| Our lab has developed ferritin proteins as
templates for the synthesis and assembly of inorganic
nanoparticles. Ferritins are a large family of 4-helix bundle
proteins that self-assemble and create a central cavity that ranges
from 4-9 nm in diameter. In order to expand the utility of
ferritin for templating stable, monodisperse, and readily
functionalized inorganic nanoparticles, the lab has focused on
manipulating reactive amino acids on the protein exterior and interior
surfaces, as well as the pores leading to the cavity. We have shown
that ferritin proteins are very amenable to large-scale
mutations. Guided by computational design, performed in
collaboration with Prof. Jeffery Saven, the lab made a ferritin-like
protein with an 86% apolar cavity surface area. This may represent the
most hydrophobic large protein cavity ever studied. More
recently, we have generated a ferritin (H8, Figure 6) with a
cysteine-rich cavity that promotes the formation of stable silver and
gold nanoparticles (Figure 7). With 192 total mutations, this
protein is the first example in which Au0 nanoparticles have been
synthesized inside ferritin. TEM data show 2-4 nm diameter Au and Ag
particles in the cavity. In collaboration with the Christianson
lab, we have obtained a 2.7-Å resolution crystal structure of H8 that
shows proper subunit assembly and the 96 non-native cysteines reduced
within the interior. Additional crystallographic experiments
should help to identify routes by which Ag+ and Au3+ gain access to the
interior sulfhydryl ligands. The channels provide a route for
controlling the synthesis of different metal particles.
|
 Figure 6. X-ray crystal structure (at 2.72-Å
resolution) of a designed human H ferritin (H8) expanded to show a
single H8 4-helix bundle subunit with mutations made to the exterior
(red) and interior (yellow) surfaces.
|
 Figure 7. Reaction of cysteine-rich ferritin 24-mer with Ag+ or Au3+ under mild reducing conditions yields nanoparticles in the protein. |
| By an
alternate strategy,
we have succeeded in using a hyperthermophilic ferritin to incorporate
with almost 100% efficiency 10-nm diameter citrate-capped gold
nanoparticles through a protein disassembly/reassembly procedure
(Figure 8). Ferritin-capped gold nanoparticles are much more
stable to salt solutions, and can be readily assembled from exactly 24
ferritin 4-helix bundle subunits. These very well-defined, monodisperse
protein-inorganic nanoparticle assemblies open exciting areas for study
in bioinorganic chemistry, bioengineering and nanomaterials
science. |
 Figure 8. Thermophilic ferritin assembly mediated by salt or Au nanoparticle. Inorganic particle is incorporated with > 95% efficiency. |