全文获取类型
收费全文 | 7318篇 |
免费 | 389篇 |
国内免费 | 2篇 |
出版年
2023年 | 28篇 |
2022年 | 52篇 |
2021年 | 106篇 |
2020年 | 49篇 |
2019年 | 74篇 |
2018年 | 134篇 |
2017年 | 130篇 |
2016年 | 183篇 |
2015年 | 234篇 |
2014年 | 258篇 |
2013年 | 368篇 |
2012年 | 654篇 |
2011年 | 847篇 |
2010年 | 403篇 |
2009年 | 188篇 |
2008年 | 565篇 |
2007年 | 542篇 |
2006年 | 480篇 |
2005年 | 435篇 |
2004年 | 359篇 |
2003年 | 349篇 |
2002年 | 276篇 |
2001年 | 78篇 |
2000年 | 155篇 |
1999年 | 91篇 |
1998年 | 63篇 |
1997年 | 25篇 |
1996年 | 24篇 |
1995年 | 28篇 |
1994年 | 23篇 |
1993年 | 20篇 |
1992年 | 25篇 |
1991年 | 22篇 |
1990年 | 16篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 21篇 |
1985年 | 21篇 |
1984年 | 29篇 |
1983年 | 10篇 |
1982年 | 19篇 |
1981年 | 13篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 20篇 |
1973年 | 12篇 |
1971年 | 11篇 |
排序方式: 共有7709条查询结果,搜索用时 31 毫秒
101.
Abstract Viscum album L. ssp. album is semi-parasitic on deciduous trees and shrubs. In order to identify hosts and map the distribution of V. album ssp. album in Croatia and Slovenia, field research was carried out, and herbaria were surveyed. In Croatia and Slovenia, V. album ssp. album occurred on 59 taxa. In Croatia, there were 52 hosts (33 autochthonous and 15 allochthonous species, two cultivars and two hybrids). In Slovenia, there were 25 hosts (21 autochthonous and four allochthonous species). There were 18 hosts common to both countries, 34 hosts were found only in Croatia, and seven hosts only in Slovenia. The hosts belonged to 13 families. The majority of these (19 species) belong to the Rosaceae, followed by Salicaceae, Aceraceae, Betulaceae, Fagaceae, Juglandaceae, Tiliaceae, Hippocastanaceae, Ulmaceae, Oleaceae, Fabaceae, Moraceae and Viscaceae. All hosts have been previously recorded in the literature, except Alnus japonica (Thunb.) Steud., Amelanchier lamarckii F.G. Schroed. and Crataegus nigra Waldst. et Kit. The distribution of this mistletoe was scattered, due to the scattered distribution of hosts, local conditions, movement of bird-vectors, etc. A continuous distribution was found only in part of the distribution area of narrow-leaved ash (Fraxinus angustifolia Vahl). 相似文献
102.
Robert Brus Dalibor Ballian Faruk Bogunić Martin Bobinac Marilena IdžOjtić 《Plant biosystems》2013,147(2):278-285
Abstract In most European countries, the service tree (Sorbus domestica L.) is a rare and threatened species and its conservation has been recognised as a priority. The aim of this study was to asses its morphologic variation in the western and central part of the Balkan Peninsula and in southern Central Europe. Three populations were analysed: one in Serbia, one in Bosnia and Herzegovina and one in Slovenia. In each population 30 trees were selected, and from each tree 30 leaves were collected for morphometric analysis based on nine leaflet morphological traits. Univariate (ANOVA) and multivariate (MANOVA) analysis of variance were used to estimate the variation within- and between populations and a discriminant analysis was performed to examine the structure of the between-population differences. The values of particular morphological traits found in our study did not differ considerably from the values reported elsewhere. The results revealed significant within- and between population variation. Variation within populations was highly significant for all the scored leaf morphological traits, while variation between populations was significant for all the studied traits except for the leaflet length. The discrimination between the three populations was significant. High percentages of correctly classified samples demonstrate good discriminating employability of the analysed leaf morphological traits and indicate differentiation of the analysed populations. 相似文献
103.
Danijela Krstić Katarina Krinulović Vesna Vasić 《Journal of enzyme inhibition and medicinal chemistry》2013,28(5):469-476
Kinetics and inhibition of Na+/K+-ATPase and Mg2+-ATPase activity from rat synaptic plasma membrane (SPM), by separate and simultaneous exposure to transition (Cu2+, Zn2+, Fe2+ and.Co2+) and heavy metals (Hg2+and Pb2+) ions were studied. All investigated metals produced a larger maximum inhibition of Na+/K+-ATPase than Mg2+-ATPase activity. The free concentrations of the key species (inhibitor, MgATP2 ? , MeATP2 ? ) in the medium assay were calculated and discussed. Simultaneous exposure to the combinations Cu2+/Fe2+ or Hg2+/Pb2+caused additive inhibition, while Cu2+/Zn2+ or Fe2+/Zn2+ inhibited Na+/K+-ATPase activity synergistically (i.e., greater than the sum metal-induced inhibition assayed separately). Simultaneous exposure to Cu2+/Fe2+ or Cu2+/Zn2+ inhibited Mg2+-ATPase activity synergistically, while Hg2+/Pb2+ or Fe2+/Zn2+ induced antagonistic inhibition of this enzyme. Kinetic analysis showed that all investigated metals inhibited Na+/K+-ATPase activity by reducing the maximum velocities (Vmax) rather than the apparent affinity (Km) for substrate MgATP2-, implying the noncompetitive nature of the inhibition. The incomplete inhibition of Mg2+-ATPase activity by Zn2+, Fe2+ and Co2+ as well as kinetic analysis indicated two distinct Mg2+-ATPase subtypes activated in the presence of low and high MgATP2 ? concentration. EDTA, L-cysteine and gluthathione (GSH) prevented metal ion-induced inhibition of Na+/K+-ATPase with various potencies. Furthermore, these ligands also reversed Na+/K+-ATPase activity inhibited by transition metals in a concentration-dependent manner, but a recovery effect by any ligand on Hg2+-induced inhibition was not obtained. 相似文献
104.
Ivana Šagud Nikolina Maček Hrvat Ana Grgičević Tena Čadež Josipa Hodak Milena Dragojević 《Journal of enzyme inhibition and medicinal chemistry》2013,28(1):460-467
Abstract The enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are primary targets in attenuating the symptoms of neurodegenerative diseases. Their inhibition results in elevated concentrations of the neurotransmitter acetylcholine which supports communication among nerve cells. It was previously shown for trans-4/5-arylethenyloxazole compounds to have moderate AChE and BChE inhibitory properties. A preliminary docking study showed that elongating oxazole molecules and adding a new NH group could make them more prone to bind to the active site of both enzymes. Therefore, new trans-amino-4-/5-arylethenyl-oxazoles were designed and synthesised by the Buchwald-Hartwig amination of a previously synthesised trans-chloro-arylethenyloxazole derivative. Additionally, naphthoxazole benzylamine photoproducts were obtained by efficient photochemical electrocyclization reaction. Novel compounds were tested as inhibitors of both AChE and BChE. All of the compounds exhibited binding preference for BChE over AChE, especially for trans-amino-4-/5-arylethenyl-oxazole derivatives which inhibited BChE potently (IC50 in µM range) and AChE poorly (IC50?100?µM). Therefore, due to the selectivity of all of the tested compounds for binding to BChE, these compounds could be applied for further development of cholinesterase selective inhibitors.
- HIGHLIGHTS
-
Series of oxazole benzylamines were designed and synthesised
-
The tested compounds showed binding selectivity for BChE
-
Naphthoxazoles were more potent AChE inhibitors
105.
S. Raić M. Pongraćić J. Vorkapić-Furać D. Vikić-Topić A. Hergold-Brundić A. Nagl 《Nucleosides, nucleotides & nucleic acids》2013,32(4):937-960
Abstract Synthesis of the novel nucleoside analogues containing exocyclic pyrrolo moiety and acyclic side chains attached to the purine ring at N-9 and N-7 is described. The site of alkylation was determined by 1H and 13C NMR on the basis of chemical shifts, C-H coupling constants and connectivity in NOESY and HETCOR spectra. The N-9 substitution of 7 was proved by its X-ray crystallographic analysis. 相似文献
106.
Recreational boats in tow between lakes are a known vector of the spread of aquatic invading species (AIS), but we have no test of the hypothesis that recreational boats are also a vector of secondary spread of AIS among freshwater ecosystems via in-water transport i.e., while boating between interconnected waterways. In this study, we surveyed recreational boaters travelling into Lake Simcoe (44°25′N, 79°20′W), Ontario, Canada, on their recreational activities, boat maintenance, and travel destinations, measured the degree of vessel fouling, and sampled all standing water and attached macrophytes associated with their vessels. A total of 321 zooplankton individuals comprising 15 different species were collected from the standing water in vessels, including veligers of the invasive zebra mussel Dreissena. The volume of water collected within the vessels significantly increased the number of zooplankton transported. Zooplankton species from pelagic habitats or with planktonic life stages were collected more frequently than species that occupy littoral or benthic habitats, likely reflecting the recreational activities of boaters. Patterns of boater activities, movements and hygiene habits, suggest recreational boating in the Lake Simcoe region is contributing to the spread of native and invasive species into nearby waterways. Our study validates the widespread assumption that recreational boats are an important in-water vector for the secondary spread of both native and invasive zooplankton species. Future management strategies to reduce the spread of AIS should be aimed at increasing awareness of boater hygiene practices, particularly the frequent draining of standing water. 相似文献
107.
Emily A. Clementi Kristina R. Wilhelm Jürgen Schleucher Ludmilla A. Morozova-Roche Anders P. Hakansson 《PloS one》2013,8(11)
HAMLET and ELOA are complexes consisting of oleic acid and two homologous, yet functionally different, proteins with cytotoxic activities against mammalian cells, with HAMLET showing higher tumor cells specificity, possibly due to the difference in propensity for oleic acid binding, as HAMLET binds 5-8 oleic acid molecules per protein molecule and ELOA binds 11-48 oleic acids. HAMLET has been shown to possess bactericidal activity against a number of bacterial species, particularly those with a respiratory tropism, with Streptococcus pneumoniae displaying the greatest degree of sensitivity. We show here that ELOA also displays bactericidal activity against pneumococci, which at lower concentrations shows mechanistic similarities to HAMLET’s bactericidal activity. ELOA binds to S. pneumoniae and causes perturbations of the plasma membrane, including depolarization and subsequent rupture, and activates an influx of calcium into the cells. Selective inhibition of calcium channels and sodium/calcium exchange activity significantly diminished ELOA’s bactericidal activity, similar to what we have observed with HAMLET. Finally, ELOA-induced death was also accompanied by DNA fragmentation into high molecular weight fragments – an apoptosis-like morphological phenotype that is seen during HAMLET-induced death. Thus, in contrast to different mechanisms of eukaryote cell death induced by ELOA and HAMLET, these complexes are characterized by rather similar activities towards bacteria. Although the majority of these events could be mimicked using oleic acid alone, the concentrations of oleic acid required were significantly higher than those present in the ELOA complex, and for some assays, the results were not identical between oleic acid alone and the ELOA complex. This indicates that the lipid, as a common denominator in both complexes, is an important component for the complexes’ bactericidal activities, while the proteins are required both to solubilize and/or present the lipid at the bacterial membrane and likely to confer other and separate functions during the bacterial death. 相似文献
108.
Martina Grdiša Sandra Babić Martina Periša Klaudija Carović‐Stanko Ivan Kolak Zlatko Liber Marija Jug‐Dujaković Zlatko Satovic 《化学与生物多样性》2013,10(3):460-472
Dalmatian pyrethrum (Tanacetum cinerariifolium (Trevir. ) Sch.Bip. ) is a plant species endemic to the east Adriatic coast. The bioactive substance of Dalmatian pyrethrum is a natural insecticide, pyrethrin, a mixture of six active components (pyrethrins I and II, cinerins I and II, and jasmolins I and II). The insecticidal potential of pyrethrin was recognized decades ago, and dried and ground flowers have traditionally been used in Croatian agriculture and households. A total of 25 Dalmatian pyrethrum populations from Croatia were studied to determine the pyrethrin content and composition, and to identify distinct chemotypes. The total pyrethrin content ranged from 0.36 to 1.30% (dry flower weight; DW) and the pyrethrin I/pyrethrin II ratio ranged from 0.64 to 3.33%. The statistical analyses revealed that the correlations between the percentage of pyrethrin I and of all the other components were significant and negative. The total pyrethrin content was positively correlated with the percentage of pyrethrin I and negatively correlated with cinerin II. The multivariate analysis of the chemical variability enabled the identification of five chemotypes among 25 Dalmatian pyrethrum populations. The chemical characterization of indigenous Dalmatian pyrethrum populations may serve as a good background for future breeding and agricultural exploitation. 相似文献
109.
Renata Jurišić Grubešić Goran Srečnik Dario Kremer Jadranka Vuković Rodríguez Toni Nikolić Sanda Vladimir‐Knežević 《化学与生物多样性》2013,10(7):1305-1316
A rapid reversed‐phase (RP) high‐performance liquid chromatography method was developed and applied for simultaneous separation, and determination of flavonoids and phenolic acids in eight Plantago L. taxa (P. altissima L., P. argentea Chaix , P. coronopus L., P. holosteum Scop . ssp. depauperata Pilger , P. holosteum ssp. holosteum, P. holosteum ssp. scopulorum (Degen) Horvati? , P. lagopus L., and P. maritima L.) growing in Croatia. Chromatographic separation was carried out on Zorbax Eclipse XDB‐C18 using gradient elution with a H2O (pH 2.5, adjusted with CF3COOH) and MeCN mixture at 30°. The contents of analyzed phenolic compounds (% of the dry weight of the leaves, dw) varied among examined species: rutin (max. 0.024%, P. argentea), hyperoside (max. 0.020%, P. lagopus), quercitrin (max. 0.013%, P. holosteum ssp. holosteum), quercetin (max. 0.028%, P. holosteum ssp. scopulorum), chlorogenic acid (max. 0.115%, P. lagopus), and caffeic acid (max. 0.046%, P. coronopus). Isoquercitrin was detected only in P. argentea (0.020%), while isochlorogenic acid content was below limit of quantification in all investigated species. Multivariate analyses (UPGMA and PCA) showed significant differences in contents of investigated polyphenolic compounds between different Plantago taxa. Accordingly, investigated substances might be employed as chemotaxonomic markers in the study of the complex genus Plantago. 相似文献
110.
Nemanja Rajčević Peđa Janaćković Srđan Bojović Vele Tešević Petar D. Marin 《化学与生物多样性》2013,10(1):144-156
The essential‐oil compositions of one Croatian and three Serbian populations of Juniperus deltoides R.P.Adams have been determined by GC/MS analysis. In total, 147 compounds were identified, representing 97.3–98.3% of the oil composition. The oils were dominated by monoterpenes, which are characteristic components for the species of the section Juniperus. Two monoterpenes, α‐pinene and limonene, were the dominant constituents, with a summed‐up average content of 49.45%. Statistical methods were used to determine the diversity of the terpene classes and the common terpenes between the newly described J. deltoides populations from Serbia and Croatia. Only reports on several specimens from this species have been reported so far, and there are no studies that treat population diversity. Cluster analysis of the oil contents of 21 terpenes showed high correlation with the geographical distribution of the populations, separating the Croatian from the Serbian populations. The comparison of the essential‐oil compositions obtained in the present study with literature data, showed the separation of J. deltoides and J. oxycedrus ssp. oxycedrus from the western Mediterranean region. 相似文献