Fluid of fused spheres as a model for protein solution

TL;DR

This paper models protein solutions using fused hard spheres with attractive sites, analyzing how non-spherical shapes influence thermodynamics and phase separation, supported by theoretical calculations and Monte Carlo simulations.

Contribution

It introduces a modified Wertheim's theory for fused-sphere molecules and compares predictions with simulations to study shape effects on phase behavior.

Findings

Non-spherical shape reduces critical density and temperature.

Model aligns with experimental data on non-spherical antibodies.

Thermodynamic properties are accurately predicted by the modified theory.

Abstract

In this work we examine thermodynamics of fluid with "molecules" represented by two fused hard spheres, decorated by the attractive square-well sites. Interactions between these sites are of short-range and cause association between the fused-sphere particles. The model can be used to study the non-spherical (or dimerized) proteins in solution. Thermodynamic quantities of the system are calculated using a modification of Wertheim's thermodynamic perturbation theory and the results compared with new Monte Carlo simulations under isobaric-isothermal conditions. In particular, we are interested in the liquid-liquid phase separation in such systems. The model fluid serves to evaluate the effect of the shape of the molecules, changing from spherical to more elongated (two fused spheres) ones. The results indicate that the effect of the non-spherical shape is to reduce the critical density and temperature. This finding is consistent with experimental observations for the antibodies of non-spherical shape.