全文获取类型
收费全文 | 3559篇 |
免费 | 309篇 |
国内免费 | 2篇 |
专业分类
3870篇 |
出版年
2023年 | 17篇 |
2022年 | 27篇 |
2021年 | 62篇 |
2020年 | 33篇 |
2019年 | 50篇 |
2018年 | 93篇 |
2017年 | 79篇 |
2016年 | 104篇 |
2015年 | 168篇 |
2014年 | 207篇 |
2013年 | 269篇 |
2012年 | 305篇 |
2011年 | 262篇 |
2010年 | 174篇 |
2009年 | 161篇 |
2008年 | 227篇 |
2007年 | 209篇 |
2006年 | 220篇 |
2005年 | 199篇 |
2004年 | 205篇 |
2003年 | 192篇 |
2002年 | 175篇 |
2001年 | 39篇 |
2000年 | 24篇 |
1999年 | 36篇 |
1998年 | 40篇 |
1997年 | 26篇 |
1996年 | 29篇 |
1995年 | 33篇 |
1994年 | 21篇 |
1993年 | 22篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 14篇 |
1989年 | 21篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1960年 | 3篇 |
排序方式: 共有3870条查询结果,搜索用时 15 毫秒
941.
Emmanuelle Sacco Jean‐Emmanuel Hugonnet Nathalie Josseaume Julie Cremniter Lionel Dubost Arul Marie Delphine Patin Didier Blanot Louis B. Rice Jean‐Luc Mainardi Michel Arthur 《Molecular microbiology》2010,75(4):874-885
Bypass of the penicillin‐binding proteins by an l ,d ‐transpeptidase (Ldtfm) confers cross‐resistance to β‐lactam and glycopeptide antibiotics in mutants of Enterococcus faecium selected in vitro. Ldtfm is produced by the parental strain D344S although it insignificantly contributes to peptidoglycan cross‐linking as pentapeptide stems cannot be used as acyl donors by this enzyme. Here we show that production of the tetrapeptide substrate of Ldtfm is controlled by a two‐component regulatory system (DdcRS) and a metallo‐d ,d ‐carboxypeptidase (DdcY). The locus was silent in D344S and its activation was due to amino acid substitutions in DdcS or DdcR that led to production of DdcY and hydrolysis of the C‐terminal d ‐Ala residue of the cytoplasmic peptidoglycan precursor UDP‐MurNAc‐pentapeptide. The T161A and T161M substitutions affected a position of DdcS known to be essential for the phosphatase activity of related sensor kinases. Complete elimination of UDP‐MurNAc‐pentapeptide, which was required specifically for resistance to glycopeptides, involved substitutions in DdcY that increased the catalytic efficiency of the enzyme (E127K) and affected its interaction with the cell envelope (I14N). The ddc locus displays striking similarities with portions of the van vancomycin resistance gene clusters, suggesting possible routes of emergence of cross‐resistance to glycopeptides and β‐lactams in natural conditions. 相似文献
942.
Sandro Bertolino Luc Wauters Aleksandar Pizzul Ambrogio Molinari Peter Lurz Guido Tosi 《Mammalian Biology》2009,74(3):210-219
Monitoring constitutes a key element in the management and conservation of many mammal species. We describe a technique to obtain population indices for red squirrels (Sciurus vulgaris) using hair-tubes and compare these indices to population estimates obtained by live trapping. Data were collected in seven study areas in the Western and Central Alps in Italy and compared with data previously collected in 11 sites in northern England. The aim was to test if hair-tube census could be used to derive a general predictive model allowing accurate predictions of squirrel numbers in different years, habitats and geographic regions. We used model equations developed from the proportion of hair-tubes visited to predict densities obtained from live-trapping. Hair-tube data gathered in the Central Alps correctly predicted squirrel densities in the Western Alps. A combined data set pooling the sites of these two regions based on the first three years successfully predicted the two successive years. In addition, a combined model derived from areas monitored for five years had a high predictive value locally (89%) and internationally (73%) when applied to the English data set. We therefore believe that the predictive model developed in this study could be of general value and be used to monitor squirrel populations in European low density conifer habitats (0.1–0.5 squirrels/ha). The approach may also be suitable for many tree squirrel populations in North America and other arboreal rodents that occur at similar densities. 相似文献
943.
944.
UDP-N-Acetylglucosamine 4-Epimerase Activity Indicates the Presence of N-Acetylgalactosamine in Exopolysaccharides of Streptococcus thermophilus Strains 总被引:1,自引:0,他引:1 下载免费PDF全文
Bart Degeest Frederik Vaningelgem Andrew P. Laws Luc De Vuyst 《Applied microbiology》2001,67(9):3976-3984
The monomer composition of the exopolysaccharides (EPS) produced by Streptococcus thermophilus LY03 and S. thermophilus Sfi20 were evaluated by high-pressure liquid chromatography with amperometric detection and nuclear magnetic resonance spectroscopy. Both strains produced the same EPS composed of galactose, glucose, and N-acetylgalactosamine. Further, it was demonstrated that the activity of the precursor-producing enzyme UDP-N-acetylglucosamine 4-epimerase, converting UDP-N-acetylglucosamine into UDP-N-acetylgalactosamine, is responsible for the presence of N-acetylgalactosamine in the EPS repeating units of both strains. The activity of UDP-N-acetylglucosamine 4-epimerase was higher in both S. thermophilus strains than in a non-EPS-producing control strain. However, the level of this activity was not correlated with EPS yields, a result independent of the carbohydrate source applied in the fermentation process. On the other hand, both the amounts of EPS and the carbohydrate consumption rates were influenced by the type of carbohydrate source used during S. thermophilus Sfi20 fermentations. A correlation between activities of the enzymes α-phosphoglucomutase, UDP-glucose pyrophosphorylase, and UDP-galactose 4-epimerase and EPS yields was seen. These experiments confirm earlier observed results for S. thermophilus LY03, although S. thermophilus Sfi20 preferentially consumed glucose for EPS production instead of lactose in contrast to the former strain. 相似文献
945.
Sam N. Chiejina Jerzy M. Behnke Pius A. Nnadi Luc A. Ngongeh Godlove A. Musongong 《Small Ruminant Research》2009,85(2-3):91-98
The West African Dwarf (WAD) goat from the humid zone of Nigeria is known for its trypanotolerance as well as for its resistance and resilience to Haemonchus contortus (haemonchotolerance). Another ecotype of WAD goat with a larger body size is found in the drier savanna zone of the country. We tested the hypothesis that the latter is less trypanotolerant, and less haemonchotolerant than the former ecotype because they have been less exposed to these infections and because of the likelihood of introgression of alleles for parasite susceptibility into the latter from neighbouring parasite-susceptible Sahelian genotypes. Two controlled experiments were carried out. In the first, we compared the responses in 8–9 month old kids of both ecotypes to subcutaneous infection with 5 × 106 Trypanosoma brucei. Infection in both ecotypes was characterised by (i) prepatent periods of 3 days; (ii) a modest peak parasitaemia 4–5 days post infection (pi), followed by rapid clearance of parasites from the blood to microscopically undetectable levels from D11 or D12 until the end of the experiment on D30 pi; (iii) a sharp but transient drop in PCV following peak parasitaemia, with no other clinical evidence of anaemia; and (iv) normal growth and a small but weakly significant change in body temperature. In a second experiment we infected groups of goats of both ecotypes with 6000 L3 of H. contortus. This infection also produced no significant changes in the PCV and body weight of the goats. Only a small percentage of the inoculum was recovered from both ecotypes at necropsy on D18 pi (Mean % recovery ± SE = 3.29 ± 0.61 for humid zone and 6.83 ± 2.72 for savanna goats) and there was no significant difference in their worm burdens. On the basis of these results we reject our hypothesis and conclude that the savanna WAD ecotype exhibits comparable, strong degrees of trypanotolerance and haemonchotolerance to its humid zone counterpart. 相似文献
946.
A set of arabinoxylan samples differing in their arabinose composition and various samples of arabino-xylo-oligosaccharide samples were analysed by Raman spectroscopy. Specific signatures for arabinose substitution were found in several spectral regions, that is, 400-600, 800-950 and 1030-1100 cm(-1). A linear relationship was observed between the peak ratio 855/895 cm(-1) of the second derivative spectra and the A/X ratio determined by chemical analysis. Moreover, spectral changes were observed in the 400-600 cm(-1) region assigned to the coupled vibrations mode in the skeleton: while the intensity of the band at 570 cm(-1) increased with the degree of substitution, that at 494 cm(-1) decreased. Similarly, a linear relationship was observed between the peak intensity ratio 570/494 cm(-1) calculated on the second derivative spectra and the composition data. Analysis of Raman spectra of arabino-xylo-oligosaccharides allowed to identify specific spectral features of disubstitution. 相似文献
947.
Fago A Mathews AJ Dewilde S Moens L Brittain T 《Journal of inorganic biochemistry》2006,100(8):1339-1343
The normally hexa coordinate ferrous form of neuroglobin binds CO by replacement of the heme-linked distal histidine residue. We have studied this reaction in detail using stopped flow techniques. The reaction time courses are complex at all the wavelengths studied. Specifically the reaction with CO occurs in two temporally separable phases, each of which shows a hyperbolic dependence of rate on CO concentration, indicating they each arise from histidine replacement by CO. Analysis of the observed rates as a function of the CO concentration, measured in the pH range 6.0-8.0, allows us to determine both the rate of histidine-heme ligand binding and dissociation for each of the two forms of the protein present in solution at each pH value. The pH dependence of the histidine association and dissociation rates is complex, as are the derived equilibrium constants for distal histidine binding. The spectral change associated with each reaction phase is very similar and independent of the CO concentration, showing that the two protein forms responsible for the two observed kinetic processes are not in equilibrium on the time scale of our investigations. Our data suggests that, unlike many other heme proteins, neuroglobin displays complex reactivity with ligands in the ferrous form due to heme rotational disorder, as has previously been reported for the ferric form of the protein. 相似文献
948.
Féart C Helmer C Fleury H Béjot Y Ritchie K Amouyel P Schraen-Maschke S Buée L Lambert JC Letenneur L Dartigues JF 《PloS one》2011,6(12):e29480
Objective
Herpes simplex virus (HSV) reactivation has been identified as a possible risk factor for Alzheimer''s disease (AD) and plasma amyloid-beta (Aβ) levels might be considered as possible biomarkers of the risk of AD. The aim of our study was to investigate the association between anti-HSV antibodies and plasma Aβ levels.Methods
The study sample consisted of 1222 subjects (73.9 y in mean) from the Three-City cohort. IgM and IgG anti-HSV antibodies were quantified using an ELISA kit, and plasma levels of Aβ1–40 and Aβ1–42 were measured using an xMAP-based assay technology. Cross-sectional analyses of the associations between anti-HSV antibodies and plasma Aβ levels were performed by multi-linear regression.Results
After adjustment for study center, age, sex, education, and apolipoprotein E-e4 polymorphism, plasma Aβ1–42 and Aβ1–40 levels were specifically inversely associated with anti-HSV IgM levels (β = −20.7, P = 0.001 and β = −92.4, P = 0.007, respectively). In a sub-sample with information on CLU- and CR1-linked SNPs genotyping (n = 754), additional adjustment for CR1 or CLU markers did not modify these associations (adjustment for CR1 rs6656401, β = −25.6, P = 0.002 for Aβ1–42 and β = −132.7, P = 0.002 for Aβ1–40; adjustment for CLU rs2279590, β = −25.6, P = 0.002 for Aβ1–42 and β = −134.8, P = 0.002 for Aβ1–40). No association between the plasma Aβ1–42-to-Aβ1–40 ratio and anti-HSV IgM or IgG were evidenced.Conclusion
High anti-HSV IgM levels, markers of HSV reactivation, are associated with lower plasma Aβ1–40 and Aβ1–42 levels, which suggest a possible involvement of the virus in the alterations of the APP processing and potentially in the pathogenesis of AD in human. 相似文献949.
Gómez-Amores L Mate A Miguel-Carrasco JL Jiménez L Jos A Cameán AM Revilla E Santa-María C Vázquez CM 《The Journal of nutritional biochemistry》2007,18(8):533-540
The present study aimed to investigate whether l-carnitine (LC) protects the vascular endothelium and tissues against oxidative damage in hypertension. Antioxidant enzyme activities, glutathione and lipid peroxidation were measured in the liver and heart of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. Nitrite and nitrate levels and total antioxidant status (TAS) were evaluated in plasma, and the expression of endothelial nitric oxide synthase (eNOS) and p22phox subunit of NAD(P)H oxidase was determined in aorta. Glutathione peroxidase activity was lower in SHR than in WKY rats, and LC increased this activity in SHR up to values close to those observed in normotensive animals. Glutathione reductase and catalase activities, which were higher in SHR, tended to increase after LC treatment. No differences were found in the activity of superoxide dismutase among any animal group. The ratio between reduced and oxidized glutathione and the levels of lipid peroxidation were respectively decreased and increased in hypertensive rats, and both parameters were normalized after the treatment. Similarly, LC was able to reverse the reduced plasma nitrite and nitrate levels and TAS observed in SHR. We found no alterations in the expression of aortic eNOS among any group; however, p22phox mRNA levels showed an increase in SHR that was reversed by LC. In conclusion, chronic administration of LC leads to an increase in hepatic and cardiac antioxidant defense and a reduction in the systemic oxidative process in SHR. Therefore, LC might increase NO availability in SHR aorta by a reduction in superoxide anion production. 相似文献
950.
Emmanuel K. Tangni Jean-Claude Motte Alfons Callebaut Anne Chandelier Marnix De Schrijver Luc Pussemier 《Mycotoxin Research》2011,27(2):105-113
A comparison of matched pairs deoxynivalenol (DON) loads in wheat samples via VERATOX for DON 5/5 performed by two laboratories
against two liquid chromatographic methods (LC-MS/MS and HPLC-UV) used by two other laboratories was carried out using biometrical
and sum of ranking differences (SRD) procedures. The Lin’s Concordance correlation coefficients, the average discrepancies,
the limits of agreement and the SRD between ELISA and reference values showed good overall agreement between VERATOX for DON
5/5 and reference methods for the two datasets. The VERATOX kits are valuable for quantitative screening and even for an initial
exposure assessment in situations when there are practical or economical reasons not to use sophisticated methods such as
HPLC or GC methods (with or without MS). However, networking of laboratories using this rapid method and laboratories with
reference analytical methods should be encouraged. 相似文献