全文获取类型
收费全文 | 3290篇 |
免费 | 147篇 |
国内免费 | 134篇 |
出版年
2024年 | 11篇 |
2023年 | 33篇 |
2022年 | 50篇 |
2021年 | 52篇 |
2020年 | 64篇 |
2019年 | 83篇 |
2018年 | 86篇 |
2017年 | 68篇 |
2016年 | 65篇 |
2015年 | 86篇 |
2014年 | 151篇 |
2013年 | 357篇 |
2012年 | 91篇 |
2011年 | 163篇 |
2010年 | 89篇 |
2009年 | 133篇 |
2008年 | 131篇 |
2007年 | 137篇 |
2006年 | 127篇 |
2005年 | 115篇 |
2004年 | 119篇 |
2003年 | 114篇 |
2002年 | 109篇 |
2001年 | 74篇 |
2000年 | 69篇 |
1999年 | 58篇 |
1998年 | 77篇 |
1997年 | 66篇 |
1996年 | 43篇 |
1995年 | 68篇 |
1994年 | 35篇 |
1993年 | 58篇 |
1992年 | 45篇 |
1991年 | 52篇 |
1990年 | 52篇 |
1989年 | 59篇 |
1988年 | 43篇 |
1987年 | 33篇 |
1986年 | 37篇 |
1985年 | 39篇 |
1984年 | 48篇 |
1983年 | 32篇 |
1982年 | 35篇 |
1981年 | 20篇 |
1980年 | 21篇 |
1979年 | 24篇 |
1978年 | 19篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有3571条查询结果,搜索用时 31 毫秒
831.
In this article, a recombinant cyclohexanone monooxygenase (CHMO), overexpressed in Escherichia coli has been used to study the oxidation of bicyclo[3.2.0]hept-2-en-6-one to its two corresponding lactones at very high enantiomeric excess. The reaction is a useful model for the study of biocatalytic oxidations to create optically pure molecules. The major limitations to a highly productive biocatalytic oxidation in this case are oxygen supply, product inhibition, and biocatalyst stability. In this article, we investigate the effects of whole cell biocatalyst concentration on the rate of reaction at a range of scales from shake flasks to 75 L bioreactors. At low cell concentrations (<2 g(dcw)/L) the maximum specific rate (0.65 g/g(dcw).h) is observed. However, at higher cell concentrations (> 2 g(dcw)/L), the reaction becomes oxygen limited and both the specific rate and absolute rate decrease with further increases in cell concentration. The role of oxygen limitation in reducing the rate of reaction with scale was investigated by increasing the maximum oxygen transfer rate in the reactor at a high cell concentration and observing the increase in product formation rate. We propose a qualitative model demonstrating the relationship between oxygen limitation, biocatalyst concentration, and the rate of reaction. This conceptual model will be a useful guide in the industrial scale-up of whole cell mediated Baeyer-Villiger biocatalysis. 相似文献
832.
Pardo-Andreu GL Dorta DJ Delgado R Cavalheiro RA Santos AC Vercesi AE Curti C 《Chemico-biological interactions》2006,159(2):141-148
Mitochondrial permeability transition (MPT) is a Ca(2+)-dependent, cyclosporin A (CsA)-sensitive, non-selective inner membrane permeabilization process. It is often associated with apoptotic cell death, and is induced by a wide range of agents or conditions, usually involving reactive oxygen species (ROS). In this study, we demonstrated that Mangifera indica L. extract (Vimang), in the presence of 20 microM Ca(2+), induces MPT in isolated rat liver mitochondria, assessed as CsA-sensitive mitochondrial swelling, closely reproducing the same effect of mangiferin, the main component of the extract, as well as MPT-linked processes like oxidation of membrane protein thiols, mitochondrial membrane potential dissipation and Ca(2+) release from organelles. The flavonoid catechin, the second main component of Vimang, also induces MPT, although to a lesser extent; the minor, but still representative Vimang extract components, gallic and benzoic acids, show respectively, low and high MPT inducing abilities. Nevertheless, following exposure to H(2)O(2)/horseradish peroxidase, the visible spectra of these compounds does not present the same changes previously reported for mangiferin. It is concluded that Vimang-induced MPT closely reproduces mangiferin effects, and proposed that this xanthone is the main agent responsible for the extract's MPT inducing ability, by the action on mitochondrial membrane protein thiols of products arising as a consequence of the mangiferin's antioxidant activity. While this effect would oppose the beneficial effect of Vimang's antioxidant activity, it could nevertheless benefit cells exposed to over-production of ROS as occurring in cancer cells, in which triggering of MPT-mediated apoptosis may represent an important defense mechanism to their host. 相似文献
833.
834.
In the phototrophic sulfur bacterium Allochromatium vinosum, sulfur of oxidation state zero stored in intracellular sulfur globules is an obligate intermediate during the oxidation of sulfide and thiosulfate. The proteins encoded in the dissimilatory sulfite reductase (dsr) locus are essential for the oxidation of the stored sulfur. DsrMKJOP form a membrane-spanning complex proposed to accept electrons from or to deliver electrons to cytoplasmic sulfur-oxidizing proteins. In frame deletion mutagenesis showed that each individual of the complex-encoding genes is an absolute requirement for the oxidation of the stored sulfur in Alc. vinosum. Complementation of the ΔdsrJ mutant using the conjugative broad host range plasmid pBBR1-MCS2 and the dsr promoter was successful. The importance of the DsrMKJOP complex is underlined by the fact that the respective genes occur in all currently sequenced genomes of sulfur-forming bacteria such as Thiobacillus denitrificans and Chlorobaculum tepidum. Furthermore, closely related genes are present in the genomes of sulfate- and sulfite-reducing prokaryotes. A phylogenetic analysis showed that most dsr genes from sulfide oxidizers are clearly separated of those from sulfate reducers. Surprisingly, the dsrMKJOP genes of the Chlorobiaceae all cluster together with those of the sulfate/sulfite-reducing prokaryotes, indicating a lateral gene transfer at the base of the Chlorobiaceae.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at . 相似文献
835.
Nrf2 regulates an adaptive response protecting against oxidative damage following diquat-mediated formation of superoxide anion 总被引:2,自引:0,他引:2
Osburn WO Wakabayashi N Misra V Nilles T Biswal S Trush MA Kensler TW 《Archives of biochemistry and biophysics》2006,454(1):7-15
Mouse embryonic fibroblasts derived from Nrf2-/- mice (N0) and Nrf2+/+ mice (WT) have been used to characterize both basal and diquat (DQ)-induced oxidative stress levels and to examine Nrf2 activation during exposure to DQ-generated superoxide anion. Microarray analysis revealed that N0 cells have similar constitutive mRNA expression of genes responsible for the direct metabolism of reactive oxygen species but decreased expression of genes responsible for the production of reducing equivalents, repair of oxidized proteins and defense against lipid peroxidation, compared to WT cells. Nonetheless, the basal levels of ROS flux and oxidative damage biomarkers in WT and N0 cells were not different. Diquat dibromide (DQ), a non-electrophilic redox cycling bipyridylium herbicide, was used to generate intracellular superoxide anion. Isolated mitochondria from both cell lines exposed to DQ produced equivalent amounts of ROS, indicating a similar cellular capacity to generate ROS. However, N0 cells exposed to DQ for 24-h exhibited markedly decreased cell viability and aconitase activity as well as increased lipid peroxidation and glutathione oxidation, relative to WT cells. 2',7'-Dichlorofluorescein fluorescence was not increased in WT and N0 cells after 30-min of DQ exposure. However, increased levels of ROS were detected in N0 cells but not WT cells after 13-h of DQ treatment. Additionally, total glutathione concentrations increased in WT, but not N0 cells following a 24-h exposure to DQ. DQ exposure resulted in activation of an antioxidant response element-luciferase reporter gene, as well as induction of Nrf2-regulated genes in WT, but not N0 cells. Thus the enhanced sensitivity of N0 cells does not reflect basal differences in antioxidative capacity, but rather an impaired ability to mount an adaptive response to sustained oxidative stress. 相似文献
836.
Zielonka J Sarna T Roberts JE Wishart JF Kalyanaraman B 《Archives of biochemistry and biophysics》2006,456(1):39-47
Hydroethidine (HE) is a cell-permeable probe used for the intracellular detection of superoxide. Here, we report the direct measurement of the rate constant between hydroethidine and superoxide radical anion using the pulse radiolysis technique. This reaction rate constant was calculated to be ca. 2 x 10(6) M(-1) s(-1) in water:ethanol (1:1) mixture. The spectral characteristics of the intermediates indicated that the one-electron oxidation product of HE was different from the one-electron reduction product of ethidium (E+). The HPLC-electrochemical measurements of incubation mixtures containing HE and the oxygenated Fenton's reagent (Fe2+/DTPA/H2O2) in the presence of aliphatic alcohols or formate as a superoxide generating system revealed 2-OH-E+ as a major product. Formation of 2-OH-E+ by the Fenton's reagent without additives was shown to be superoxide dismutase-sensitive and we attribute the formation of superoxide radical anion to the one-electron reduction of oxygen by the DTPA-derived radical. Addition of tert-butanol, DMSO, and potassium bromide to the Fenton's system caused inhibition of 2-OH-E+ formation. Results indicate that reducing and oxidizing radicals have differential effects on the formation of 2-OH-E+. 相似文献
837.
838.
Sulfamate is an analogue of thiosulfate, and the sodium and potassium salts of sulfamic acid inhibited the chemolithoautotrophic growth on thiosulfate of Acidithiobacillus ferrooxidans and Halothiobacillus neapolitanus. The chemo-organotrophic growth of Paracoccus versutus on sucrose was similarly inhibited by sulfamate. Thiosulfate oxidation by suspensions of H. neapolitanus was, however, unaffected by sulfamate, showing that sulfamate did not directly affect thiosulfate uptake, activation or oxidation. Inhibition of P. versutus was not relieved by cysteine and methionine, indicating that sulfate uptake and sulfur amino acid biosynthesis were not directly affected by sulfamate. Sulfamate was not degraded by any of the bacteria, and so could not serve as an alternative to thiosulfate as an energy-yielding substrate. Sulfamate is also an analogue of ammonia and might act like hydrazine by inhibiting ammonium uptake or an essential enzyme activity. 相似文献
839.
The effect of vitamin C (ascorbate) on oxidative DNA damage was examined by first incubating cells with dehydroascorbate, which boosts the intracellular concentration of ascorbate, and then exposing cells to H2O2. Oxidative DNA damage was estimated by the analysis of 5-hydroxy-2′-deoxycytidine (oh5dCyd) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (oxo8dGuo). The presence of a high concentration of ascorbate (30 mM), compared to the absence of ascorbate in cells, when exposed to H2O2 (200 μM), resulted in a remarkable sensitization of oh5dCyd from 2.7 ± 0.6 to 40.8 ± 6.1 lesions /106 dCyd (15-fold). In contrast, the level of oxo8dGuo increased from 8.4 ± 0.4 to 12.1 ± 0.5 lesions/106 dGuo (50%). The formation of oh5dCyd was also observed at lower concentrations of intracellular ascorbate and exogenous H2O2. Additional studies showed that replacement of H2O2 with tert-butyl hydroperoxide completely abolished damage, and that preincubation with iron and desferroxamine increased and decreased this damage, respectively. The latter studies suggest that a Fenton reaction is involved in the mechanism of damage. In conclusion, we report a novel model system in which ascorbate sensitizes H2O2-induced oxidative DNA damage in cells, leading to elevated levels of oh5dCyd and oxo8dGuo, with a strong bias toward the formation of oh5dCyd. 相似文献
840.
The patterns of the growth and oxidation of different types of natural pyrites were studied for the three microbial species adapted to these substrates and belonging to phylogenetically remote groups: gram-negative bacterium Acidithiobacillus ferrooxidans, gram-positive bacterium Sulfobacillus thermotolerans, and the archaeon Ferroplasma acidiphilum. For both A. ferrooxidans strains, TFV-1 and TFBk, pyrite 4 appeared to be the most difficult to oxidize and grow; pyrite 5 was oxidized by both strains at an average rate, and pyrite 3 was the most readily oxidized. On each of the three pyrites, growth and oxidation by TFBk were more active than by TFV-1. The effectiveness of the adaptation of S. thermotolerans Kr1T was low compared to the A. ferrooxidans strains; however, the adapted strain Kr1T showed the highest growth rate on pyrite 3 among all the cultures studied. No adaptation of strain Kr1T to pyrite 5 was observed; the rates of growth and pyrite oxidation in the third transfer were lower than in the first transfer. The strain F. acidiphilum YT was not adapted to pyrites 3 and 5; the rates of growth and pyrite oxidation were the same in the first five transfers. The strains of three species of the microorganisms studied, A. ferrooxidans, S. thermotolerans, and F. acidiphilum, grew on pyrite 3 (holetype (p) conductivity) and oxidized it better than pyrite 5 (mixed-type (n-p) conductivity). The most readily oxidized were the pyrites with a density of 5.6–5.7 g/cm3 and high resistance values (ln R = 8.8). The pyrite oxidation rate did not depend on the type of conductivity. Changes in the chromosomal DNA structure were revealed in strain TFBk on adaptation to pyrites 3 and 4 and in the TFV-1 plasmid profile on adaptation to pyrite 3. Correlation between genetic variability and adaptive capabilities was shown for A. ferrooxidans. No changes in the chromosomal DNA structure were found in S. thermotolerans Kr1T and F. acidiphilum YT on adaptation to pyrites 3 and 5. Plasmids were absent in the cells of these cultures. 相似文献