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Theoretical Estimates (theoretical + estimate)
Selected AbstractsSelf-Assembly of Rod-Like Copolymers into Monolayers: A Simple Theoretical Estimate of Molecular Recognition QualityMACROMOLECULAR THEORY AND SIMULATIONS, Issue 7-8 2008Anatoly V. Berezkin Abstract A discrete model of a monolayer, consisting of identical rod-like copolymer molecules, is suggested. The influence of the copolymer's composition and sequence on its self-assembly was studied. Thermodynamic quantities of monolayers were calculated. It is shown that the system undergoes an "order-disorder" transition upon temperature increase. The most regular monolayers are formed by copolymers with quasi-random sequences. Nevertheless, the monomer composition of such "good" sequences can vary over a wide range. It is shown that homopolymers, copolymers with a predominance of one-type monomer units and copolymers consisting of a small number of large blocks have a reduced ability to self-assembly. [source] Liposome transport of hydrophobic drugs: Gel phase lipid bilayer permeability and partitioning of the lactone form of a hydrophobic camptothecin, DB-67JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 1 2008Vijay Joguparthi Abstract The design of liposomal delivery systems for hydrophobic drug molecules having improved encapsulation efficiency and enhanced drug retention would be highly desirable. Unfortunately, the poor aqueous solubility and high membrane binding affinity of hydrophobic drugs necessitates extensive validation of experimental methods to determine both liposome loading and permeability and thus the development of a quantitative understanding of the factors governing the encapsulation and retention/release of such compounds has been slow. This report describes an efflux transport method using dynamic dialysis to study the liposomal membrane permeability of hydrophobic compounds. A mathematical model has been developed to calculate liposomal membrane permeability coefficients of hydrophobic compounds from dynamic dialysis experiments and partitioning experiments using equilibrium dialysis. Also reported is a simple method to study the release kinetics of liposome encapsulated camptothecin lactone in plasma by comparing the hydrolysis kinetics of liposome entrapped versus free drug. DB-67, a novel hydrophobic camptothecin analogue has been used as a model permeant to validate these methods. Theoretical estimates of DB-67 permeability obtained from the bulk solubility diffusion model and the "barrier-domain" solubility diffusion model are compared to the experimentally observed value. The use of dynamic dialysis in drug release studies of liposome and other nanoparticle formulations is further discussed and experimental artifacts that can arise without adequate validation are illustrated through simulations. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:400,420, 2008 [source] Electrostatic Stabilization of Ultrafine Titania in EthanolJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2002Johnny Widegren The colloidal stability of a suspension of very fine titania (TiO2) powder in ethanol has been investigated. The electrokinetic behavior and stability ratios were determined as a function of operational pH and salt concentration. We show that the stability is strongly dependent on ionic strength; the TiO2suspensions will aggregate relatively rapidly at ionic strengths >10,3M, while suspensions exhibiting long-term stability can be made at ,10,4M, providing that the zeta potential of the TiO2particles is sufficiently high, i.e., |,|= 40,50 mV. Theoretical estimates of the stability based on electrostatic repulsion show a relatively good correlation with the experimental results, in particular in the acidic operational pH (pH*) range. [source] A magnetic field controlled negative-index microwave lensMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 11 2008R. V. Petrov Abstract A three-dimensional lens with magnetic field control of the index of refraction and focal length has been designed and characterized over 2-18 GHz. The lens is made of a lattice of cross-wire resonators and ferrite cubes. For the lattice without the ferrite cubes, a refractive index n , ,1 is measured at 11.25 GHz. With the introduction of ferrite cubes in the lattice, n could be tuned over the range ,1.2 to +1.8 for nominal bias magnetic fields H. The focal point for the lens with n , ,1, for example, could be displaced by 2 cm by tuning H from 4100 to 4125 Oe. Theoretical estimates of the lens characteristics are in agreement with the data. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2804,2807, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23847 [source] Coronal activity from dynamos in astrophysical rotatorsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000Eric G. Blackman We show that a steady mean-field dynamo in astrophysical rotators leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle and wind acceleration in a corona. The connection between energy and magnetic helicity arises because mean-field generation is linked to an inverse cascade of magnetic helicity. To maintain a steady state in large magnetic Reynolds number rotators, there must then be an escape of relative magnetic helicity associated with the mean field, accompanied by an equal and opposite contribution from the fluctuating field. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be estimated. Steady coronal activity including the dissipation of magnetic energy, and formation of multi-scale helical structures therefore necessarily accompanies an internal dynamo. This highlights the importance of boundary conditions which allow this to occur for non-linear astrophysical dynamo simulations. Our theoretical estimate of the power delivered by a mean-field dynamo is consistent with that inferred from observations to be delivered to the solar corona, the Galactic corona, and Seyfert 1 AGN coronae. [source] A new look at the quantum mechanics of the harmonic oscillatorANNALEN DER PHYSIK, Issue 7-8 2007H.A. Kastrup Abstract In classical mechanics the harmonic oscillator (HO) provides the generic example for the use of angle and action variables and I > 0 which played a prominent role in the "old" Bohr-Sommerfeld quantum theory. However, already classically there is a problem which has essential implications for the quantum mechanics of the (,,I)-model for the HO: the transformation is only locally symplectic and singular for (q,p) = (0,0). Globally the phase space {(q,p)} has the topological structure of the plane ,2, whereas the phase space {(,,I)} corresponds globally to the punctured plane ,2 -(0,0) or to a simple cone with the tip deleted. From the properties of the symplectic transformations on that phase space one can derive the functions h0 = I, h1 = Icos , and h2 = - Isin , as the basic coordinates on {(,,I)}, where their Poisson brackets obey the Lie algebra of the symplectic group of the plane. This implies a qualitative difference as to the quantum theory of the phase space {(,,I)} compared to the usual one for {(q,p)}: In the quantum mechanics for the (,,I)-model of the HO the three hj correspond to the self-adjoint generators Kj, j = 0,1,2, of certain irreducible unitary representations of the symplectic group or one of its infinitely many covering groups, the representations being parametrized by a (Bargmann) index k > 0. This index k determines the ground state energy of the (,,I)-Hamiltonian . For an m -fold covering the lowest possible value for k is k = 1/m, which can be made arbitrarily small by choosing m accordingly! This is not in contradiction to the usual approach in terms of the operators Q and P which are now expressed as functions of the Kj, but keep their usual properties. The richer structure of the Kj quantum model of the HO is "erased" when passing to the simpler (Q,P)-model! This more refined approach to the quantum theory of the HO implies many experimental tests: Mulliken-type experiments for isotopic diatomic molecules, experiments with harmonic traps for atoms, ions and BE-condensates, with charged HOs in external electric fields and the (Landau) levels of charged particles in external magnetic fields, with the propagation of light in vacuum, passing through strong external electric or magnetic fields. Finally it may lead to a new theoretical estimate for the quantum vacuum energy of fields and its relation to the cosmological constant. [source] Bacteriorhodopsin-Monolayer-Based Planar Metal,Insulator,Metal Junctions via Biomimetic Vesicle Fusion: Preparation, Characterization, and Bio-optoelectronic Characteristics,ADVANCED FUNCTIONAL MATERIALS, Issue 8 2007D. Jin Abstract A reliable and reproducible method for preparing bacteriorhodopsin (bR)-containing metal,biomolecule,monolayer-metal planar junctions via vesicle fusion tactics and soft deposition of Au top electrodes is reported. Optimum monolayer and junction preparations, including contact effects, are discussed. The electron-transport characteristics of bR-containing membranes are studied systematically by incorporating native bR or artificial bR pigments derived from synthetic retinal analogues, into single solid-supported lipid bilayers. Current,voltage (I,V) measurements at ambient conditions show that a single layer of such bR-containing artificial lipid bilayers pass current in solid electrode/bilayer/solid electrode structures. The current is passed only if retinal or its analogue is present in the protein. Furthermore, the preparations show photoconductivity as long as the retinal can isomerize following light absorption. Optical characterization suggests that the junction photocurrents might be associated with a photochemically induced M-like intermediate of bR. I,V measurements along with theoretical estimates reveal that electron transfer through the protein is over four orders of magnitude more efficient than what would be estimated for direct tunneling through 5,nm of water-free peptides. Our results furthermore suggest that the light-driven proton-pumping activity of the sandwiched solid-state bR monolayer contributes negligibly to the steady-state light currents that are observed, and that the orientation of bR does not significantly affect the observed I,V characteristics. [source] Numerical approximation of generalized Newtonian fluids using Powell,Sabin,Heindl elements: I. theoretical estimatesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2003S.-S. Chow Abstract In this paper we consider the numerical approximation of steady and unsteady generalized Newtonian fluid flows using divergence free finite elements generated by the Powell,Sabin,Heindl elements. We derive a priori and a posteriori finite element error estimates and prove convergence of the method of successive approximations for the steady flow case. A priori error estimates of unsteady flows are also considered. These results provide a theoretical foundation and supporting numerical studies are to be provided in Part II. Copyright © 2003 John Wiley & Sons, Ltd. [source] ,-Dynamics free energy simulation methodsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2009Jennifer L. Knight Abstract Free energy calculations are fundamental to obtaining accurate theoretical estimates of many important biological phenomena including hydration energies, protein-ligand binding affinities and energetics of conformational changes. Unlike traditional free energy perturbation and thermodynamic integration methods, ,-dynamics treats the conventional "," as a dynamic variable in free energy simulations and simultaneously evaluates thermodynamic properties for multiple states in a single simulation. In the present article, we provide an overview of the theory of ,-dynamics, including the use of biasing and restraining potentials to facilitate conformational sampling. We review how ,-dynamics has been used to rapidly and reliably compute relative hydration free energies and binding affinities for series of ligands, to accurately identify crystallographically observed binding modes starting from incorrect orientations, and to model the effects of mutations upon protein stability. Finally, we suggest how ,-dynamics may be extended to facilitate modeling efforts in structure-based drug design. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009 [source] Discontinuous Galerkin methods for periodic boundary value problemsNUMERICAL METHODS FOR PARTIAL DIFFERENTIAL EQUATIONS, Issue 3 2007Kumar Vemaganti Abstract This article considers the extension of well-known discontinuous Galerkin (DG) finite element formulations to elliptic problems with periodic boundary conditions. Such problems routinely appear in a number of applications, particularly in homogenization of composite materials. We propose an approach in which the periodicity constraint is incorporated weakly in the variational formulation of the problem. Both H1 and L2 error estimates are presented. A numerical example confirming theoretical estimates is shown. © 2006 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2007 [source] Spin polarization and electronic structure of ferromagnetic Mn5Ge3 epilayersPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2005R. P. Panguluri Abstract Germanium-based alloys hold great promise for future spintronics applications, due to their potential for integration with conventional Si-based electronics. High-quality single phase Mn5Ge3(0001) films, grown by solid-phase epitaxy on Ge(111) and GaAs(111), exhibit strong ferromagnetism up to the Curie temperature TC , 296 K. Point Contact Andreev Reflection (PCAR) measurements on Mn5Ge3 epilayers reveal a spin-polarization P = 42 ± 5% for both substrates. We also calculate the spin polarization of bulk Mn5Ge3 in the diffusive and ballistic regimes using density-functional theory (DFT). The measured spin polarization exceeds the theoretical estimates of PDFT = 35 ± 5% and 10 ± 5% for the diffusive and ballistic limits, respectively. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dipole Moment Enhancement in Molecular Crystals from X-ray Diffraction DataCHEMPHYSCHEM, Issue 14 2007Mark A. Spackman Prof. Abstract Although reliable determination of the molecular dipole moment from experimental charge density analyses on molecular crystals is a challenging undertaking, these values are becoming increasingly common experimental results. We collate all known experimental determinations and use this database to identify broad trends in the dipole moment enhancements implied by these measurements as well as outliers for which enhancements are pronounced. Compelling evidence emerges that molecular dipole moments from X-ray diffraction data can provide a wealth of information on the change in the molecular charge distribution that results from crystal formation. Most importantly, these experiments are unrivalled in their potential to provide this information in such detail and deserve to be exploited to a much greater extent. The considerable number of experimental determinations now available has enabled us to pinpoint those studies that merit further attention, either because they point unequivocally to a considerable enhancement in the crystal (of 50,% or more), or because the experimental determinations suggest enhancements of 100,% or more,much larger than independent theoretical estimates. In both cases further detailed experimental and theoretical studies are indicated. [source] | |||||||||||||||||||