Myoglobin molecule charging in electrolyte solutions

Abstract

All atom molecular dynamic modeling was applied in order to determine water molecule and electrolyte ion concentration profiles around and inside the myoglobin molecule at various pH values. Significant penetration of counter ions into the molecule was confirmed. The electric potential distribution within and outside the molecule was quantitatively described using the non-linear Poisson–Boltzmann (PB) approach. Using this model, calculations were performed, yielding the surface and zeta potential for various physicochemical parameters, comprising pH, the electric permittivity, the ion penetration depth and the protein volume fraction (crowding effect). The theoretical results were used for the interpretation of experimental data acquired under different ionic strengths and temperatures by electrophoretic mobility measurements. It is confirmed that the experimental data are adequately reflected for acidic pH values by the non-linear PB model where the nominal molecule charge was calculated from the H++ model. The deviations occurring for larger pH values were accounted for by considering additional non-electrostatic interactions stemming from the van der Waals and ion-induced dipole forces. In this way, it is both experimentally and theoretically confirmed that the effective charge of the myoglobin molecule in electrolyte solutions is considerably smaller than the nominal, structure-based, predicted charge. As a result, under physiological conditions prevailing, e.g. in skeletal muscles, the effective charge of the myoglobin molecule should practically vanish. One can expect that the approach developed in this work can be applied for predicting charging mechanisms of other protein molecules characterized by an analogous charge vs. pH characteristic, e.g., the SARS-CoV-2 virus spike proteins, and for soft particles with pH responsive characteristics.