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   4853篇
  免费   181篇
  国内免费   1篇
  5035篇
  2023年   15篇
  2022年   14篇
  2021年   33篇
  2020年   17篇
  2019年   31篇
  2018年   83篇
  2017年   78篇
  2016年   96篇
  2015年   73篇
  2014年   107篇
  2013年   196篇
  2012年   420篇
  2011年   627篇
  2010年   275篇
  2009年   72篇
  2008年   417篇
  2007年   392篇
  2006年   333篇
  2005年   303篇
  2004年   233篇
  2003年   231篇
  2002年   170篇
  2001年   68篇
  2000年   146篇
  1999年   75篇
  1998年   47篇
  1997年   13篇
  1996年   12篇
  1995年   16篇
  1994年   10篇
  1993年   10篇
  1992年   20篇
  1991年   14篇
  1990年   13篇
  1989年   13篇
  1987年   17篇
  1985年   19篇
  1984年   22篇
  1983年   9篇
  1982年   17篇
  1981年   9篇
  1980年   18篇
  1979年   14篇
  1978年   13篇
  1977年   9篇
  1976年   17篇
  1975年   12篇
  1974年   18篇
  1973年   9篇
  1971年   10篇
排序方式: 共有5035条查询结果,搜索用时 9 毫秒
81.
This paper deals with the application of scientometric parameters in the evaluation of scientists, either as individuals or in small formal groups. The parameters are divided into two groups: parameters of scientific productivity and citation parameters. The scientific productivity was further subdivided into three types of parameters: (i) total productivity, (ii) partial productivity, and (iii) productivity in scientific fields and subfields. These citation parameters were considered: (i) impact factors of journals, (ii) impact factors of scientific fields and subfields, (iii) citations of individual papers, (iv) citations of individual authors, (v) expected citation rates and relative citation rates, and (vi) self-citations, independent citations and negative citations. Particular attention was payed to the time-dependence of the scientometric parameters. If available, numeric values of the world parameters were given and compared with the data about the scientific output of Croatian scientists.  相似文献   
82.
The annual reproduction cycle of gilt sardine, Sardinella aurita, based on gonad maturity stages, gonad weight and gonadosomatic index was the subject of this study. A total of 2033 gilt sardines (983 males, 1021 females and 29 undetermined) were analysed. Fish were collected monthly from commercial purse seiners between November 2007 and January 2009 in the eastern middle Adriatic Sea (mesh size 8 mm/bar length/; sampling: five boats per month). Based on the monthly evolution of gonad maturation stages, gonad weights and gonadosomatic index, the peak spawning season was determined to be from June to August. Variations in sea surface temperature (SST) coincided with monthly variations of the gonadosomatic index. Highest monthly average values for both analysed parameters were recorded in July (GSI = 3.38; T = 26.5°C). Fifty per cent (L50) of males and females reached sexual maturity at TL 15.8 cm and at 16.6 cm, respectively. Absolute fecundity ranged from 8458 to 48 032 (mean 34 565 ± 10 310), whereas relative fecundity was from 171 to 722 (mean 385 ± 104.35). Mean value of the oocyte size was 0.53 ± 0.10 mm.  相似文献   
83.
The mannosylated derivative of adamant‐1‐yl tripeptide (D ‐(Ad‐1‐yl)Gly‐L ‐Ala‐D ‐isoGln) was prepared to study the effects of mannosylation on adjuvant (immunostimulating) activity. Mannosylated adamant‐1‐yl tripeptide (Man‐OCH2CH(Me)CO‐D ‐(Ad‐1‐yl)Gly‐L ‐Ala‐D ‐isoGln) is a non‐pyrogenic, H2O‐soluble, and non‐toxic compound. Adjuvant activity of mannosylated adamantyl tripeptide was tested in the mouse model with ovalbumin as an antigen and in comparison to the parent tripeptide and peptidoglycan monomer (PGM, β‐D ‐GlcNAc‐(1→4)‐D ‐MurNAc‐L ‐Ala‐D ‐isoGln‐mesoDAP(εNH2)‐D ‐Ala‐D ‐Ala), a well‐known effective adjuvant. The mannosylation of adamantyl tripeptide caused the amplification of its immunostimulating activity in such a way that it was comparable to that of PGM.  相似文献   
84.
Since recently, it is possible, using noninvasive cortical stimulation, such as the protocol of paired associative stimulation (PAS), to induce the plastic changes in the motor cortex, in humans that mimic Hebb's model of learning. Application of TMS conjugated with peripheral electrical stimulation at strictly coherent temporal manner lead to convergence of inputs in the sensory-motor cortex, with the consequent synaptic potentiation or weakening, if applied repetitively. However, when optimal interstimulus interval (ISI) for induction of LTP-like effects is applied as a single pair, Motor evoked potential (MEP) amplitude inhibition is observed, the paradigm known as short-latency afferent inhibition (SLAI). Aiming to resolve this paradox, PAS protocols were applied, with 200 repetitions of TMS pulses paired with median nerve electrical stimulation, at ISI equal to individual latencies of evoked response of somatosensory cortex (N(20)) (PAS(LTP)), and at ISI of N(20) shortened for 5 msec (PAS(LTD)) - protocols that mimic LTP-like changes in the human motor cortex. MEP amplitudes before, during and after interventions were measured as an indicator based on output signals originating from the motor system. Post-intervention MEP amplitudes following the TMS protocols of PAS(LTP) and PAS(LTD) were facilitated and depressed, respectively, contrary to MEP amplitudes during intervention. During PAS(LTP) MEP amplitudes were significantly decreased in case of PAS(LTP), while in the case of PAS(LTD) an upward trend was observed. In conclusions, a possible explanation for the seemingly paradoxical effect of PAS can be found in the mechanism of homeostatic modulation of plasticity. Those findings indicate the existence of complex relationships in the development of plasticity induced by stimulation, depending on the level of the previous motor cortex excitability.  相似文献   
85.
Abstract Human dipeptidyl peptidase III (DPP III) is a member of the metallopeptidase family M49 with an implied role in the pain-modulatory system and endogenous defense against oxidative stress. Here, we report the heterologous expression of human DPP III and the site-directed mutagenesis results which demonstrate a functional role for Tyr318 at the active site of this enzyme. The substitution of Tyr318 to Phe decreased kcat by two orders of magnitude without altering the binding affinity of substrate, or of a competitive hydroxamate inhibitor designed to interact with S1 and S2 subsites. The results indicate that the conserved tyrosine could be involved in transition state stabilization during the catalytic action of M49 peptidases.  相似文献   
86.
87.

Background and Aims

In the genus Anemone two small groups of taxa occur with the highest ploidy levels 2n = 6x = 48, belonging to the closely related clades: the montane/alpine Baldensis clade and the more temperate Multifida clade. To understand the formation of polyploids within these groups, the evolution of allohexaploid A. baldensis (AABBDD, 2n = 6x = 48) from Europe and allotetraploid Anemone multifida (BBDD, 2n = 4x = 32) from America was analysed.

Methods

Internal transcribed spacer and non-transcribed spacer sequences were used as molecular markers for phylogenetic analyses. Cytogenetic studies, including genomic in situ hybridization with genomic DNA of potential parental species as probe, fluorescence in situ hybridization with 5S and 18S rDNA as probes and 18S rDNA restriction analyses, were used to identify the parental origin of chromosomes and to study genomic changes following polyploidization.

Key Results

This study shows that A. multifida (BBDD, 2n= 4x = 32) and A. baldensis (AABBDD, 2n = 6x = 48) are allopolyploids originating from the crosses of diploid members of the Multifida (donor of the A and B subgenomes) and Baldensis groups (donor of the D subgenome). The A and B subgenomes are closely related to the genomes of A. sylvestris, A. virginiana and A. cylindrica, indicating that these species or their progeny might be the ancestral donors of the B subgenome of A. multifida and A and B subgenomes of A. baldensis. Both polyploids have undergone genomic changes such as interchromosomal translocation affecting B and D subgenomes and changes at rDNA sites. Anemone multifida has lost the 35S rDNA loci characteristic of the maternal donor (B subgenome) and maintained only the rDNA loci of the paternal donor (D subgenome).

Conclusions

It is proposed that A. multifida and A. baldensis probably had a common ancestor and their evolution was facilitated by vegetation changes during the Quaternary, resulting in their present disjunctive distribution.  相似文献   
88.
Composition of the essential oils of Rosmarinus officinalis of ten populations from the Balkan Peninsula were determined by GC/FID and GC/MS. The main constituents were 1,8-cineole, camphor, α-pinene, and borneol. Multivariate statistical analysis (UPGMA cluster analysis and principal-component analysis (PCA)) revealed two major types of rosemary oil, i.e., 1,8-cineole and camphor-type, and two intermediate types, i.e., camphor/1,8-cineole/borneol type and 1,8-cineole/camphor type. The regression analyses (simple linear regression and stepwise multiple regression) have shown that, with respect to basic geographic, orographic, and 19 bioclimatic characteristics of each population, bioclimatic factor temperature of habitat represented the dominant abiogenetic factor, which, in chemical sense, led to differentiation of populations in the studied region. Also, the regression analysis have shown that some constituents of essential oils are independent of any single bioclimatic factors. However, some constituents display statistically significant correlations with some abiotic factors.  相似文献   
89.
The aim of this research was to investigate the prevalence of obesity and high blood pressure and to prove which of three anthropometric indicators of obesity - waist circumference, body mass index (BMI) waist-to-hip ratio - is better predictor for the development of hypertension in women population of the island of Cres. We approached separately groups of women with measured high blood pressure and with previously diagnosed. The research was preformed within the research project "Genetic and biomedical characteristics of the population of the island of Cres". This was the cross sectional study and data were obtained on the sample of 247 females over 18 years old that voluntarily participated in this study. In our study group the prevalence of overweight was 39.0%, obesity 27.5%, increased waist circumference was present in 69.4% while increased blood pressure was found in 53.0% examinees. Our results indicate that age, BMI, impaired glucose concentration and serum cholesterol could be considered as predictors for the development of arterial hypertension, whether measured or previously diagnosed.  相似文献   
90.
Herein we report an assessment of 24 1,2,3,4-tetrahydroisoquinoline derivatives for potential DNase I (deoxyribonuclease I) inhibitory properties in vitro. Four of them inhibited DNase I with IC50 values below 200 μM. The most potent was 1-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-1-yl)propan-2-one ( 2 ) (IC50=134.35±11.38 μM) exhibiting slightly better IC50 value compared to three other active compounds, 2-[2-(4-fluorophenyl)-1,2,3,4-tetrahydroisoquinolin-1-yl]-1-phenylethan-1-one ( 15 ) (IC50=147.51±14.87 μM), 2-[2-(4-fluorophenyl)-1,2,3,4-tetrahydroisoquinolin-1-yl]cyclohexan-1-one ( 18 ) (IC50=149.07±2.98 μM) and 2-[6,7-dimethoxy-2-(p-tolyl)-1,2,3,4-tetrahydroisoquinolin-1-yl]cyclohexan-1-one ( 22 ) (IC50=148.31±2.96 μM). Cytotoxicity assessment of the active DNase I inhibitors revealed a lack of toxic effects on the healthy cell lines MRC-5. Molecular docking and molecular dynamics simulations suggest that interactions with Glu 39, His 134, Asn 170, Tyr 211, Asp 251 and His 252 are an important factor for inhibitors affinity toward the DNase I. Observed interactions would be beneficial for the discovery of new active 1,2,3,4-tetrahydroisoquinoline-based inhibitors of DNase I, but might also encourage researchers to further explore and utilize potential therapeutic application of DNase I inhibitors, based on a versatile role of DNase I during apoptotic cell death.  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