Sorption of proteins to charged microgels: characterizing binding isotherms and driving forces

TL;DR

This study develops Langmuir binding models incorporating electrostatic cooperativity to analyze protein sorption to charged microgels, revealing that binding is mainly electrostatic and influenced by osmotic effects, with implications for interpreting binding isotherms.

Contribution

The paper introduces a novel Langmuir-based model that accounts for electrostatic cooperativity and substrate charge modifications during protein sorption, enabling more accurate analysis of binding mechanisms.

Findings

Intrinsic binding affinity is approximately 7 kBT.

Electrostatic interactions dominate total binding affinity.

Osmotic deswelling significantly affects sorption behavior.

Abstract

We present a set of Langmuir binding models in which electrostatic cooperativity effects to protein sorption is incorporated in the spirit of Guoy-Chapman-Stern models, where the global substrate (microgel) charge state is modified by bound reactants (charged proteins). Application of this approach to lysozyme sorption to oppositely charged core-shell microgels allows us to extract the intrinsic, binding affinity of the protein to the gel, which is salt-concentration independent and mostly hydrophobic in nature. The total binding affinity is found to be mainly electrostatic in nature, changes many orders of magnitude during the sorption process, and is significantly influenced by osmotic deswelling effects. The intrinsic binding affinity is determined to be about 7 kBT for our system. We additionally show that Langmuir binding models and those based on excludedvolume interactions are formally equivalent for low to moderate protein packing, if the nature of the bound state is consistently defined. Having appreciated this, a more quantitative interpretation of binding isotherms in terms of separate physical interactions is possible in future for a wide variety of experimental approaches.