Viscoelastic Phase Separation of Protein Solutions

In addition to the known behavior of normal phase separation and gelation, we report novel phase-separation behavior of protein solutions as their intermediate case. A network structure of the protein-rich phase may be formed even if it is the minority phase, contrary to the conventional wisdom. This behavior is characteristic of viscoelastic phase separation found in polymer solutions. This kinetic pathway may play crucial roles in the complex phase ordering of protein solutions, in particular, protein network formation in biological systems and foods.

Figure 1

Phase diagram of protein solutions in the $c\mathrm{\text{−}}T$ plane. ○: droplet phase separation [see Fig. 2a]. ●: network phase separation [see Figs. 2b, 2c]. □: phase separation accompanying physical gelation [see Fig. 2d].

Figure 2

Temporal change in 2D phase-separation patterns observed for protein solutions of $c=200\mathrm{mg}/\mathrm{ml}$. The width of the images corresponds to $230\mu \mathrm{m}$.

Figure 3

3D pattern evolution observed at $T=42°\mathrm{C}$ for $c=175\mathrm{mg}/\mathrm{ml}$ and 7.5 wt % NaCl ($160\times 160\times 40\mu {\mathrm{m}}^3$).

Figure 4

(a) Temporal change in $q_p$ for phase separation of a lysozyme solution of $c=200\mathrm{mg}/\mathrm{ml}$ and 7.5 wt % NaCl at $T=38$ (□), 37 (●), and 36 °C (○). $q_p$ decreases with time as $q_p\propto t^{-1/2}$, different from NPS. (b) Temporal change in ${\Phi }_V$ and ${\chi }_E$ for the 3D pattern evolution shown in Fig. 3.