Adsorption of pH responsive ampholytic ions into weak Polyelectrolyte brush: A simulation study

Weak polyelectrolytic (PE) brush mediated protein adsorption has gained significant attention in the recent past. Especially beyond the isoelectric point, where a similar charge PE-protein attraction is seen. While, one plausible argument is the presence of low effective pH inside the brush, leading to subsequent charge reversal in the ampholyte protein. Other studies on patchy protein attributes it to the attraction of oppositely charged patches on protein surfaces and a counterion release force. To corroborate these further, we employ an explicit ion based molecular dynamic simulations to study a two phase weak PE brush system in contact with a reservoir consisting pH-responsive ampholytic entities. This simualtion framework accounts for both acid-base equilibria as well as the exchange of ions across the two phases within a grand canonical framework. Strong Donan partitioning of ions as well as the electrostatic effects together is seen to cause a significant shift in effective pH inside the brush, impacting the charge regulation and net uptake of adsorbing ampholyte ions. Insights from this study are extended to advanced models mimicking globular proteins, such as macroscopic particles with ionizable surface groups. Understanding charge regulation coupled with the structural arrangement of surface groups reveals intriguing facets of the protein adsorption mechanism under changing physiological conditions.