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Previous Experimental Work (previous + experimental_work)
Selected AbstractsOrigin of Bends in Unperturbed Vinyl Polymers: An Illustration with PolystyreneMACROMOLECULAR THEORY AND SIMULATIONS, Issue 7 2007Yergou B. Tatek Abstract Previous experimental works have shown that dendronized vinyl polymers exhibit bends when adsorbed onto a surface. Two different mechanisms are believed to be responsible for the formation of these bends. These mechanisms are the temperature dependent random fluctuations of torsional bond states on one hand, and the intramolecular interactions due to the randomness in the stereochemical sequence of side chains on the other hand. Investigation of the amplitude and scope of the above mechanisms has been made by studying the conformational space of PS chains via RIS based Monte Carlo sampling. It was found that at low temperature bend formation is due to tacticity, whereas it was thermally driven at high temperature. The existence of a transition temperature between these two bend formation modes was demonstrated. It was also shown that for atactic chains, the maximum of bend formation occurs at Pm,,,0.7. [source] Matrix Models as a Tool for Understanding Invasive Plant and Native Plant InteractionsCONSERVATION BIOLOGY, Issue 3 2005DIANE M. THOMSON competencia; invasión biológica; plantas invasoras; modelo matricial; perturbación Abstract:,Demographic matrix models are an increasingly standard way to evaluate the effects of different impacts and management approaches on species of concern. Although invasive species are now considered among the greatest threats to biodiversity, matrix methods have been little used to explore and integrate the potentially complicated effects of invasions on native species. I developed stage-structured models to assess the impacts of invasive grasses on population growth and persistence of a federally listed (U.S.A.) endemic plant, the Antioch Dunes evening primrose (Oenothera deltoides subsp. howellii [Munz] W. Klein). I used these models to evaluate two frequently made assumptions: (1) when rare plant populations decline in invaded habitats, invasive species are the cause and (2) invasive plants suppress rare plants primarily through direct resource competition. I compared two control and two removal matrices based on previous experimental work that showed variable effects of invasive grasses on different life-history stages of O. deltoides. Matrix analysis showed that these effects translated into substantial changes in population growth rates and persistence, with control matrices predicting a mean stochastic population growth rate (,) of 0.86 and removal matrices predicting growth rates from 0.92 to 0.93. Yet even the most optimistic invasive removal scenarios predicted rapid decline and a probability of extinction near one in the next 100 years. Competitive suppression of seedlings had much smaller effects on growth rates than did lowered germination, which probably resulted from thatch accumulation and reduced soil disturbance. These results indicate that although invasive grasses have important effects on the population growth of this rare plant, invasion impacts are not solely responsible for observed declines and are likely to be interacting with other factors such as habitat degradation. Further, changes in the disturbance regime may be as important a mechanism creating these impacts as direct resource competition. My results highlight the value of demographic modeling approaches in creating an integrated assessment of the threats posed by invasive species and the need for more mechanistic studies of invasive plant interactions with native plants. Resumen:,Los modelos demográficos matriciales son una forma cada vez más utilizada para evaluar los efectos de diferentes impactos y métodos de gestión sobre las especies en cuestión. Aunque actualmente se considera a las plantas invasoras entre las mayores amenazas a la biodiversidad, los modelos matriciales han sido poco utilizados para explorar e integrar los efectos potencialmente complicados de las invasiones sobre las especies nativas. Desarrollé modelos estructurados por etapas para evaluar los impactos de pastos invasores sobre el crecimiento poblacional y la persistencia de una especie de planta endémica, enlistada federalmente (E.U.A.), Oenothera deltoides ssp. howellii [Munz] W. Klein. Utilicé estos modelos para evaluar dos suposiciones frecuentes: (1) cuando las poblaciones de plantas raras declinan en hábitats invadidos, las especies invasoras son la causa y (2) las plantas invasoras suprimen a las plantas raras principalmente mediante la competencia directa por recursos. Comparé dos matrices de control y dos de remoción con base en trabajo experimental previo que mostró efectos variables de los pastos invasores sobre las diferentes etapas de la historia de vida de O. deltoides. El análisis de la matriz mostró que estos efectos se tradujeron en cambios sustanciales en las tasas de crecimiento y persistencia de la población, las matrices de control predijeron una tasa media de crecimiento poblacional estocástica (,) de 0.86 y las matrices de remoción predijeron tasas de crecimiento de 0.92-0.93. Aun los escenarios más optimistas de remoción de invasores predijeron una rápida declinación y una probabilidad de extinción en 100 años cerca de uno. La supresión competitiva de plántulas tuvo mucho menor efecto sobre las tasas de crecimiento que la disminución en la germinación, que probablemente resultó de la acumulación de paja y reducción en la perturbación del suelo. Estos resultados indican que, aunque los pastos invasores tienen efectos importantes sobre el crecimiento poblacional de esta planta rara, los impactos de la invasión no son los únicos responsables de las declinaciones observadas y probablemente están interactuando con otros factores como la degradación del hábitat. Más aun, los cambios en el régimen de perturbación pueden ser un mecanismo tan importante en la creación de estos impactos como la competencia directa por recursos. Mis resultados resaltan el valor del enfoque de los modelos demográficos para la evaluación integral de las amenazas de especies invasoras y la necesidad de estudios más mecanicistas de las interacciones de plantas invasoras con plantas nativas. [source] Combustion of a substitution fuel made of cardboard and polyethylene: influence of the mix characteristics,modelingFIRE AND MATERIALS, Issue 7 2008S. Salvador Abstract The model proposed in this paper describes the combustion of a porous medium subjected to a radiative heat flux at its surface. There is no forced convection of air through the medium; hence this situation corresponds to the one encountered at the surface of fuel elements such as pellets, bricks or ballots, inside a furnace or kiln. Ash is not removed from the surface. No assumption is made a priori in terms of the limiting phenomena. The medium is composed of cardboard and polyethylene (PE). Based on previous experimental work (Fuel 2004; 83:451,462), the material is assumed to be a macroscopically homogeneous porous medium. Local thermal equilibrium is also assumed. Most of the parameters required for the modeling were determined from specific experiments. Good predictions of the sample mass evolution and of the temperature levels inside the sample body were obtained for a large range of densities and PE content. A devolatilization front of about 20,mm first propagates inside the medium. The volatile matter flux is advected to the surface, which leads to the formation of the flame above the surface. Then a second char oxidation front propagates, starting from the surface. The front thickness is approximately 25,mm under the experimental conditions. Copyright © 2008 John Wiley & Sons, Ltd. [source] Computational studies of electron-transfer processes in old yellow enzymeINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001Ginger M. Chateauneuf Abstract Old Yellow Enzyme (OYE) is a flavoenzyme that was first isolated from brewer's bottom yeast. Homologues have been identified in other strains of yeast, bacteria, and plants. In plants, the OYE homologue functions enzymatically in the synthesis of plant hormones, but the biological function of OYE in yeast is still unknown. Flavin mononucleotide (FMN) is the cofactor that is noncovalently bound in the enzyme. OYE binds several phenolic ligands that serve as models for reactive biological substrates. These complexes have broad long-wavelength absorption bands, which have been ascribed to charge-transfer interactions, with the phenolate anion acting as the electron donor and the FMN as the acceptor [Abramovitz, A. S.; Massey, V. J Bio Chem 1976, 251, 5327,5336]. The computational characterization of these electronic transitions in the active site will help in understanding the biological processes in the enzyme. It was found that at several levels of computational methods, and through computationally mutating relevant amino acids, a charge-transfer process is occurring. This result agrees with previous experimental work and is consistent with all ultraviolet,visible spectrophotometric data. The preliminary results for the computational studies of these electron-transfer processes will be presented. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Molecular polarizability of fullerenes and endohedral metallofullerenesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2002Francisco Torrens Abstract The interacting induced dipoles polarization model implemented in our program POLAR is used for the calculation of the molecular dipole µ and tensor quadrupole moments and also the dipole,dipole polarizability . The method is tested with Scn, Cn (fullerene and graphite) and endohedral Scn@Cm clusters. The polarizability is an important quantity for the identification of clusters with different numbers of atoms and even for the separation of isomers. The results for the polarizability are of the same order of magnitude as from reference calculations performed with our version of the program PAPID. The bulk limit for the polarizability is estimated from the Clausius,Mossotti relationship. The polarizability trend for these clusters as a function of size is different from what one might have expected. The clusters are more polarizable than what one might have inferred from the bulk polarizability. Previous theoretical work yielded the same trend for Sin, Gen and GanAsm small clusters. However, previous experimental work yielded the opposite trend for Sin, GanAsm and GenTem larger clusters. At present, the origin of this difference is problematic. One might argue that smaller clusters need not behave like those of intermediate size. The high polarizability of small clusters is attributed to dangling bonds at the surface of the cluster. In this respect, semiconductor clusters resemble metallic clusters. Copyright © 2002 John Wiley & Sons, Ltd. [source] Human Instability in Flood Flows,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 5 2008S.N. Jonkman Abstract:, Loss of human stability in flood flows and consequent drowning are a high personal hazard. In this paper, we review past experimental work on human instability. The results of new experiments by the Flood Hazard Research Centre (FHRC) are also reported. These new results show that low depth/high velocity flood waters are more dangerous than suggested based on previous experimental work. It is discussed how human instability can be related to two physical mechanisms: moment instability (toppling) and friction instability (sliding). Comparison of the test results with these physical mechanisms suggests that the occurrence of instability in the tests by FHRC is related to friction instability. This mechanism appears to occur earlier than moment instability for the combination of shallow depth and high flow velocity. Those concerned to identify locations where high flood flows could be a threat to human life need to modify their hazard assessments accordingly. [source] Retardation of the unfolding process by single N-glycosylation of ribonuclease A based on molecular dynamics simulationsBIOPOLYMERS, Issue 2 2008Youngjin Choi Abstract The conformational characteristics of glycosylated- and unglycosylated bovine pancreatic ribonuclease A (RNaseA) were traced with unfolding molecular dynamics simulations using CHARMM program at 470 K. The glycosylated RNase (Glc_RNase) possesses nearly identical protein structure with RNaseA, differing only by presence of a single acetylglucosamine residue N-linked to Asn34 in the RNaseA. Attaching of monomeric N -acetylglucosamine residue to the Asn34 in RNaseA resulted in a change of denaturing process of Glc_RNase. Simulations showed that the unfolding of RNaseA involved significant weakening of nonlocal interactions whereas the glycosylation led Glc_RNase to preserve the nonlocal interactions even in its denatured form. Even in simulations over 8 ns at 470 K, Glc_RNase remained relatively stable as a less denatured conformation. However, conformation of RNaseA was changed to a fully unfolded state before 3 ns of the simulations at 470 K. This difference was due to fact that formation of hydrogen bond bridges and nonlocal contacts induced by the attached N -acetylglucosamine of Glc_RNase showing in the unfolding simulations. These high-temperature unfolding MD simulations provided a theoretical basis for the previous experimental work in which Glc_RNase showed slower unfolding kinetics compared with unglycosylated RNaseA, suggesting that single N-glycosylation induced retardation of unfolding process of the ribonuclease protein. © 2007 Wiley Periodicals, Inc. Biopolymers 89: 114,123, 2008. This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] | |||||||||||||