Why Do Protein Crystals Grown With the Aid of Porous Materials Show Better Diffraction Quality?

Well-diffracting protein crystals are indispensable for X-ray diffraction analysis, which is still the most powerful method for structure-function studies of biomolecules. A promising approach to growing such crystals is by using porous nucleation-inducing materials. However, while protein crystal nucleation in pores has been thoroughly considered, little attention has been paid to the subsequent growth of the crystals. Although the nucleation stage is decisive, it is the subsequent growth of the crystals outside the pore that determines their diffraction quality. The molecular-scale mechanism of growth of protein crystals in and outside pores is here considered theoretically. Due to the metastable conditions, the crystals that emerge from the pores grow slowly, which is a prerequisite for better diffraction. This expectation has been corroborated by experiments carried out with several types of porous material, such as Bioglass (“Naomi’s Nucleant”), Buckypaper, porous gold and porous silicon. Protein crystals grown with the aid of Bioglass and Buckypaper yielded significantly better diffraction quality compared with crystals grown conventionally. In all cases, visually superior crystals are usually obtained. We furthermore conclude that heterogeneous nucleation of a crystal outside the pore is an exceptional case. Rather, the protein crystals nucleating inside the pores continue growing outside them.