Protein container scaffolds for the assembly of structured nanomaterials

  • Proteincontainergerüste für den Aufbau von strukturierten Nanomaterialien 

Künzle, Matthias; Simon, Ulrich (Thesis advisor); Schwaneberg, Ulrich (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Structured nanomaterials are an emerging class of novel materials composed of individual nanoscale building blocks. By programmed assembly of these building blocks, the materials’ properties can be tuned. In this regard, the structural organization of the materials must be precisely controlled. Biomolecules can be used as versatile scaffolds for the assembly of inorganic components in a highly ordered superlattice. Moreover, biomolecules add biological features and functionalities to the generated biohybrid material. In particular, protein containers, with their highly symmetrical shape and inherent ability to encapsulate cargo molecules, are perfect building blocks. Here, a novel strategy for the precise assembly of inorganic nanoparticles in a crystalline superlattice using protein containers as scaffolds is presented. First, two variants of the ferritin protein container, engineered with opposite surface charge, and the native encapsulin container from Thermotoga maritima were produced biochemically and purified to high purity. Subsequently, the protein containers were loaded with inorganic nanoparticles by two different loading approaches. In the nanoreactor approach, three types of metal oxide nanoparticles were synthesized directly within the container cavities. Here, the protein shell acted as a size-constraining reaction vessel for nanoparticle growth. In the second approach, the highly specific cargo-loading mechanism of the encapsulin nanocompartment was employed for the encapsulation of presynthesized nanoparticles. For this purpose, gold nanoparticles were decorated with a small number of encapsulin cargo-loading peptides (CLP). By lock-and-key interaction between the peptides and the peptide-binding pockets on the inner container surface, nanoparticles were encapsulated with high efficiency. Moreover, peptide-directed encapsulation was independent from the ionic strength of solution, in contrast to encapsulation of cationic nanoparticles by electrostatic interactions with negatively charged patches on the inner container surface. After nanoparticle loading, oppositely charged ferritin containers were crystallized in highly ordered superlattices with different nanoparticle combinations. Due to favorable electrostatic interactions, binary protein crystals with a tetragonal lattice in an unusual CuAu-like arrangement were obtained. The structure type could be changed by increasing the magnesium concentration in the crystallization condition, resulting in a unitary superlattice composed of only negatively charged ferritins. As demonstrated by X ray structure determination, magnesium ions play a central role in the crystal interface of unitary crystals by complex formation between two adjacent protein subunits. Importantly, nanoparticle loading did not influence the structure of the crystals as the protein container overwrote any imperfections of the nanoparticle cargo.

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