全文获取类型
收费全文 | 2739篇 |
免费 | 264篇 |
国内免费 | 9篇 |
专业分类
3012篇 |
出版年
2023年 | 11篇 |
2022年 | 33篇 |
2021年 | 53篇 |
2020年 | 38篇 |
2019年 | 52篇 |
2018年 | 57篇 |
2017年 | 42篇 |
2016年 | 72篇 |
2015年 | 146篇 |
2014年 | 175篇 |
2013年 | 204篇 |
2012年 | 229篇 |
2011年 | 223篇 |
2010年 | 138篇 |
2009年 | 139篇 |
2008年 | 152篇 |
2007年 | 144篇 |
2006年 | 145篇 |
2005年 | 121篇 |
2004年 | 121篇 |
2003年 | 78篇 |
2002年 | 93篇 |
2001年 | 63篇 |
2000年 | 59篇 |
1999年 | 52篇 |
1998年 | 36篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 29篇 |
1991年 | 23篇 |
1990年 | 17篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 16篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 13篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1975年 | 9篇 |
1974年 | 11篇 |
1972年 | 6篇 |
排序方式: 共有3012条查询结果,搜索用时 0 毫秒
11.
Serge Fermandjian Constantin Sakarellos Franlois Piriou Michel Juy Flavio Toma Hung Lam Thanh Karl Lintner Mahesh C. Khosla Robert R. Smeby F. Merlin Bumpus 《Biopolymers》1983,22(1):227-231
The conformation–biological activity relationships in a series of angiotensin II analogs substituted in position 5 were studied. Results indicated that only analogs with β-branched residue in position 5 possess spectral and biological properties identical to that of parent angiotensin II. 相似文献
12.
Determination of nanogram quantities of osmium-labeled nucleic acids by stripping (inverse) voltammetry 总被引:3,自引:0,他引:3
Modification of nucleic acids with OSO4 in the presence of pyridine results in a formation of a covalently bound electroactive center in a polynucleotide chain detectable by polarographic (voltammetric) methods. It has been shown that DNA modified with osmium (DNA-Os) accumulates at the hanging mercury-drop electrode during a waiting time in a wide range of potentials between 0 and -1.0 V (against the saturated calomel electrode) and produce at neutral pH a well-developed reduction peak at about -1.2 V due to scanning in the cathodic direction. Using the differential-pulse stripping (inverse) voltammetry, nanogram quantities of single-stranded DNA-Os can be determined at relatively short waiting times (1-3 min). Double-stranded DNA is modified with osmium to a much lesser extent as compared to single-stranded polynucleotides. The degree of modification of double-helical DNA is influenced by the presence of single-stranded and distorted double-stranded regions in the DNA molecules and by the environmental conditions which influence the DNA conformation. Osmium can thus be used as a probe of the DNA structure, and a few micrograms of double-helical DNA sample suffice for the voltammetric analysis. 相似文献
13.
Hung Cao Danh Margherita Strolin Benedetti Philippe Dostert 《Journal of neurochemistry》1983,41(3):618-622
Abstract: Aldehyde dehydrogenase (ALDH) activity was measured in brains, livers, and hearts of 23–26-month-old and 3-month-old rats. A significant increase of ALDH activity was found in whole brain of old rats with both acetaldehyde (39%) and propionylaldehyde (15%) used as substrates. In different brain areas of old rats, with acetaldehyde used as substrate, a significant increase of ALDH activity was found in striatum (30–50%) and cerebral cortex (37%). However, no significant difference in ALDH activity was found in livers and hearts of young and old rats. Preliminary experiments showed a significant increase of aldehyde reductase activity (52%) with p -nitrobenzaldehyde used as substrate in whole brain of old rats compared with young rats. The present work indicates that an increase of ALDH activity in brain of old rats may be an adaptive phenomenon. 相似文献
14.
15.
16.
Michele I. Van Dyke Hung Lee Ph.D. Jack T. Trevors Ph.D. 《Journal of industrial microbiology & biotechnology》1989,4(4):299-306
Summary The toxicity of germanium dioxide (GeO2) to 21 bacterial and 13 yeast strains was investigated in liquid broth medium to obtain information on strains tolerant to high (1 to 2 mg/ml) GeO2 concentrations.Arthrobacter sp. NRC 32005,enterobacter aerogenes NRC 2926,Klebsiella aerogenes NCTC 418 andPseudomonas putida NRC 5019 were tolerant to 1 mg/ml GeO2.Bacillus sp. RC607 was able to grow in the presence of 2 mg/ml GeO2 at pH 10 in broth culture. The yeastsCandida guilliermondii, Candida shehatae andPachysolen tannophilus were the most sensitive to GeO2 as evidenced by their diminished growth rates at a GeO2 concentration as low as 0.1 mg/ml. None of the yeast strains tested exhibited growth in the presence of 1 mg/ml GeO2. The high pH of the medium containing germanium may be partially responsible for the growth inhibition of the yeast cultures. Select bacterial cultures previously exposed to 1 mg/ml GeO2 could tolerate and grow better at 2 mg/ml GeO2, suggesting the existence of very efficient adaptive mechanisms. The pH of the medium could modulate GeO2 tolerance and this effect was found to be strain-dependent. 相似文献
17.
18.
Experimental determination of wall shear rate in canine carotid arteries perfused in vitro 总被引:1,自引:0,他引:1
The mathematical model of Hung (Tsai and Hung, 1984) is employed to determine the wall shear rate acting on canine carotid arteries perfused in vitro. Model equations for pulsatile flow in a deformable vessel are coupled with experimental data of dynamic pressure drop, flow rate, vessel radius and radial wall motion. Derived quantities, e.g. velocity profiles and wall shear, are obtained for vessels exposed to 'normotensive' hemodynamics, 'hypertension' simulations and perfusions in which the compliance of the vessel wall is deliberately altered. Our results indicate that wall shear varies markedly as a function of the hemodynamic environment. The effects of vessel radius vs flow rate on the development of wall shear are also demonstrated. It is found that convective processes correlate with the magnitude of wall shear in the 'hypertension' simulations. The present findings and complementary published data may explain, at least in part, the variations in vessel wall transport and endothelial cell biology we observe as a function of the hemodynamic environment. For example we have documented that the exposure of canine carotids to 'hypertensive' (vs 'normotensive') hemodynamics is associated with an increased flux of lipoproteins (LDL) into the intima and luminal media. Alternations in wall compliance, on the other hand, profoundly influence endothelial shape, orientation and cytoskeletal array. 相似文献
19.
Plasma samples from 106 pregnant Karan Swiss (Brown Swiss x Sahiwal) cows and 108 Murrah buffalo were tested for thyroxine (T(4)) levels to determine the relationship between the hormonal changes and advancing pregnancy in the two species. All samples were collected within 2 months (January and February) to avoid seasonal interference on T(4) levels. In pregnant cows, the concentration of T(4) increased sharply during the first trimester, reaching a peak at the third month of gestation followed by a gradual decline until the last month of pregnancy. In pregnant buffalo, peripheral plasma T(4) levels fluctuated slightly throughout pregnancy without exhibiting a specific trend. Statistical analysis revealed that the magnitude of T(4) levels was significantly lower in buffalo (P < 0.01) than in the cows throughout pregnancy and that the hormonal patterns of the two species were significantly different (P < 0.05) during gestation. It was hypothesized from this study that T(4) requirements for the fetal buffalo calf may be lower than that for the fetal cattle calf since the buffalo gestation period is a month longer and the metabolic rate lower vis-a-vis the cow. 相似文献
20.
Magnetically controlled targeted micro-carrier systems 总被引:11,自引:0,他引:11
Magnetically controlled targeted drug delivery systems are aimed at concentrating drugs at a defined target site, with the aid of a magnetic field. This technique has been developed specifically for directing drugs away from the reticuloendothelial system (RES). Literature on this topic suggests that these delivery systems are capable of altering the distribution of chemotherapeutic agents in the body. Hence these delivery devices offer the possibility of improving the therapeutic efficacy of the associated drugs. This paper reviews the work done to date towards the development and evaluation of biodegradable and non-biodegradable magnetic targeted drug delivery systems and outlines their future prospects and limitations in cancer chemotherapy. 相似文献