A comparison between a mean held theory of chain packing in membranes and micelles and Monte Carlo simulations is presented for model lipid bilayers. In both approaches the `'lipids'' are modeled as freely jointed (but self-avoiding) chains of spherical segments. The first segment of the chain represents the head group, anchored to the bilayer interface by a harmonic binding potential. The simulations are performed for symmetric bilayers composed of 200 chains, with periodic boundary conditions. Both pure and mixed bilayers (composed of long and short chains) are analyzed. In the simulation nonbonded segments interact via Lennard-Jones potentials, ensuring nearly uniform segment density in the bilayer core, as assumed in the mean held theory. The lateral pressure profiles governing the probability distribution of chain conformations in the mean field theory are related and compared to the tangential pressure profiles calculated from the simulations using Kirkwood-Buff's molecular theory. The two pressure profiles show very good agreement. We also calculate two conformational chain properties: end-segment distributions and orientational bond order parameters. The end-segment distributions calculated by the two approaches show excellent agreement. The order parameters compare somewhat less satisfactorily, yet we found that the order parameters derived from the simulations depend rather sensitively on the details of the interaction potential. In general, the results of the simulations support the use of the mean held theory as a (simple) tool for studying conformational chain statistics in confined environments and related thermodynamic properties, such as membrane curvature elasticity. (C) 1997 American Institute Physics.
Abraham Neyman. 1997. “Cooperation, Repetition and Automata.” In Cooperation: Game-Theoretic Approaches, NATO ASI Series, edited by sergiu hart and Andreu Mas-Colell, 155: Pp. 233--255.
Analyzes the model proposed for the diffusion of copper, hydrogen and titanium through potassium lithium tantalate niobate crystals. Occupation of potassium/lithium vacant sites by copper and hydrogen ions; Effect of titanium on the ability of hydrogen ions to stay at the sites; Reduction in the hydrogen ion concentration.
Shlomo. Magdassi. 1997. “Delivery Systems in Cosmetics..” Colloids Surf., AColloids and Surfaces, A: Physicochemical and Engineering Aspects, 123-124, Pp. 671 - 679. Abstract
A review with 29 refs. Cosmetic delivery systems such as different types of emulsions, liposomes, niosomes, and microcapsules are discussed. [on SciFinder(R)]
A highly sensitive assay for the detn. of Cr(VI), in the presence of Cr(III) and in the presence of other cations by cyclic voltammetry. The detn. is carried out with a gold or gold-plated electrode with a self-assembled monolayer of a pyridine deriv. of the kind of 4-(2-ethanethiol) pyridine. Detns. can be effected at extremely low levels of the order of 1 ppt (part per trillion, 1012). [on SciFinder(R)]
Visualized fingerprints on unfired brass cartridge cases developed by a novel method of palladium deposition were examd. by Auger electron spectroscopy (AES), SEM, and electron probe microscopy (EPMA). In spite of the strong contrast of colors between the ridges and the valleys of the developed images, which suggests a selective coating of palladium on the valleys, palladium was found to deposit onto both valleys and ridges. Lumps of org. material were scattered mostly along the ridges. The study suggests a model for the deposition of metals on brass cartridges on which sebaceous fingerprints were impressed. [on SciFinder(R)]
Otefin is a 45-kDa nuclear envelope protein with no apparent homology to other known proteins. It includes a large hydrophilic domain, a single carboxyl-terminal hydrophobic sequence of 17 amino acids, and a high content of serine and threonine residues. Cytological labeling located otefin on the nucleoplasmic side of the nuclear envelope. Chemical extraction of nuclei from Drosophila embryos revealed that otefin is a peripheral protein whose association with the nuclear envelope is stronger than that of lamin. Deletion mutants of otefin were expressed in order to identify regions that direct otefin to the nuclear envelope. These experiments revealed that the hydrophobic sequence at the carboxyl terminus is essential for correct targeting to the nuclear envelope, whereas additional regions in the hydrophilic domain of otefin are required for its efficient targeting and stabilization in the nuclear envelope.
A molecular level theory is presented for the thermodynamic stability of two (similar) types of structural complexes formed by (either single strand or supercoiled) DNA and cationic liposomes, both involving a monolayer-coated DNA as the central structural unit. In the `'spaghetti'' complex the central unit is surrounded by another, oppositely curved, monolayer, thus forming a bilayer mantle. The `'honeycomb'' complex is a bundle of hexagonally packed DNA-monolayer units. The formation free energy of these complexes, starting from a planar cationic/neutral lipid bilayer and bare DNA, is expressed as a sum of electrostatic, bending, mixing, and (for the honeycomb) chain frustration contributions. The electrostatic free energy is calculated using the Poisson-Boltzmann equation. The bending energy of the mixed lipid layers is treated in the quadratic curvature approximation with composition-dependent bending rigidity and spontaneous curvature. Ideal lipid mixing is assumed within each lipid monolayer. We found that the most stable monolayer-coated DNA units are formed when the charged/neutral lipid composition corresponds (nearly) to charge neutralization; the optimal monolayer radius corresponds to close DNA-monolayer contact. These conclusions are also valid for the honeycomb complex, as the chain frustration energy is found to be negligible. Typically, the stabilization energies for these structures are on the order of 1 k(B)T/Angstrom of DNA length, reflecting mainly the balance between the electrostatic and bending energies. The spaghetti complexes are less stable due to the additional bending energy of the external monolayer. A thermodynamic analysis is presented for calculating the equilibrium lipid compositions when the complexes coexist with excess bilayer.
High molecular Rydberg states, whose time evolution exhibits multiple time scales are discussed as an example of a system which can be examined in detail. Three bottlenecks to the sampling of phase space, associated with an incomplete mixing of the zeroth order quantum numbers, are identified. The physics of all three is that the number of open ionization channels is smaller than the number of quasi isoenergetic zero order discrete states and that an electron with a high orbital angular momentum, which is far from the core, cannot effectively couple to it. States trapped behind the bottlenecks have a high resilience to decay and such states are possible even high above the ionization threshold and in the presence of external perturbations. On the other hand, states that are directly coupled to the ionization channels decay promptly, with less sampling of the dense manifold of isoenergetic delayed states. We conclude that the decay of high molecular Rydberg states provides a useful analogue for unimolecular reactions from a dense set of states which typically results in a prompt and a delayed decay.
The orbital control of stereochemistry is discussed with special reference to the Na (3p P-2) + H-2 collision. As seen by H-2, the p orbital of the electronically excited Na atom is like a quadrupole, which may or may not lock along the molecular axis. Quantum mechanically, variations in the alignment of the orbital represent changes in the electronic state of the system and so dynamical methods which allow for such interstate transitions must be used. A new, time dependent quantum mechanical method for propagating the wave function on several electronic states is used to study these interstate transitions. Particular attention is given to the question of orbital following. The computational method is fully quantum mechanical but it uses a basis set which takes full account of the classical motion on any given electronic state while the solution of the Schrodinger equation addresses the electronic-state-changing transitions. We pay specific attention to the orbital alignment for both cold and rotationally warm H-2 and for low and high impact parameters throughout the course of the collision. It is concluded that orbital locking is not necessarily instantaneous and can lag behind the faster nuclear motion, including the (fast) rotational motion of H-2.
Felsenstein D. 1997. “Economic Development and the Community Organization.” In Shmidt H. (ed) Community Organizations: Trends and Change, Pp. 605-623. Tel Aviv: HaHevra LaMatnasim and Yediot Ahronot (Hebrew).
Examines the formation of Barkhausen spikes in volume phase holograms of potassium lithium tantalate niobate crystals. Effect of ferroelectric domain reversal induced by photorefractive space charge fields; Measurement of displacement currents; Changes in diffraction efficiency during a single macrodomain switching.
