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531.
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533.
Sung In Jeong Melissa D. Krebs Christopher A. Bonino Saad A. Khan Eben Alsberg 《Macromolecular bioscience》2010,10(8):934-943
Alginate, a natural polysaccharide that has shown great potential as a cell scaffold for the regeneration of many tissues, has only been nominally explored as an electrospun biomaterial due to cytotoxic chemicals that have typically been used during nanofiber formation and crosslinking. Alginate cannot be electrospun by itself and is often co‐spun with poly(ethylene oxide) (PEO). In this work, a cell adhesive peptide (GRGDSP) modified alginate (RA) and unmodified alginate (UA) were blended with PEO at different concentrations and blending ratios, and then electrospun to prepare uniform nanofibers. The ability of electrospun RA scaffolds to support human dermal fibroblast cell attachment, spreading, and subsequent proliferation was greatly enhanced on the adhesion ligand‐modified nanofibers, demonstrating the promise of this electrospun polysaccharide material with defined nanoscale architecture and cell adhesive properties for tissue regeneration applications.
534.
Norbert Strter William N. Lipscomb Thomas Klabunde Bernt Krebs 《Angewandte Chemie (International ed. in English)》1996,35(18):2024-2055
Numerous studies, both in enzymatic and nonenzymatic catalysis, have been undertaken to understand the way by which metal ions, especially zinc ions, promote the hydrolysis of phosphate ester and amide bonds. Hydrolases containing one metal ion in the active site, termed mononuclear metallohydrolases, such as carboxypeptidase. A and thermolysin were among the first enzymes to have their structures unraveled by X-ray crystallography. In recent years an increasing number of metalloenzymes have been identified that use two or more adjacent metal ions in the catalysis of phosphoryl-transfer reactions (R-OPO3 + R′-OH → R′-OPO3 + R-OH; in the case of the phosphatase reaction R′-OH is a water molecule) and carbonyl-transfer reactions, for example, in peptidases or other amidases. These dinuclear metalloenzymes catalyze a great variety of these reactions, including hydrolytic cleavage of phosphomono-, -di- and -triester bonds, phosphoanhydride bonds as well as of peptide bonds or urea. In addition, the formation of the phosphodiester bond of RNA and DNA by polymerases is catalyzed by a two-metal ion mechanism. A remarkable diversity is also seen in the structures of the active sites of these di- and trinuclear metalloenzymes, even for enzymes that catalyze very similar reactions. The determination of the structure of a substrate, product, stable intermediate, or a reaction coordinate analogue compound bound to an active or inactivated enzyme is a powerful approach to investigate mechanistic details of enzyme action. Such studies have been applied to several of the metalloenzymes reviewed in this article; together with many other biochemical studies they provide a growing body of information on how the two (or more) metal ions cooperate to achieve efficient catalysis. 相似文献
535.
The crystal and molecular structure of hexaphenylditin selenide (C6H5)3SnSeSn(G6H5)3 was determined by X-ray diffraction data and was refined to R 0.055. The compound is monoclinic, space group P21, with a 9.950(4), b 18.650(7), c 18.066(6) Å, β 106.81(4)°, Z 4. The two molecules in the asymmetric unit differ slightly in their conformations, both having approximate C2 symmetry. Bond lengths and angles are: SnSe 2.526 (2.521(3) ? 2.538(3)) Å; SnC 2.138 (2.107(16)?2.168(19)) Å; SnSeSn 103.4(1)°, 105.2(1)°. There are only slight angular distortions at the SnSeC3 tetrahedra (SeSnC angles: 104.3(5)?114.8(4)°). The bond data indicate essentially single bonds around the Sn atoms. 相似文献
536.
Sascha Holzhauer Friedrich Krebs Andreas Ernst 《Computational & Mathematical Organization Theory》2013,19(2):128-150
Social networks have become an important part of agent-based models, and their structure may have remarkable impact on simulation results. We propose a simple and efficient but empirically based approach for spatial agent-based models which explicitly takes into account restrictions and opportunities imposed by effects of baseline homophily, i.e. the influence of local socio-demography on the composition of one’s social network. Furthermore, the algorithm considers the probability of links that depends on geographical distance between potential partners. The resulting network reflects social settings and furthermore allows the modeller to influence network properties by adjusting agent type specific parameters. Especially the parameter for distance dependence and the probability of distant links allow for control of clustering and agent type distribution of personal networks. 相似文献
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538.
Dongmei Zhou Yongnan Zheng K. Matsuta M. Mihara M. Fukuda D. Nishimura J. Komurasaki D. Ishikawa R. Matsumiya T. Nagatomo T. Izumikawa S. Takahashi H. Hirano T. Ohtsubo S. Momota Y. Nojiri A. Kitagawa M. Kanazawa M. Torikoshi S. Sato T. Minamisono J. R. Alonso G. F. Krebs T. J. M. Symons Daqing Yuan Yi Zuo Ping Fan T. Suzuki Xizhen Zhang Shengyun Zhu 《Hyperfine Interactions》2007,180(1-3):37-42
The magnetic moment of 28P (I π = 3?+?, T1/2 = 270.3 ms) in the ground state has been measured by the $\upbeta $ -nuclear magnetic resonance method for the first time. The measured magnetic moment of $\vert \upmu (^{28}$ P)∣ = 0.309(9) $\upmu _{{\rm N}}$ is well reproduced by the shell model value of +0.306 $\upmu _{{\rm N} }$ . The shell model calculation also yields a proton density distribution with a long tail. The present results provide a strong confirmation of the configuration of the 2s 1/2 proton which should lead to the proton halo. 相似文献
539.
Ernst-Peter Krebs 《Helvetica chimica acta》1981,64(4):1023-1025
Deprotonation of ethyl (E)-2-alkenoates 1 , 3 and 4 yields after protonation the double bond migrated (3 Z)-isomers 5 , 7 and 9 as major products. In contrast, deprotonation and reprotonation of ethyl (Z)-2-pentenoate ( 2 ) gives the (3 E)-isomer 6 exclusively. These findings are explained by reaction paths starting from different ester conformations. 相似文献
540.
Maas G Gettwert V Krebs F Schmidtberg G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(9):1646-1655
Thermally induced intra-/intermolecular [3 + 2] cycloaddition reaction sequences of alpha-(alkynyloxy)silyl-alpha-diazoacetates 1 lead to [3.3](1,4)pyrazolophanes (2)2 and higher cyclooligomers thereof [(2)n, n > 2]. In most cases, the cyclodimer was isolated by crystallization, while a complete separation of the mixture of the higher cyclooligomers was not possible. Solid state structures of cyclodimers (2b)2 and (2c)2, cyclotrimer (2b)3, and cyclotetramer (2e)4 were determined by X-ray diffraction analysis. Field-desorption mass spectra were used to characterize the cyclooligomer mixtures. The relative amounts of the cyclooligomers depend on the substitution pattern of the diazo compound. The cyclooligomerization reactions reported herein demonstrate, for the first time, the involvement of diazo functions in macrocyclization reactions via 1,3-dipolar cycloaddition. 相似文献