An entropy-based study of effects of neutral polymer additives on protein crystallization

Crystallization of proteins and colloidal particles is important in both fundamental and applied chemistry. To promote the crystallization, some polymer additives are used. This study investigates the effects of polymer additives on protein crystallization from entropic perspectives. From the study, it was found that the polymer additives simultaneously satisfy that (1) selective adsorption of the protein on the crystal surface; (2) thermo-dynamical facilitation of the crystallization; (3) preservation of the protein's structural stability. Recently, we developed the non-additive Asakura-Oosawa (NAO) theory which accounts for size non-additivities of particles. The NAO theory explained selective adsorption of the protein onto the crystal surface. While crystallization the mixing entropy of the solution decreases. However, it was found that the mixing entropy loss is inhibited when there are polymer additives in the solution. Maintaining protein structural stability is also critical in the crystallization. Our simple lattice model predicts that changes in conformational entropy upon unfolding of the protein with and without the polymer additives are the same. Furthermore, to verify the validity of theoretical result of the conformational entropy, we investigated the thermal stability of a protein by measuring UV-Vis absorbance and particle size distribution, where we used hemoglobin as a protein model and soluble starch as a polymer additive model. These experimental studies show that the polymers do not negatively affect thermal and structural stabilities of the protein. These findings provide insights for applications in structural biology, soft-matter physics, and materials science.