Interaction between β‐lactoglobuline and a weak polyelectrolyte chain: a Monte Carlo simulation.

QUIROGA, EVELINA; BOERIS, VALERIA; PAOLA B. TORRES; CLAUDIO F. NARAMBUENA

Wageningen

Simposio; 12th International Symposium of Polyelectrolytes; 2018

Wageningen University and Research (WUR)

β‐lactoglobuline (BLG) is the main protein on serum lactum, and has an important nutritional value. The purification of the BLG through simple, fast and economic methods is the principal objective for their industrial application. In the presence of a polyelectrolyte (PE), a complex with the BLG can beobtained. This complex is formed in specific conditions and it is insoluble and easily separated. In this work we study at molecular level the interaction between a protein molecule of BLG and a weak polyelectrolyte chain. The methodology used consists on a coarse grain model with a minimum number of parameter that allows us to represent the physicochemical essence of the process. The simulations are performed with the Monte Carlo method. We focused on the electrostatic interaction and charge regulation mechanism after the complex formation of BLG‐PE is formed. The net charge of the isolated protein as a function of the solution pH was analyzed. Moreover, the dissociation degree and linear charge density of the isolated polyelectrolyte as a function of the pH was also studied. In the PE model, the equilibrium distance parameter l0 was modified and it took three different values: 0.25nm, 0.50nm and 0.75nm. The weak polyelectrolyte was modeled as an anionic chain with intrinsic pka value. The isolated results of the PE were compared with the ideal values as a function of the pH. The results from the simulation showed that the isolated PE had a lower dissociation degree compared to the ideal. This was due to the fact that the electrostatic repulsion between negatively charged monomers increases the protonation of acidic groups. This generates a shift, so the effective pKa moves to the right of the intrinsic pKa. The adsorption of the polyelectrolyte on the protein surface was molecularly quantified with a structural criterion. The amount of ionic pairs, which are a charged monomer with an oppositely charged residue of the protein, was quantified. Results showed that the interaction protein ? weak PE was favored at pH below the isoelectric point of the protein. At extreme acidic conditions the adsorption was low since the polyelectrolyte had a low dissociation degree. This produces a peak on the adsorption at pH 3.5 in the three cases studied. The maximum adsorption obtained was ~10 which was similar to 0.25nm, 0.50nm and 0.75nm. The peak observed in the interaction can be explained analyzing the protein net charge and dissociation degree. The protein net charge due to the presence of the anionic polyelectrolyte becomes more positively charged at pH below the isoelectric point. This increase of the positive charge leads to the dissociation of titratable groups of the polyelectrolyte chain.