Magnetite Mineralization inside Cross-Linked Protein Crystals

Abstract

Crystallization in confined spaces is a widespread process in nature that also has important implications for the stability and durability of many man-made materials. It has been reported that confinement can alter essential crystallization events, such as nucleation and growth and, thus, have an impact on crystal size, polymorphism, morphology, and stability. Therefore, the study of nucleation in confined spaces can help us understand similar events that occur in nature, such as biomineralization, design new methods to control crystallization, and expand our knowledge in the field of crystallography. Although the fundamental interest is clear, basic models at the laboratory scale are scarce mainly due to the difficulty in obtaining well-defined confined spaces allowing a simultaneous study of the mineralization process outside and inside the cavities. Herein, we have studied magnetite precipitation in the channels of cross-linked protein crystals (CLPCs) with different channel pore sizes, as a model of crystallization in confined spaces. Our results show that nucleation of an Fe-rich phase occurs inside the protein channels in all cases, but, by a combination of chemical and physical effects, the channel diameter of CLPCs exerted a precise control on the size and stability of those Fe-rich nanoparticles. The small diameters of protein channels restrain the growth of metastable intermediates to around 2 nm and stabilize them over time. At larger pore diameters, recrystallization of the Fe-rich precursors into more stable phases was observed. This study highlights the impact that crystallization in confined spaces can have on the physicochemical properties of the resulting crystals and shows that CLPCs can be interesting substrates to study this process.

Porous protein crystals: synthesis and applications

“…139 Magnetite nucleation is achieved through a combination of iron binding within a confined region, and localized iron supersaturation. 140 Following nucleation within the crystal pores, metastable growth of larger nanoparticles and nanorods can be initiated by adjusting metal ion concentration, pH, or other solution conditions. Templating enables control over the size and composition of the resulting nanoparticles and facilitates the development of functional nanomaterials, in particular nanoparticles, nanorods, and nanowires.…”

“…Previous methods involving the use of nanoporous protein scaffolds as templates for nanorod and nanoparticle production have relied on a diverse range of protein-based scaffolds, including hen egg white lysozyme (HEWL) crystals, 140,142,143 bacterial polyisoprenoid-binding proteins, 141 modified ferritin cages, 144,145 S-layer protein arrays 146 and BSA adsorbed to titanium oxide. 147 Ferritin cages, in particular, continue to be heavily investigated due to well-established protocols for heterologous expression, purification, and in vitro assembly.…”