Interplay between Spinodal Decomposition and Glass Formation in Proteins Exhibiting Short-Range Attractions

We investigate the competition between spinodal decomposition and dynamical arrest using aqueous solutions of the globular protein lysozyme as a model system for colloids with short-range attractions. We show that quenches below a temperature $T_a$ lead to gel formation as a result of a local arrest of the protein-dense phase during spinodal decomposition. The rheological properties of these gels allow us to use centrifugation experiments to determine the local densities of both phases and to precisely locate the gel boundary and the attractive glass line close to and within the unstable region of the phase diagram.

Figure 1

(a) $T\mathrm{\text{−}}\varphi$ plane of the phase diagram of aqueous lysozyme solutions ($20\mathrm{m}M$ Hepes buffer, $p\mathrm{H}=7.8$, $0.5M$ NaCl). Liquid-liquid coexistence curve (○), spinodal (□). Also shown are state points in the unstable region investigated with rheology where liquidlike (*) and solidlike (×) behavior has been observed. (b) Phase contrast micrographs of samples at $\varphi =0.11$ showing the coarsening at $16.8°\mathrm{C}$ (*) and the freezing at $13°\mathrm{C}$ (×) of the bicontinuous texture in the spinodal region.

Figure 2

(a) Elastic modulus $G^{\prime }(\nu )$ (full symbols) and loss modulus $G^{\prime \prime }(\nu )$ (open symbols) for a gel formed in the spinodal region at $\varphi =0.148$ and a quench temperature $T=13°\mathrm{C}<T_a$. (b) Corresponding inverse creep compliances $J^{-1}(t)$.

Figure 3

(a) Evolution of the relative height of the interface, $h(t)$, with centrifugation time for different quenched temperatures. (b) Dependence of the plateau value $h_{\mathrm{pl}}$ relative to imposed acceleration. Same symbols as in (a).

Figure 4

Kinetic phase diagram of aqueous lysozyme solutions showing regions of complete demixing (I), gel formation (II), and glasses (III). Full symbols stand for the results of the centrifugation experiments: (●) arrested dense phase, (◆) dilute phase.