The porosity, order, biocompatibility, and chirality of protein crystals has motivated interest from diverse research domains including materials science, biotechnology, and medicine. Porous protein crystals have the unusual potential to organize guest molecules within highly ordered scaffolds, enabling applications ranging from biotemplating and catalysis to biosensing and drug delivery. Significant research has therefore been directed toward characterizing protein crystal materials in hopes of optimizing crystallization, scaffold stability, and application efficacy. In this overview article, we describe recent progress in the field of protein crystal materials with special attention given to applications in nanomedicine and nanobiotechnology.

The binding and release of guest fluorescent proteins inside a protein crystal with 13 nm axial pores is controlled. Spatially segregated guest protein loading is achieved via sequential binding and release stages. Additionally, selective stabilization of the crystal exterior results in hollow crystalline shells.

With rapidly growing interest in therapeutic macromolecules, targeted drug delivery, and in vivo biosensing comes the need for new nanostructured biomaterials capable of macromolecule storage and metered release that exhibit robust stability and cytocompatibility. One novel possibility for such a material are engineered large-pore protein crystals (LPCs). Here, various chemically stabilized LPC derived biomaterials were generated using three cross-linking agents: glutaraldehyde, oxaldehyde, and 1-ethyl-3-(3-(dimethylamino)­propyl)­carbodiimide. LPC biostability and in vitro mammalian cytocompatibility was subsequently evaluated and compared to similarly cross-linked tetragonal hen egg white lysozyme crystals. This study demonstrates the ability of various cross-linking chemistries to physically stabilize the molecular structure of LPC materialsincreasing their tolerance to challenging conditions while exhibiting minimal cytotoxicity. This approach produces LPC-derived biomaterials with promising utility for diverse applications in biotechnology and nanomedicine.

DNA assemblies have been used to organize inorganic nanoparticles into 3D arrays, with emergent properties arising as a result of nanoparticle spacing and geometry. We report here the use of engineered protein crystals as an alternative approach to biologically mediated assembly of inorganic nanoparticles. The protein crystal's 13 nm diameter pores result in an 80% solvent content and display hexahistidine sequences on their interior. The hexahistidine sequence captures Au25(glutathione)∼17 (nitrilotriacetic acid)∼1 nanoclusters throughout a chemically crosslinked crystal via the coordination of Ni(ii) to both the cluster and the protein. Nanoparticle loading was validated by confocal microscopy and elemental analysis. The nanoparticles may be released from the crystal by exposure to EDTA, which chelates the Ni(ii) and breaks the specific protein/nanoparticle interaction. The integrity of the protein crystals after crosslinking and nanoparticle capture was confirmed by single crystal X-ray crystallography.

Eu(TTA)3phen was synthesized inside of crosslinked protein crystals. And we characterized the volumetric changes quantitatively induced by DMSO.