Nucleation of protein crystals in pores and their growth

X-ray diffraction enables determination of biomolecular structure but requires well-diffracting crystals that are notoriously difficult to grow. Porous materials can aid the crystallization of refractory proteins and, since knowledge of the mode of action of such materials may contribute to finding new crystallization inducers, this process has been studied thoroughly. It was established that, even under conditions where heterogeneous nucleation on flat surfaces is absent, a synergistic diffusion-adsorption effect inside a sufficiently narrow pore can increase the protein concentration to a level sufficient for crystal nucleation. The formation of a protein crystal in a pore begins with the assembly of molecules into a crystalline layer of monomolecular thickness, which is stabilized by its cohesion with the pore wall. We highlight thermodynamic considerations that provide an estimate of the importance of the protection due to the pore walls for crystal stability. In addition, molecular-kinetic considerations reveal further details of protein crystal nucleation assisted by porous materials. The observation that protein crystals nucleated by means of porous materials often display improved X-ray diffraction is of practical importance for structural studies. It is hoped that this review will guide scientists in their efforts to grow crystals of target proteins, complementing the usual trial-and-error strategies.