首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   363篇
  免费   25篇
  388篇
  2023年   2篇
  2022年   4篇
  2021年   4篇
  2020年   2篇
  2019年   10篇
  2018年   17篇
  2017年   17篇
  2016年   22篇
  2015年   22篇
  2014年   24篇
  2013年   32篇
  2012年   27篇
  2011年   24篇
  2010年   15篇
  2009年   16篇
  2008年   21篇
  2007年   23篇
  2006年   17篇
  2005年   19篇
  2004年   15篇
  2003年   9篇
  2002年   7篇
  2001年   2篇
  2000年   1篇
  1999年   2篇
  1998年   2篇
  1997年   2篇
  1995年   1篇
  1993年   2篇
  1991年   2篇
  1990年   1篇
  1989年   2篇
  1988年   1篇
  1986年   2篇
  1985年   4篇
  1983年   4篇
  1982年   1篇
  1981年   2篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
  1974年   1篇
  1973年   1篇
  1968年   1篇
  1966年   1篇
排序方式: 共有388条查询结果,搜索用时 15 毫秒
291.
Porphyrins and chlorins such as Foscan® have a natural proclivity to accumulate in cancer cells. This trait has made them good candidates for photosensitizers and as imaging agents in phototherapy. In order to improve on cellular selectivity to lower post-treatment photosensitivity bile acid porphyrin bioconjugates have been prepared and investigated in esophageal cancer cells. Bile acids which are known to selectively bind to, or be readily taken up by cancer cells were chosen as targeting moieties. Synthesis of the conjugates was achieved via selective nucleophilic monofunctionalization of 5,10,15,20-tetrahydroxyphenylporphyrins with propargyl bromide followed by Cu(I) mediated cycloaddition with bile acid azides in good yields. The compounds were readily taken up by esophageal cancer cells but showed no PDT activity.  相似文献   
292.
Lycopodium represents a phylogenetically distinct clade of basal vascular plants with anatomical characters that have no parallel in other lineages. Thus, knowledge of lycopod structure and development may reveal important information about the common ancestors of all vascular plants. Here we report the unique architecture of the conducting system in Lycopodium annotinum and Lycopodium clavatum. Based on multiple series of anatomical sections, we reconstructed spatial relationships between microphylls and the stelar system. Analysis revealed that protoxylem ribs (PXR) were vertical, regardless of type of phyllotaxis, and their numbers were variable. Microphyll traces (MTr) were randomly distributed between ribs, resulting in the absence of defined sympodia and varied lengths of MTr. Dichotomous branching contributed to additional features, for example occurrence of mesarch protoxylem, affecting stele structure and PXR numbers. Our data showed limited interrelationships between lycopod vasculature and microphyll phyllotaxis. This may suggest that both systems developed independently, then evolved together to form the integrated supply system. Thus vasculature in extant lycophytes may be less functionally efficient than in seed plants, where consistent leaf-trace lengths guarantee predictable energy utilization during ontogeny. Differences may result from the phylogenetically different origin of microphylls, and the level of vascular complexity.  相似文献   
293.
Three triticale cDNAs encoding inhibitors of cysteine endopeptidases, belonging to phytocystatins, have been identified and designated as TrcC-1, TrcC-4 and TrcC-5. Full-length cDNAs of TrcC-1 (617?bp) and TrcC-4 (940?bp), as well as a fragment of TrcC-5 cDNA (369?bp), were obtained. A high-level identity of the deduced amino acid sequence of TrcCs with other known phytocystatins, especially with wheat and barley, has been observed. Moreover, the presence of conserved domain, containing the G and W residues, the sequence of QxVxG and the sequence of LARFAV, characteristic for plant cysteine endopeptidase inhibitors, has been noted. The profiles of TrcC-1 and TrcC-5 mRNA levels in the developing seeds of two triticale cultivars that differ in their resistance to preharvest sprouting (Zorro and Disco) were similar. However, the expression of TrcC-4 was, higher in the developing seeds, and in the scutellum of germinating seeds of a cultivar more resistant to preharvest sprouting (Zorro) than in the less resistant (Disco). Additionally, the expression of TrcC-4 remained longer in developing seeds of Zorro as compared to Disco. The performed studies suggest that TrcC-4 might have an influence on the higher resistance of Zorro cultivar to preharvest sprouting.  相似文献   
294.
The Mediterranean Basin is typified by a high degree of species rarity and endemicity that reflects its position, geomorphology, and history. Although the composition and cryptic variation of the bat faunas from the Iberian and Balkan Peninsulas are relatively well studied, data from the Apennine Peninsula are still incomplete. This is a significant shortfall, given the presumed refugial role of this region in the context of Europe's Pleistocene phylogeography. It was thus our aim to supplement the phylogeographical information from the region, generating mitochondrial sequences and reviewing published data, with a focus on the dispersal and diversification patterns characterizing taxa with different life strategies. Site‐specific lineages were ascertained, especially in the genera Myotis and Plecotus and amongst the pipistrelloid bats, representing speciose radiations. It was possible to observe disjunct ranges with patches isolated south of the Alps in several species, corresponding with evolution of elevated genetic distance. The genetic subdivision within the continuous Italian range into northern and southern lineages in several taxa indicated the possible past substructure of the refugium. Several shared lineages between the Apennine and Ibero‐Maghrebian regions were observed, indicating connectivity between the Adriatic and Atlantic?Mediterranean refuges, and raising questions as to which region these clades originated from and what was the direction of faunal exchange between them. In contrast to Europe's other two main refugia, the Apennine Peninsula is a smaller region with simpler phylogeographical patterns. Nevertheless, our results support the idea that the region generated novel lineages. Whereas diversification in sedentary bats may have been driven through the generation of in situ adaptations, specialization, and niche differentiation, the emergence of species with a tramp strategy could have entailed the utilization of faunal drift and the taxon cycle. © 2015 The Linnean Society of London  相似文献   
295.
The aim of this work was to compare the basic food ingredient level and some nutritional status indices between the two groups of adolescents: the first one from the urban environment and the other one from the rural environment. A series of tests were conducted on a 400-teenager-group (200 girls and 200 boys), which was divided into two age groups: 10.5-year-olds and 13.5-year-olds. Nutritional status was estimated on the basis of the following anthropometric measurements: body height, body mass index, upper arm circumference, triceps skinfold thickness. Food intake was assessed by means of a 24-h dietary recall. The analysis of the results of the investigation showed: rare overweight and obesity occur in rural children aged 10.5 years and a higher risk of undernutrition among rural children, especially boys; more frequent overweight and obesity in rural girls and urban boys aged 13.5 years; a lower protein consumption, especially animal protein, and a lower percentage of the accomplishment of the norm for many mineral components and vitamins in rural girls and boys.  相似文献   
296.
We carried out translocations of three rare plants that inhabit mineral islands in the marshy Biebrza Valley in order to create alternative populations and facilitate connectivity between existing subpopulations. The species chosen were Iris aphylla and two orchids: Cypripedium calceolus and Cephalanthera rubra. Thirty soil monoliths with vegetative orchid plants or parts of I. aphylla rhizomes were dug out and transplanted to three different sites on new mineral islands (half in 2012 and half in 2013). Prior to translocation, we measured soil moisture and pH and took phytosociological characteristics in 68 potential recipient sites for orchids and 15 for I. aphylla. Then, we monitored the number of shoots for 4 years and retrospectively conducted principal component analysis (PCA) in order to compare the similarity of donor sites and chosen recipient sites. Three years after transplantation we found new C. calceolus populations in good condition, I. aphylla populations in moderate, while C. rubra transplants emerged only in the first and second season. All newly established populations of C. calceolus and I. aphylla survived. In the case of the first species, fruiting was observed in two populations, while a marked increase in shoot number was observed in one population. The most successful C. calceolus translocation site was also the most similar to the donor sites according to PCA.  相似文献   
297.
298.

Background

Recipient NK cells may detect the lack of recipient''s (i.e., self) HLA antigens on donor renal tissue by means of their killer cell immunoglobulin-like receptors (KIRs). KIR genes are differently distributed in individuals, possibly contributing to differences in response to allogeneic graft.

Methodology/Principal Findings

We compared frequencies of 10 KIR genes by PCR-SSP in 93 kidney graft recipients rejecting allogeneic renal transplants with those in 190 recipients accepting grafts and 690 healthy control individuals. HLA matching results were drawn from medical records. We observed associations of both a full-length KIR2DS4 gene and its variant with 22-bp deletion with kidney graft rejection. This effect was modulated by the HLA-B,-DR matching, particularly in recipients who did not have glomerulonephritis but had both forms of KIR2DS4 gene. In contrast, in recipients with glomerulonephritis, HLA compatibility seemed to be much less important for graft rejection than the presence of KIR2DS4 gene. Simultaneous presence of both KIR2DS4 variants strongly increased the probability of rejection. Interestingly, KIR2DS5 seemed to protect the graft in the presence of KIR2DS4fl but in the absence of KIR2DS4del.

Conclusions/Significance

Our results suggest a protective role of KIR2DS5 in graft rejection and an association of KIR2DS4 with kidney rejection, particularly in recipients with glomerulonephritis.  相似文献   
299.

This study evaluates the changes in bacterial and archaeal community structure during the gradual evaporation of water from the brine (extracted from subsurface Jurassic deposits) in the system of graduation towers located in Ciechocinek spa, Poland. The communities were assessed with 16S rRNA gene sequencing (MiSeq, Illumina) and microscopic methods. The microbial cell density determined by direct cell count was at the order of magnitude of 107 cells/mL. It was found that increasing salt concentration was positively correlated with both the cell counts, and species-level diversity of bacterial and archaeal communities. The archaeal community was mostly constituted by members of the phylum Euryarchaeota, class Halobacteria and was dominated by Halorubrum-related sequences. The bacterial community was more diverse, with representatives of the phyla Proteobacteria and Bacteroidetes as the most abundant. The proportion of Proteobacteria decreased with increasing salt concentration, while the proportion of Bacteroidetes increased significantly in the more concentrated samples. Representatives of the genera Idiomarina, Psychroflexus, Roseovarius, and Marinobacter appeared to be tolerant to changes of salinity. During the brine concentration, the relative abundances of Sphingobium and Sphingomonas were significantly decreased and the raised contributions of genera Fabibacter and Fodinibius were observed. The high proportion of novel (not identified at 97% similarity level) bacterial reads (up to 42%) in the 16S rRNA gene sequences indicated that potentially new bacterial taxa inhabit this unique environment.

  相似文献   
300.
Phenylalanine aminopeptidase (Phe-AP) was isolated from the shoots of 3-week-old pea plants and purified to molecular homogeneity using a four-step purification procedure (ammonium sulphate precipitation, chromatography on DEAE-cellulose, phenyl-sepharose HP, and Protein-Pak Q 8HR HPLC columns). The enzyme was purified 513-fold with a recovery of 8%. The molecular weight of the purified enzyme as determined by SDS-PAGE and gel filtration was approximately 60 kDa, and the enzyme appeared to be a monomer. Its pH and temperature optimum were pH 7.5 and 37°C, respectively. The enzyme prefers substrates with Phe at the N-terminus, although a high activity for substrates with N-terminal Tyr, Trp, Leu, and Met was also observed. The activity with Leu-β-naphthylamide was at least two times lower than that with Phe-β-naphthylamide (Phe-β-NA). The K m value for activity with Phe-β-NA was the lowest amongst the substrates tested, and it was 7.5 × 10−5 M. The activity of Phe-AP was not inhibited by EDTA, 1,10-phenanthroline and pepstatin A. The most effective inhibitors were pHMB and E-64, which modify sulphydryl groups; however, a significant inhibition in the presence of DFP and PMSF, both of which are serine protease inhibitors, was also observed. By applying mass spectrometry analysis, the peptides derived from the purified Phe-AP were assigned to amino acid sequences of the leucine aminopeptidases of N-type (LAPs-N).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号