We follow the gelation process of protein microparticles directly in real space. Unlike the individual proteins, the protein microparticles are large enough (around two micrometers) to be seen with an optical microscope, and yet, they exhibit protein-like interactions.
We use cold-set acid-induced gelation to separate the early pre-aggregation stages from the later clustering and gelation stages. This allows us to follow the advanced stages of aggregation and the gelation process alone. To obtain insight into how the gel emerges and obtain its stability, we follow the process in three dimensions with particle-scale resolution. The pictures above show three-dimensional reconstructions of the system after 6 (left) and 9 hours (right). Colour indicates the local connectivity (number of bonded neighbours) of a particle. This connectivity increases, while the structure coarsens.
Cluster analysis reveals that this process proceeds as a typical percolation process: the largest cluster keeps growing and successively incorporates the smaller clusters until it spans the entire field of view, as shown in Fig. 1. Remarkably, the cluster growth follows the scaling predictions from percolation theory, as predicted by our model viewing the gelation process as a dynamical critical point (see our paper in Nature Comm (2021)). This is shown in Fig. 2, where we plot the evolution of the fraction fp of particles in the largest cluster (Fig. 2a) and the cluster correlation length (Fig. 2b) as a function of average coordination number, <Z>. Critical scaling with the exponents -1.6 and -0.8 is observed, in agreement with the model.
Fig. 1 Protein cluster growth and gelation
(a) Growth of the largest (red dots) and second largest cluster (orange crosses). The largest cluster takes over at the expense of the second largest cluster, until it spans the field of view. (b) Reconstructions showing the largest (red) and second largest clusters (orange) after 300, 500 and 610min of growth.
Fig. 2 Critical scaling upon approaching the gel point
Evolution of the fraction fp of particles in the largest cluster (a), and the cluster correlation length (b) as a function of average coordination number, <Z>. Insets: scaling of both quantities upon approaching the critical coordination number.