Protein-Polymer Mixtures in the Colloid Limit: Aggregation, Sedimentation and Crystallization

TL;DR

This study explores the phase behavior of non-spherical eGFP proteins with added polymer, revealing aggregation and crystallization patterns similar to hard spherocylinders, and confirms theoretical predictions with experimental data.

Contribution

It introduces a non-spherical protein model and demonstrates that protein-polymer mixtures behave like hard spherocylinders with ideal polymers, aligning experimental results with theoretical predictions.

Findings

eGFP undergoes dimerization forming spherocylinders

Phase behavior shows aggregation and crystallization

Experimental results agree with theoretical models

Abstract

While proteins have been treated as particles with a spherically symmetric interaction, of course in reality the situation is rather more complex. A simple step towards higher complexity is to treat the proteins as non--spherical particles and that is the approach we pursue here. We investigate the phase behavior of enhanced green fluorescent protein (eGFP) under the addition of a non--adsorbing polymer, polyethylene glycol (PEG). From small angle x-ray scattering we infer that the eGFP undergoes dimerization and we treat the dimers as spherocylinders with aspect ratio $L/D-1 = 1.05$. Despite the complex nature of the proteins, we find that the phase behaviour is similar to that of hard spherocylinders with ideal polymer depletant, exhibiting aggregation and, in a small region of the phase diagram, crystallization. By comparing our measurements of the onset of aggregation with predictions for hard colloids and ideal polymers [S.V. Savenko and M. Dijkstra, J. Chem. Phys 124, 234902 (2006) and F. lo Verso et al., Phys. Rev. E 73, 061407 (2006)] we find good agreement, which suggests that the eGFP proteins are consistent with hard spherocylinders and ideal polymer.