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   1046篇
  免费   42篇
  2022年   6篇
  2021年   14篇
  2020年   5篇
  2019年   13篇
  2018年   20篇
  2017年   15篇
  2016年   34篇
  2015年   33篇
  2014年   52篇
  2013年   61篇
  2012年   76篇
  2011年   74篇
  2010年   43篇
  2009年   34篇
  2008年   60篇
  2007年   60篇
  2006年   60篇
  2005年   49篇
  2004年   51篇
  2003年   59篇
  2002年   64篇
  2001年   10篇
  2000年   8篇
  1999年   10篇
  1998年   16篇
  1997年   6篇
  1996年   7篇
  1995年   7篇
  1994年   5篇
  1993年   7篇
  1992年   9篇
  1991年   4篇
  1990年   3篇
  1988年   7篇
  1986年   11篇
  1985年   9篇
  1984年   13篇
  1983年   4篇
  1982年   7篇
  1981年   7篇
  1980年   5篇
  1979年   3篇
  1978年   3篇
  1977年   4篇
  1975年   4篇
  1973年   6篇
  1972年   3篇
  1971年   3篇
  1967年   3篇
  1953年   2篇
排序方式: 共有1088条查询结果,搜索用时 15 毫秒
971.
972.
Eleven oxytocin analogues substituted in position 4, 5 or 9 by tetrazole analogues of amino acids were prepared using solid-phase peptide synthesis method and tested for rat uterotonic in vitro and pressor activities, as well as for their affinity to human oxytocin receptor. The tetrazolic group has been used as a bioisosteric substitution of carboxylic, ester or amide groups in structure-activity relationship studies of biologically active compounds. Replacement of the amide groups of Gln(4) and Asn(5) in oxytocin by tetrazole analogues of aspartic, glutamic and alpha-aminoadipic acids containing the tetrazole moiety in the side chains leads to analogues with decreased biological activities. Oxytocin analogues in which the glycine amide residue in position 9 was substituted by tetrazole analogues of glycine had diminished activities as well. The analysis of differences in rat uterotonic activity and in the affinity to human oxytocin receptors of analogues containing either an acidic 5-substituted tetrazolic group or a neutral 1,5- or 2,5-tetrazole nucleus makes it possible to draw some new conclusions concerning the role of the amide group of amino acids in positions 4, 5 and 9 of oxytocin for its activity. The data suggest that the interaction of the side chain of Gln(4) with the oxytocin receptor is influenced mainly by electronic effects and the hydrogen bonding capacity of the amide group. Steric effects of the side chain are minor. Substitution of Asn(5) by its tetrazole derivative gave an analogue of very low activity. The result suggests that in the interaction between the amide group of Asn(5) and the binding sites of oxytocic receptor hydrogen bonds are of less importance than the spatial requirements for this group.  相似文献   
973.
Vindoline and its analogues are important constituents of the Madagascan periwinkle Catharanthus roseus, and some of them are valuable chemotherapy drugs used in treatment for some types of cancer, including leukaemia, lymphoma, breast and lung cancer. The search for semi-synthetic congeners of natural substances is still an important task for organic chemistry. In this communication we report the synthesis of five new vindoline derivatives, 15-(2-methoxyphenyl)vindoline 11, 15-(3-methoxyphenyl)vindoline 12, 15-(2-nitrophenyl)vindoline 13, 15-(3-cyanophenyl)vindoline 15, and 15-(4-cyanophenyl)vindoline 16 using the Suzuki-Miyaura reaction as the key step. X-Ray analysis of compound 16 is also reported.  相似文献   
974.
The aim of this study was to evaluate microflora of the vaginal part of the cervix uteri (including high-oncogenic HPV types and C. trachomatis) in women with ASCUS, LSIL comparing with women with normal cytology. The results demonstrated that infections with high-oncogenic HPV types and C. trachomatis don't reduce significantly Lactobacillus spp.  相似文献   
975.
In our previous study, using the micronucleus (MN) assay, a hyper-radiosensitivity (HRS)-like phenomenon was observed after single low doses for fibroblasts from two and keratinocytes from four of the 40 patients studied. In this paper, we report the response of primary keratinocytes from 23 and fibroblasts from 21 of these cancer patients to multiple low-dose irradiations and answer the question regarding whether the patients with an HRS-like response after single low doses also demonstrate chromosomal hypersensitivity after multiple low doses. The cells were irradiated with three doses of 0.25 Gy separated by 4-h intervals, and MN induction was compared with that after the same total dose given as a single fraction of 0.75 Gy. Similarly, the effect of three doses of 0.5 Gy was compared with that of a single dose of 1.5 Gy. For fibroblasts from two and keratinocytes from four patients who demonstrated a single-dose HRS-like response, a significant inverse effect of fractionation (greater MN induction after three doses of 0.25 Gy than after a single dose of 0.75 Gy) was observed, which suggests a repeated hypersensitive response after each dose of 0.25 Gy. Such an effect was not seen for the cells from 19 patients who were single-dose HRS-like negative. In conclusion, an inverse fractionation effect for MN induction that was observed in fibroblasts from two and keratinocytes from four patients after three doses of 0.25 Gy (but not 3 x 0.5 Gy) reflects the chromosomal hyper-radiosensitivity seen in the same patients in response to single low doses.  相似文献   
976.
The effect of the pH of an electrolyte solution on the electric surface charge of the liposome membrane was studied. The membrane of vesicles contained egg phosphatidylcholine (PC) with different proportions of stearylamine (ST). The surface charge density of the membrane was determined as a function of pH from electrophoretic mobility measurements. A six equilibria model describing the solution ions adsorption on the PC-ST liposome membrane surface was presented in this paper. The knowledge of the association constants of the -PO(-) and -N(+)CH(3)(3) groups of PC with H(+), OH(-), Na(+), Cl(-) ions: K(A(1)H), K(B(1)OH), K(A(1)Na), K(B(1)Cl), that had been presented earlier, allowed to determine the association constants of the -N(+)H(3) group of ST with OH(-) and Cl(-) ions: K(B(2)OH), K(B(2)Cl). The proposed model has been proved to be correct by comparing the resulting theoretic charge variation curves of the PC-ST liposomal membrane with the experimental data.  相似文献   
977.
Mucin 1 (MUC1) is overexpressed in various cancer cells especially in breast cancer cells. There are known research works on the use of anti-MUC1 antibody with docetaxel in ovarian cancer, but there are no data about combined therapy platinum compounds with anti-MUC1 in breast cancer. The aim of the study was to evaluate the antiproliferative properties of a new dinuclear platinum(II) complex (Pt12) used with anti-MUC1 in human breast cancer cells. The dinuclear platinum(II) complex (Pt12) has been synthesized, and its cytotoxicity with anti-MUC1 has been tested in both MCF-7 and MDA-MB-231 breast cancer cells. In this study, the effects of Pt12 with anti-MUC1 on collagen and DNA biosynthesis in human breast cancer cells were compared to those evoked by cisplatin and cisplatin with anti-MUC1. The mechanism of action of Pt12 with anti-MUC1 was studied employing flow cytometry assessment of annexin V binding assay. It was found that Pt12 with anti-MUC1 was more active inhibitor of DNA and collagen synthesis as well more cytotoxic agent than Pt12 alone and cisplatin with anti-MUC1. Cytotoxicity of Pt12 with anti-MUC1 against breast cancer cells is due to apoptotic cell death as well as necrotic cell death. These results indicate that the use of Pt12 with anti-MUC1 may constitute a novel strategy in the chemotherapy of breast cancer tumors.  相似文献   
978.
Modified mRNA cap analogs aid in the study of mRNA-related processes and may enable creation of novel therapeutic interventions. We report the synthesis and properties of 11 dinucleotide cap analogs bearing a single boranophosphate modification at either the α-, β- or γ-position of the 5′,5′-triphosphate chain. The compounds can potentially serve either as inhibitors of translation in cancer cells or reagents for increasing expression of therapeutic proteins in vivo from exogenous mRNAs. The BH3-analogs were tested as substrates and binding partners for two major cytoplasmic cap-binding proteins, DcpS, a decapping pyrophosphatase, and eIF4E, a translation initiation factor. The susceptibility to DcpS was different between BH3-analogs and the corresponding analogs containing S instead of BH3 (S-analogs). Depending on its placement, the boranophosphate group weakened the interaction with DcpS but stabilized the interaction with eIF4E. The first of the properties makes the BH3-analogs more stable and the second, more potent as inhibitors of protein biosynthesis. Protein expression in dendritic cells was 2.2- and 1.7-fold higher for mRNAs capped with m27,2′-OGppBH3pG D1 and m27,2′-OGppBH3pG D2, respectively, than for in vitro transcribed mRNA capped with m27,3′-OGpppG. Higher expression of cancer antigens would make mRNAs containing m27,2′-OGppBH3pG D1 and m27,2′-OGppBH3pG D2 favorable for anticancer immunization.  相似文献   
979.

Background

Disseminated cancer cells (DCCs) and circulating tumor cells (CTCs) are extremely rare, but comprise the precursors cells of distant metastases or therapy resistant cells. The detailed molecular analysis of these cells may help to identify key events of cancer cell dissemination, metastatic colony formation and systemic therapy escape.

Methodology/Principal Findings

Using the Ampli1™ whole genome amplification (WGA) technology and high-resolution oligonucleotide aCGH microarrays we optimized conditions for the analysis of structural copy number changes. The protocol presented here enables reliable detection of numerical genomic alterations as small as 0.1 Mb in a single cell. Analysis of single cells from well-characterized cell lines and single normal cells confirmed the stringent quantitative nature of the amplification and hybridization protocol. Importantly, fixation and staining procedures used to detect DCCs showed no significant impact on the outcome of the analysis, proving the clinical usability of our method. In a proof-of-principle study we tracked the chromosomal changes of single DCCs over a full course of high-dose chemotherapy treatment by isolating and analyzing DCCs of an individual breast cancer patient at four different time points.

Conclusions/Significance

The protocol enables detailed genome analysis of DCCs and thereby assessment of the clonal evolution during the natural course of the disease and under selection pressures. The results from an exemplary patient provide evidence that DCCs surviving selective therapeutic conditions may be recruited from a pool of genomically less advanced cells, which display a stable subset of specific genomic alterations.  相似文献   
980.

Background

Loss of function COQ2 mutations results in primary CoQ10 deficiency. Recently, recessive mutations of the COQ2 gene have been identified in two unrelated Japanese families with multiple system atrophy (MSA). It has also been proposed that specific heterozygous variants in the COQ2 gene may confer susceptibility to sporadic MSA. To assess the frequency of COQ2 variants in patients with MSA, we sequenced the entire coding region and investigated all exonic copy number variants of the COQ2 gene in 97 pathologically-confirmed and 58 clinically-diagnosed MSA patients from the United States.

Results

We did not find any homozygous or compound heterozygous pathogenic COQ2 mutations including deletion or multiplication within our series of MSA patients. In two patients, we identified two heterozygous COQ2 variants (p.S54W and c.403?+?10G?>?T) of unknown significance, which were not observed in 360 control subjects. We also identified one heterozygous carrier of a known loss of function p.S146N substitution in a severe MSA-C pathologically-confirmed patient.

Conclusions

The COQ2 p.S146N substitution has been previously reported as a pathogenic mutation in primary CoQ10 deficiency (including infantile multisystem disorder) in a recessive manner. This variant is the third primary CoQ10 deficiency mutation observed in an MSA case (p.R387X and p.R197H). Therefore it is possible that in the heterozygous state it may increase susceptibility to MSA. Further studies, including reassessing family history in patients of primary CoQ10 deficiency for the possible occurrence of MSA, are now warranted to resolve the role of COQ2 variation in MSA.
  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