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Dielectric Medium (dielectric + medium)
Selected AbstractsA 3-D dielectrophoretic filter chipELECTROPHORESIS, Issue 7 2007Ciprian Iliescu Dr. Abstract The paper presents a 3-D filter chip employing both mechanical and dielectrophoretic (DEP) filtration, and its corresponding microfabrication techniques. The device structure is similar to a classical capacitor: two planar electrodes, made from a stainless steel mesh, and bonded on both sides of a glass frame filled with round silica beads. The solution with the suspension of particles flows through both the mesh-electrodes and silica beads filter. The top stainless steel mesh (with openings of 60,,m and wires of 30,,m-thickness) provides the first stage of filtration based on mechanical trapping. A second level of filtration is based on DEP by using the nonuniformities of the electric field generated in the capacitor due to the nonuniformities of the dielectric medium. The filter can work also with DC and AC electric fields. The device was tested with yeast cells (Saccharomyces cerevisae) and achieved a maximal trapping efficiency of 75% at an applied AC voltage of 200,V and a flow rate of 0.1,mL/min, from an initial concentration of cells of 5×105 cells/mL. When the applied frequency was varieted in the range between 20 and 200,kHz, a minimal value of capture efficiency (3%) was notticed at 50,kHz, when yeast cells exhibit negative DEP and the cells are repelled in the space between the beads. [source] Quantum chemical modeling of enzymatic reactions: The case of histone lysine methyltransferaseJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2010Polina Georgieva Abstract Quantum chemical cluster models of enzyme active sites are today an important and powerful tool in the study of various aspects of enzymatic reactivity. This methodology has been applied to a wide spectrum of reactions and many important mechanistic problems have been solved. Herein, we report a systematic study of the reaction mechanism of the histone lysine methyltransferase (HKMT) SET7/9 enzyme, which catalyzes the methylation of the N-terminal histone tail of the chromatin structure. In this study, HKMT SET7/9 serves as a representative case to examine the modeling approach for the important class of methyl transfer enzymes. Active site models of different sizes are used to evaluate the methodology. In particular, the dependence of the calculated energies on the model size, the influence of the dielectric medium, and the particular choice of the dielectric constant are discussed. In addition, we examine the validity of some technical aspects, such as geometry optimization in solvent or with a large basis set, and the use of different density functional methods. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Investigation of penetratin peptides Part 1.JOURNAL OF PEPTIDE SCIENCE, Issue 4 2002The environment dependent conformational properties of penetratin, two of its derivatives Abstract The homeodomain, the DNA-binding domain of Antennapedia homeoprotein, is composed of three ,-helices and one ,-turn between helices II and III. Its third helix from the N -terminal (helix III) can translocate through the cell membrane into the nucleus and can be used as an intracellular vehicle for the delivery of oligopeptides and oligonucleotides. To the best of our knowledge, this helix III, called penetratin, which consists of 16 amino acids, is internalized by cells in a specific, non-receptor-mediated manner. For a better understanding of the mechanism of the transfer, the structure of penetratin was examined in both extracellular matrix-mimetic and membrane-mimetic environments; 1H-NMR and CD spectroscopic measurements were performed in mixtures of TFE/water with different ratios. The molecular conformations of two analogue peptides [(6,14-Phe)-penetratin and a 12 amino acid penetratin derivative (peptide 3)] were also studied. An atomic level comprehensive analysis of penetratin and its two analogues was performed. In a membrane-mimetic solvent system (TFEd2/water = 9 : 1), on the basis of 553 distance restraints, the 4,12 region of penetratin exhibits a bent, irregular helical structure on NMR examination. Interactions between hydrophobic amino acid residues in conjunction with H-bonds stabilize the secondary structure of the molecule. Thus, both derivatives adopt a helix-like conformation. However, while (6,14-Phe)-penetratin displays both ,-helical and 310 -helical features, the structure of peptide 3 is predominantly a 310 -helix. Of the three peptides, surprisingly (6,14-Phe)-penetratin has the largest helical content. An increase in the polarity of the molecular environment gradually disintegrates these helix-like secondary structures. In a highly aqueous molecular system (TFEd2/water = 1 : 9), the fast exchange of multiple conformers leads to too few distance restraints being extracted, therefore the NMR structures can no longer be determined. The NMR data show that only short-range order can be traced in these peptides. Under these conditions, the molecules adopt nascent helix-like structures. On the other hand, CD spectra could be recorded at any TFE/water ratio and the conformational interconversion could therefore be monitored as a function of the polarity of the molecular environment. The CD data were analysed comprehensively by the quantitative deconvolution method (CCA+). All three penetratin peptides display helical conformational features in a low dielectric medium, with significant differences as a function of their amino acid composition. However, these conformational features are gradually lost during the shift from an apolar to a polar molecular environment. Copyright © 2002 European Peptide Society and John Wiley & Sons, Ltd. [source] Gain enhancement in embedded microstrip antennasMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 2 2008M. Fallah-Rad Abstract A microstrip antenna embedded inside a dielectric medium representing the building material is investigated for its gain. It is shown that coupling the embedded antenna to a parasitic patch, placed outside the embedding material, can increase the antenna gain significantly. In addition, by properly selecting the antenna depth inside the embedded material its radiation characteristics can be made less sensitive to the material parameters of the surrounding. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 269,271, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23070 [source] Surface phonons in CdS1,xSex nanoparticles embedded in a dielectric mediumPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2009Yu. M. Azhniuk Abstract Raman scattering by surface phonons in CdS1,xSex nanocrystals grown in a borosilicate glass matrix, is studied. CdSe-like and CdS-like surface phonon frequencies are shown to increase with the relevant component content in the nanocrystal composition. No clear size dependence of the surface phonon frequencies has been observed. On the other hand, surface phonons in CdS1,xSex nanocrystals exhibit resonant dependence on the Raman excitation wavelength. It is emphasized that measurements of Raman scattering by surface phonons in CdS1,xSex nanocrystals give no reason to judge upon the nanocrystal shape. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dispersion relation of the optical phonon frequencies in AlN/GaN superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005S.K. Medeiros Abstract In this work we study the dispersion relation of the phonon frequencies in heterojunctions composed by III-V nitride materials (GaN and AlN). We are concerned with the superlattice structure, namely /substrate /AlN /AlxGa1,xN/GaN/AlxGa1,xN/,, where the substrate is here considered to be a transparent dielectric medium like sapphire. We make use of a model based on the Fröhlich Hamiltonian, taking into account the macroscopic theory known as the continuum dielectric model. The optical phonon modes are modelled considering only the electromagnetic boundary conditions, in the absence of charge transfer between ions. Numerical results of the confined optical phonon dispersion are presented, characterizing three distinct optical phonon classes designated as interface (IF), half-space (HS) and propagating (PR) modes. Furthermore, due to the dielectric anisotropy presented in the nitride, some additional peculiarities will be presented, like dispersive confined modes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||