全文获取类型
收费全文 | 7525篇 |
免费 | 818篇 |
国内免费 | 609篇 |
专业分类
化学 | 5522篇 |
晶体学 | 29篇 |
力学 | 19篇 |
综合类 | 92篇 |
数学 | 97篇 |
物理学 | 683篇 |
综合类 | 2510篇 |
出版年
2024年 | 13篇 |
2023年 | 95篇 |
2022年 | 368篇 |
2021年 | 345篇 |
2020年 | 357篇 |
2019年 | 224篇 |
2018年 | 212篇 |
2017年 | 214篇 |
2016年 | 330篇 |
2015年 | 330篇 |
2014年 | 407篇 |
2013年 | 447篇 |
2012年 | 487篇 |
2011年 | 401篇 |
2010年 | 382篇 |
2009年 | 483篇 |
2008年 | 410篇 |
2007年 | 399篇 |
2006年 | 432篇 |
2005年 | 406篇 |
2004年 | 380篇 |
2003年 | 294篇 |
2002年 | 238篇 |
2001年 | 182篇 |
2000年 | 200篇 |
1999年 | 189篇 |
1998年 | 127篇 |
1997年 | 129篇 |
1996年 | 84篇 |
1995年 | 74篇 |
1994年 | 70篇 |
1993年 | 52篇 |
1992年 | 46篇 |
1991年 | 53篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 7篇 |
1986年 | 12篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有8952条查询结果,搜索用时 0 毫秒
111.
N. S. Papageorgiou 《Journal of Optimization Theory and Applications》1991,68(2):305-320
In this paper, we consider a sequence of abstract optimal control problems by allowing the cost integrand, the partial differential operator, and the control constraint set all to vary simultaneously. Using the notions of -convergence of functions,G-convergence of operators, and Kuratowski-Mosco convergence of sets, we show that the values of the approximating problems converge to that of the limit problem. Also we show that a convergent sequence of optimal pairs for the approximating problems has a limit which is optimal for the limit problem. A concrete example of parabolic optimal control problems is worked out in detail.This research was supported by NSF Grant No. DMS-88-02688. 相似文献
112.
El-B. Yallaoui 《Periodica Mathematica Hungarica》1991,23(2):109-120
Let X be an abstract set and L a lattice of subsets of X. To each lattice regular measure µ, we associate two induced measures
and
on suitable lattices of the Wallman space IR(L) and another measure µ on the spaceIR(L). We will investigate the reflection of smoothness properties of p onto
,
and µ; and try to set some new criterion for repleteness and measure repleteness. 相似文献
113.
《Arabian Journal of Chemistry》2022,15(12):104368
Protein hydrolysates have the potential to be natural and safer sources of bioactive peptides. In this study, two proteases were used to hydrolyze Chinese sturgeon (Acipenser sinensis) protein, and the hydrolysates were then purified to yield antioxidant peptides. The degree of hydrolysis of 23.56 % and 18.14 % was obtained using papain and alcalase 2.4L, respectivly, and hydrolysates had 96.80 % and 87.24 % total amino acid content, respectivly. The papain hydrolysate (PH) and alcalase 2.4L hydrolysate (AH) showed good antioxidant activity against DPPH? (IC50 of 3.64 and 3.15 mg/mL) and ABTS?+ (IC50 of 1.92 and 1.58 mg/mL), respectively. The low-molecular-weight (<1000 Da) fraction of both hydrolysates demonstrated the highest antiradical activity (IC50 of 2.59 and 2.31 mg/mL, DPPH) and (IC50 of 1.54 and 1.36 mg/mL, ABTS), respectively. Nine peptides were separated from both hydrolysates using reverse phase high performance liquid chromatography (RP-HPLC). The IC50 for ABTS?+ scavenging activity of peptide P5 with valine, glycine and asparagine (MW of 282.13 Da) from PH, and peptide P3 with histidine, glycine and alanine (MW of 302.74 Da) from AH was 0.89 and 0.72 mg/mL, respectively. The fractions and purified peptides obtained from Chinese sturgeon hydrolysates could be utilized as natural antioxidant substitutes in pharmaceuticals and food products. 相似文献
114.
115.
Noncovalent interactions play a pivotal role in regulating protein conformation, stability and dynamics. Among the quantum mechanical (QM) overlap-based noncovalent interactions, is the best understood with studies ranging from small molecules to -turns of model proteins such as GB1. However, these investigations do not explore the interplay between multiple overlap interactions in contributing to local structure and stability. In this work, we identify and characterize all noncovalent overlap interactions in the -turn, an important secondary structural element that facilitates the folding of a polypeptide chain. Invoking a QM framework of natural bond orbitals, we demonstrate the role of several additional interactions such as and that are energetically comparable to or larger than . We find that these interactions are sensitive to changes in the side chain of the residues in the -turn of GB1, suggesting that the may not be the only component in dictating -turn conformation and stability. Furthermore, a database search of and in the PDB reveals that they are prevalent in most proteins and have significant interaction energies (∼1 kcal/mol). This indicates that all overlap interactions must be taken into account to obtain a comprehensive picture of their contributions to protein structure and energetics. Lastly, based on the extent of QM overlaps and interaction energies, we propose geometric criteria using which these additional interactions can be efficiently tracked in broad database searches. 相似文献
116.
Chieu D. Tran Dora Ilieva Santhosh Challa 《Journal of Sol-Gel Science and Technology》2004,32(1-3):207-217
Concentration distributions and conformations of bovine serum albumin (BSA) entrapped in sol–gels were successfully determined for the first time by use of the recently developed NIR multispectral imaging instrument. It was found that BSA molecules were inhomogeneously distributed within the sol–gel matrix, independent of its concentration. At relatively high concentration (366 mg/mL) the encapsulation process does not seem to produce any observable changes in the conformation of BSA. However, when the concentration of BSA was decreased to 220 mg/mL, pronounced changes in the spectra of the protein were observed as a function of (sol–gel reaction) time. The observed inhomogenity might be traced to both a non-uniform concentration distribution of BSA and changes in protein native conformation resulting from interactions between the charged protein and the silicate anions. Results obtained seem to suggest that the protein molecules might adapt themselves to accommodate evolving geometry and sites of the silica network. 相似文献
117.
118.
SARS冠状病毒N蛋白的表达及二级结构预测分析 总被引:2,自引:1,他引:2
通过RT-PCR获得SARS冠状病毒N蛋白基因,分别克隆到原核表达载体pET21a,pET32a和pGEX-4T-1中,将3种重组质粒pET2la-N,pET32a-N和pGEX-4T-1-N分别转化大肠杆菌BL21(DE3),经IPTG诱导,细菌中分别表达出约46kD的重组N蛋白、约60kD的6xHis-N融合蛋白和约70kD的GST-N融合蛋白,表达量分别达总蛋白的45%、40%和30%.进一步的分析表明:6xHis-N融合蛋白在大肠杆菌中为可溶性表达,该可溶性组分占细菌裂解液的70%左右,且能被6xHis抗体所识别.用蛋白分析软件对N蛋白进行了序列分析和二级结构预测.SARS冠状病毒N蛋白在大肠杆菌中的高效可溶性表达,有助于进一步结晶后进行X射线晶体衍射分析其结构与功能. 相似文献
119.
一个在梅山猪×大白猪杂交组合的背最长肌中差异表达的新基因的分离、cDNA 序列分析及组织表达谱 总被引:1,自引:0,他引:1
为了揭示猪杂种优势的分子机理,用mRNA差异显示技术从梅山猪×大白猪杂交组合的背最长肌中分离到一个差异表达的基因,并用半定量RT-PCR进行鉴定,随后通过cDNA末端快速扩增法(RACE)得到该基因的cDNA全长.经与GenBank进行Blast比较,这个基因与猪的已知的基因没有明显的同源性.基因预测显示这个基因编码一个由188个氨基酸组成的蛋白质,且该蛋白质具有保守的PRA1 family protein的结构域,同时该蛋白质和人、大鼠、小鼠的PRA1 family protein 3分别具有88%,88%,87%的同源性,因此将这个基因命名为猪的PRA1 family protein 3基因.进化树分析表明猪的PRA1family protein 3和人的PRA1family protein 3具有比大鼠、小鼠的PRA1family protein 3更近的亲缘关系.组织表达谱分析显示这个基因在肌肉与脂肪组织中表达丰富,在脾中中等表达,在心、肾、卵巢、肺中少量表达,在肝与小肠中几乎不表达.同时对这个基因的功能及与猪的杂种优势的关系进行了讨论. 相似文献
120.
通过对人(Homo sapiens)的117个蛋白质和大肠杆菌(E.coli)的87个蛋白质的统计分析,发现mRNA序列中的同义密码子与氨基酸的上下文关联是和蛋白质二级结构有关的,并给出了各二级结构中有意义的上下文关联型.讨论了该结果对蛋白质结构预测的改进意义及其在基因工程领域可能的应用。 相似文献