3D Visualization of Proteins within Metal-Organic Frameworks via Ferritin-Enabled Electron Microscopy

Electron tomography holds great promise as a tool for investigating the 3D morphologies and internal structures of metal-organic framework-based protein biocomposites (protein@MOFs). Understanding the 3D spatial arrangement of proteins within protein@MOFs is paramount for developing synthetic methods to control their spatial localization and distribution patterns within the biocomposite crystals. In this study, the naturally occurring iron oxide mineral core of the protein horse spleen ferritin (Fn) is leveraged as a contrast agent to directly observe individual proteins once encapsulated into MOFs by electron microscopy techniques. This methodology couples scanning electron microscopy, transmission electron microscopy, and electron tomography to garner detailed 2D and 3D structural interpretations of where proteins spatially lie in Fn@MOF crystals, addressing the significant gaps in understanding how synthetic conditions relate to overall protein spatial localization and aggregation. These findings collectively reveal that adjusting the ligand-to-metal ratios, protein concentration, and the use of denaturing agents alters how proteins are arranged, localized, and aggregated within MOF crystals. Protein@MOF biocomposites are desirable materials for biocatalysis and the delivery of therapeutic proteins. The iron oxide mineral core in horse spleen ferritin enables direct protein visualization within protein@MOFs using electron microscopy methods. Electron tomography enables the quantification of the 3D protein spatial localization and aggregation in protein@MOF crystals with single protein resolution.image