Formation of dielectric layers and charge regulation in protein adsorption at biomimetic interfaces

Protein charge is an important parameter in the understanding of protein interactions and function. Proteins are subject to dynamic charge regulation, that is, the influence of the local environment (such as charged interfaces and biopolymers) on protein charge. Charge regulation is governed by differences in the dielectric and electrostatic environment between adsorbed protein and the free protein in bulk solution. In this work protein charge regulation is addressed experimentally by employing electrochemistry at interfaces between two immiscible electrolyte solutions (ITIES) as well as theoretically by developing a new protein adsorption model at ITIES. Electrochemistry at ITIES is shown to be particularly well suited to study protein charge regulation as the adsorbed protein experiences a different dielectric environment compared to the bulk phase and the external control of the water/oil potential difference allows systematic studies on how potential induced ion gradients affect protein charge. The theoretical model incorporates all the features of the experimental system and specifically takes into account protein charge regulation at ITIES as well as the impact of the formation of dielectric layers on the experimentally observed impedance. The model parameters include the protein charge-pH profile, bulk pH, and the overall potential difference. It is shown that the formation of a dielectric layer and the associated charge regulation are the main factors dictating the observed experimental behavior. Finally, the theoretical model is used to interpret literature results, and the consistency between the model and the relatively large data set suggests that the model may be used more generally for understanding and predicting protein adsorption.