El borat de zinc com a retardant de flama

Els borats són minerals que podem trobar a la natura i que contenen bor, el cinquè element de la taula periòdica. N'hi ha traces a roques, al sòl i a l'aigua. Tant el bor com el zinc són micronutrients essencials pel creixement de les plantes. Així mateix, les persones també els necessiten com a part important d'una dieta saludable i com un ingredient essencial en molts productes necessaris per tenir un estàndard de vida acceptable

Els plastòmers

Un plastòmer és un material polimèric que presenta propietats d'elastòmers termoplàstics, és a dir, presenta tant deformació plàstica com elastomèrica. Els plastòmers són una classe de copolímers que van sorgir amb els avenços en la polimerització amb catalitzadors metal·locènics i estan fabricats amb etilè i una alfa-olfina. Típicament, les alfa-olefines emprades són l'1-propè, l'1-butè, l'1-hexè i l'1-octè (el més fet servir actualment)

El carbonat de calci: molt més que un filler

El carbonat de calci és un dels compostos químics més emprats, més útils i més versàtils coneguts per l'home. La seva fórmula és CaCO3, té una aparença de pols blanca, no fa olor i és molt poc soluble en aigua i molt soluble en àcids. És la sal de calci més abundant a la naturalesa, a més d'ésser el component principal de les roques; representa el 15% dels sediments i de les roques sedimentàries de la Terra, i el 4% de l'escorça terrestre amb diferents formes i graus de puresa. Les plantes i els animals el necessiten per viure i poder formar els seus esquelets. L'ésser humà l'ha transformat en un producte bàsic en el seu desenvolupament industrial, mèdic, etc.; tant és així que gairebé tots els productes que ens envolten en la nostra vida quotidiana en contenen

El racó de la història: Cent anys del tercer principi de la termodinàmica: 1906-2006

l Fons Històric de Física i Química de la Biblioteca de les facultats de Química i Física de la Universitat de Barcelona va néixer el 2005 arran de l'Any Mundial de la Física dedicat al centenari de tres de les principals aportacions d'Albert Einstein, publicades el 1905. Per aprofitar el gran nombre de fonts primàries, bàsicament llibres i revistes, que la Biblioteca disposa relatives a les diverses disciplines de la física i la química, així com d'un elevat nombre de monografies escrites pels principals científics que han contribuït al desenvolupament d'aquestes, es va crear una mostra permanent de les principals fonts primàries a partir del 1820, ja que les anteriors es troben al fons de reserva de la Biblioteca de l'edifici històric de la Universitat de Barcelona, relacionades amb les principals disciplines científiques en què es pot dividir la física i la química. Junt amb la mostra es va realitzar una exposició temporal dedicada a l'annus mirabilis d'Einstein relatiu a les seves brillants contribucions sobre la relativitat especial, l'efecte..

Unravelling Constant pH Molecular Dynamics in Oligopeptides with Explicit Solvation

An accurate description of the protonation state of amino acids is essential to correctly simulate the conformational space and the mechanisms of action of proteins or other biochemical systems. The pH and the electrochemical environments are decisive factors to define the effective pKa of amino acids and, therefore, the protonation state. However, they are poorly considered in Molecular Dynamics (MD) simulations. To deal with this problem, constant pH Molecular Dynamics (cpHMD) methods have been developed in recent decades, demonstrating a great ability to consider the effective pKa of amino acids within complex structures. Nonetheless, there are very few studies that assess the effect of these approaches in the conformational sampling. In a previous work of our research group, we detected strengths and weaknesses of the discrete cpHMD method implemented in AMBER when simulating capped tripeptides in implicit solvent. Now, we progressed this assessment by including explicit solvation in these peptides. To analyze more in depth the scope of the reported limitations, we also carried out simulations of oligopeptides with distinct positions of the titratable amino acids. Our study showed that the explicit solvation model does not improve the previously noted weaknesses and, furthermore, the separation of the titratable amino acids in oligopeptides can minimize them, thus providing guidelines to improve the conformational sampling in the cpHMD simulations

On the use of the discrete constant pH molecular dynamics to describe the conformational space of peptides

Solvent pH is an important property that defines the protonation state of the amino acids and, therefore, modulates the interactions and the conformational space of the biochemical systems. Generally, this thermodynamic variable is poorly considered in Molecular Dynamics (MD) simulations. Fortunately, this lack has been overcome by means of the Constant pH Molecular Dynamics (CPHMD) methods in the recent decades. Several studies have reported promising results from these approaches that include pH in simulations but focus on the prediction of the effective pKa of the amino acids. In this work, we want to shed some light on the CPHMD method and its implementation in the AMBER suitcase from a conformational point of view. To achieve this goal, we performed CPHMD and conventional MD (CMD) simulations of six protonatable amino acids in a blocked tripeptide structure to compare the conformational sampling and energy distributions of both methods. The results reveal strengths and weaknesses of the CPHMD method in the implementation of AMBER18 version. The change of the protonation state according to the chemical environment is presumably an improvement in the accuracy of the simulations. However, the simulations of the deprotonated forms are not consistent, which is related to an inaccurate assignment of the partial charges of the backbone atoms in the CPHMD residues. Therefore, we recommend the CPHMD methods of AMBER program but pointing out the need to compare structural properties with experimental data to bring reliability to the conformational sampling of the simulations

Effect of charge regulation and conformational equilibria in the 2 stretching properties of weak polyelectrolytes

Weak polyelectrolytes can modulate their charge in response to external perturbations, such as changes in the pH, ionic strength (I), or electrostatic interactions with other charged species, a phenomenon known as charge regulation (CR). On the other hand, it is well established that CR is highly coupled with the conformational degrees of freedom. In this paper, the influence of CR in the stretching properties of weak polyelectrolytes is analyzed, and the possibility of CR induced by mechanical stretching is explored. With this aim, we make use of a minimal model, which captures the fundamental aspects present in the stretching of a flexible weak linear polyelectrolyte: internal angle rotation, bond stretching, bond bending, and proton binding, which is the paradigmatic mechanism of CR. The angle rotation is described by using the rotational isomeric state approximation, while for protonation, the site binding model is assumed. Mechanical stretching is studied by performing semi-grand canonical Monte Carlo simulations at different pH and ionic strength conditions. The simulations simultaneously provide both conformational (bond state probabilities, persistence length lP, and chain elongation) and protonation properties (degree of protonation θ and the effective protonation constant Kc). The obtained force−extension curves suggest that the pH value and the ionic strength I have a significant effect on polyelectrolyte stretching. Three different force regimes can be observed. For large forces (F > 100 pN for typical force constants), the force−extension curve is almost independent of the pH and I. For low forces, the persistence length lP is force-independent, although it strongly increases with the pH value. Under this regime, linear and Pincus scaling behaviors are observed. Finally, in the intermediate-force regime, both rotational and protonation degrees of freedom are mechanically activated, and the picture becomes more complicated. It is found that lP increases with F and, under certain conditions, a significant increase of θ with F is observed, indicating that CR could in principle be induced by means of mechanical stretching. This fact can be explained by analyzing the coupling between θ and the probability of a bond to be in the gauche state P(g). P(g) decreases with F as the bonds adopt the trans conformation so that the electrostatic repulsion is reduced and θ increases. Finally, the intricate interplay between short-range and long-range interactions is analyzed, leading to apparently contradictory behaviors (P(g) and lp simultaneously decrease with I), which can only be explained by CR and the presence of complex spatial correlations