Universal effective interactions of globular proteins close to liquid-liquid phase separation: corresponding-states behavior reflected in the structure factor

TL;DR

This study demonstrates that the extended law of corresponding states (ELCS) applies to globular protein solutions near liquid-liquid phase separation, with the structure factor showing universal behavior when normalized by critical temperature.

Contribution

The paper shows that the ELCS can describe the structure factor of protein solutions near LLPS, validated through SAXS data and effective colloidal models, revealing universal behavior.

Findings

Structure factor fits Baxter's adhesive hard-sphere model.

Normalized second virial coefficient $b_2$ is temperature-independent.

$b_2$ follows a universal trend when scaled by critical temperature.

Abstract

Intermolecular interactions in protein solutions in general contain many contributions. If short-range attractions dominate, the state diagram exhibits liquid-liquid phase separation (LLPS) that is metastable with respect to crystallization. In this case, the extended law of corresponding states (ELCS) suggests that thermodynamic properties are insensitive to details of the underlying interaction potential. Using lysozyme solutions, we investigate the applicability of the ELCS to the static structure factor and in how far effective colloidal interaction models can help to rationalize the phase behavior and interactions of protein solutions in the vicinity of the LLPS binodal. The (effective) structure factor has been determined by small-angle X-ray scattering (SAXS). It can be described by Baxter's adhesive hard-sphere model, which implies a single fit parameter from which the normalized second virial coefficient $b_2$ is inferred and found to quantitatively agree with previous results from static light scattering. The $b_2$ values are independent of protein concentration, but systematically vary with temperature and solution composition, i.e. salt and additive content. If plotted as a function of temperature normalized by the critical temperature, the values of $b_2$ follow a universal behaviour. These findings validate the applicability of the ELCS to globular protein solutions and indicate that the ELCS can also be reflected in the structure factor.