Theoretical Study of the electrostatic behavior of Saccharomyces cerevisiae yeast cell using Poisson-Boltzmann equation

Abstract

The investigation of the electrostatic behavior and surface properties of cell surfaces in biological systems is of great importance for understanding biological process as metal uptake from solution, biofilm formations, biocorrosion processes, among others. Yeast cells are present in nature and in a number of scientific and industrial processes where electrostatic forces are predominant, such as adhesion processes, flotation and aggregation. The cell wall surface of yeast cells is composed of proteins and glycoproteins that contain many acid-base functional groups – e.g. carboxyl, phosphoryl, hydroxyl, and amine – that under different conditions can be ionized. The protonation or deprotonation of these groups influences the surface charge and promotes a charge distribution between the medium and the cell surface, creating a charge regulated surface. In this work, the electrostatic potential of the Saccharomyces cerevisiae yeast cell was studied and a value for the zeta potential was estimated. Calculations were performed using two models based on the Poisson-Boltzmann equation, the charge regulation on the surface model and the charge regulated volumetric density charge model. The two models differ in the way they describe the charge on the yeast cell surface, while the first considers a charge on the cell wall surface area the second model considers the charge varying over the volume of the cell wall. The zeta potential calculated using both models were then compared to experimental results to check for the accuracy of the models. Calculations were performed for aqueous solutions with two different ionic strengths, 1 mM and 10 mM. Results show that both models are able to describe experimental zeta potential points for different pH values, with the volumetric charge density model showing a slightly better fit than the charge regulation on the surface model. In general, the results for ionic strength of 10 mM for both models could describe better the shape of experimental points than the results for 1 mM. Some important parameters of the model related to the characterization of the yeast cell wall (functional groups) were estimated using the least-squares method, and results of the model with the estimated parameters showed great improvement in most cases. The same parameters were then further analyzed using a Monte Carlo simulation and a QR decomposition of the sensitive matrix of the charge regulation model to check the relation between the parameters and the model. In the analysis it was found a range for which the parameters can best describe experimental points and it was also possible to identify which parameters the model are most and least sensitive to.