全文获取类型
收费全文 | 19493篇 |
免费 | 1897篇 |
国内免费 | 8篇 |
专业分类
21398篇 |
出版年
2024年 | 27篇 |
2023年 | 118篇 |
2022年 | 273篇 |
2021年 | 630篇 |
2020年 | 347篇 |
2019年 | 450篇 |
2018年 | 510篇 |
2017年 | 451篇 |
2016年 | 712篇 |
2015年 | 1196篇 |
2014年 | 1257篇 |
2013年 | 1322篇 |
2012年 | 1788篇 |
2011年 | 1802篇 |
2010年 | 1113篇 |
2009年 | 965篇 |
2008年 | 1244篇 |
2007年 | 1251篇 |
2006年 | 1089篇 |
2005年 | 988篇 |
2004年 | 935篇 |
2003年 | 749篇 |
2002年 | 667篇 |
2001年 | 146篇 |
2000年 | 99篇 |
1999年 | 124篇 |
1998年 | 122篇 |
1997年 | 86篇 |
1996年 | 80篇 |
1995年 | 65篇 |
1994年 | 52篇 |
1993年 | 58篇 |
1992年 | 50篇 |
1991年 | 33篇 |
1990年 | 33篇 |
1989年 | 43篇 |
1988年 | 35篇 |
1987年 | 29篇 |
1986年 | 29篇 |
1985年 | 50篇 |
1984年 | 34篇 |
1983年 | 33篇 |
1982年 | 26篇 |
1981年 | 24篇 |
1980年 | 20篇 |
1979年 | 22篇 |
1978年 | 23篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1960年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Methylation of (R,S)-DOPA with diazomethane gave the trimethyl derivative in which the phenolic hydroxy groups and the carboxy group were methylated. N-Methylated side products were also formed. N-Acylation of the racemic trimethyl derivative with (S)-α-methoxy-α-trifluoromethylphenylacetyl chloride gave two diastereomeric amides which were resolved by gas chromatography, the diastereomer derived from (S)-(−)-DOPA cluting first. The procedure was also applied to α-methyl-DOPA. 相似文献
82.
83.
The ability of glucocorticoids to modify lymphokine-induced macrophage proliferation, an in vitro correlate of cellular immunity in the guinea pig, was investigated. Lymphocyte production of macrophage mitogenic factor (MMF) was decreased in the presence of physiological concentrations of glucocorticoids. Inhibition was concentration dependent (IC50 of triamcinolone acetonide (TA): 2 × 10?9M), glucocorticoid specific, and reversed by cortexolone. In contrast, pharmacological concentrations of glucocorticoids were necessary to inhibit macrophage proliferation induced by suboptimal dilutions of MMF. This inhibition was concentration dependent (IC50 of TA: 4 × 10?7M), glucocorticoid specific, and reversed by cortexolone. At supraoptimal dilutions of MMF, glucocorticoids caused a twofold potentiation of MMF-induced macrophage proliferation. Potentiation was concentration dependent (EC50 of TA: 3 × 10?8M), glucocorticoid specific, reversed by glucocorticoid antagonists, and occurred in the presence of indomethacin. Thus, glucocorticoids regulate both the initiation and effector phases of this in vitro model of delayed hypersensitivity. However, the results indicate that the major mechanism of glucocorticoid-mediated anti-inflammatory action occurs at the level of the MMF-producing lymphocyte rather than at the effector macrophage, as MMF-induced proliferation is likely controlled by opposing glucocorticoid-sensitive mechanisms. 相似文献
84.
Paul W. Brandt-Rauf Gary Bomzer David Belford Matthew R. Pincus 《The protein journal》1991,10(4):437-441
Thebcr-abl chimeric gene of Philadelphia chromosome positive chronic myelogenous leukemias is only weakly transforming. This transformation activity is greatly enhanced by a Lys-for-Glu substitution at position 832 in the c-abl gene, as occurs in the highly transforming v-abl genes. It has been suggested that this mutation results in a significant structural change in the encoded protein product. Using conformational energy analysis, we have determined the allowed low-energy conformations for residues 828–836 of this protein with Lys and Glu at position 832. In both cases, the overwhelmingly preferred conformation for this region is a bend-helix motif. The helix terminates at residue 836, and there are no discernible differences in conformation between the Lys- and Glu-containing sequences. These results suggest that the activating amino acid substitution at position 832 in the c-abl protein product does not produce its effect via a local conformational change. 相似文献
85.
86.
Matthew R. Wright Saeed Sattari Dion R. Brocks Fakhreddin Jamali 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1992,583(2)
A rapid, inexpensive and sensitive high-performance liquid chromatographic method for the quantitation of ibuprofen enantiomers from a variety of biological fluids is reported. This method uses a commercially available internal standard and has significantly less interference from endogenous co-extracted solutes than do previously reported methods. The method involves the acid extraction of drug and internal standard [(±)-fenoprofen] from the biological fluid with isooctane—isopropanol (95:5) followed by evaporation and derivatization with enthylchloroformate and R-(+)-α-phenylethylamine. Excellent linearity was observed between the peak-area ratio and enantiomer concentration (r > 0.99) over a concentration range of 0.25–50 μg/ml. This method is suitable for the quantitation of ibuprofen from single-dose pharmacokinetic studies involving either rats or humans. 相似文献
87.
The modified Tanford-Kirkwood theory of Shire et al. [Shire, S. J., Hanania, G.I.H., & Gurd, F.R.N. (1974) Biochemistry 13, 2967] for electrostatic interactions was applied to the hydrogen ion equilibria of human deoxyhemoglobin and oxyhemoglobin. Atomic coordinates for oxyhemoglobin were generated by the application of the appropriate rigid rotation function to alpha and beta chains of the deoxyhemoglobin structure [Fermi, G. (1975) J. Mol. Biol. 97, 237]. The model employs two sets of parameters derived from the crystalline protein structures, the atomic coordinates of charged amino acid residues and static solvent accessibility factors to reflect their individual degrees of exposure to solvent. Theoretical titration curves based on a consistent set of pKint values compared closely with experimental potentiometric curves. Theoretical pK values at half-titration for individual protein sites corresponded to available observed values for both quaternary states. The results bring out the cumulative effects of numerous electrostatic interactions in the tetrameric structures and the major effects of the quaternary transition that result from changes in static solvent accessibility of certain ionizable groups. 相似文献
88.
Types of beta-lactamase determined by plasmids in gram-negative bacteria. 总被引:22,自引:0,他引:22 下载免费PDF全文
Two species of beta-lactamase determined by plasmids in enteric bacteria that show some resemblance to TEM enzymes are described. Both are distinct from all other plasmid-mediated beta-lactamases and differ from the TEM beta-lactamases in ability to hydrolyze some substrates, in isoelectric point, in immunological specificity, and in susceptibility to inhibition. One of the enzyme species, mediated by plasmid p453, has been briefly described previously. We have discovered that this beta-lactamase, designated SHV-1, is unique in its response to inhibition by the sulfhydryl group reagent p-chloromercuribenzoate, because the hydrolysis of cephaloridine but not that of benzylpenicillin is affected. This enzyme is found in a variety of plasmid types which were transferred from several bacterial species collected from a wide geographic range. The other enzyme species is novel; only a single plasmid determining this kind of beta-lactamase (designated HMS-1) has been detected. 相似文献
89.
A detailed model of reverse transcription and tests of crucial aspects 总被引:48,自引:0,他引:48
90.
J B Matthew S H Friend L H Botelho L D Lehman G I Hanania F R Gurd 《Biochemical and biophysical research communications》1978,81(2):416-421
The modified Tanford-Kirkwood theory of Shire et al. for intramolecular electrostatic interactions has been applied to hydrogen ion equilibria of sperm whale ferrimyoglobin, human hemoglobin α-chain and horse cytochrome c. The model employs two sets of parameters derived from the crystalline protein structures, first, the atomic coordinates of charged amino acid residues and, second, static accessibility factors to reflect their solvent exposure. In addition, a consistent set of intrinsic pK values (pKint) for the individual groups is employed. The theoretical pK values at half-titration for individual groups in each protein correspond to the available observed pK values, and the theoretical titration curves compare closely with experimental potentiometric curves. 相似文献