全文获取类型
收费全文 | 407905篇 |
免费 | 37575篇 |
国内免费 | 942篇 |
专业分类
446422篇 |
出版年
2018年 | 14109篇 |
2017年 | 12833篇 |
2016年 | 10902篇 |
2015年 | 5626篇 |
2014年 | 6258篇 |
2013年 | 8910篇 |
2012年 | 13614篇 |
2011年 | 22048篇 |
2010年 | 18059篇 |
2009年 | 13813篇 |
2008年 | 17893篇 |
2007年 | 19782篇 |
2006年 | 8614篇 |
2005年 | 8560篇 |
2004年 | 9102篇 |
2003年 | 8874篇 |
2002年 | 8542篇 |
2001年 | 14524篇 |
2000年 | 14475篇 |
1999年 | 11496篇 |
1998年 | 4026篇 |
1997年 | 4151篇 |
1996年 | 3936篇 |
1995年 | 3800篇 |
1994年 | 3804篇 |
1993年 | 3718篇 |
1992年 | 9825篇 |
1991年 | 9423篇 |
1990年 | 9218篇 |
1989年 | 8946篇 |
1988年 | 8364篇 |
1987年 | 7998篇 |
1986年 | 7213篇 |
1985年 | 7320篇 |
1984年 | 6050篇 |
1983年 | 5319篇 |
1982年 | 4177篇 |
1981年 | 3692篇 |
1980年 | 3512篇 |
1979年 | 6094篇 |
1978年 | 4548篇 |
1977年 | 4272篇 |
1976年 | 3968篇 |
1975年 | 4300篇 |
1974年 | 4651篇 |
1973年 | 4716篇 |
1972年 | 4590篇 |
1971年 | 4324篇 |
1970年 | 3440篇 |
1969年 | 3334篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
801.
802.
803.
804.
Purification and partial characterization of pyruvate decarboxylase from Oryza sativa L 总被引:2,自引:0,他引:2
Pyruvate decarboxylase(PyrDC) was purified from rice bran to a specific activity of 1 mu kat/mg and partially characterized. The holoenzyme is a tetramer of two types of subunits with molecular masses 64 kDa and 62 kDa. Purified rice PyrDC exhibits positive cooperative kinetics with respect to pyruvate and functions with a significant lag phase. When compared to other plant PyrDC, the lag phase was shorter at low pyruvate concentrations and the S0.5 was smaller. The optimum pH (6.25) was also less acidic and the enzyme retained 30% of its maximal activity at neutral pH. In contrast to other plant PyrDC, rice PyrDC could be active at the onset of anoxia and would be activated by small changes in pyruvate concentration. 相似文献
805.
B. Maestra T. Naranjo 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1999,98(5):744-750
Chromosome pairing at metaphase-I was analyzed in F1 hybrids among T. turgidum (AABB), T. aestivum (AABBDD), and T. timopheevii (AtAtGG) to study the chromosome structure of T. timopheevii relative to durum (T. turgidum) and bread (T. aestivum) wheats. Individual chromosomes and their arms were identified by means of C-banding. Homologous pairing between the A-genome
chromosomes was similar in the three hybrid types AAtBG, AAtBGD, and AABBD. However, associations of B-G were less frequent than B-B. Homoeologous associations were also observed, especially
in the AAtBGD hybrids. T. timopheevii chromosomes 1At, 2At, 5At, 7At, 2G, 3G, 5G, and 6G do not differ structurally from their counterpart in the A and B genomes. Thus, these three polyploid
species inherited translocation 5AL/4AL from the diploid A-genome donor. Chromosome rearrangements that occurred at the tetraploid
level were different in T. turgidum and T. timopheevii. Translocation 4AL/7BS and a pericentric inversion of chromosome 4A originated only in the T. turgidum lineage. The two lines of T. timophevii studied carry four different translocations, 6AtS/1GS, 1GS/4GS, 4GS/4AtL, and 4AtL/3AtL, which most likely arose in that sequence. These structural differences support a diphyletic origin of polyploid wheats.
Received: 15 June 1998 / Accepted: 19 August 1998 相似文献
806.
B Müller D Besser P Kleinw?chter O Arad S Reissmann 《The journal of peptide research》1999,54(5):383-393
To improve the assembly of backbone cyclic peptides, N-functionalized dipeptide building units were synthesized. The corresponding N-aminoalkyl or N-carboxyalkyl amino acids were formed by alkylation or reductive alkylation of amino acid benzyl or tert-butyl esters. In the case of N-aminoalkyl amino acid derivatives the aldehydes for reductive alkylation were obtained from N,O-dimethyl hydroxamates of N-protected amino acids by reduction with LiAlH4. N-carboxymethyl amino acids were synthesized by alkylation using bromoacetic acid ester and the N-carboxyethyl amino acids via reductive alkylation using aldehydes derived from formyl Meldrums acid. Removal of the carboxy protecting group leads to free N-alkyl amino acids of very low solubility in organic solvents, allowing efficient purification by extraction of the crude product. These N-alkyl amino acids were converted to their tetramethylsilane-esters by silylation with N,O-bis-(trimethylsilyl)acetamide and could thus be used for the coupling with Fmoc-protected amino acid chlorides or fluorides. To avoid racemization the tert-butyl esters of N-alkyl amino acids were coupled with the Fmoc-amino acid halides in the presence of the weak base collidine. Both the N-aminoalkyl and N-carboxyalkyl functionalized dipeptide building units could be obtained in good yield and purity. For peptide assembly on the solid support, the allyl type protection of the branching moiety turned out to be most suitable. The Fmoc-protected N-functionalized dipeptide units can be used like any amino acid derivative under the standard conditions for Fmoc-solid phase synthesis. 相似文献
807.
808.
809.
Khakass families can be considered as a fundamental ethnic determinant. Based on the family subdivision the analysis of intensity and genetic effectiveness of marriage migrations has revealed the speed of formation of a new ethnic-territorial community--Khakass nationality--on the territory of the Minusinsk hollow. 相似文献
810.
The cause of the pregnancy condition preeclampsia (PE) is thought to be endothelial dysfunction caused by oxidative stress. As abnormal glucose tolerance has also been associated with PE, we use a fluorinated-mimic of this metabolite to establish whether any oxidative damage to lipids and proteins in the erythrocyte membrane has increased cell membrane permeability. Data were acquired using 19F Dynamic-NMR (DNMR) to measure exchange of 3-fluoro-3-deoxyglucose (3-FDG) across the membrane of erythrocytes from 10 pregnant women (5 healthy control women, and 5 from women suffering from PE). Magnetisation transfer was measured using the 1D selective inversion and 2D EXSY pulse sequences, over a range of time delays. Integrated intensities from these experiments were used in matrix diagonalisation to estimate the values of the rate constants of exchange and membrane permeability. No significant differences were observed for the rate of exchange of 3-FDG and membrane permeability between healthy pregnant women and those suffering from PE, leading us to conclude that no oxidative damage had occurred at this carrier-protein site in the membrane. 相似文献