全文获取类型
收费全文 | 46935篇 |
免费 | 3186篇 |
国内免费 | 4815篇 |
专业分类
54936篇 |
出版年
2023年 | 749篇 |
2022年 | 1026篇 |
2021年 | 1165篇 |
2020年 | 1139篇 |
2019年 | 1799篇 |
2018年 | 1610篇 |
2017年 | 1295篇 |
2016年 | 1177篇 |
2015年 | 1135篇 |
2014年 | 2303篇 |
2013年 | 2904篇 |
2012年 | 1771篇 |
2011年 | 2170篇 |
2010年 | 1641篇 |
2009年 | 2141篇 |
2008年 | 2208篇 |
2007年 | 2433篇 |
2006年 | 2178篇 |
2005年 | 1770篇 |
2004年 | 1507篇 |
2003年 | 1486篇 |
2002年 | 1328篇 |
2001年 | 1149篇 |
2000年 | 960篇 |
1999年 | 833篇 |
1998年 | 792篇 |
1997年 | 751篇 |
1996年 | 739篇 |
1995年 | 739篇 |
1994年 | 725篇 |
1993年 | 637篇 |
1992年 | 640篇 |
1991年 | 629篇 |
1990年 | 479篇 |
1989年 | 500篇 |
1988年 | 445篇 |
1987年 | 438篇 |
1986年 | 415篇 |
1985年 | 663篇 |
1984年 | 895篇 |
1983年 | 722篇 |
1982年 | 754篇 |
1981年 | 606篇 |
1980年 | 611篇 |
1979年 | 546篇 |
1978年 | 432篇 |
1977年 | 405篇 |
1976年 | 371篇 |
1974年 | 243篇 |
1973年 | 255篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
982.
983.
984.
985.
3-(2-Carboxyethyl)thymine (3-CET) was synthesized from β-propiolactone (BPL) and dThd5′P at pH 9.0–9.5 via the intermediate 3-(2-carboxyethyl)thymidine-5′-monophosphoric acid (3-CEdThd5′P). 3-CEdThd5′P was converted to 3-CET by hydrolysis in 1.5 N HCl at 100°C for 2 h. The structure of 3-CET was assigned on the basis of UV spectra, electron impact (EI) and isobutane chemical ionization mass spectra and the EI mass spectrum of a trimethylsilyl derivative of 3-CET. BPL was reacted in vitro with calf thymus DNA at pH 7.5. 100 A units of BPL-reacted DNA yielded, following perchloric acid hydrolysis and preparative paper chromatography, 3 A units of 3-CET. Reaction of BPL with the phosphodiester thymidylyl-(3′-5′)thymidine gave 3-(2-carboxyethyl)thymidylyl-(3′-5′)-3-(2-carboxyethyl)thymidine (~3%). Phosphotriester formation was not detected. 相似文献
986.
Preparation and spectroscopic characterization of molecular species of brain phosphatidylserines 总被引:2,自引:0,他引:2
This study describes the first preparation and spectroscopic characterization of naturally occurring phospholipids separated according to degree of unsaturation. Phosphatidylserines (PS) have been prepared from bovine brain and shown to be pure by extensive thin layer chromatographic analysis as well as by infrared spectroscopy and fatty acid analysis. The PS has been separated according to degree of unsaturation and prepared using AgNO3-impregnated silica gel H thin-layer chromatography. Fatty acid analysis of the two principal PS subfractions indicates that they are enriched in the molecular species 1-octadecanoyl-2-docosahexaenoyl-sn-glycero-3-phosphorylserine and 1-octadecanoyl-2-octadecenoyl-sn-glycero-3-phosphorylserine. The identity of the two PS subfractions was further verified by rechromatographing on several thin layer systems and by infrared spectroscopy. With the use of a 100 MHz Fourier transform nuclear magnetic resonance (NMR) spectrometer, the spectra of bovine whole brain, white matter, gray matter, monoenoic, and hexaenoic PS were obtained. Distinct proton resonances were assigned to double bond protons, protons adjacent to a double bond, and protons between two double bonds, using fatty acid methyl ester standards. The various PS preparations gave different intensities of the various proton resonances which correlated with differences in fatty acid composition. The method provides a convenient, non-destructive spectroscopic method for distinguishing monoenoic and polyunsaturated species of intact phospholipids. Electron spin resonance studies of nitroxide-labelled cholestane in sonicated PS vesicles showed greater probe motion as the unsaturation of the acyl chains was increased. The hexaenoic PS vesicles were more fluid than monoenoic PS vesicles at all temperatures in the range 10-55 degrees C. These results suggest that neuronal membranes are more fluid than myelin membranes as neuronal membranes contain more hexaenoic phospholipids. 相似文献
987.
988.
Arnold M. Katz Charles F. Louis Doris I. Repke Gary Fudyma Priscilla A. Nash-Adler Robert Kupsaw Munekazu Shigekawa 《生物化学与生物物理学报:生物膜》1980,596(1):94-107
Unfractionated and low buoyant density sarcoplasmic reticulum vesicles released calcium spontaneously after ATP- or acetyl phosphate-supported calcium uptake when internal Ca2+ was stabilized by the use of 50 mM phosphate as calcium-precipitating anion. This spontaneous calcium release could not be attributed to falling Ca2+ concentration outside the vesicles (Ca02+), substrate depletion, ADP accumulation, nonspecific membrane deterioration or the attainment of a high vesicular calcium content. Instead, spontaneous calcium release was directly proportional to Ca02+ at the time that calcium content was maximal. A causal relationship between high Ca02+ and spontaneous calcium release was suggested by the finding that elevation of Ca02+ from less than 1 μM to 3–5 μM increased the rate and extent of calcium release.The spontaneous calcium release was due both to acceleration of calcium efflux and slowing of calcium influx that was not accompanied by a significant change in the rate of ATP hydrolysis. Neither reversal of the transmembrane KCl gradient nor incubation with cation and proton ionophores abolished the spontaneous calcium release. The persistence of calcium release under conditions where the membrane was permeable to both anions and cations makes it unlikely that this phenomenon is due to a changing transmembrane potential. 相似文献
989.
The pH dependence of proton uptake upon binding of NADH to porcine heart mitochondrial malate dehydrogenase (l-malate: NAD+ oxidoreductase, EC 1.1.1.37) has been investigated. The enzyme has been shown to exhibit a pH-dependent uptake of protons upon binding NADH at pH values from 6.0 to 8.5. Enzyme in which one histidine residue has been modified per subunit by the reagent iodoacetamide (E. M. Gregory, M. S. Rohrbach, and J. H. Harrison, 1971, Biochim. Biophys. Acta253, 489–497) was used to establish that this specific histidine residue was responsible for the uptake of a proton upon binding of NADH to the native enzyme. It has also been established that while there is no enhancement of the nucleotide fluorescence upon addition of NADH to the iodoacetamide-modified enzyme, NADH is nevertheless binding to the modified enzyme with the same stoichiometry as with native enzyme. The data are discussed in relation to the involvement of the essential histidine residue in the catalytic mechanism of “histidine dehydrogenases” recently proposed by Lodola et al. (A. Lodola, D. M. Parker, R. Jeck, and J. J. Holbrook, 1978, Biochem. J.173, 597–605) and the catalytic mechanism of “malate dehydrogenases” recently proposed by L. H. Bernstein and J. Everse (1978, J. Biol. Chem.253, 8702–8707). 相似文献
990.
An adoptive transfer system is described to measure serum helper activity in the primary antibody response to sheep red blood cells (SRBC). Mice injected with a high dose of cyclophosphamide and reconstituted with rabbit anti-thymocyte serum-treated spleen cells were used as recipients. Serum obtained 9 hr after ip injection of normal mice with 2 × 108 SRBC (S(SRBC)) injected i.v. in the recipients caused a significant enhancement of the antibody response to 2 × 107 SRBC. The serum helper activity was not generated in thymectomized animals and could be absorbed from S(SRBC) by normal and formalinized SRBC. The SRBC-specific serum helper activity (SSHA) is heat labile (30 min 56 °C) and shows allogeneic restriction. Another test system described in literature for measuring T-cell help in vivo was less suited to measure SSHA in the response to 2 × 107 SRBC. A system using normal mice injected with 105 SRBC for determining specific immune response-enhancing factor (SIREF), demonstrated SIREF activity in S(SRBC). It did, however, not measure SSHA, as absorption of S(SRBC) with formalinized SRBC did not abolish the activity in that system. 相似文献