Acid-base thermochemistry of gaseous aliphatic α-aminoacids

Acid-base thermochemistry of isolated aliphatic amino acids (denoted AAA): glycine, alanine, valine, leucine, isoleucine and proline has been examined theoretically by quantum chemical computations at the G3MP2B3 level. Conformational analysis on neutral, protonated and deprotonated species has been used to identify the lowest energy conformers and to estimate the population of conformers expected to be present at thermal equilibrium at 298 K. Comparison of the G3MP2B3 theoretical proton affinities, PA, and ΔH(acid) with experimental results is shown to be correct if experimental thermochemistry is re-evaluated and adapted to the most recent acidity-basicity scales. From this point of view, a set of evaluated proton affinities of 887, 902, 915, 916, 919 and 941 kJ mol(-1), and a set of evaluated ΔH(acid) of 1433, 1430, 1423, 1423, 1422 and 1426 kJ mol(-1), is proposed for glycine, alanine, valine, leucine, isoleucine and proline, respectively. Correlations with structural parameters (Taft's σ(α) polarizability parameter and molecular size) suggest that polarizability of the side chain is the major origin of the increase in PA and decrease in ΔH(acid) along the homologous series glycine, alanine, valine and leucine/isoleucine. Heats of formation of gaseous species AAA, AAAH(+) and [AAA-H](-) were computed at the G3MP2B3 level. The present study provides previously unavailable Δ(f)H°(298) for the ionized species AAAH(+) and [AAA-H](-). Comparison with Benson's estimate, and correlation with molecular size, show that several experimental Δ(f)H°(298) values of neutral or gaseous AAA might be erroneous.     

Network formation as a cluster-cluster diffusion-limited aggregation process, 2. Modelling the polyethylene crosslinking process using Monte Carlo and graph techniques

A Monte Carlo simulation of the network formation of polyethylene radicals has been carried out using the reaction modelling scheme and a graph exploration algorithm based on the breadth-first search technique. The results are obtained in a three dimensional cubic continuum space simulation with periodic boundary conditions. Results for three different polyethylene concentrations are reported. The structural evolution of the studied system was followed in terms of: number of reactions, molecular mass, aggregate dimension and fractal dimension analysis.     

Flavodoxin as an Electronic Donor in Photosynthetic Inorganic Nitrogen Assimilation by Iron-deficient Chlorella fusca Cells

Two ferredoxin-dependent enzymes involved in the photosynthetic assimilation of nitrogen, nitrite reductase (EC 1.7.7.1) and glutamate synthase (EC 1.4.7.1) from Chlorella fusca have been shown to use either ferredoxin or flavodoxin (from the same organism) as electron donor. Nitrite reductase showed K _m, values of 13.8 ± 0.9 μ M for ferredoxin and 30.4 ± 7.8 μ M for flavodoxin. Glutamate synthase exhibited a Km of 6.6 ± 1.2 μ M for ferredoxin compared to a calculated K _m of 22.4 ± 6.4 μ M for flavodoxin.     

Preparation and properties of the polyester made from 2.2-bis(4-hydroxycyclohexyl)propane and adipic acid

Poly[2,2-bis(4-oxycyclohexyl)propane adipate] was formed by melt polymerizing adipic acid and 2,2-bis(4-hydroxycyclohexyl)propane in the presence of n-butylhydroxyoxostannane. This polyester possessed a melting temperature (T_m) of 200°C and a glass transition temperature (T_g) of 75°C. An excellent notched impact strength of 22 ft. lbs./in. was its best physical property.     

Bulk-type all-solid-state batteries with mechanically prepared LiCoPO4 composite cathodes

Journal of Solid State Electrochemistry - All-solid-state Li batteries are suitable for the application of 5 V-class cathode materials that can provide high energy densities. However, it is hard to...     

Multiple Histogram Method and Static Monte Carlo Sampling

Summary: We describe an approach to use multiple‐histogram methods in combination with static, biased Monte Carlo simulations. To illustrate this, we computed the force‐extension curve of an athermal polymer from multiple histograms constructed in a series of static Rosenbluth Monte Carlo simulations. From the complete histogram of the distribution function of the end‐to‐end vectors of the polymer chain, we can efficiently compute the polymer force‐extension curve.

Comparison of the stress‐strain curves for the stress ensemble (symbols) and the strain ensemble (lines). Results obtained for N = 100, 200, 400, and 600. For small x, f(x) = −F′(x) was computed by aproximating F(x) by a second degree polynomial and then taking the derivative. For large x, f(x) = −F′(x) was computed numerically.