acid sequencing, to the eight N-glycosylated asparagines in the molecule; only Asn334 and Asn341 could not be separated. The oligosaccharides liberated from each glycopeptide by endo-beta-N-acetylglucosaminidase H, or by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, were fractionated and subjected to structural analysis by one- and two-dimensional 1H NMR, as well as by methylation/gas-liquid-chromatography/mass-fragmentography. At each glycosylation site, the substituents were found to be heterogeneous including, at Asn334/341 and Asn410, substitution by different classes of N-glycans: oligomannosidic oligosaccharides, mainly Man alpha 1----6(Man alpha 1----3)Man alpha 1----6(Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4GlcNAc beta 1----, were detected at Asn168, Asn334/341 and Asn410. Hybrid species, partially sialylated, intersected and (proximally) funcosylated Man alpha 1----6(Man alpha 1----3)Man alpha 1----6 and Man alpha 1----3Man alpha 1----6 and Man alpha 1----3Man alpha 1----6(Gal beta 1----4GlcNAc beta 1----2Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4GlcNAc beta 1----, were found at Asn12, as previously published [Schlüter, M., Linder, D., Geyer, R., Hunsmann, H., Schneider, J. & Stirm, S. (1984) FEBS Lett. 169, 194-198] and at Asn334/341. N-Acetyllactosaminic glycans, mainly partially intersected and fucosylated NeuAc alpha 2----3 or Gal alpha 1----3Gal beta 1----4GlcNAc beta 1----2Man alpha 1----6(NeuAc alpha 2----6 or NeuAc alpha 2----3Gal-beta 1----4GlcNAc beta 1----2Man alpha 1----3)Man beta 1----4GlcNac beta 1----4GlcNAc beta 1---- with some bifurcation at ----6Man alpha 1----6, were obtained from Asn266, Asn302, Asn334/341, Asn374 and Asn410. In addition, Thr268, Thr277, Thr279, Thr304/309, as well as Ser273 and Ser275, were found to be O-glycosidically substituted by Gal beta 1----3GalNAc alpha 1----, monosialylated or desialylated at position 3 of Gal or/and position 6 of GalNAc. 相似文献
62.
In vivo and in vitro analysis of structure-function relationships in ribosomal protein L25 from Saccharomyces cerevisiae 总被引:2,自引:0,他引:2
C A Rutgers P J Schaap J van 't Riet C L Woldringh H A Raué 《Biochimica et biophysica acta》1990,1050(1-3):74-79
We have developed a combination of in vivo and in vitro methods which allows us to determine the effect of practically every structural change, deletions as well as point mutations, on various biological functions of a ribosomal protein (r-protein). We have used this approach to delineate the functional domains of r-protein L25 from Saccharomyces cerevisiae. By analysis of the intracellular distribution of fusion proteins carrying various portions of L25 linked to Escherichia coli beta-galactosidase we traced the nuclear localization signal(s) of L25 to the region encompassing the N-terminal 61 amino acids of the protein. On the other hand, using in vitro prepared fragments of L25 we located the domain responsible for its specific binding to 26S rRNA to the region between amino acids 61 and 135. In order to be able to analyze the effect of mutations in L25 on ribosome biogenesis and function in vivo we constructed a mutant yeast strain in which the chromosomal L25 gene is placed under control of the inducible yeast GAL promoter. Since this strain is unable to grow on glucose as a carbon source the L25 gene must be essential for cell viability. Growth on glucose can be restored by introduction of a wild-type L25 gene on a plasmid, demonstrating that under these conditions the cells are dependent upon an extrachromosomally supplied copy of the gene. 相似文献
63.
Previous experiments have indicated that enrichment of subgingival plaque in human serum can lead to the accumulation of Treponema denticola. T. denticola depends on bacterial interactions for its growth in serum. Aim of the present study was to identify specific microorganisms involved in the growth stimulation of T. denticola. To this end, strains isolated from previous plaque enrichment cultures were tested for growth stimulation in co-cultures with T. denticola. In addition, growth of T. denticola was tested in culture filtrates of the same strains, Bacteroides intermedius, Eubacterium nodatum, Veillonella parvula and Fusobacterium nucleatum were found to enhance growth of T. denticola in co-cultures. A continuous co-culture of T. denticola, F. nucleatum and B. intermedius in human serum gave very high levels of T. denticola, up to 3.10(9).ml-1. Mechanisms involved in growth stimulation may include the ability of B. intermedius and E. nodatum to cleave the protein-core of serum (glyco-)proteins, making these molecules accessible for degradation by T. denticola. In addition, E. nodatum was found to produce a low-molecular weight growth-factor for T. denticola, that was heat-stable and acid as well as alkaline resistant. V. parvula may provide peptidase activities complementary to those of T. denticola. The nature of the growth enhancing activity of F. nucleatum is yet unknown. The data support the dependency of T. denticola on other bacterial species for growth in the periodontal pocket. 相似文献
64.
L. Schachinger U. Hagen H. Klöter Ch. Schippel M. Jaenicke 《Radiation and environmental biophysics》1982,21(2):97-107
Summary In a previous publication it has been shown that the radiation induced physiological inactivation of dibutyryl-cAMP was far more pronounced than the chemical modification (Schachinger et al. 1981). In this paper it will be shown, that irradiation of dib-cAMP in solution resulted in the formation of monobutyryl cAMP and other not yet completely identified hydroxylated derivatives as well as a decomposition of the purine structure. Moreover, irradiated dib-cAMP inhibited the physiological activity of not irradiated dib-cAMP on the smooth muscle. From the data an effectiveK
m
-value for dib-cAMP of 2.4 × 10–5 M was determined and an effectiveK
I
-value of 1.3 × 10–6 M was found for the irradiation products, i.e., a tenfold affinity of the latter compared to unirradiated dib-cAMP. The results are discussed with respect to a better understanding of dose-response curves for chemical and physiological inactivation. 相似文献
65.
V. P. Claassen L. F. Oltmann C. E. M. Vader J. van 't Riet A. H. Stouthamer 《Archives of microbiology》1982,133(4):283-288
Molybdenum cofactor was extracted from membranes of Proteus mirabilis by three methods: acidification, heat treatment and heat treatment in the presence of sodium-dodecylsulphate (SDS). Extracts prepared by the latter method contained the highest concentration of molybdenum cofactor. In these extracts molybdenum cofactor was present in a low molecular weight form. It could not penetrate an YM-2 membrane during ultrafiltration suggesting a molecular weight above 1000. During aerobic incubation of cofactor extracts from membranes at least four fluorescent species were formed as observed in a reversed-phase high performance liquid chromatography (HPLC) system. The species in the first peak was inhomogeneous while the species in the others seem to be homogenous. In water, all fluorescent products had an excitation maximum at 380 nm and an emission maximum at 455 nm. Their absorption spectra showed maxima at around 270 nm and 400 nm. Fluorescent compounds present in the first peak could penetrate an YM-2 membrane during ultrafiltration, whereas the compounds in the other peaks hardly did. Using xanthine oxidase from milk as source of molybdenum cofactor apparently identical cofactor species were found. Cytoplasmic nor membrane extracts of the chlorate resistant mutant chl S 556 of P. mirabilis could complement nitrate reductase of Neurospora crassa nit-1 in the presence of 20 mM molybdate. However, fluorescent species with identical properties as found for the wild-type were formed during aerobic incubation of extracts from membranes of this mutant.Non-common Abbreviations HPLC
high performance liquid chromatography
- I.D.
internal diameter
- SDS
sodium dodecyl sulphate 相似文献
66.
Summary Double heterozygosity of pyruvate kinase (PK) deficiency associated with hereditary hemolytic anemia is emphasized by studies of a kindred harboring two distinct mutant forms of this enzyme. The hematologically unaffected parents exhibit slightly reduced PK activity, a normal Hill coefficient, and a normal thermodynamic dissociation constant for the overall reaction. The paternal enzyme is characterized by normal substrate affinities and decreased activities with the substrate analogues CDP and GDP, whereas the maternal enzyme shows normal affinity for PEP, but an increased affinity for ADP and low thermostability. It is assumed that the erythrocytes of the parents contain a mixture of normal PK and a functionally abnormal isoenzyme, the latter differing between the parents. The two children suffer from hereditary hemolytic anemia. Their PK must be a combination of the mutant paternal and maternal isoenzymes, and their activities are reduced to about 30%. These enzymes are characterized by an increased affinity for PEP and a decreased affinity for ADP, a Hill coefficient of about 1 (indicating lack of cooperativity due to a loss of its allosteric properties), a decreased overall catalytic activity, and a higher resistance to heat denaturation. Further differences are observed in the SDS-gel electrophoresis between the two patients' enzymes. From the enzymological point of view it is impossible to characterize true PK variants in such double heterozygous cases which contain a combination of two different isoenzymes. The cause of chronic hemolysis appears to depend mainly on the loss of the allosteric properties, i.e., the lack of enzyme cooperativity. 相似文献
67.
Summary Leaf protoplasts of Nicotiana tabacum SR1 (2n=4x=48) treated with iodoacetate (10 mM; 25 C; 30 min) and consequently unable to divide, and untreated leaf protoplasts of Nicotiana sylvestris (2n=2x=24) were fused using polyethylene glycol (PEG). The SR1 line is resistant to streptomycin because of a maternally inherited mutation, and has streptomycin-insensitive chloroplast ribosomes.After 1 month of growth in the absence of streptomycin protoplast-derived calli were plated into selective medium (1,000 g ml-1 streptomycin) and the resistant clones were isolated. Out of 106 PEG-treated protoplasts (1:1 mixture of parental types) 137 resistant (green) clones were obtained, whereas in the same number of parental cells, not subjected to fusion induction, no resistant callus was found.At least four plants were regenerated from each of the clones. The regenerates were identified as somatic hybrids (H), N. sylvestris (Ns) or N. tabacum (Nt) by looking at esterase and peroxidase isoenzymes and morphology. The three types of regenerates were distributed amongst the clones as follows: H only (105 clones); Ns (16 clones); Ns+H (6 clones); Nt only (3 clones); Nt+H (6 clones); Nt+Ns (1 clone). The high proportion of hybrid regenerates indicates that nuclear fusion has occured in the overwhelming majority of the heterokaryocytes. Cytoplasmic mutations in combination with inactivation by iodoacetate, therefore, are suitable markers to produce somatic hybrids. Segregation of nuclei after fusion resulted in new combinations of organelles and nuclei, the final outcome being the transfer of resistant chloroplasts into N. sylvestris, some of which have the original diploid (2n=24) chromosome number. Data suggest that segregants were in most cases obtained from multiple fusions. Streptomycin resistance was inherited maternally in the N. sylvestris (six clones) tested and the hybrid (three clones) regenerates. 相似文献
68.
Summary In touch preparations and tissue sections from normal rat hepatocytes and -amanitine- and actinomycind-poisoned liver cells the extent of chromatin condensation relatively to the degree of ploidy and the chromatin distribution were studied by means of computer aided cytophotometry. It could be found, that the relationship of the condensed and decondensed chromatin is independent of the degree of ploidy and first of all dependent upon the intoxicant. Therefore, the extent of chromatin condensation can be utilized also in sections as additional parameter for automated cell screening.Supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 105 相似文献
69.
70.
Mass culture experiments with Brachionus Rubens 总被引:1,自引:1,他引:0
M. Schlüter 《Hydrobiologia》1980,73(1-3):45-50
In order to develop the optimum conditions for a mass culture of Brachionus rubens, eight strains of phytoplankton were tested as food for the rotifers. The optimum food concentration as well as the concentration of algal medium tolerated by R. rubens, and the influence of nitrite, sodium chloride, extreme pH-values and low oxygen concentrations on the reproduction of B. rubens were determined. 相似文献