全文获取类型
收费全文 | 662943篇 |
免费 | 74748篇 |
国内免费 | 517篇 |
出版年
2016年 | 6945篇 |
2015年 | 9921篇 |
2014年 | 11583篇 |
2013年 | 16557篇 |
2012年 | 18565篇 |
2011年 | 18638篇 |
2010年 | 12530篇 |
2009年 | 11575篇 |
2008年 | 16512篇 |
2007年 | 17382篇 |
2006年 | 16333篇 |
2005年 | 15754篇 |
2004年 | 15410篇 |
2003年 | 14771篇 |
2002年 | 14822篇 |
2001年 | 31136篇 |
2000年 | 31777篇 |
1999年 | 24990篇 |
1998年 | 8279篇 |
1997年 | 8696篇 |
1996年 | 8180篇 |
1995年 | 7811篇 |
1994年 | 7670篇 |
1993年 | 7550篇 |
1992年 | 20670篇 |
1991年 | 20143篇 |
1990年 | 19401篇 |
1989年 | 18780篇 |
1988年 | 17662篇 |
1987年 | 16795篇 |
1986年 | 15722篇 |
1985年 | 15763篇 |
1984年 | 12961篇 |
1983年 | 11142篇 |
1982年 | 8709篇 |
1981年 | 7926篇 |
1980年 | 7279篇 |
1979年 | 12607篇 |
1978年 | 9856篇 |
1977年 | 9003篇 |
1976年 | 8416篇 |
1975年 | 9543篇 |
1974年 | 10307篇 |
1973年 | 10202篇 |
1972年 | 9459篇 |
1971年 | 8347篇 |
1970年 | 7385篇 |
1969年 | 7137篇 |
1968年 | 6479篇 |
1967年 | 5574篇 |
排序方式: 共有10000条查询结果,搜索用时 531 毫秒
991.
992.
R Mendelsohn J W Brauner L Faines H H Mantsch R A Dluhy 《Biochimica et biophysica acta》1984,774(2):237-246
Glycophorin has been isolated in pure form from human erythrocyte membranes and reconstituted into lipid vesicles composed of binary mixtures of bovine brain phosphatidylserine (PS) and acyl-chain perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62). The effect of protein on lipid melting behavior and order has been monitored with differential scanning calorimetry and Fourier transform infrared spectroscopy (FT-IR). The phase diagram for PS/DPPC-d62 is consistent with that previously reported for PS/DPPC (Stewart et al. (1979) Biochim. Biophys. Acta 556, 1-16) and indicates that acyl chain perdeuteration does not greatly alter the lipid mixing characteristics. The use of deuterated lipid allows the examination of lipid order by FT-IR of each lipid component in the binary mixtures as well as in the ternary (lipid/lipid/protein) systems. Addition of glycophorin to a 30:70 PS/DPPC-d62 binary lipid mixture results in a preferential glycophorin/PS interaction leading to bulk lipid enriched in DPPC-d62. This is revealed in two ways: first, through cooperative calorimetric transitions increased in temperature from the binary lipid system and second, through FT-IR melting curves of the DPPC-d62 component which shows transitions increased in both onset and completion temperatures in the presence of protein. In addition, non-cooperative melting events are observed at temperatures below the onset of phase separation. The FT-IR data are used to assign these non-cooperative events to the melting of the PS component. For the 50:50 lipid mixture with protein, two transitions are observed in the DSC experiments. The IR results indicate that both lipid components are involved with the lower temperature event. 相似文献
993.
Comparison of physicochemical properties of purified mucus glycoproteins isolated from respiratory secretions of cystic fibrosis and asthmatic patients 总被引:10,自引:0,他引:10
The major nonreduced mucus glycoproteins (mucins) from sputa of cystic fibrosis (CF) and asthmatic patients have been purified to electrophoretic homogeneity and subjected to physical and chemical characterization. The sputum specimens were solubilized in buffer containing 0.22 M KSCN and fractionated on Bio-Gel A-5m, followed by digestion with DNase, rechromatography on the same column, and chromatography on hydroxylapatite. Sodium dodecyl sulfate gel electrophoresis of purified mucins gave a single band. Carbohydrate analyses of the purified mucins showed no significant differences in the sugar components from the two mucins. However, the CF mucin contained substantially higher (11%) sulfate content than that observed for the asthmatic mucin (5.9%). Amino acid analyses indicated that the CF mucin had higher levels of serine plus threonine (35%) as compared to the asthmatic mucin (29%). In contrast, CF mucin contained a lower content of aspartate, glutamate, and glycine than that observed for the asthmatic mucin. Molecular weights of 3.8 X 10(6) and 3.5 X 10(6) were obtained for CF and asthmatic mucins, respectively, from light-scattering studies of mucins in the presence of 6 M guanidine hydrochloride. Reduction of the disulfide bonds of the two mucins did not alter their molecular weights. Liquid chromatographic studies on Sepharose CL2B showed that CF mucin forms aggregates sufficiently large to be excluded from the gel. As compared to the CF mucin, the asthmatic mucin formed fewer of these large aggregates under identical experimental conditions. Reduction and alkylation of the mucins resulted in their inability to form aggregates. The higher state of aggregation of CF mucin may influence the viscoelastic properties of the CF lung's mucus secretions. 相似文献
994.
H. -D. Pfannenstiel 《Helgoland Marine Research》1985,39(1):49-53
The dye Lucifer yellow CH was iontophoresed into recently fertilized eggs and early blastomeres ofHydractinia echinata. Iontophoresis was carried out on the stage of an inverted microscope in order to follow filling of the injected cells by short pulses of epifluorescent illumination. Lucifer yellow proved to be nontoxic and development in embryos with injected blastomeres proceeded normally. When zygotes were injected all the cells of the forming embryo contained dye. When one of the first two blastomeres was injected all the progeny of the injected cell also contained dye. Dye-coupling between injected and uninjected blastomeres did not occur in two-cell embryos nor between descendants of either line. Development of Lucifer-yellow-filled blastomeres or zygotes could be stopped by blue light irradiation. In a number of injected cells, the dye tended to accumulate forming brightly shining spots. The dye did not penetrate the nuclear envelope of injected cells. 相似文献
995.
Mary H. Moran 《American anthropologist》1998,100(1):235-236
Wives and Warriors: Women and the Military in the United States and Canada. Laurie Weinstein and Christie C. White. eds. Westport, CT: Bergin and Garvey, 1997. 252 pp. 相似文献
996.
997.
998.
999.
Fluorometric determination of histamine with OPT: optimum reaction conditions and tests of identity 总被引:8,自引:0,他引:8
Histamine reacts with OPT at an alkaline pH giving fluorescent conjugation products. Optimum fluorophore formation was observed at pH 12.5 after 40 min reaction at 0°C under continuous gassing with nitrogen. After acidification with sulfuric acid to pH 2–5 the fluorescence was stable for hours. Reagent blanks were reduced by the lowering of the reaction temperature and by decreasing the amount of OPT added. These modifications permitted the determination of 2 ng histamine/ml and gave far better reproducibility than the original procedure of Shore, Burkhalter, and Cohn. The fluorometric assay for histamine is nonspecific; the major interfering OPT-reactive tissue component is believed to be spermidine. Specificity was secured by adding formaldehyde before acidification, thus abolishing the fluorescence of histamine but not that of spermidine, or by adding CdCl2 or SrCl2 together with OPT, thus preventing the formation of the spermidine fluorophore but not that of the histamine fluorophore. 相似文献
1000.
A number of drugs cause marked increases in the steroid hydroxylase activity of hepatic microsomes. Beginning 2 days after estrus, 117 mature ewes were each given 14 injections over a 27-day period of phenobarbital sodium, diphenylhydantoin, chlorcyclizine HCl or phenylbutazone. Blood samples for luteinizing hormone (LH) and progesterone determination by radioimmunoassay (RIA) were taken on day 10 of the first estrous cycle (day 18 if no heat was observed) and on days 5 and 10 of the second cycle. On day 10 of the second cycle, the ewes were given an intravenous injection of 1 ml of 6% solution of pentobarbitol sodium anesthetic per 4.5 kg body weight, and the length of anesthetic sleep time was measured. The ewes were then killed and corpora lutea and liver were weighed.In 33 ewes treated with either phenobarbitol sodium or phenylbutazone, sleep time was shortened (18 min 29 min in untreated controls, P<.01), indicating that enzyme induction had occurred. For 41 ewes treated with either chlorcyclizine HCl or diphenylhydantoin, sleep time was lengthened to 93 min (P<.01 controls), indicating impaired liver function. Electron micrographs of liver cells verified that enzyme induction or hepatic degeneration had occurred. 相似文献