全文获取类型
收费全文 | 135篇 |
免费 | 6篇 |
专业分类
化学 | 100篇 |
数学 | 4篇 |
物理学 | 37篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 2篇 |
2018年 | 3篇 |
2017年 | 1篇 |
2016年 | 6篇 |
2015年 | 3篇 |
2014年 | 2篇 |
2013年 | 3篇 |
2012年 | 12篇 |
2011年 | 13篇 |
2010年 | 5篇 |
2009年 | 2篇 |
2008年 | 4篇 |
2007年 | 7篇 |
2006年 | 6篇 |
2005年 | 9篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2001年 | 1篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有141条查询结果,搜索用时 15 毫秒
101.
Reactions proceeding through open-shell, single-electron pathways offer attractive alternative outcomes to those proceeding through closed-shell, two-electron mechanisms. In this context, samarium diiodide (SmI(2) ) has emerged as one of the most important and convenient-to-use electron-transfer reagents available in the laboratory. Recently, significant progress has been made in the reductive chemistry of other divalent lanthanides which for many years had been considered too reactive to be of value to synthetic chemists. Herein, we illustrate how new samarium(II) complexes and nonclassical lanthanide(II) reagents are changing the landscape of modern reductive chemistry. 相似文献
102.
We report the in vitro selection of an RNA-based ATP aptamer with the ability to discriminate between adenosine ligands based on their 5' phosphorylation state. Previous selection of ATP aptamers yielded molecules that do not significantly discriminate between ligands at the 5' position. By applying a selective pressure that demands recognition of the 5' triphosphate, we obtained an aptamer that binds to ATP with a Kd of approximately 5 muM, and to AMP with a Kd of approximately 5.5 mM, a difference of 1100-fold. This aptamer demonstrates the ability of small RNAs to interact with negatively charged moieties. 相似文献
103.
104.
105.
The role of divalent cations in the mechanism of pyrophosphate-activated, template-directed oligoribonucleotide ligation has been investigated. The dependence of the reaction rate on Mg2+ concentration suggests a kinetic scheme in which a Mg2+ ion must bind before ligation can proceed. Mn2+, Ca2+, Sr2+, and Ba2+ can also catalyze the reaction. Although Pb2+ and Zn2+ do not catalyze the reaction in the absence of other divalent ions, they significantly modulate the reaction rate when added in the presence of Mg2+, with Pb2+ stimulating the reaction (up to 65-fold) and Zn2+ inhibiting the reaction. The logarithm of the ligation rate increases linearly, with slope of 0.95, as a function of pH, indicating that the reaction involves a single critical deprotonation step. The ligation rates observed with the different divalent metal ion catalysts (Mn2+ > Mg2+ > Ca2+ > Sr2+ = Ba2+) vary inversely with the pKa values of their bound water molecules. The pH profile and these relative ligation rates suggest a mechanism in which a metal-bound hydroxide ion located near the ligation junction promotes catalysis, most likely by deprotonation of the hydroxl nucleophile. The effects of changing either the leaving group or the attacking hydroxyl, together with the large delta S(++) value for oligonucleotide ligation (about -20 eu), are consistent with an associative transition state. 相似文献
106.
Karpińska J Szostak J 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(5):975-981
Two methods for spectrophotometric determination of chlorprothixene and amitryptyline hydrochlorides were proposed. One of them is based on spectral analysis of their derivative spectra. The measurement of the value at 316.0 nm of first derivative was used for construction of calibration graph for chlorprothixene. The Beer law was obeyed in the concentration range 0.5-50.0 microg ml(-1). The amplitude of the second derivative at 261.4 nm was used for determination of amitryptyline in the range 0.5-75.0 microg ml(-1). The second proposed method is utilized the use of solid sorbent for simultaneous preconcentration and assay of studied compounds. For this purpose the filtration gel Sephadex G100 was applied. The elaborated solid-phase spectrophotometric method was used for determination of chlorprothixene at 268.0 nm in the range 2.5-75.0 microg ml(-1) and amitryptyline at 238.0 nm in the concentration range 10.0-75.0 microg ml(-1). 相似文献
107.
RNA aptamers that bind flavin and nicotinamide redox cofactors 总被引:1,自引:0,他引:1
RNA molecules that specifically bind riboflavin (Rb) and beta-nicotinamide mononucleotide (NMN) have been isolated by in vitro selection. A simple structural motif containing intramolecular G-quartets was found to bind tightly to oxidized riboflavin (Kd = 1-5 micromolar). DNA versions of the consensus sequence also bind, but with weaker affinity. DMS protection experiments show that the quartet structure of these aptamers is stabilized by interaction with the flavin. As a measure of their redox specificity, the aptamers do not show significant differential binding between oxidized and reduced forms of a 5-deazariboflavin derivative that is a close structural analog of riboflavin. In contrast to the lack of redox specificity of the riboflavin aptamers, RNAs selected for binding to the nicotinamide portion of NAD discriminate between NAD and NADH in solution by over an order of magnitude. A mutagenized pool based on one of the NMN aptamer sequences was used to reselect for NMN binding. Comparison of the reselected sequences led to the identification of the binding region of the aptamer. A complex secondary structure containing two interacting stem-loops is proposed for the minimal NMN-binding RNA. The same mutagenized pool was used to select for increased discrimination between NMN and NMNH. From these reselected sequences, a mutation within the binding region was identified that increases specificity for NMN. These experiments show that RNA can bind these cofactors with low micromolar affinity and, in the case of nicotinamide cofactors, can discriminate between the two redox states. These cofactor binding motifs may provide a framework for generating new ribozymes that catalyze redox reactions similar to those found in basic metabolic pathways. 相似文献
108.
Chen IA Salehi-Ashtiani K Szostak JW 《Journal of the American Chemical Society》2005,127(38):13213-13219
We are engaged in a long-term effort to synthesize chemical systems capable of Darwinian evolution, based on the encapsulation of self-replicating nucleic acids in self-replicating membrane vesicles. Here, we address the issue of the compatibility of these two replicating systems. Fatty acids form vesicles that are able to grow and divide, but vesicles composed solely of fatty acids are incompatible with the folding and activity of most ribozymes, because low concentrations of divalent cations (e.g., Mg(2+)) cause fatty acids to precipitate. Furthermore, vesicles that grow and divide must be permeable to the cations and substrates required for internal metabolism. We used a mixture of myristoleic acid and its glycerol monoester to construct vesicles that were Mg(2+)-tolerant and found that Mg(2+) cations can permeate the membrane and equilibrate within a few minutes. In vesicles encapsulating a hammerhead ribozyme, the addition of external Mg(2+) led to the activation and self-cleavage of the ribozyme molecules. Vesicles composed of these amphiphiles grew spontaneously through osmotically driven competition between vesicles, and further modification of the membrane composition allowed growth following mixed micelle addition. Our results show that membranes made from simple amphiphiles can form vesicles that are stable enough to retain encapsulated RNAs in the presence of divalent cations, yet dynamic enough to grow spontaneously and allow the passage of Mg(2+) and mononucleotides without specific macromolecular transporters. This combination of stability and dynamics is critical for building model protocells in the laboratory and may have been important for early cellular evolution. 相似文献
109.
110.
Guangrong Meng Lazaros Kakalis Steven P. Nolan Michal Szostak 《Tetrahedron letters》2019,60(4):378-381
The σ-donor properties of NHC ligands (NHC?=?N-heterocyclic carbene) are crucial in controlling their interaction with transition metals, and as a consequence, to determine the selectivity and reactivity of NHCs in transition-metal-catalysis. Herein, we report a simple NMR method for estimating the σ-donor properties of NHC ligands based on a straightforward 1H NMR measurement of ligand precursors. We present evaluation of σ-donating properties for a range of NHC ligands varied by structure and electronics that are relevant to transition-metal-catalysis. We expect that the simple measurement of σ-donating properties of NHCs, together with the known methods for evaluating sterics and π-backbonding, will enhance the understanding of the properties of NHCs in transition-metal-catalysis. 相似文献