Intermolecular interactions on amine‐cured epoxy matrices with different crosslink densities. Influence on the hole and specific volumes and the mechanical behavior

The architecture of an epoxy matrix was modified by curing the resin with mono‐/diamine mixtures having identical chemical structures. Both hole volume and specific volume variations were studied by positron annihilation lifetime spectroscopy and pressure‐volume‐temperature/density measurements, respectively. The average hole volume of the networks at room temperature slightly increased when the monoaminic chain extender content increased. The increment in the intermolecular interactions between functional groups of the networks chains, due to the less hindered nitrogen introduced by the monoamine, appears to be the responsible for the observed behavior. Besides, only small variations on the specific volume were observed on increasing the monoamine content, which points out that for a cured epoxy system, the chemical structure of the curing agent is mainly responsible for chain packing in the networks. On the other hand, intermolecular interactions between chains were considered as the key factor for fixing stiffness and strength. Thus, it was observed that the increase of the intermolecular interactions with the monoamine content produced a decrease in the sub‐T_g small‐range cooperative motions, which increased the low‐deformation mechanical properties at temperatures between β and α relaxations. This conclusion could be applied to previous investigations with epoxy matrices not fully crosslinked (nonstoichiometric or noncompletely cured formulations). Finally, it was found that fracture properties do not significantly depend either on the hole volume or on the intermolecular interactions. Fracture properties are more dependent on the crosslink density and the glass transition temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1240–1252, 2009     

Cu, Zn Superoxide dismutase: distorted active site binds substrate without significant energetic cost

Copper, Zinc superoxide dismutase (CuZnSOD) catalyzes the dismutation of the toxic superoxide radical into molecular oxygen and hydrogen peroxide. Dismutation is achieved by reduction and re-oxidation of the active site copper ion, where the superoxide substrate binds. This enzyme is considered to be a perfect enzyme, as the catalytic rate is very high and diffusion controlled. The redox active copper ion is coordinated by four histidine residues in a distorted square planar geometry. Much has been written about the biological significance of the geometry distortion. It is sometimes considered that it should help to tune the redox potential of the copper ion in order to efficiently reduce the first superoxide molecule and oxidize the second one. In this work we present a series of high level theoretical calculations using realistic models, which demonstrate that the distorted geometry is fundamental for the catalytic efficiency of the enzyme by allowing substrate binding without extensive geometric reorganization of the copper complex, upon changing from four to five ligands. A lower limit for the reorganization energy is calculated here in 22 kcal/mol, which would slow down the reaction kinetics by more than 13 orders of magnitude, transforming a perfect enzyme into an inefficient one.     

On the microbial reduction of ethyl α-methylacetoacetate

In the present work several microorganisms were screened in the reduction of methyl and ethyl acetoacetates. Good to excellent ees were obtained with up to quantitative substrate conversions. These microorganisms were also tested in the reduction of ethyl α-methylacetoacetate with good des and excellent ees. It was noticed that Kluyveromyces marxianus and Trichoderma harzianum were found to lead to products of opposite configuration with the use of methyl or ethyl acetoacetates, which is a very unusual finding.     

Determination of Caffeic Acid in Red Wine by Voltammetric Method

The electrochemical oxidation of 3,4‐dihydroxycinnamic acid, caffeic acid, leads to a stable electroactive poly(caffeic acid) thin film containing quinone moiety on a preactivated glassy polymeric carbon electrode. The properties of the deposited films as well as the stability study under different experimental conditions were investigated. Taking advantage of the electrochemical behavior, an analytical method based on differential pulse voltammetry for determination of caffeic acid in red wine was proposed.     

Optimization strategies in credit portfolio management

This paper focuses on the application of an original global optimization algorithm, based on the hybridization between a genetic algorithm and a semi-deterministic algorithm, for the resolution of various constrained optimization problems for realistic credit portfolios. Results are analyzed from a financial point of view in order to confirm their relevance.     

Determination of periodic orbits of nonlinear oscillators by means of piecewise-linearization methods

An approximate method based on piecewise linearization is developed for the determination of periodic orbits of nonlinear oscillators. The method is based on Taylor series expansions, provides piecewise analytical solutions in three-point intervals which are continuous everywhere and explicit three-point difference equations which are P-stable and have an infinite interval of periodicity. It is shown that the method presented here reduces to the well-known Störmer technique, is second-order accurate, and yields, upon applying Taylor series expansion and a Padé approximation, another P-stable technique whenever the Jacobian is different from zero. The method is generalized for single degree-of-freedom problems that contain the velocity, and (approximate) analytical solutions are presented. Finally, by introducing the inverse of a vector and the vector product and quotient, and using Taylor series expansions and a Padé approximation, the method has been generalized to multiple degree-of-freedom problems and results in explicit three-point finite difference equations which only involve vector multiplications.     

An almost L‐stable BDF‐type method for the numerical solution of stiff ODEs arising from the method of lines

A new BDF‐type scheme is proposed for the numerical integration of the system of ordinary differential equations that arises in the Method of Lines solution of time‐dependent partial differential equations. This system is usually stiff, so it is desirable for the numerical method to solve it to have good properties concerning stability. The method proposed in this article is almost L‐stable and of algebraic order three. Numerical experiments illustrate the performance of the new method on different stiff systems of ODEs after discretizing in the space variable some PDE problems. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007