全文获取类型
收费全文 | 10166篇 |
免费 | 949篇 |
国内免费 | 280篇 |
出版年
2023年 | 89篇 |
2022年 | 125篇 |
2021年 | 372篇 |
2020年 | 201篇 |
2019年 | 313篇 |
2018年 | 328篇 |
2017年 | 300篇 |
2016年 | 398篇 |
2015年 | 598篇 |
2014年 | 601篇 |
2013年 | 715篇 |
2012年 | 877篇 |
2011年 | 796篇 |
2010年 | 511篇 |
2009年 | 428篇 |
2008年 | 563篇 |
2007年 | 533篇 |
2006年 | 448篇 |
2005年 | 413篇 |
2004年 | 354篇 |
2003年 | 313篇 |
2002年 | 344篇 |
2001年 | 214篇 |
2000年 | 242篇 |
1999年 | 184篇 |
1998年 | 78篇 |
1997年 | 49篇 |
1996年 | 59篇 |
1995年 | 62篇 |
1994年 | 52篇 |
1993年 | 52篇 |
1992年 | 90篇 |
1991年 | 72篇 |
1990年 | 75篇 |
1989年 | 67篇 |
1988年 | 57篇 |
1987年 | 43篇 |
1986年 | 57篇 |
1985年 | 50篇 |
1984年 | 37篇 |
1983年 | 33篇 |
1982年 | 17篇 |
1981年 | 21篇 |
1980年 | 15篇 |
1979年 | 19篇 |
1978年 | 18篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1975年 | 13篇 |
1974年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Investigation of the role of the disulphide bond in the activity and structure of staphylococcal enterotoxin C1 总被引:13,自引:0,他引:13
Carolyn J. Hovde James C. Marr Marcy L. Hoffmann Sean P. Hackett Young-in Chi Kimberlee K. Crum Dennis L. Stevens Cynthia V. Stauffacher Gregory A. Bohach 《Molecular microbiology》1994,13(5):897-909
The goal of this study was to Investigate the role of the disulphide bond of staphylococcal enterotoxin C1 (SEC1) in the structure and activity of the toxin. Mutants unable to form a disulphide bond were generated by substituting alanine or serine for cysteine at positions 93 and/or 110. Although we did not directly investigate the residues between the disulphide linkage, tryptic lability showed that significant native structure in the cystine loop is preserved in the absence of covalent bonding between residues 93 and 110. Since no correlation was observed between the behaviour of these mutants with regard to toxin stability, emesis and T cell proliferation, we conclude that SEC1 -induced emesis and T cell proliferation are dependent on separate regions of the molecule. The disulphide bond itself is not an absolute requirement for either activity. However, conformation within or adjacent to the loop is important for emesis. Although mutants with alanine substitutions were not emetic, those with serine substitutions retained this activity, suggesting that the disulphide linkage stabilizes a crucial conformation but can be replaced by residues which hydrogen bond. 相似文献
92.
Gary B. Quistad Quyen Nguyen Paul Bernasconi Douglas J. Leisy 《Insect biochemistry and molecular biology》1994,24(10):955-961
The potency of venom from Bracon hebetor against lepidopterous larvae has been known for over 40 years, but previous attempts to purify and characterize individual protein toxins have been largely unsuccessful. Three protein toxins were purified from venom of this small parasitic wasp and the amino acid sequences of 22–31 consecutive residues at the amino-terminus were determined. These relatively large toxins (apparent molecular mass 73 kDa) were labile under many isolation techniques, but anion-exchange chromatography allowed purification with retention of biological activity. Two purified toxins were quite insecticidal (LD50 < 0.3μg/g) when injected into six species of lepidopterous larvae. On a molar basis, one toxin (Brh-I) has the highest known biocidal activity against Heliothis virescens (LD50 = 2 pmol/g). 相似文献
93.
生淀粉高浓度酒精发酵的研究 总被引:15,自引:1,他引:14
本研究利用国内常用的糖化酶制剂糖化生玉米面中的淀粉,同时接种酵母菌,在30℃下,探讨了玉米淀粉的高浓度酒精发酵工艺。选择到了一株产高浓度酒精酵母菌,H0菌株。发酵温度为30℃、pH4一s、加糖化酶量为每克原料300单位、酵母接种量3%(v/v)和原料加量为33.0%(w/v)时,这株酵母菌在70小时内可产生17.5%(v/v)的乙醇。如果原料加量为36.0%(w/v)时,该菌株在96小时内可以产生18.O%(v/v)的乙醇。在前一种加料情况下,成熟发酵醪中的pH为5、残还原糖为O.19%、残总糖为3.5“。在后一种加料情况下,成熟发酵醪中的pH为5、残还原糖为0.81%、残总糖为5.1%。 相似文献
94.
Qiyuan Chen Melanie Smith Tam Nguyen Darryl W. Maher Peter Hersey 《Cancer immunology, immunotherapy : CII》1994,38(6):385-393
Previous studies have shown that recognition of melanoma by cytotoxic T lymphocytes may be restricted by HLA-A1, A2 and other HLA antigens. The present study examined the cytotoxic specificity and major histocompatibility complex restriction of cloned cytotoxic T lymphocytes (CTL) isolated from a patient with the HLA phenotype A3,31 who had been immunized with a vaccine prepared from HLA-A1,3 melanoma cells. Cytotoxic assays against HLA-typed allogeneic melanoma cells indicated that cloned CTL from the patient were able to kill allogeneic melanoma cells expressing HLA-A1 but not other HLA-A1-positive cells. Studies on a representative clone indicated that proliferation and cytokine (tumour necrosis factor ) production in response to melanoma cells was also associated with HLA-A1 on melanoma cells. Response to the melanoma cells was associated with interleukin-4 (IL-4) rather than IL-2 production. The antigen recognized in the context of HLA-A1 on allogeneic melanoma cells was detected in cytotoxic assays on cells from 9 of 12 HLA-A1+ melanoma cell lines and did not appear to be the product of the MAGE-1 or-3 genes. These findings suggest that T cells can recognize melanoma antigens in the context of alloantigens and that allogeneic vaccines containing immunodominant alloantigens may generate CTL that are ineffective against autologous melanoma. The study does not, however, exclude the possibility that CTL with specificity to the latter may be activated by allogeneic vaccines, and further studies are needed to answer this question. 相似文献
95.
Olivier Bornet Gérard Lancelot Luc Chanteloup Nguyen T. Thuong Jean-Marie Beau 《Journal of biomolecular NMR》1994,4(4):575-580
Summary We present NMR studies of an intramolecular triple helix, the three strands of which have been linked by a hexaethylene glycol chain. To overcome the generally encountered difficulties of assignment in the homopyrimidine strands, the carbon C1 of the pyrimidines were selectively 13C-enriched. Assignments of the aromatic and sugar protons were obtained from NOESY-HMQC and TOCSY-HMQC spectra. We show that the recognition of a DNA duplex by a third strand via triplex formation is easily carried out in solution by observing the changes of the 1H1–13C1 connectivities as a function of pH. Furthermore, the conformation of the sugars has been found to be C2-endo, on the basis of the coupling constant values directly measured in an HSQC spectrum. 相似文献
96.
用浅麻醉的Wistar大鼠40只,以辐射热照尾作为伤害性刺激,经玻璃微电极引导尾核痛兴奋神经元(PEN)和痛抑制神经元(PIN)的放电,同时测量甩尾反射潜伏期(TFL)作为甩尾痛阈的指标。结果表明:(1)自然状态下辐射热照尾引起尾核中PEN放电频率增加和PIN放电频率减少与甩尾反射(TF)相伴行。放电变化发生在TF之前,结束在TF之后,PEN和PIN放电变化与TFL呈高度正相关。(2)辐射热照尾同时引起一侧尾核PEN放电频率增加和另一侧PIN放电频率减少,二者协同活动,并发生在TF之前,结束在TF之后。(3)侧脑室注射55μg/10μl吗啡呈镇痛作用时,尾核PEN放电频率减少和PIN放电频率增加,放电变化仍发生在TF之前,结束在TF之后。结果提示,尾核中PEN、PIN放电和TF是中枢不同水平同时对痛觉调制所产生的结果。 相似文献
97.
Glucocorticoids regulate the expression of many liver-specific genes via glucocorticoid receptors. The presence of glucocorticoid receptors in liver has been reported in many mammalian species but not in nude mice. In the present study, we demonstrate the presence of specific glucocorticoid receptors in nude mouse liver. The binding of ligands to these receptors could be completely inhibited by RU486, and partially blocked by hydrocortisone and progesterone, whereas estrogen and testosterone had no effect. Hydrocortisone down-regulated the level of glucocorticoid receptors in livers of nude mice and correspondingly enhanced the activities of tyrosine aminotransferase and -glutamyltransferase. Our results indicate that glucocorticoid receptors in nude mouse liver are specific, fully functional, and present at levels 28.5-fold higher than in the liver of normal inbred mice. We suggest that the nude mouse is a valuable model for studies of hepatic glucocorticoid action and may provide a clue to a putative hepatic-thymic interaction. 相似文献
98.
Increased levels of a 65-kDa stress protein (Msp65) were observed in group B Neisseria meningitidis grown under stationary-growth conditions. Electron microscopy showed two apposing rings of seven subunits, a structure typical of Escherichia coli GroEL. Msp65 was not found in either the periplasmic space or the outer membrane. Several important differences between the GroEL analogs of N. meningitidis and Neisseria gonorrhoeae are discussed. 相似文献
99.
Nguyen T. Buu 《Journal of neurochemistry》1993,61(4):1557-1560
Abstract: The neurotoxin N -methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes, via its metabolite 1-methyl-4-phenylpyridinium (MPP+ ), parkinsonism in humans, monkeys, and mice but not in rats. When incubated with mouse brain homogenates, [3 H]-MPP+ is recovered in relatively large concentrations in the brain cell nucleus. Although isolated cell nuclei from rat and mouse brain contain uptake systems for dopamine (DA), only brain cell nuclei from mice avidly take up [3 H]MPP+ . This nuclear uptake is ATP dependent and can be blocked by ouabain and Nethylmaleimide. It is not, however, affected by neuronal and vesicular blockers such as benztropine. mazindol, and reserpine. Selective uptake of MPP+ into brain cell nuclei may provide a new avenue for future investigation into the complex modes of action of the neurotoxin MPTP. 相似文献
100.
Monoclonal antibodies against the muscle-specific N-terminus of dystrophin: characterization of dystrophin in a muscular dystrophy patient with a frameshift deletion of exons 3-7. 总被引:2,自引:2,他引:0 下载免费PDF全文
T T Le T M Nguyen D R Love T R Helliwell K E Davies G E Morris 《American journal of human genetics》1993,53(1):131-139