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   1702篇
  免费   82篇
  1784篇
  2023年   35篇
  2022年   17篇
  2021年   33篇
  2020年   34篇
  2019年   22篇
  2018年   75篇
  2017年   63篇
  2016年   79篇
  2015年   47篇
  2014年   56篇
  2013年   118篇
  2012年   122篇
  2011年   131篇
  2010年   79篇
  2009年   49篇
  2008年   90篇
  2007年   113篇
  2006年   87篇
  2005年   77篇
  2004年   91篇
  2003年   94篇
  2002年   83篇
  2001年   30篇
  2000年   43篇
  1999年   18篇
  1998年   8篇
  1997年   5篇
  1996年   3篇
  1995年   4篇
  1994年   5篇
  1993年   2篇
  1992年   3篇
  1991年   3篇
  1985年   2篇
  1984年   7篇
  1983年   4篇
  1982年   2篇
  1981年   3篇
  1980年   3篇
  1978年   4篇
  1977年   2篇
  1975年   7篇
  1974年   3篇
  1973年   4篇
  1972年   2篇
  1971年   3篇
  1969年   4篇
  1967年   2篇
  1966年   2篇
  1964年   2篇
排序方式: 共有1784条查询结果,搜索用时 0 毫秒
1.
Allopatric isolation in glacial refugia has caused differentiation and speciation in many taxa globally. In this study, we investigated the nuclear and mitochondrial genetic differentiation of the long fingered bat, Myotis capaccinii during the ice ages in south-eastern Europe and Anatolia. The mitochondrial DNA (mtDNA) analyses indicated a suture zone similar to those recorded in other animal species, including bats, suggesting the association of more than one refugium with the region. Contrary to most of the other species where a suture zone was seen in Anatolia, for M. capaccinii the geographical location of the genetic break was in south-eastern Europe. This mitochondrial differentiation was not reflected in the nuclear microsatellites, however, suggesting that the lack of contact during the ice ages did not result in reproductive isolation. Hence taxonomically, the two mitochondrial clades cannot be treated as separate species.  相似文献   
2.
This study seeks to explore whether neuroticism, agreeableness, and conscientiousness moderate the influence of relationship conflict experienced in groups on changes in group members'' evaluative cognitions related to teamwork quality (teamwork-related mental models). Data from 216 students, nested in 48 groups were analyzed using a multilevel modeling approach. Our results show that the experience of relationship conflict leads to a negative shift from the pre-task to the post-task teamwork-related mental models. Moreover, the results indicate that conscientiousness buffered the negative association between relationship conflict and the change in teamwork-related mental models. Our results did not support the hypothesized moderating effect of agreeableness and show that the detrimental effect of relationship conflict on the shift in teamwork-related mental models is accentuated for group members scoring low rather than high on neuroticism. These findings open new research venues for exploring the association between personality, coping styles and change in teamwork-related mental models.  相似文献   
3.
The antiinflammatory activity of three hydrosoluble aryldimethyltriazenes has been examined on the carrageenin induced edema in guinea pig. The administration of equitoxic dosages of p-(3,3-dimethyl-1-triazeno)benzoic acid potassium salt (DM-COOK) and p-(3,3-dimethyl-1-triazeno)sulfonic acid sodium salt (DM-SO3Na) 1 h after carrageenin application, causes 4 h later a similar and statistically significant reduction of paw swelling by about 40% whereas, p-alanylphenyl-3,3-dimethyl-1-triazeno (DM-ALA(OH)) is inactive. Of the two active compounds, DM-COOK displays interesting properties, being rapidly active and causing a peak of inhibition higher than that caused by DM-SO3Na. The antiinflammatory activity of DM-COOK is comparable with that caused by 5 mg/kg indomethacin and 200 mg/kg phenylbutazone. However, DM-COOK, unlike indomethacin, causes an inhibition of leukocyte migration into the peritoneal cavity induced by casein treatment, thus indicating a different mechanism of action. This effect needs clarification and seems not to be correlated to cytotoxicity of the drug for migrating white blood cells, as evidenced by "in vitro' examination.  相似文献   
4.
A new dihydroflavonol, pallasiin, together with kaempferol, quercetin, isorhamnetin, mearnsetin, aromadendrin, eriodictyol and taxifolin, has been isolated from the bark of Rhamnus pallasii and its structure elucidated as 2,3-dihydromyricetin 4′-O-methyl ether.  相似文献   
5.
Summary Trichophyton quinckeanum was isolated from a spontaneous infection in rabbit. The hairs were also invaded by the fungus, exibiting a yellowish fluorescence in Wood's light. White mice inoculation of the isolate produced typical scutula with hair penetration fluorescing in green colour. The type of animal hair invasion is also discussed. The morphologic features ofTr. quinckeanum, together with its ability of producing scutula while inoculating the white mice, must be emphasized, when proving its separate identity.  相似文献   
6.
In this study, three new axially disubstituted silicon phthalocyanines ( SiPc1–3 ) and their quaternized phthalocyanine derivatives ( QSiPc1–3 ) were prepared and characterized. The biological properties (antioxidant, antimicrobial, antibiofilm, and microbial cell viability activities) of the water-soluble silicon phthalocyanines were examined, as well. A 1 % DMSO diluted with pure water was used as a solvent in biological activity studies. All the compounds exhibited high antioxidant activity. They displayed efficient antimicrobial and antimicrobial photodynamic therapeutic properties against various microorganisms, especially Gram (+) bacteria. Additionally, they demonstrated high antibiofilm activities against S. aureus and P. aeruginosa. In addition, 100 % bacterial reduction was obtained for all the studied phthalocyanines against E. coli viable cells. Besides, the DNA cleavage and binding features of compounds ( QSiPc1–3 ) were studied using pBR322 DNA and CT-DNA, respectively. Furthermore, the human topoisomerase I enzyme inhibition activities of compounds QSiPc1 – 3 were studied. Anticancer properties of the water-soluble compounds were investigated using cell proliferation MTT assay. They exhibited anticarcinogenic activity against the human colon cancer cell line (DLD-1). Compounds QSiPc1 and QSiPc3 displayed a high anticarcinogenic effect on the DLD-1 cell line. The obtained results indicated that all the studied compounds may be effective biological agents and anticancer drugs after further investigations.  相似文献   
7.
Two strains of Aeromonas hydrophila (the type strain ATCC 7966 and a food-derived strain JAH4) were pre-incubated at 5°C in Brain Heart Infusion (BHI) broth with pH adjusted to 6.0 or 7.0, and then incubated at the same temperature in BHI broth with pH adjusted to 6.0, 6.5, 7.0 and 7.5. Growth kinetics during incubation were determined by two methods: viable count (VC) and measurement of optical density (O.D.). Pre-incubation at different pH values did not significantly affect the maximum specific growth rates of the strains during incubation, but the lag phases were shorter after pre-incubation at pH 6.0 than at pH 7.0. The VC method was more sensitive than O.D. measurements for assessing lag phase.  相似文献   
8.
Summary Equitoxic doses of 5-(3-3-dimethyl-1-triazeno)imidazole-4-carboxamide (DTIC) and aryl-triazene derivatives (compounds all capable of inducing a marked increase in murine tumor cell immunogenicity) were studied for their effects on the host immune system. At different times after drug exposure the animals were tested for allograft responses, competence in producing lymphocytes active in lethal graft-versus-host disease, delayed-type hypersensitivity, humoral antibody production, and mitogen responsiveness. While some of the aryl-triazenes tested (DM-COOK DM-NO2) showed a pattern of immunodepression similar to that of DTIC, others were less (MIC, MM-COOK, MM-Cl) or far less (DM-Cl, MM-NO2) active than DTIC in impairing host immunocompetence, although all retained or even augmented their ability to induce chemical xenogenization.This work was supported by Progetto Finalizzato Controllo della Crescita Neoplastica contracts no. 83.00815.96 and no. 83.00838.96 (CNR, Rome, Italy)  相似文献   
9.
Summary The antitumor and antimetastatic activities of the water-soluble peptidoglycan monomer GlnNAc-Mur-NAc-L-Ala-D-iso-Gln-meso-diamminopimelic acid (-NH2)-D-Ala-D-Ala (PGM), which has immunostimulant effects, have been evaluated in CBA mice bearing MCa mammary carcinoma. The antineoplastic effects of PGM depend strictly on the dosage and treatment schedule used. Though a significant inhibition of the primary tumor growth is observed over a wide range of dosage, only the IV administration of daily doses of 50 mg/kg/day on days 1, 5, 10, 15 inhibits spontaneous lung metastasis formation and in parallel prolongs the survival time of the treated mice. The magnitude of the antimetastatic effects of PGM depends on the degree of dissemination of the tumor, and is greater when the number of metastatic foci is low. Examination of the therapeutic potential of PGM in combination with surgery has further indicated that the timing of administration plays an important role in the overall effectiveness of this substance. A 5-day interval is necessary between two consecutive injections for the induction of significant increases of the survival times.  相似文献   
10.
The temperature dependence of the hydrolysis of p-nitrophenyl carboxylates with general formula H(CH2)nCOOC6H4NO2 catalyzed by alkaline mesentericopeptidase has been studied (n varying from 1 to 7, temperature range 2–30°C, pH 8.80, 5 vol% dimethylsulfoxide). The activation parameters of the deacylation step depend on the length of the hydrophobic side chain of the substrate molecule ( , , and decrease by 2.0 kcal/mol, 4.9 kcal/mol, and 10 eu, respectively, as the length of the acyl carbon chain increases from n = 1 to n = 4). The following criteria were applied to establish a chemical enthalpy-entropy compensation effect: (a) Exner's plot of log vs : (b) Petersen's plot of log, k/T vs 1/T; (c) Exner's statistical treatment in coordinates log k vs 1/T; (d) according to Krug et al. (ΔH vs ΔGThm). By use of all the above-mentioned criteria the existence of a chemical enthalpy-entropy compensation effect was proved with an isokinetic temperature β of about 470°K, which is significantly higher than the average experimental temperature.  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