全文获取类型
收费全文 | 2958篇 |
免费 | 319篇 |
专业分类
3277篇 |
出版年
2023年 | 11篇 |
2022年 | 34篇 |
2021年 | 52篇 |
2020年 | 47篇 |
2019年 | 57篇 |
2018年 | 69篇 |
2017年 | 56篇 |
2016年 | 101篇 |
2015年 | 130篇 |
2014年 | 138篇 |
2013年 | 165篇 |
2012年 | 192篇 |
2011年 | 197篇 |
2010年 | 119篇 |
2009年 | 122篇 |
2008年 | 129篇 |
2007年 | 133篇 |
2006年 | 137篇 |
2005年 | 103篇 |
2004年 | 103篇 |
2003年 | 107篇 |
2002年 | 89篇 |
2001年 | 89篇 |
2000年 | 84篇 |
1999年 | 85篇 |
1998年 | 32篇 |
1997年 | 23篇 |
1996年 | 20篇 |
1995年 | 24篇 |
1994年 | 24篇 |
1993年 | 23篇 |
1992年 | 50篇 |
1991年 | 57篇 |
1990年 | 54篇 |
1989年 | 56篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 35篇 |
1985年 | 29篇 |
1984年 | 24篇 |
1983年 | 29篇 |
1982年 | 23篇 |
1981年 | 17篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1976年 | 9篇 |
1974年 | 12篇 |
1971年 | 11篇 |
排序方式: 共有3277条查询结果,搜索用时 15 毫秒
81.
Jacob D. Berson Francisco Garcia‐Gonzalez Leigh W. Simmons 《Journal of evolutionary biology》2019,32(11):1186-1193
A role for sexual selection in the evolution of insect cuticular hydrocarbons (CHCs) is suggested by observations of selection acting on male CHCs during female mate choice. However, evidence that CHCs evolve in response to sexual selection is generally lacking, and there is a need to extend our understanding beyond well‐studied taxa. Experimental evolution offers a powerful approach to investigate the effect of sexual selection on the evolution of insect CHCs. We conducted such an experiment using the dung beetle, Onthophagus taurus. After six, 12 and 21 generations of experimental evolution, we measured the CHCs of beetles from three populations subject to sexual selection and three populations within which sexual selection had been removed via enforced monogamy. We found that the male CHC profile responded to the experimental removal of sexual selection. Conversely, the CHC profile of females responded to the presence of sexual selection but not to its removal. These results show that sexual selection can be an important mechanism affecting the evolution of insect CHCs and that male and female CHCs can evolve independently. 相似文献
82.
D'Avolio A Sciandra M Siccardi M Baietto L de Requena DG Bonora S Di Perri G 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2007,848(2):374-378
A simple method for the quantification of tipranavir, the first non-peptidic HIV protease inhibitor, was developed and validated. Quinoxaline, as internal standard, was added to 50 microl of plasma before a liquid-liquid extraction by 600 microl of protein precipitation solution. The extracts were diluted before being injected in the chromatographic system. Chromatographic separation was made on a C18 column using potassium phosphate buffer (pH 3.2) and acetonitrile with gradient. Detection was performed by an UV detector at 260 nm. Relative error at three control quality concentrations ranged from -1.81 to 1.72%. Intra-day (CV%) and inter-day (CV%) precision ranged from 0.94 to 2.55% and from 3.07 to 4.24%, respectively. LOQ and LOD were 0.090 microg/ml and 0.035 microg/ml, respectively. Mean recovery was 87.1%+/-2.4%. Calibration curve was linear up to 180 microg/ml. Concentration range when optimized (0.703-180 microg/ml) proved to be adequate to measure tipranavir concentration in HIV-1-positive patients, therefore this method could be suitable for therapeutic drug monitoring of this drug. 相似文献
83.
Simon Gonzalez Florian Gallier Sabrina Kellouche Franck Carreiras Ettore Novellino Alfonso Carotenuto Gérard Chassaing Paolo Rovero Jacques Uziel Nadège Lubin-Germain 《生物化学与生物物理学报:生物膜》2019,1861(1):50-61
Over the past decades, membranotropic peptides such as positively charged cell-penetrating peptides (CPPs) or amphipathic antimicrobial peptides (AMPs) have received increasing interest in order to improve therapeutic agent cellular uptake.As far as we are concerned, we were interested in studying HCV fusion peptides as putative anchors. Two peptides, HCV6 and HCV7, were identified and conjugated to a fluorescent tag NBD and tested for their interaction with liposomes as model membranes. DSC and spectrofluorescence analyses demonstrate HCV7 propensity to insert or internalize in vesicles containing anionic lipids DMPG whereas no activity was observed with zwitterionic DMPC. This behavior could be explained by the peptide sequence containing a cationic arginine residue. On the contrary, HCV6 did not exhibit any membranotropic activity but was the only sequence able to induce liposomes' fusion or aggregation monitored by spectrofluorescence and DLS. This two peptides mild activity was related to their inefficient structuration in contact with membrane mimetics, which was demonstrated by CD and NMR experiments.Altogether, our data allowed us to identify two promising membrane-active peptides from E1 and E2 HCV viral proteins, one fusogenic (HCV6) and the other membranotropic (HCV7). The latter was also confirmed by fluorescence microscopy with CHO cells, indicating that HCV7 could cross the plasma membrane via an endocytosis process. Therefore, this study provides new evidences supporting the identification of HCV6 as the HCV fusion peptide as well as insights on a novel membranotropic peptide from the HCV-E2 viral protein. 相似文献
84.
Reyes-Martínez Antonio Valle-Aguilera Juan Roberto Antunes-Ricardo Marilena Gutiérrez-Uribe Janet Gonzalez Carmen Santos-Díaz María del Socorro 《Plant Cell, Tissue and Organ Culture》2019,139(1):119-129
Plant Cell, Tissue and Organ Culture (PCTOC) - Pyrostegia venusta (Ker Gawl.) Miers is a plant used for the treatment of respiratory diseases, diarrhea, vitiligo, jaundice and to attenuate... 相似文献
85.
Effects of themenstrual cycle on heat loss and heat production(M) and core and skin temperatureresponses to cold were studied in six unacclimatized female nonsmokers(18-29 yr of age). Each woman, resting supine, was exposed to acold transient (ambient temperature = mean radiant temperature = 20 to5°C at 0.32°C/min, relative humidity = 50 ± 2%, wind speed = 1 m/s) in the follicular (F) phase(days 2-6) and midluteal (L)phase (days 19-23) of her menstrual cycle. Clothed in each of two ensembles with different thermal resistances, women performed multiple experiments in the F andL phases. Thermal resistance was 0.2 and 0.4 m2 · K · W1for ensembles A andB, respectively. Esophagealtemperature (Tes), mean weightedskin temperature(sk),finger temperature (Tfing), andarea-weighted heat flux were recorded continuously. Rate of heat debt(S) and integrated mean bodytemperature(b,i)were calculated by partitional calorimetry throughout the cold ramp. Extensive peripheral vasoconstriction in the F phase during early periods of the ramp elevated Tesabove thermoneutral levels. Shivering thermogenesis(M = M Mbasal,W /m2) was highly correlated withdeclines insk andTfing(P <0.0001). There was a reducedslope in M as a function ofb,i inthe L phase with ensembles A(P < 0.02) andB (P < 0.01). Heat flux was higher andS was less in the L phases withensemble A(P < 0.05). An analytic modelrevealed thatsk andTes contribute as additive inputsand Tfing has a multiplicativeeffect on the total control of Mduring cold transients(R2 = 0.9).Endogenous hormonal levels at each menstrual cycle phase, coretemperature andskinputs, vascular responses, and variations in body heat balance must beconsidered in quantifying thermoregulatory responses in women duringcold stress. 相似文献
86.
Prostaglandin E2 production in astrocytes: regulation by cytokines, extracellular ATP, and oxidative agents 总被引:5,自引:0,他引:5
Xu J Chalimoniuk M Shu Y Simonyi A Sun AY Gonzalez FA Weisman GA Wood WG Sun GY 《Prostaglandins, leukotrienes, and essential fatty acids》2003,69(6):437-448
Upregulation and activation of phospholipases A2 (PLA2) and cyclooxygenases (COX) leading to prostaglandin E2(PGE2) production have been implicated in a number of neurodegenerative diseases. In this study, we investigated PGE2 production in primary rat astrocytes in response to agents that activate PLA2 including pro-inflammatory cytokines (IL-1beta, TNFalpha and IFNgamma), the P2 nucleotide receptor agonist ATP, and oxidants (H2O2 and menadione). Exposure of astrocytes to cytokines resulted in a time-dependent increase in PGE2 production that was marked by increased expression of secretory sPLA2 and COX-2, but not COX-1 and cytosolic cPLA2. Although astrocytes responded to ATP or phorbol ester (PMA) with increased cPLA2 phosphorylation and arachidonic acid release, ATP or PMA only caused a small increase in levels of PGE2. However, when astrocytes were first treated with cytokines, further exposure to ATP or PMA, but not H2O2 or menadione, markedly increased PGE2 production. These results suggest that ATP release during neuronal excitation or injury can enhance the inflammatory effects of cytokines on PGE2 production and may contribute to chronic inflammation seen in Alzheimer's disease. 相似文献
87.
Cryptosporidium and giardia recoveries in natural waters by using environmental protection agency method 1623 总被引:1,自引:0,他引:1
Relatively few studies have examined recoveries from source waters by using Environmental Protection Agency method 1623 with organism spike doses that are environmentally realistic and at turbidity levels commonly found in surface waters. In this study, we evaluated the filtration capacities and recovery efficiencies of the Gelman Envirochek (standard filter) and the Gelman Envirochek high-volume (HV) sampling capsules under environmental conditions. We also examined the performance of method 1623 under ambient conditions with matrix spike experiments using 10 organisms/liter. Under turbid conditions, the HV capsule filtered approximately twice the volume filtered by the standard filter, but neither could filter 10 liters without clogging. In low-turbidity waters, oocyst, but not cyst, recoveries were significantly higher when the HV capsule was used. In turbid waters, organism recoveries were lower than those in nonturbid waters and were not significantly different for the different filters. When the HV capsule was used, Cryptosporidium recoveries ranged from 36 to 75%, and Giardia recoveries ranged from 0.5 to 53%. For both organisms, recoveries varied significantly by site. Turbidity could explain variation in Giardia recoveries (r(2) = 0.80) but not variation in Cryptosporidium recoveries (r(2) = 0.16). The inconsistent recoveries across sites suggested that the background matrix of the ambient water affected recovery by method 1623. A control sample collected at the height of the winter rainy season detected one organism, highlighting the difficulty of using this method to accurately measure pathogen abundance under natural conditions. Our findings support the use of the HV filter under field conditions but suggest that designing a cost-effective and statistically valid monitoring program to evaluate sources and loads of protozoan pathogens may be difficult. 相似文献
88.
C. Betancor R. Freire A.G. Gonzalez J.A. Salazar C. Pascard T. Prange 《Phytochemistry》1980,19(9):1989-1993
The investigation of stems and leaves of Catha cassinoides afforded, in addition to sitosterol, β-amyrin, ursolic acid, lup-20(29)-en-3β,30-diol and friedelin, three new pentacyclic triterpenes: 30-hydroxyfriedelan-3-one, 29-hydroxyfriedelan-3-one and 3-oxo-friedelan-29-oic acid. The structures ofthese were determined by spectral studies and correlations, and were confirmed by X-ray analysis of 29-hydroxyfriedelan-3-one acetate. 相似文献
89.
Chowdhury SF Guerrero RH Brun R Ruiz-Perez LM Pacanowska DG Gilbert IH 《Journal of enzyme inhibition and medicinal chemistry》2002,17(5):293-302
Dihydrofolate reductase is a drug target that has not been thoroughly investigated in leishmania and trypanosomes. Work has previously shown that 5-benzyl-2,4-diaminopyrimidines are selective inhibitors of the leishmanial and trypanosome enzymes. Modelling predicted that alkyl/aryl substitution on the 6-position of the pyrimidine ring should increase enzyme activity of 5-benzyl-2,4-diaminopyrimidines as inhibitors of leishmanial and trypanosomal dihydrofolate reductase. Various compounds were prepared and evaluated against both the recombinant enzymes and the intact organisms. The presence of a substituent had a small or negative effect on activity against the enzyme or intact parasites compared to unsubstituted compounds. 相似文献
90.
Manuel Gonzalez Sierra Oreste A. Mascaretti Vera M. Merkuza Evangelina L. Tosti Edmundo A. Ruveda Ching-Jer Chang 《Phytochemistry》1974,13(12):2865-2869
Two new components, the peptide alkaloid scutianine D and scutianene C have been isolated from Scutia buxifolia and their structures elucidated. The configuration of some of the asymmetric centers of scutianine A has been determined by gas chromatography. 相似文献