全文获取类型
收费全文 | 2347篇 |
免费 | 117篇 |
国内免费 | 5篇 |
出版年
2024年 | 5篇 |
2023年 | 29篇 |
2022年 | 35篇 |
2021年 | 108篇 |
2020年 | 67篇 |
2019年 | 179篇 |
2018年 | 119篇 |
2017年 | 86篇 |
2016年 | 84篇 |
2015年 | 123篇 |
2014年 | 128篇 |
2013年 | 186篇 |
2012年 | 177篇 |
2011年 | 159篇 |
2010年 | 93篇 |
2009年 | 82篇 |
2008年 | 98篇 |
2007年 | 73篇 |
2006年 | 107篇 |
2005年 | 83篇 |
2004年 | 70篇 |
2003年 | 57篇 |
2002年 | 41篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 20篇 |
1998年 | 8篇 |
1997年 | 14篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 17篇 |
1984年 | 6篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 8篇 |
排序方式: 共有2469条查询结果,搜索用时 437 毫秒
31.
Mycelium of Fusarium oxysporum grown on a glucose-containing medium lacked fructosyl transferase and invertase activities. Synthesis of fructosyl t 相似文献
32.
Ketan Shah Iqbal Mehdi A. W. Khan V. C. Vora 《Applied microbiology and biotechnology》1980,10(1-2):167-169
Summary
Arthrobacter
simplex, Serratia
marcescens, Fusarium and Mycobacterium were tested for their ability to transform phytosterol to Androsta 1, 4 diene 3, 17 dione (ADD). Arthrobacter
simplex ATCC 6946 was found to be more efficient than the other species tested. 相似文献
33.
This report describes a simple method to measure the activity of dihydrofolate reductase using the substrate [3H]dihydrofolate, which is generated by preincubation of [3H]folic acid for 10 min with dithionite before the enzymatic reaction. The procedure then measures the direct reduction of [3H]dihydrofolate to [3H]tetrahydrofolate by coprecipitating the unreduced substrate with excess unlabeled folic acid and acidified zinc sulfate. The advantage of this method is that [3H]dihydrofolate, which is not commercially available, can be generated from high specific activity [3H]folic acid, which is commercially available, immediately before initiating the enzymatic reaction. By this modification, the two important advantages of radioenzymatic assays for dihydrofolate reductase can be more easily exploited; namely, increased sensitivity because much less substrate need be used, and the ability to measure enzyme activity in crude tissue preparations without interference by precipitating proteins or nucleotide oxidases. 相似文献
34.
Raman scattering and infrared spectroscopic techniques were used to study the vibrational spectrum and conformation of the membrane channel protein gramicidin A in the solid state, in organic solutions and, using Raman scattering only, in a phospholipid environment. The investigation also includes measurements on head- and tail-group-modifled gramicidin A and a potassium thiocyanate-gramicidin A complex. Tentative identification of the molecular vibrations is proposed on the basis of the data on model compounds. The existence of four distinct conformations of the gramicidin A chain is established: conformation I present in the solid state, and CH3OH and CD3OD solutions; conformation II present in films cast from CHCl3 solution; conformation III present in (CH3)2SO and (CD3)2SO solutions at concentrations below 0.5 m gramicidin A; and conformation IV present in the potassium thiocyanate-gramicidin A complex. The data obtainable on a gramicidin A-phospholipid suspension indicate a gramicidin A conformation in this environment corresponding either to the conformation I or II. The details of the spectra in the amide I region are shown to be consistent with a β-parallel hydrogen-bonded πLD helix for conformational I, in terms of the polypeptide vibrational calculations of Nevskaya and co-workers. Conformation II is found to be consistent with an antiparallel double-stranded πLD helix, while conformations III and IV probably have π-helical structures with larger channel diameters. The data on head- and tail-modified gramicidin A molecules indicate that their conformations are only slightly different from that of gramicidin A in conformation I. 相似文献
35.
Brian Greenway M J Iqbal P J Johnson Roger Williams 《BMJ (Clinical research ed.)》1983,286(6359):93-95
The detection of high activities of two sex-steroid biosynthetic enzymes—aromatase and 5-alpha-reductase—in pancreatic carcinoma tissue suggested the possibility that these enzymes may influence the circulating concentrations of sex-steroid hormones. Mean serum testosterone concentrations in 22 men and women with exocrine carcinoma of the pancreas were significantly lower than those in 20 healthy controls and 27 patients with other known malignancies, who were of similar age and sex composition. Low serum testosterone concentrations may be due to uptake by and metabolism within the tumour; alternatively, patients with low androgen concentrations may be at greater risk of developing pancreatic carcinoma. 相似文献
36.
Peptide NH chemical shifts and their temperature dependences have been monitored as a function of concentration for the decapeptide, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aib-OMe in CDCl3 (0.001–0.06M) and (CD3)2SO (0.001–0.03M). The chemical shifts and temperature coefficients for all nine NH groups show no significant concentration dependence in (CD3)2SO. Seven NH groups yield low values of temperature coefficients over the entire range, while one yields an intermediate value. In CDCl3, the Aib(1) NH group shows a large concentration dependence of both chemical shift and temperature coefficient, in contrast to the other eight NH groups. The data suggest that in (CD3)2SO, the peptide adopts a 310 helical conformation and is monomeric over the entire concentration range. In CDCl3, the 310 helical peptide associates at a concentration of 0.01M, with the Aib(1) NH involved in an intermolecular hydrogen bond. Association does not disrupt the intramolecular hydrogen-bonding pattern in the decapeptide. 相似文献
37.
Mazhar N. Malik Laurie A. Meyers Khalid Iqbal Ashfaq M. Sheikh Lois Scotto Henryk M. Wisniewski 《Life sciences》1981,29(8):795-802
A Ca2+ activated protease(s) capable of hydrolyzing several polypeptides at neutral pH including cytoskeletal proteins, actin, myosin, tubulin and neurofilament triplet was identified in calf brain cortex. The enzyme activity precipitates at 75 mM KCl, pH 6.5 – 7.0 and is inhibited by the sulfhydryl inhibitors, N-ethylmaleimide and para-chloromercuribenzoate and the protease inhibitors, antipain, pepstatin and leupeptin, leupeptin being the most effective. 相似文献
38.
Sensitivity of glutamine synthetase assay by the hydroxamate method can be doubled by replacing ferric chloride with ferric nitrate or ferric perchlorate, each with the appropriate acid. 相似文献
39.
Calmodulin, a calcium-dependent regulatory protein has been isolated from mammalian nerve. The protein has similarities to the calcium-binding protein earlier shown to be transported at a fast rate in the nerve fibers. The implication is that calmodulin, which has been shown to be involved in various key cellular processes, may have a relation to axoplasmic transport. 相似文献
40.
Chemical pathology of neurofibrils. Neurofibrillary tangles of Alzheimer's presenile-senile dementia. 总被引:3,自引:0,他引:3
K Iqbal H M Wisniewski I Grundke-Iqbal J K Korthals R D Terry 《The journal of histochemistry and cytochemistry》1975,23(7):563-569
A subcellular fraction enriched in twisted tubules was obtained by differential centrifugation of a homogenate of neurons isolated from areas of the brain with many neurofibrillary tangles from patients with Alzheimer's presenile-senile dementia. A unique protein (molecular weight 50,000 daltons) which does not co-migrate with either of the two tubulin monomers of the major neurofilament protein, both purified from human brain, was found in this subcellular fraction on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Similarly processed tissue from areas of the brain poor in neurofibrillary tangles contained low levels of this new protein. The new protein band could not be seen in control patients. 相似文献