全文获取类型
收费全文 | 932篇 |
免费 | 77篇 |
国内免费 | 1篇 |
专业分类
1010篇 |
出版年
2023年 | 2篇 |
2022年 | 15篇 |
2021年 | 36篇 |
2020年 | 26篇 |
2019年 | 26篇 |
2018年 | 24篇 |
2017年 | 27篇 |
2016年 | 23篇 |
2015年 | 42篇 |
2014年 | 57篇 |
2013年 | 61篇 |
2012年 | 65篇 |
2011年 | 68篇 |
2010年 | 32篇 |
2009年 | 37篇 |
2008年 | 30篇 |
2007年 | 34篇 |
2006年 | 30篇 |
2005年 | 41篇 |
2004年 | 24篇 |
2003年 | 41篇 |
2002年 | 39篇 |
2001年 | 8篇 |
2000年 | 21篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 12篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1983年 | 21篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1956年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有1010条查询结果,搜索用时 0 毫秒
111.
Hosia W Bark N Liepinsh E Tjernberg A Persson B Hallén D Thyberg J Johansson J Tjernberg L 《Biochemistry》2004,43(16):4655-4661
The tetrapeptide KFFE is one of the shortest amyloid fibril-forming peptides described. Herein, we have investigated how the structural environment of this motif affects polymerization. Using a turn motif (YNGK) or a less rigid sequence (AAAK) to fuse two KFFE tetrapeptides, we show by several biophysical methods that the amyloidogenic properties are strongly dependent on the structural environment. The dodecapeptide KFFEAAAKKFFE forms abundant thick fibril bundles. Freshly dissolved KFFEAAAKKFFE is monomeric and shows mainly disordered secondary structure, as evidenced by circular dichroism, NMR spectroscopy, hydrogen/deuterium exchange measurements, and molecular modeling studies. In sharp contrast, the dodecapeptide KFFEYNGKKFFE does not form fibrils but folds into a stable beta-hairpin. This structure can oligomerize into a stable 12-mer and multiples thereof, as shown by size exclusion chromatography, sedimentation analysis, and electrospray mass spectrometry. These data indicate that the structural context in which a potential fibril forming sequence is present can prevent fibril formation by favoring self-limiting oligomerization over polymerization. 相似文献
112.
We studied the effects of fungal endophyte infection of meadow ryegrass (Lolium pratense=Festuca pratensis) on the frequency of the barley yellow dwarf virus (BYDV). The virus is transferred by aphids, which may be deterred by endophyte-origin alkaloids within the plant. In our experiment, we released viruliferous aphid vectors on endophyte-infected and endophyte-free plants in a common garden. The number of aphids and the percentage of BYDV infections were lower in endophyte-infected plants compared to endophyte-free plants, indicating that endophyte infection may protect meadow ryegrass from BYDV infections. 相似文献
113.
Impaired cerebral energy metabolism may be a major contributor to the secondary injury cascade that occurs following traumatic brain injury (TBI). To estimate the cortical energy metabolic state following mild and severe controlled cortical contusion (CCC) TBI in rats, ipsi-and contralateral cortical tissues were frozen in situ at 15 and 40 min post-injury and adenylate (ATP, ADP, AMP) levels were analyzed using high-performance liquid chromatography (HPLC) and the energy charge (EC) was calculated. At 15 min post-injury, mildly brain-injured animals showed a 43% decrease in cortical ATP levels and a 2.4-fold increase in AMP levels (P < 0.05), and there was a significant reduction of the ipsilateral cortical EC when compared to sham-injured animals (P < 0.05). At 40 min post-injury, the ipsilateral adenylate levels and EC had recovered to the values observed in the sham-injury group. In the severe CCC group, there was a 51% decrease in ipsilateral cortical ATP levels and a 5.3-fold increase in AMP levels with a significant reduction of cortical EC at 15 min post-injury (P < 0.05). At 40 min post-injury, a 2.6-fold ipsilateral increase in AMP levels and an 11% and 44% decrease in EC and ATP levels, respectively, remained (P < 0.05). A 37–38% reduction of the total adenylate pool was observed ipsilaterally in both CCC severity groups at the early time-point, and a 19% and 28% decrease remained in the mild and severe CCC groups, respectively, at 40 min post-injury. Significant contralateral ATP and EC changes were only observed in the severe CCC group at 40 min post-injury (P < 0.05). The energy-requiring secondary injury cascades that occur early post-injury do not challenge the brain tissue to the extent of ATP depletion and may provide a window of opportunity for therapeutic intervention. 相似文献
114.
Otterhag L Gustavsson N Alsterfjord M Pical C Lehrach H Gobom J Sommarin M 《Biochimie》2006,88(1):11-21
The Arabidopsis thaliana protein kinase AtPDK1 was identified as a homologue of the mammalian 3-phosphoinositide-dependent protein kinase-1 (PDK1), which is involved in a number of physiological processes including cell growth and proliferation. We now show that AtPDK1, expressed in E. coli as a recombinant protein, undergoes autophosphorylation at several sites. Using mass spectrometry, three phosphorylated amino acid residues, Ser-177, Ser-276 and Ser-382, were identified, followed by mutational analyses to reveal their roles. These residues are not conserved in mammalian PDK1s. Mutation of Ser-276 in AtPDK1 to alanine resulted in an enzyme with no detectable autophosphorylation. Autophosphorylation was significantly reduced in the Ser177Ala mutant but was only slightly reduced in the Ser382Ala mutant. Other identified sites of importance for autophosphorylation and/or activity of AtPDK1 were Asp-167, Thr-176, and Thr-211. Sites in the mammalian PDK1 corresponding to Asp-167 and Thr-211 are essential for PDK1 autophosphorylation and activity. Autophosphorylation was absent in the Asp167Ala mutant while the Thr176Ala and The211Ala mutants exhibited very low but detectable autophosphorylation, pointing to both similarity and difference between mammalian and plant enzymes. We also demonstrate that AtS6k2, an A. thaliana homologue to the mammalian S6 kinases, is an in vitro target of AtPDK1. Our data clearly show that Asp-167, Thr-176, Ser-177, Thr-211, and Ser-276 in AtPDK1 are important for the downstream phosphorylation of AtS6k2. The results confirm that AtPDK1, like mammalian PDK1, needs phosphorylation at several sites for full downstream phosphorylation activity. Finally, we investigated A. thaliana 14-3-3 proteins as potential AtPDK1 regulatory proteins and the effect of phospholipids on the AtPDK1 activity. Nine of the 12 14-3-3 isoforms tested enhanced AtPDK1 activity whereas one isoform suppressed the activity. No significant effects on AtPDK1 activity by the various phospholipids (including phosphoinositides) were evident. 相似文献
115.
116.
Biomass partitioning and leaf N,P - stoichiometry: comparisons between tree and herbaceous current-year shoots 总被引:1,自引:0,他引:1
We compare the biomass partitioning patterns and the nitrogen/phosphorus (N,P) stoichiometry of the current-year shoots of tree and herbaceous species and ask whether they scale in the same ways. Our analyses indicate that few statistically significant differences exist between the shoot biomass partitioning patterns of the two functional species-groups. In contrast, statistically significant N,P - stoichiometric differences exist between the two functional groups. Across all species, dry leaf mass scales nearly as the square of basal stem diameter and isometrically with respect to dry stem mass. However, total leaf N scales as the 1.37-power and as the 1.09-power of total leaf P across herbaceous and tree shoots, respectively. Therefore, tree shoots can be viewed as populations of herbs elevated by their older, woody herbaceous cohorts. However, tree leaf stoichiometry cannot be modelled in terms of herbaceous N,P - leaf stoichiometry. 相似文献
117.
Peña C Wahlberg N Weingartner E Kodandaramaiah U Nylin S Freitas AV Brower AV 《Molecular phylogenetics and evolution》2006,40(1):29-49
We have inferred the first empirically supported hypothesis of relationships for the cosmopolitan butterfly subfamily Satyrinae. We used 3090 base pairs of DNA from the mitochondrial gene COI and the nuclear genes EF-1alpha and wingless for 165 Satyrinae taxa representing 4 tribes and 15 subtribes, and 26 outgroups, in order to test the monophyly of the subfamily and elucidate phylogenetic relationships of its major lineages. In a combined analysis, the three gene regions supported an almost fully resolved topology, which recovered Satyrinae as polyphyletic, and revealed that the current classification of suprageneric taxa within the subfamily is comprised almost completely of unnatural assemblages. The most noteworthy findings are that Manataria is closely related to Melanitini; Palaeonympha belongs to Euptychiina; Oressinoma, Orsotriaena and Coenonympha group with the Hypocystina; Miller's (1968). Parargina is polyphyletic and its components group with multiple distantly related lineages; and the subtribes Elymniina and Zetherina fall outside the Satyrinae. The three gene regions used in a combined analysis prove to be very effective in resolving relationships of Satyrinae at the subtribal and tribal levels. Further sampling of the taxa closely related to Satyrinae, as well as more extensive sampling of genera within the tribes and subtribes for this group will be critical to test the monophyly of the subfamily and establish a stronger basis for future biogeographical and evolutionary studies. 相似文献
118.
? Premise of the study: Theory and empirical studies have shown that, on average, belowground biomass (M(B)) scales one-to-one (isometrically) with aboveground biomass (M(A)) within and across plant species both at the individual and population level, i.e., M(B) ∝ M(A)(α=1), where α is the scaling exponent. However, little is known about how domestication affects this relationship. ? Methods: To examine the effects of domestication, we investigated the root vs. shoot biomass relationship during the first 30 d of growth of four wheat genotypes: two older genotypes, MO4 (T. monococcum, a diploid) and DM31 (T. dicoccum, a tetraploid) and two more recent genotypes, DX24 and L8275 (T. aestivum, both hexaploids). ? Results: Biomass allocation to roots scaled more or less isometrically with respect to shoot biomass allocation during the first 30 d of growth for both of the older genotypes, whereas shoot biomass allocation exceeded root allocation for the two more recent genotypes. This difference was attributable to the first 15 d of growth. Although root biomass allocation exceeded shoot biomass allocation during the first 15 d of growth for the two older genotypes, shoot biomass exceeded root biomass allocation during this critical phase of development for the two more recent genotypes. ? Conclusions: Based on a very limited sample of wheat genotypes, these results indicate that domestication has resulted in an increased biomass allocation to shoots compared to root biomass allocation. This shift possibly reflects artificial selection under agricultural conditions (for which water and nutrients are not limiting) favoring higher crop yields. 相似文献
119.
Brodehl A Hedde PN Dieding M Fatima A Walhorn V Gayda S Šarić T Klauke B Gummert J Anselmetti D Heilemann M Nienhaus GU Milting H 《The Journal of biological chemistry》2012,287(19):16047-16057
Mutations in the DES gene coding for the intermediate filament protein desmin may cause skeletal and cardiac myopathies, which are frequently characterized by cytoplasmic aggregates of desmin and associated proteins at the cellular level. By atomic force microscopy, we demonstrated filament formation defects of desmin mutants, associated with arrhythmogenic right ventricular cardiomyopathy. To understand the pathogenesis of this disease, it is essential to analyze desmin filament structures under conditions in which both healthy and mutant desmin are expressed at equimolar levels mimicking an in vivo situation. Here, we applied dual color photoactivation localization microscopy using photoactivatable fluorescent proteins genetically fused to desmin and characterized the heterozygous status in living cells lacking endogenous desmin. In addition, we applied fluorescence resonance energy transfer to unravel short distance structural patterns of desmin mutants in filaments. For the first time, we present consistent high resolution data on the structural effects of five heterozygous desmin mutations on filament formation in vitro and in living cells. Our results may contribute to the molecular understanding of the pathological filament formation defects of heterozygous DES mutations in cardiomyopathies. 相似文献
120.
Niklas J Priesnitz C Rose T Sandig V Heinzle E 《Applied microbiology and biotechnology》2012,93(4):1637-1650
Metabolic responses of the new neuronal human cell line AGE1.HN to various substrate levels were analyzed in this study showing
that reduced substrate and especially pyruvate load improves metabolic efficiency, leading to improved growth and α1-antitrypsin (A1AT) production. The adaptation of the metabolism to different pyruvate and glutamine concentrations was analyzed
in detail using a full factorial design. The most important finding was an increasingly inefficient use of substrates as well
as the reduction of cell proliferation with increasing pyruvate concentrations in the medium. Cultivations with different
feeding profiles showed that the highest viable cell density and A1AT concentration (167% of batch) was reached in the culture
with the lowest glucose level and without pyruvate feeding. Analysis of metabolic fluxes in the differently fed cultures revealed
a more efficient metabolic phenotype in the cultures without pyruvate feeding. The measured in vitro enzyme activities of
the selected enzymes involved in pyruvate metabolism were lower in AGE1.HN compared with CHO cells, which might explain the
higher sensitivity and different adaptation of AGE1.HN to increased pyruvate concentrations. The results indicate on the one
hand that increasing the connectivity between glycolysis and the TCA cycle might improve substrate use and, finally, the production
of A1AT. On the other hand, a better balanced substrate uptake promises a reduction of energy spilling which is increased
with increasing substrate levels in this cell line. Overall, the results of this study provide important insights into the
regulation of primary metabolism and into the adaptation of AGE1.HN to different substrate levels, providing guidance for
further optimization of production cell lines and applied process conditions. 相似文献