Search results for " method"
showing 10 items of 10455 documents
Direct monitoring of spin state in dinuclear iron(II) coordination compounds
2001
So far there has been no direct method to determine the spin state of molecules in dinuclear iron(II) compounds. The molecular fractions of high-spin (HS) and low-spin (LS) species have been deduced from magnetic susceptibility and zero-field Mossbauer spectroscopy data irrespective of whether they belong to LS–LS, LS–HS and HS–HS pairs. However, the distinction of pairs becomes possible if Mossbauer measurements are carried out in an external magnetic field. The proposed method opens new possibilities in the study of spin crossover phenomena in dinuclear compounds.
Monte Carlo methods for polymer chains in two - dimensional geometries (polymers at surfaces and interfaces)
1993
Coarse-grained models of polymers at interfaces can be defined such that their treatment by Monte Carlo simulation is most convenient and efficient for the problem at hand. This simulation strategy is briefly illustrated with three examples: (1) The orientational ordering of rigid rod-like polymers grafted to a surface, where “table methods” can be used, applying a fine discretization of the angles describing rod orientation. (2) Surface enrichment of one species in a polymer blend is treated by a semi-grand-canonical technique. (3) The number of configurations and structure of a star polymer attached with its center to a wall is studied by a “growth technique” generalizing simple sampling …
Chain linear dimensions in the surface-enriched layer of polymer mixtures
1992
We calculate the mean-square end-to-end distances and mean-square gyration radii using the bond fluctuation model for a binary polymer blend in the presence of a wall by Monte Carlo simulation. In the bulk, the size of the minority, low-concentration polymer species is compressed compared to the majority one. In the vicinity of the wall, where the minority polymer concentration is enriched due to attraction from the wall, the dimensions of the two types of polymers are approximately equal and are essentially the same as in an athermal polymer melt. Thus, the geometric constraint is more important to the structure of the polymers than the polymer-polymer and polymer-wall interactions.
Semidilute and Concentrated Polymer Solutions near Attractive Walls: Dynamic Monte Carlo Simulation of Density and Pressure Profiles of a Coarse-Gra…
1997
Using a bead−spring model of flexible polymer chains, we study polymer adsorption from solutions onto attractive planar walls, varying both the strength of the adsorption potential e and the concentration of the solution over a wide range. Treating the case of good solvents, the profiles of density and pressure are computed and it is shown that thermal equilibrium between the adsorbed layer and the bulk solution is obtained. The case of a wall with purely repulsive potential under otherwise identical conditions is treated for comparison. It is shown that for the strongly adsorbing wall there is a pronounced layering, while a layered structure at the repulsive wall occurs only for high conce…
Vapor–liquid equilibria in dendrimer and hyperbranched polymer solutions: experimental data and modeling using UNIFAC-FV
2004
Abstract This work evaluates the potential of the group contribution method universal functional activity coefficients-free-volume (UNIFAC-FV) to predict vapor–liquid equilibria (VLE) in dendrimer-solvent and hyperbranched polymer-solvent systems. The VLE of hydroxyl-functional dendritic polymers (polyethers, polyesters, polyamidoamine), dissolved in polar solvents (ethanol and/or water) are studied both experimentally and theoretically. A new approach is suggested to account for the contribution of selected types of polymer structural units (i.e. linear, dendritic and terminal units) to the solvent residual activity. The results of calculations are in a good agreement with experiment. Furt…
Experimental and estimated saturated vapour pressures of aroma compounds
1999
Abstract The saturated vapour pressure of d-linalool, 2-nonanone, d-limonene and isoamyl acetate were measured using a static method at different temperatures from 223 to 468 K. From the experimental values, Antoine's constants were determined to enable the calculations of the saturated vapour pressures at a given temperature. The saturated vapour pressure of the four aroma compounds at 298 K were respectively 27, 59, 200 and 733 Pa. These results were compared with those obtained using different estimation methods (Antoine–Grain, Watson, Lee–Kesler, Gomez–Thodos, Grain and Mackay). Gomez–Thodos' model was found to be the most accurate method for the estimation of the saturated vapour press…
RELAXATION METHODS FOR STUDYING MACROMOLECULAR MOTION IN THE BULK
1982
Abstract Macromolecular motion in amorphous and partially crystalline polymers is discussed in the light of recent relaxation experiments with particular emphasis on NMR methods. Polystyrene and polyethylene serve as pertinent examples where a considerable amount of new experimental data provides a bysis for better understanding molecular processes below and above the glass transition, and in the melt.
Stereoelectronic properties ofN-acetyl-α,β-dehydroamino acidN′-methylamides
1998
α,β-Dehydroamino acids are useful peptide modifiers. However, their stereoelectronic properties still remain insufficiently recognized. Based on FTIR experiments in the range ofvs(N-H), AI, AII andvs(Cα=Cβ) and ab initio calculations with B3LYP/6–31G*, we studied the solution conformational preferences and the amide electron density perturbation of Ac-ΔXaa-NHMe, where ΔXaa=ΔAla, (E)-ΔAbu, (Z)-ΔAbu, (Z)-ΔLeu, (Z)-ΔPhe and ΔVal. Each of these dehydroamides adopts a C5 structure, which in Ac-ΔAla-NHMe is fully extended and accompanied by the strong C5 hydrogen bond. Interaction with bond Cα=Cβ lessens the amidic resonance within the flanking amide groups. TheN-terminal C=O bond is noticeably s…
Why Are Some Enzymes Dimers? Flexibility and Catalysis in Thermotoga maritima Dihydrofolate Reductase
2019
[Image: see text] Dihydrofolate reductase from Thermotoga maritima (TmDFHFR) is a dimeric thermophilic enzyme that catalyzes the hydride transfer from the cofactor NADPH to dihydrofolate less efficiently than other DHFR enzymes, such as the mesophilic analogue Escherichia coli DHFR (EcDHFR). Using QM/MM potentials, we show that the reduced catalytic efficiency of TmDHFR is most likely due to differences in the amino acid sequence that stabilize the M20 loop in an open conformation, which prevents the formation of some interactions in the transition state and increases the number of water molecules in the active site. However, dimerization provides two advantages to the thermophilic enzyme: …
Effect of konjac glucomannan addition on aroma release in gels containing potato starch.
2014
International audience; The present study aimed to measure the retention of aroma compounds (ethyl acetate, ethyl hexanoate and carvacrol) in dispersions based on konjac glucomannan and/or potato starch, and to highlight the influence of konjac glucomannan on the mechanisms involved in aroma retention. Publications on the effect of konjac glucomannan on aroma release are scarce. Konjac glocomannan is a polysaccharide used as a food additive for its viscous and emulsifying properties. Retention of aroma compounds in dispersions was calculated from partition coefficients which were measured using the phase ratio variation method. This method, consisting of analyses of the headspace at equilib…