全文获取类型
收费全文 | 96433篇 |
免费 | 511篇 |
国内免费 | 1170篇 |
专业分类
98114篇 |
出版年
2024年 | 10篇 |
2023年 | 48篇 |
2022年 | 112篇 |
2021年 | 213篇 |
2020年 | 137篇 |
2019年 | 168篇 |
2018年 | 11963篇 |
2017年 | 10740篇 |
2016年 | 7576篇 |
2015年 | 790篇 |
2014年 | 552篇 |
2013年 | 575篇 |
2012年 | 4535篇 |
2011年 | 13054篇 |
2010年 | 12131篇 |
2009年 | 8350篇 |
2008年 | 9933篇 |
2007年 | 11472篇 |
2006年 | 381篇 |
2005年 | 605篇 |
2004年 | 1041篇 |
2003年 | 1107篇 |
2002年 | 856篇 |
2001年 | 296篇 |
2000年 | 215篇 |
1999年 | 86篇 |
1998年 | 44篇 |
1997年 | 52篇 |
1996年 | 36篇 |
1995年 | 31篇 |
1994年 | 32篇 |
1993年 | 52篇 |
1992年 | 52篇 |
1991年 | 62篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 32篇 |
1987年 | 29篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 19篇 |
1975年 | 7篇 |
1972年 | 246篇 |
1971年 | 274篇 |
1970年 | 6篇 |
1965年 | 13篇 |
1962年 | 25篇 |
1944年 | 12篇 |
1940年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
Roongsawang N Thaniyavarn J Thaniyavarn S Kameyama T Haruki M Imanaka T Morikawa M Kanaya S 《Extremophiles : life under extreme conditions》2002,6(6):499-506
Twenty-three halotolerant and biosurfactant producing strains were collected from salty conditions in central Thailand. One of the strains designated BBK-1 produced the biosurfactants with the highest activity. BBK-1 was isolated from fermented foods and was identified as B. subtilis based on its physiological characteristics and 16S rRNA gene sequence. We show that the strain grows in media containing NaCl up to 16% (w/v) and produces biosurfactants in NaCl up to 8%. We found that B. subtilis BBK-1 produces three kinds of surface-active lipopeptides simultaneously. By their respective molecular weights and amino acid compositions, it is indicated that these lipopeptides are bacillomycin L, plipastatin, and surfactin. In order to analyze the production mechanism of lipopeptides further in the strain, a generally important biosynthetic gene encoding 4'-phosphopantetheinyl transferase was cloned and sequenced. The gene existed in a single copy in the genome and the deduced amino acid sequence was almost identical to that of Lpa-14 from B. subtilis strain RB14, which co-produces iturin A and surfactin. 相似文献
912.
913.
Polo-like kinase (Plk) plays a central role in centrosome cycle and is closely associated with the oncogenesis of lung cancer. The protein consists of a catalytic kinase domain (KD) and a regulatory polo-box domain (PBD); either direct inhibition of the KD’s catalytic activity or indirect disruption of the PBD–substrate interaction can be used to potentially suppress the pathological activation of lung cancer Plk. Here, we reported a successful molecular design and engineering of phosphopeptide ligands to target Plk PBD domain by integrating in silico modeling and in vitro assay. In the procedure, a helical peptide segment hps was derived from dimerization interface of the complex crystal structure of domain dimer using bioinformatics approach, which was then used as sequence template to generate potent phosphopeptide binders of Plk PBD domain in terms of a systematic residue mutation profile. Fluorescence anisotropy assays were conducted to substantiate the findings and conclusions obtaining from the molecular engineering. Consequently, three helical phosphopeptides, including the native hps and its two mutants hps-m 1 and hps-m 2, were successfully designed that can independently rebind to Plk PBD domain with a moderate or high affinity (K d = 127, 26, and 5 μM, respectively). These peptide ligands can be considered as potent self-competitors to disrupt PBD dimerization in lung cancer metastasis. Structural and energetic analysis revealed that hydrophobic forces and van der Waals contacts confer strong stability for domain–peptide complex system, while hydrogen bonds and electrostatic interactions contribute specificity and selectivity to the complex recognition. 相似文献
914.
In the present study Cervatana and Almagra models from decision support system, MicroLEIS DSS, were applied to segregation
of arable land surfaces from the marginal ones and suitability evaluation of wheat (Triticum aestivum), maize (Zea mays) and alfalfa (Medicago sativa) in Souma area with approximately 4100 ha extension in West Azarbaijan. Obtained results from both models are presented and
discussed in this research work. Soil morphological and analytical data were collected from 35 soil profiles, representative
of the study area and stored in SDBm plus database. The control or vertical section of soil for applying and running the models
for annual selected crops, was calculated by soil layer generator 0.0–50 cm in depth, or between the surface and the limit
of useful depth when the latter is between 0.0 and 50 cm. According to results, 80.49% of the total area was good capable
for agricultural uses and 19.51% must be reforested and not dedicated to agriculture. The lands with good capability for agricultural
uses is classified as highly suitable area (S2) for wheat, maize and alfalfa, but results in 822 ha for maize and in 126 ha
for alfalfa refers to an excellent suitable (S1) and moderately suitable (S3) classes respectively. The most important limitation
factors are soil texture and carbonate alone or together and maize — wheat — alfalfa can be selected as the best crop rotation.
A simple map subsystem (ArcView GIS) was used for basic data and models result demonstration on a map. 相似文献
915.
KOJI NAKANISHI STEVEN BLOBSTEIN MAKOTO FUNAMIZU NOBUO FURUTACHI GEORGE VAN LEAR DEZIDER GRUNBERGER KARL W. LANKS I. BERNARD WEINSTEIN 《Nature: New biology》1971,234(47):107-109
WE wish to report that reconstituted sperm whale myoglobin prepared by the method of Breslow1 (except that pH 2 was found sufficient to remove all the haem) (I) crystallizes2 in a different habit from those prepared by the method of Rossi-Fanelli et al.3 (II) using haemin of Sigma lot 77B-0220 and our own 57Fe photoporphyrin preparation and the native myoglobin (III). Although all three form type A3 monoclinic prisms, the best developed plane is [001] for II and III, it is [100] for I. There seems to be great interest in reconstituted haemoproteins4,5, so it is important that crystallization habit may be a sensitive test for subtle changes in protein structures. 相似文献
916.
Transactive response DNA/RNA-binding protein 43-kDa (TDP-43) C-terminal fragments, such as a 25-kDa fragment (TDP-25), have been identified as a ubiquitinated and phosphorylated components of inclusion bodies (IBs) in motor neurons from amyotrophic lateral sclerosis patients. Cells contain proteins that function as molecular chaperones and prevent aggregate formation of misfolded and aggregation-prone proteins. Recently, we reported that heat shock protein (HSP)70, an abundant molecular chaperone, binds to TDP-25 in an ATP-dependent manner; however, whether HSP70 can prevent the formation of TDP-25-related IBs remains unknown. Here, we showed that HSP70 prevented TDP-25 aggregation according to green fluorescent protein-tagged TDP-25 (G-TDP-25) colocalization in the cytoplasm with mCherry-tagged HSP70 (HSP70-R). The mobile fraction of HSP70-R in the cytoplasmic IBs associated with G-TDP-25 increased relative to that of G-TDP-25, suggesting that HSP70 strongly bound to G-TDP-25 in the IBs, whereas a portion remained dissociated from the IBs. Importantly, the proportion of G-TDP-25 IBs was significantly decreased by HSP70-R overexpression; however, G-TDP-25 levels in the insoluble fraction remained unchanged by HSP70-R overexpression, suggesting that G-TDP-25 formed aggregated species that cannot be dissolved, even in the presence of strong detergents. These results indicated that HSP70 prevented the accumulation of G-TDP-25 aggregates in cytoplasmic IBs, but was insufficient for G-TDP-25 disassembly and solubilization. 相似文献
917.
Baker PJ Costanzo JP Lee RE 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2007,177(8):875-883
Hatchlings of the painted turtle, Chrysemys picta, hibernate terrestrially and can survive subfreezing temperatures by supercooling or by tolerating the freezing of their
tissues. Whether supercooled or frozen, an ischemic hypoxia develops because tissue perfusion is limited by low temperature
and/or freezing. Oxidative stress can occur if hatchlings lack sufficient antioxidant defenses to minimize or prevent damage
by reactive oxygen species. We examined the antioxidant capacity and indices of oxidative damage in hatchling C. picta following survivable, 48 h bouts of supercooling (−6°C), freezing (−2.5°C), or hypoxia (4°C). Samples of plasma, brain, and
liver were collected after a 24 h period of recovery (4°C) and assayed for Trolox-equivalent antioxidant capacity (TEAC),
thiobarbituric acid reactive substances (TBARS), and carbonyl proteins. Antioxidant capacity did not vary among treatments
in any of the tissues studied. We found a significant increase in TBARS in plasma, but not in the brain or liver, of frozen/thawed
hatchlings as compared to untreated controls. No changes were found in the concentration of TBARS or carbonyl proteins in
supercooled or hypoxia-exposed hatchlings. Our results suggest that hatchling C. picta have a well-developed antioxidant defense system that minimizes oxidative damage during hibernation. 相似文献
918.
919.
Tang TT Zhu ZF Wang J Zhang WC Tu X Xiao H Du XL Xia JH Dong NG Su W Xia N Yan XX Nie SF Liu J Zhou SF Yao R Xie JJ Jevallee H Wang X Liao MY Shi GP Fu M Liao YH Cheng X 《PloS one》2011,6(9):e24272
Objective
Animal studies suggest that regulatory T (Treg) cells play a beneficial role in ventricular remodeling and our previous data have demonstrated defects of Treg cells in patients with chronic heart failure (CHF). However, the mechanisms behind Treg-cell defects remained unknown. We here sought to elucidate the mechanism of Treg-cell defects in CHF patients.Methods and Results
We performed flow cytometry analysis and demonstrated reduced numbers of peripheral blood CD4+CD25+FOXP3+CD45RO−CD45RA+ naïve Treg (nTreg) cells and CD4+CD25+FOXP3+CD45RO+CD45RA− memory Treg (mTreg) cells in CHF patients as compared with non-CHF controls. Moreover, the nTreg/mTreg ratio (p<0.01), CD4+CD25+FOXP3+CD45RO− CD45RA+CD31+ recent thymic emigrant Treg cell (RTE-Treg) frequency (p<0.01), and T-cell receptor excision circle levels in Treg cells (p<0.01) were lower in CHF patients than in non-CHF controls. Combined annexin-V and 7-AAD staining showed that peripheral Treg cells from CHF patients exhibited increased spontaneous apoptosis and were more prone to interleukin (IL)-2 deprivation- and CD95 ligand-mediated apoptosis than those from non-CHF individuals. Furthermore, analyses by both flow cytometry and real-time polymerase chain reaction showed that Treg-cell frequency in the mediastinal lymph nodes or Foxp3 expression in hearts of CHF patients was no higher than that of the non-CHF controls.Conclusion
Our data suggested that the Treg-cell defects of CHF patients were likely caused by decreased thymic output of nascent Treg cells and increased susceptibility to apoptosis in the periphery. 相似文献920.
Habitat fragmentation is known to generally reduce the size of plant populations and increase their isolation, leading to
genetic erosion and increased between-population genetic differentiation. In Flanders (northern Belgium) Primula vulgaris is very rare and declining. Populations have incurred strong fragmentation for the last decades and are now restricted to
a few highly fragmented areas in an intensively used agricultural landscape. Previous studies showed that small populations
of this long-lived perennial herb still maintained high levels of genetic variation and low genetic differentiation. This
pattern can either indicate recent gene flow or represent historical variation. Therefore, we used polymorphic microsatellite
loci to investigate genetic variation and structure in adult (which may still reflect historical variation) and seedling (recent
generation, thus affected by current processes) life stages. The recent generation (seedlings) showed a significant loss of
observed heterozygosity (H
o) together with lower expected heterozygosity (H
e), a trend for higher inbreeding levels (F
IS) and higher differentiation (F
ST) between populations compared to the adult generation. This might result from (1) a reduction in effective population size,
(2) higher inbreeding levels in the seedlings, (3) a higher survival of heterozygotes over time due to a higher fitness of
heterozygotes (heterosis) and/or a lower fitness of homozygotes (inbreeding depression), (4) overlapping generations in the
adult life stage, or (5) a lack of establishment of new (inbred) adults from seedlings due to degraded habitat conditions.
Combining restoration of both habitat quality and gene flow between populations may be indispensable to ensure a sustainable
conservation of fragmented populations. 相似文献