全文获取类型
收费全文 | 6448篇 |
免费 | 815篇 |
国内免费 | 511篇 |
专业分类
化学 | 6482篇 |
晶体学 | 36篇 |
力学 | 59篇 |
综合类 | 175篇 |
数学 | 115篇 |
物理学 | 907篇 |
出版年
2024年 | 13篇 |
2023年 | 108篇 |
2022年 | 486篇 |
2021年 | 440篇 |
2020年 | 393篇 |
2019年 | 252篇 |
2018年 | 210篇 |
2017年 | 215篇 |
2016年 | 334篇 |
2015年 | 329篇 |
2014年 | 355篇 |
2013年 | 466篇 |
2012年 | 442篇 |
2011年 | 367篇 |
2010年 | 343篇 |
2009年 | 394篇 |
2008年 | 339篇 |
2007年 | 318篇 |
2006年 | 301篇 |
2005年 | 296篇 |
2004年 | 256篇 |
2003年 | 199篇 |
2002年 | 134篇 |
2001年 | 100篇 |
2000年 | 108篇 |
1999年 | 115篇 |
1998年 | 86篇 |
1997年 | 76篇 |
1996年 | 42篇 |
1995年 | 43篇 |
1994年 | 46篇 |
1993年 | 32篇 |
1992年 | 26篇 |
1991年 | 20篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 5篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1959年 | 3篇 |
排序方式: 共有7774条查询结果,搜索用时 15 毫秒
121.
The nanometer scale is a special place where all sciences meet and develop a particularly strong interdisciplinarity. While biology is a source of inspiration for nanoscientists, chemistry has a central role in turning inspirations and methods from biological systems to nanotechnological use. DNA is the biological molecule by which nanoscience and nanotechnology is mostly fascinated. Nature uses DNA not only as a repository of the genetic information, but also as a controller of the expression of the genes it contains. Thus, there are codes embedded in the DNA sequence that serve to control recognition processes on the atomic scale, such as the base pairing, and others that control processes taking place on the nanoscale. From the chemical point of view, DNA is the supramolecular building block with the highest informational content. Nanoscience has therefore the opportunity of using DNA molecules to increase the level of complexity and efficiency in self-assembling and self-directing processes. 相似文献
122.
Avian eggshell matrix proteins were studied by two analytical approaches. Peptide mapping was done by trypsin and pepsin followed by collagenase cleavage; analyses were carried out by capillary electrophoresis and reversed-phase high-performance liquid chromatography (HPLC). Comparison of peptide maps obtained by both methods revealed a complex mixture of peptides in the insoluble layers of the eggshell; it was concluded that there are at least three different insoluble protein/peptide layers in the avian eggshell (cuticle, palisade, and mammillary layer). Partial characterization of peptides in each layer was made by HPLC-mass spectrometry analysis. There is an evidence that the eggshell insoluble proteins contain species susceptible to collagenase cleavage, however, the sequences split by this enzyme probably are not those typical for the main triple-helical core of collagenous proteins. It is proposed that the action of collagenase upon eggshell proteins is caused by the side effect of collagenase described previously with synthetic peptides. Some of the proteins present are probably glycosylated. Fatty acid content in the insoluble eggshell layers (after decalcification) was in the range of 2-4% (which reflected both lipid and lipoproteins bound fatty acids). Porphyrin pigments are dominant in the cuticle layer. 相似文献
123.
The synthesis of an asymmetric glycocluster 1 has been achieved by coupling of a sugar unit with the β-alanine polypeptide, the principal chain, and combining a carbohydrate chain with the side chain causing it to branch from the N terminal. The synthesis of this side chain multivalent ligands is based on the scaffolding of some ω-amino acid (glycine, β-alanine, and GABA) derivatives. This method facilitated the synthesis of the cluster, of which the length of each unit differs. 相似文献
124.
Nguyen Huyen Rao A. Madhusudhan Phillips J. B. John Vijay T. Reed Wayne F. 《Applied biochemistry and biotechnology》1991,(1):843-853
We describe a technique to modify protein solubility and optimize enzyme activity in reversed micellar solutions. The technique
is based on the ability of hydrates of natural gas to form in the micro-aqueous phase. Clathrate hydrates are crystalline
inclusions of water and gas, and their formation in bulk water has traditionally been studied with relevance to natural gas
recovery. We have found that hydrates can form in the environment of the microaqueous pools of reversed micelles, and that
their extent of formation can be well controlled through the thermodynamic variables of temperature and pressure. Additionally,
formation of hydrates affects the size and aggregation number of the micelles, and thus influences the solubility and conformation
of encapsulated proteins. We demonstrate how the concept can be used in two applications: (i) protein extraction into reversed
micelles and subsequent recovery, and (ii) optimization of enzyme activity in reversed micelles. 相似文献
125.
Biochemistry is a branch subject of chemistry, which is the study of chemical structures and processes associated with living organisms. COVID-19 pandemic is a problem for human beings. From the perspective of biochemistry, this paper demonstrates the chemical structure and interactions of SARS-CoV-2's spike protein and its receptor (human angiotensin converting enzyme 2), and summarizes the related research progresses. The authors hope to provide insights for the development of COVID-19 vaccine. 相似文献
126.
Plant cells have been immobilized in alginate, where they have been shown to retain their biological activity. Such systems
can be utilized for bioconversions. 相似文献
127.
Titmuss SJ Cummins PL Rendell AP Bliznyuk AA Gready JE 《Journal of computational chemistry》2002,23(14):1314-1322
QM/MM methods have been developed as a computationally feasible solution to QM simulation of chemical processes, such as enzyme-catalyzed reactions, within a more approximate MM representation of the condensed-phase environment. However, there has been no independent method for checking the quality of this representation, especially for highly nonisotropic protein environments such as those surrounding enzyme active sites. Hence, the validity of QM/MM methods is largely untested. Here we use the possibility of performing all-QM calculations at the semiempirical PM3 level with a linear-scaling method (MOZYME) to assess the performance of a QM/MM method (PM3/AMBER94 force field). Using two model pathways for the hydride-ion transfer reaction of the enzyme dihydrofolate reductase studied previously (Titmuss et al., Chem Phys Lett 2000, 320, 169-176), we have analyzed the reaction energy contributions (QM, QM/MM, and MM) from the QM/MM results and compared them with analogous-region components calculated via an energy partitioning scheme implemented into MOZYME. This analysis further divided the MOZYME components into Coulomb, resonance and exchange energy terms. For the model in which the MM coordinates are kept fixed during the reaction, we find that the MOZYME and QM/MM total energy profiles agree very well, but that there are significant differences in the energy components. Most significantly there is a large change (approximately 16 kcal/mol) in the MOZYME MM component due to polarization of the MM region surrounding the active site, and which arises mostly from MM atoms close to (<10 A) the active-site QM region, which is not modelled explicitly by our QM/MM method. However, for the model where the MM coordinates are allowed to vary during the reaction, we find large differences in the MOZYME and QM/MM total energy profiles, with a discrepancy of 52 kcal/mol between the relative reaction (product-reactant) energies. This is largely due to a difference in the MM energies of 58 kcal/mol, of which we can attribute approximately 40 kcal/mol to geometry effects in the MM region and the remainder, as before, to MM region polarization. Contrary to the fixed-geometry model, there is no correlation of the MM energy changes with distance from the QM region, nor are they contributed by only a few residues. Overall, the results suggest that merely extending the size of the QM region in the QM/MM calculation is not a universal solution to the MOZYME- and QM/MM-method differences. They also suggest that attaching physical significance to MOZYME Coulomb, resonance and exchange components is problematic. Although we conclude that it would be possible to reparameterize the QM/MM force field to reproduce MOZYME energies, a better way to account for both the effects of the protein environment and known deficiencies in semiempirical methods would be to parameterize the force field based on data from DFT or ab initio QM linear-scaling calculations. Such a force field could be used efficiently in MD simulations to calculate free energies. 相似文献
128.
Bhaskar M Gnanamani A Ganeshjeevan RJ Chandrasekar R Sadulla S Radhakrishnan G 《Journal of chromatography. A》2003,1018(1):117-123
Extracellular fluid protein (ECFP) of Streptomyces species SS07 has been used to reduce water soluble azo dyes and the carcinogenic amines released have been compared with that from chemical reduction. The effect of temperature, pH and contact time on the recovery of amines using ECFP was studied. The ECFP releases carcinogenic amines at a pH of 9.2 and a temperature of 37 degrees C for a contact period of 24 h. The reduction products were analyzed with HPLC and their structures confirmed by LC-MS and GC-MS. It was observed that both the ECFP and chemical reduction methods released similar type of amine products. In the case of dye samples, compared to chemical reduction, 5-20% increase in the release of carcinogenic amines by ECFP was observed. The percentage of amine products released by chemical reduction was higher for leather garment samples compared to ECFP treatment. 相似文献
129.
130.
肿瘤的生长依赖于血管的生成,新生血管不仅为肿瘤生长提供必需的营养物质,而且为肿瘤细胞扩散提供了重要的途径。1997年哈佛大学的O'Reilly等发现了一种内源性新血管生成抑制因子内皮抑素(Endoscatin),显示出特异抑制激活的血管内皮细胞增殖和肿瘤新血管生成的生物学活性,其抗肿瘤作用具有高效、低毒、无耐药性的优点。目前,内皮抑素的研究引起了国内外广泛的兴趣,在美国已进行以安全性为目的的I期临床实验,国内也有多家公司对内皮抑素进行了抗肿瘤研究并申报一类新药。内皮抑素有望成为医治肿瘤而又没有化疗和放疗的毒副作用的一种新的治疗方法,但是否能作为药物应用于临床,尚需对内皮抑素的结构特点及抑制肿瘤和内皮细胞的作用机制等方面进行许多深入的研究。 相似文献