Inside cells, proteins may be found in crowded solution, at high concentrations or immobilized on surfaces, without losing their activity. In contrast, in highly concentrated bulk solutions, proteins tend to aggregate and crash out. This is the case, for example, for myoglobin and lysozyme, two small proteins whose activity in the cytoplasm of a cell is controlled through their interaction with a chaperonin for proper folding and unfolding. Now, Siefker et al. have shown that aggregation of both myoglobin and lysozyme is also prevented inside the pores of two mesoporous silica materials, SBA-15 and KIT-6.
The researchers load their mesoporous silica with either protein at increasing concentrations and carry out small-angle neutron scattering experiments. At low loading, the scattering signals are indistinguishable from those of the unloaded silica. Because of low signal contrast, the presence of a small amount of protein cannot be reliably distinguished from the roughness of the pore walls. However, at high loading, high enough to approach the maximum packing density of the proteins, a broad feature starts to appear in the scattering spectrum that is characteristic of species showing intense, not sterically hindered, intermolecular interactions, but not aggregation. This liquid-like behaviour is due to enhanced interactions among proteins, and between the proteins and the pore walls, which promote stabilization rather than aggregation, similar to what is observed inside the cytosol.