Modulation of the electrostatic potential around α-lactalbumin using oligoelectrolyte chains, pH and salt concentration.

In this study, we conducted a comprehensive computational investigation of the interaction between α-lactalbumin, a small globular protein, and strong anionic oligoelectrolyte chains with a polymerization degree from 2 to 9. Both the protein and oligoelectrolyte chains are represented using coarse-grained models, and their properties were calculated by the Monte Carlo method under constant pH conditions. We were able to estimate the effects of this interaction on the electrostatic potential around the protein. At acidic pH, the protein had a net positive charge; therefore, the electrostatic potential around it was also positive. To neutralize or reverse this electrostatic potential, oligoelectrolyte chains with a minimum size of six monomers were necessary. Simultaneously, low salt concentrations were required as elevated salt levels led to a significant attenuation of the electrostatic interactions and the corresponding electrostatic potential.

Order and disorder in the adsorption model of repulsively interacting binary mixtures on triangular lattices: theory and Monte Carlo simulations.

The problem of interacting binary mixtures adsorbed on triangular lattices has been studied by means of ground-state (GS) calculations, Monte Carlo (MC) simulations and exact counting of configurations on finite cells [the so-called cluster-exact approximation (CA)]. We focus on the case of repulsive intraspecies couplings and null interspecies interaction, where a rich variety of ordered phases is found in the adsorbed layer. Each surface structure is separated from a disordered state by a phase transition occurring at a finite temperature. The ordered phases are identified by means of GS analysis, and their dependence on temperature is studied by MC simulations and CA. Total and partial adsorption isotherms are obtained for values of the lateral interactions in the different regions of the phase diagram. MC results are compared with CA calculations. A very good coincidence is obtained between both methods, supporting the validity of the exact counting of states on finite cells as an adequate approach to describe the behavior of multicomponent competitive adsorption with adsorbate-adsorbate interactions.

Adsorption of interacting particles on bivariate diffusion-limited aggregates.

The adsorption of pairwise interacting particles on fractal surfaces has been studied by grand canonical Monte Carlo simulations. The substrate is built from a mixture of two types of objects: (i) objects with two bonds and (ii) objects with four bonds. These objects move on a square lattice, according to diffusion-limited aggregation (DLA) rules, and stick to each other only if they have a free bond pointing at each other and, of course, are first neighbors of each other. The resulting substrate, which is named as bivariate diffusion-limited aggregate (BDLA), is a fractal structure composed by two bonds units with fraction [Formula: see text] and four bonds units with concentration [Formula: see text]. Different surface morphologies are obtained by varying [Formula: see text] and [Formula: see text]. In the limit case of [Formula: see text] and [Formula: see text], the standard DLA model is recovered. In addition, repulsive lateral interactions between adsorbed particles are considered. Adsorption isotherms and differential heats of adsorption are calculated for different values of the parameters of the system. In the case of high repulsive couplings, a wide variety of structural orderings are observed in the adlayer. The main characteristics of these ordered phases are discussed in terms of the topological properties of the bivariate aggregates.

Protonation of ß-lactoglobulin in the presence of strong polyelectrolyte chains: a study using Monte Carlo simulation.

In this work, the molecular interaction between the protein ß-lactoglobulin and strong polyelectrolyte chains was studied using Monte Carlo simulations. Different coarse-grained models were used to represent the system components. Both net charge and protonation of the isolated dimeric protein were analyzed as a function of pH. The acid-base equilibrium of each titratable group was distinctively modified by the presence of polyanion or polycation chains. The complexation on the wrong side of pI was more evident with the polycation than with the polyanion. It was mainly due to a charge regulation mechanism, where the reversion in net charge of the protein was more pronounced at the left of isoelectric point of the protein. The glutamic and aspartic groups play a key role in this charge reversion. Both polyanion and polycation were spatially adsorbed in different region on the protein surface, suggesting the importance of the surface charge distribution of the protein.