全文获取类型
收费全文 | 27703篇 |
免费 | 2117篇 |
国内免费 | 223篇 |
出版年
2023年 | 237篇 |
2022年 | 409篇 |
2021年 | 793篇 |
2020年 | 531篇 |
2019年 | 595篇 |
2018年 | 825篇 |
2017年 | 697篇 |
2016年 | 972篇 |
2015年 | 1384篇 |
2014年 | 1464篇 |
2013年 | 1822篇 |
2012年 | 2039篇 |
2011年 | 1966篇 |
2010年 | 1243篇 |
2009年 | 1116篇 |
2008年 | 1465篇 |
2007年 | 1372篇 |
2006年 | 1236篇 |
2005年 | 1036篇 |
2004年 | 1028篇 |
2003年 | 882篇 |
2002年 | 806篇 |
2001年 | 659篇 |
2000年 | 588篇 |
1999年 | 502篇 |
1998年 | 261篇 |
1997年 | 166篇 |
1996年 | 173篇 |
1995年 | 194篇 |
1994年 | 156篇 |
1993年 | 142篇 |
1992年 | 267篇 |
1991年 | 286篇 |
1990年 | 218篇 |
1989年 | 224篇 |
1988年 | 202篇 |
1987年 | 181篇 |
1986年 | 160篇 |
1985年 | 174篇 |
1984年 | 152篇 |
1983年 | 118篇 |
1982年 | 87篇 |
1981年 | 94篇 |
1980年 | 89篇 |
1979年 | 105篇 |
1978年 | 106篇 |
1977年 | 77篇 |
1975年 | 82篇 |
1974年 | 65篇 |
1973年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
G. Negrón B. Quiclet-Sire Y. Diaz R. Gaviño R. Cruz 《Nucleosides, nucleotides & nucleic acids》2013,32(7):1539-1543
Abstract The preparation of 3-alkyl D4T derivatives has been carried out starting from the corresponding 5′-O-t-butyldimethylsilyl-3′-O-methanesulfonylthymidine 2 by way of deprotection-elimination and succesive alkylation reactions. 相似文献
992.
Dijana Saftić Ljubica Glavaš-Obrovac Mirosława Studzińska Edyta Paradowska Zbigniew J. Leśnikowski 《Nucleosides, nucleotides & nucleic acids》2013,32(7):397-414
AbstractAs a part of the research aimed on identification of new nucleobase derivatives with improved biological properties, a series of novel 8-substituted acyclovir derivatives were synthesized. The 8-azidoguanosine 4 and novel 8-azidoacyclovir 9 were synthesized from commercially available guanosine 1 and acyclovir 6 which were transformed into 8-bromopurine derivatives 2 and 7 and hydrazine derivatives 3 and 8, respectively. 8-Triazolylguanosine 5 and 8-triazolylacyclovir analogs 10–12 were successfully synthesized via the Cu(I) catalyzed 1,3-dipolar cycloaddition reaction of azides 4 and 9 with propargyl alcohol, 4-pentyn-1-ol and 5-hexyn-1-ol. The novel 1,4-disubstituted 1,2,3-triazolyl compounds 5, 10–12 were evaluated for antiviral activity against selected DNA and RNA viruses and cytostatic activity against normal Madine Darby canine kidney (MDCK I) cells, and seven tumor cell lines (HeLa, CaCo-2, NCI-H358, Jurkat, K562, Raji and HuT78). While tested compounds exerted no antiviral activity at nontoxic concentrations, the 8-triazolyl acyclovir derivative 10, with the shortest alkyl substituent at the C-4 of triazole ring, was found to be the most active against the CaCo-2 cell line. 相似文献
993.
994.
995.
Tam Dinh Le Vo Ji-seun Ko Seung Hwan Lee Si Jae Park Soon Ho Hong 《Biotechnology and Bioprocess Engineering》2013,18(6):1062-1066
This study investigated the effect of glutamate decarboxylase from Neurospora crassa OR74A on GABA production in Escherichia coli. GABA is one of the inhibitory neurotransmitters in the mammalian central nervous system, and can be used as a precursor of promising biopolymer Nylon 4. E. coli that overexpressed N. crassa glutamate decarboxylase was cultured at various pH levels and temperatures to determine optimum conditions for GABA production. When the recombinant E. coli strain was cultured at 30°C and pH 3, a final GABA concentration of 5.26 g/L was obtained from 10 g/L of monosodium glutamate (MSG), corresponding to a GABA yield of 86.23%. 相似文献
996.
María José Buitrago Cristina Elena Canteros Guadalupe Frías De León Ángel González Manoel Marques-Evangelista De Oliveira César O. Muñoz José Antonio Ramirez Adriana Isabel Toranzo Rosely Zancope-Oliveira Manuel Cuenca-Estrella 《Revista iberoamericana de micología》2013,30(4):256-260
BackgroundA multicenter study was conducted. A panel containing DNA from Histoplasma capsulatum, as well as negative and cross-reaction controls, was sent to five different laboratories, members of the MICOMOL network from CYTED Program.AimsThe objective was to assess the accuracy of different PCR protocols to detect H. capsulatum DNA.MethodsSeven different PCR protocols were tested. They were based on PCR techniques and used unicopy and multicopy targets.ResultsMost of these protocols (4/7) were able to detect the smallest amounts of fungal DNA (102 fg/μl). Overall sensitivity was 86% and specificity was 100%. The protocol based on a unicopy target (SCAR220) presented lower sensitivity (43%) but 100% specificity. The real-time protocols tested were highly reproducible, sensitive, and specific. Neither false positives nor cross-reactions were detected in any protocol.ConclusionsAll laboratories were able to amplify H. capsulatum DNA, and real-time PCR seems to be a promising tool to efficiently detect this pathogen in clinical samples. 相似文献
997.
Laçine Aksoy Erdi Kolay Yasin Ağılönü Zeyneb Aslan Mustafa Kargıoğlu 《Saudi Journal of Biological Sciences》2013,20(3):235-239
Thermopsis turcica, endemic to Turkey, is in danger of extinction. Studies on this species are very few due to the fact that it was only discovered in 1983 and grows in a small circumscribed area in Turkey. In this study, free radical scavenging activity, total phenolic content, total oxidant status (TOS), and total antioxidant status (TAS) of methanol (TTM) and acetone (TTA) extracts of T. turcica were measured spectroscopically. Free radical scavenging activity was determined according to the elimination of DPPH radicals and total phenol content was determined by the Folin–Ciocalteu reaction. Total oxidant status (TOS) and total antioxidant status (TAS) were measured with commercially available kits. Methanol and acetone extracts of T. turcica were found to have a specific radical scavenging effect. This effect was found to be related to the total phenolic content of the extracts. Since the TTA had a higher phenolic content than the methanol extract, it had a stronger radical scavenging effect. In addition, the total antioxidant capacity of the methanol extract was observed to be higher than that of its acetone counterpart. As a result, due to its antioxidative properties, T. turcica is thought to be a natural source of antioxidants. 相似文献
998.
Mathias Ahii Chia Ana Teresa Lombardi Maria da Graça Gama Melão 《Phycological Research》2013,61(4):286-291
Quantification of the calorific content of microalgae is critical in studies of energy flow, trophic partitioning, plant/herbivore interactions in aquaculture and biomass production for biofuels. We investigated the calorific value and biochemical composition of Chlorella vulgaris at different phosphorus (P) concentrations (6.0 × 10?7, 2.3 × 10?6 and 2.3 × 10?4 mol L?1 P). As expected, the control (2.3 × 10?4 mol L?1 P) supported better growth than P limited treatments. Biomolecules like total carbohydrates and lipids accumulated under P limitation, which significantly correlated with high calorific values. Lipid class composition showed that triacylglycerols were the most accumulated under P limited conditions. The calorific value reported under control conditions (13.78 kJ g?1) was less than those obtained under P limitation (30.47–33.07 kJ g?1). The highest calorific value with less growth retardation was obtained at 2.3 × 10?6 mol L?1 P. 相似文献
999.
João de Pina‐Cabral 《The journal of the Royal Anthropological Institute》2013,19(1):75-101
This article is a study of personal naming using drawings made by secondary school students from two small towns in coastal Bahia (Brazil) of the persons who gave them their names. The article explores the relation between namer and named as one of continued identity created by imaginative processes of surrogation akin to pretend‐play. In early ontogeny, the person is constituted through a series of mutualities of being that come to constitute a core of affects. The article aims to contribute towards a better understanding of the processes of constitution of personhood in the context of human relatedness. 相似文献
1000.
Yuanhong Zheng Vanminh Le Zhuoan Cheng Sheng Xie Hegeng Li Jianhui Tian Jianwen Liu 《Cell stress & chaperones》2013,18(2):203-213
Photodynamic therapy (PDT) is a regulatory-approved modality for treating a variety of malignant tumors. It induces tumor tissue damage via photosensitizer-mediated oxidative cytotoxicity. The heat shock protein 70 (HSP70-1) is a stress protein encoded by the HSPA1A gene and is significantly induced by oxidative stress associated with PDT. The aim of this study was to identify the functional region of the HSPA1A promoter that responds to PDT-induced oxidative stress and uses the stress responsiveness of HSPA1A expression to establish a rapid and cost-effective photocytotoxic assessment bioassay to evaluate the photodynamic potential of photosensitizers. By constructing luciferase vectors with a variety of hspa1a promoter fractions and examining their relative luciferase activity, we demonstrated that the DNA sequence from −218 to +87 of the HSPA1A gene could be used as a functional promoter to detect the PDT-induced oxidative stress. The maximal relative luciferase activity level of HSPA1A (HSP70-1) induced by hypericin-PDT was nearly nine times that of the control. Our results suggest that the novel reporter gene assay using a functional region of the HSP70A1A promoter has significant advantages for the detection of photoactivity in terms of both speed and sensitivity, when compared with a cell viability test based on ATP quantification and ROS levels. Furthermore, phthalocyanine zinc and methylene blue both induced significantly elevated levels of relative luciferase activity in a dose-dependent manner. 相似文献