全文获取类型
收费全文 | 244837篇 |
免费 | 22480篇 |
国内免费 | 259篇 |
专业分类
267576篇 |
出版年
2018年 | 2635篇 |
2017年 | 2578篇 |
2016年 | 3426篇 |
2015年 | 3659篇 |
2014年 | 4576篇 |
2013年 | 6550篇 |
2012年 | 7204篇 |
2011年 | 7900篇 |
2010年 | 5381篇 |
2009年 | 4787篇 |
2008年 | 6868篇 |
2007年 | 7081篇 |
2006年 | 6712篇 |
2005年 | 6411篇 |
2004年 | 6315篇 |
2003年 | 6151篇 |
2002年 | 6009篇 |
2001年 | 12005篇 |
2000年 | 11962篇 |
1999年 | 9143篇 |
1998年 | 2669篇 |
1997年 | 2743篇 |
1996年 | 2692篇 |
1995年 | 2477篇 |
1994年 | 2424篇 |
1993年 | 2316篇 |
1992年 | 7174篇 |
1991年 | 6978篇 |
1990年 | 7056篇 |
1989年 | 6841篇 |
1988年 | 6355篇 |
1987年 | 6009篇 |
1986年 | 5353篇 |
1985年 | 5661篇 |
1984年 | 4462篇 |
1983年 | 3861篇 |
1982年 | 2669篇 |
1981年 | 2493篇 |
1980年 | 2303篇 |
1979年 | 4107篇 |
1978年 | 3134篇 |
1977年 | 2881篇 |
1976年 | 2826篇 |
1975年 | 3268篇 |
1974年 | 3496篇 |
1973年 | 3524篇 |
1972年 | 3058篇 |
1971年 | 2842篇 |
1970年 | 2530篇 |
1969年 | 2299篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
111.
112.
Phase-sensitive two-dimensional nuclear Overhauser effect spectra of [d(GGTATACC)]2 in aqueous deuterium oxide solution at four mixing times were quantified to give all nonoverlapping cross-peak intensities. A structural model for [d(GGTATACC)]2 was built in which the GG- and -CC moieties were in the B-DNA form, while the middle -TATA- moiety was in the wrinkled-D form (BDB model). This model was subjected to energy refinement by molecular mechanics calculations with the program AMBER. Counterions (Na+) were added to neutralize the charges, and water molecules were placed bridging across the minor groove. A complete relaxation matrix analysis was used to calculate two-dimensional nuclear Overhauser effect spectra of [d(GGTATACC)]2 from the above models (before and after energy refinement) and from four other [d(GGTATACC)]2 structural models: regular A, crystalline A, regular B, and energy-minimized B. Among them, the energy-minimized BDB model yielded a set of theoretical spectra that gave the best fit to the experimental spectra. It was also the energetically most stable. Therefore, it is a good representation of the ensemble- and time-averaged structure of the octamer in solution. This model has backbone torsion angles similar to those of B-form DNA in the GG- and -CC moieties and torsion angles similar to those of wrinkled D form DNA in the -TATA- moiety. The base stacking and base pairing are not interrupted at the junctions between the two structural moieties. Its minor groove is narrower than that of B DNA, and the solvent-accessible surface of the minor groove forms a closed hydration tunnel in the middle -TATA- segment. 相似文献
113.
N A Markovich T I Vorotyntseva M I Zil'berman V K Antonov 《Biokhimii?a (Moscow, Russia)》1985,50(5):795-803
Cathepsin D was purified from the lactating rabbit mammary gland by a rapid procedure, which included fractionation with (NH4)2SO4, acid precipitation, double affinity chromatography on pepstatin-Sepharose 4B and gel filtration on Sephadex G-100, resulting in approximately 360-fold purification of the enzyme over the homogenate and approximately 16% recovery. After isoelectric focusing, the enzyme dissociated into four (pI 5.8, 6.3, 6.5 and 7.2) multiple forms, but appeared homogeneous on polyacrylamide gel electrophoresis. Cathepsin D has a Mr of 45 kDa as determined by Sephadex G-100 column chromatography. On sodium dodecylsulfate/polyacrylamide gel electrophoresis the enzyme gave a single protein band, corresponding to Mr of 45 kDa. The amino acid composition of the enzyme is similar to that of cathepsins D from other tissues. A single N-terminal amino acid was glycine. Cathepsin D contains 6.4% carbohydrates consisting of mannose, galactose, fucose and glucosamine at a ratio of 3:9:2:2. Cathepsin D is inhibited by pepstatin with Ki of 2.5 X 10(-9) M and irreversibly by N-diazoacetyl-N'-2.4-dinitrophenyl-ethylene diamine. The enzyme hydrolyzes bovine hemoglobin with the maximal activity at pH 3.0 with Km = 10(-5) M and HLeu-Ser-Phe(NO2)-Nle-Ala-Leu-OMe with Km = 4 X 10(-5) M and Rcat = 0.95 s-1. The major cleavage sites were Leu15-Tyr16, Phe24-Phe25 and Phe25-Tyr26 during hydrolysis of the oxidized insulin B-chain by cathepsin D. 相似文献
114.
V E Kagan E A Serbinova A A Minin V M Savov K N Novikov 《Biokhimii?a (Moscow, Russia)》1985,50(6):986-991
The localization and mechanism of generation of active oxygen species in the enzymatic NADPH-dependent lipid peroxidation system in liver microsomes were studied. Using the spin-trapping method, the key role of active oxygen species in the initiation of NADPH-dependent enzymatic lipid peroxidation was confirmed. It was shown that active oxygen species are generated via consecutive one-electron reduction of the oxygen molecule by NADPH-cytochrome P-450 reductase. 相似文献
115.
Summary A major chromosomal abnormality was observed in 10.3% of subfertile men in this study. This result is similar to a previous survey using the same criteria for selection of probands. The high frequency of chromosomal abnormalities emphasizes the importance of cytogenetic examination in subfertile men. The detection of such an abnormality should be followed by chromosome analysis in the patient's family. Prenatal diagnosis is indicated if a subfertile man with an abnormal karyotype fathers a child. 相似文献
116.
The effects of nucleoside triphosphates (ATP and GTP) on phosphodiesterase (PDE) of brain and outer segments of the retina enriched or devoid of protein modulators were studied. In the case of retinal outer segment PDE the enzyme activity was considerably inhibited by both nucleosides only when the enzyme was separated from the inhibitor. In case of brain PDE, on the contrary, the effect of the nucleosides was much more pronounced in the enzyme preparation coupled with the protein activator, calmodulin. The latter when added to brain PDE devoid of the activator in the presence of ATP and GTP considerably reduced the enzyme activity. An addition of the inhibitor simultaneously with GTP to the purified PDE of outer segments increased the PDE activity. The constants for the inhibition of brain PDE coupled with calmodulin and retinal outer segment PDE separated from the inhibitor by ATP and GTP were determined. 相似文献
117.
N A Voskanian E K Dzhikidze M A Pochkhura 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1986,(3):62-65
The work presents the results of investigations, carried out in different monkey species, on the physiological norms as regards the bactericidal factors of neutrophilic polymorphonuclear leukocytes in the blood, nonenzymatic lysosomal cationic proteins and myeloperoxidase, as well as on changes in these characteristics in monkeys at different periods of their acclimatization at the Institute of Experimental Pathology and Therapy, Sukhumi. The possibility of correcting the characteristics under study by means of the immunostimulating agent levamisole is shown. 相似文献
118.
The activity of cAMP-dependent protein kinases, cAMP binding and the spectrum of cAMP-binding proteins in renal papillary cytosol of intact rats and of rats kept on a water-deprived diet for 24 hours were investigated. It was found that the stimulation of protein kinases by 10(-6) M cAMP in the experimental group was significantly higher than in the control one. On DEAE-cellulose chromatography, the position of peaks of the specific cAMP binding corresponded to those of the regulatory cAMP-dependent protein kinases type I and II. Under these conditions, more than 80% of the binding activity in intact animals was localized in peak II, whereas in rats kept on a water-deprived diet over 60% of the binding activity was localized in peak I. The total binding activity of cytosol in experimental animals remained unchanged is compared to intact rats. It is suggested that in renal papilla dehydration is accompanied by the induction of synthesis of regulatory subunits of cAMP-dependent protein kinase type I. 相似文献
119.
120.
D G Astaf'ev I V Maianskaia A K Akatov A N Maianski? 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1986,(7):48-51
In investigations based on the use of a highly sensitive test system permitting the detection of normal human antibodies to S. aureus peptidoglycan, the antigenic relationships between the peptidoglycans of S. aureus and other representatives of the genus Staphylococcus have been studied. Among other staphylococcal species, S. simulans, S. xylosus, S. hyicus, S. cohnii, S. hyicus s. s. chromogenes have been found to possess peptidoglycans most closely related to S. aureus peptidoglycans, while S. warneri and S. epidermidis peptidoglycans have proved to be least closely related to it. 相似文献