Light scattering and phase behavior of Lysozyme-PEG mixtures

TL;DR

This study investigates how different molecular weights of PEG affect lysozyme solutions, revealing that PEG induces attractive interactions contrary to depletion theory, with implications for understanding protein-polymer phase behavior.

Contribution

The paper demonstrates that lysozyme-PEG interactions are driven by energetic attraction rather than entropic depletion, supported by light scattering and phase transition measurements.

Findings

Low molecular weight PEG stabilizes lysozyme solutions.

High molecular weight PEG destabilizes lysozyme solutions.

Attractive lysozyme-PEG interactions are confirmed by virial coefficient analysis.

Abstract

Measurements of liquid-liquid phase transition temperatures (cloud points) of mixtures of a protein (lysozyme) and a polymer, poly(ethylene glycol) (PEG) show that the addition of low molecular weight PEG stabilizes the mixture whereas high molecular weight PEG was destabilizing. We demonstrate that this behavior is inconsistent with an entropic depletion interaction between lysozyme and PEG and suggest that an energetic attraction between lysozyme and PEG is responsible. In order to independently characterize the lysozyme/PEG interactions, light scattering experiments on the same mixtures were performed to measure second and third virial coefficients. These measurements indicate that PEG induces repulsion between lysozyme molecules, contrary to the depletion prediction. Furthermore, it is shown that third virial terms must be included in the mixture's free energy in order to qualitatively capture our cloud point and light scattering data. The light scattering results were consistent with the cloud point measurements and indicate that attractions do exist between lysozyme and PEG.