Biomineralisation - Biological Inorganic Chemistry - page 367

Ferritin as a Supramolecular Template in Nanotechnology
As one might have anticipated, the extraordinary way in which biological systems manage to control the
morphology, the particular form or crystal structure which is selected, and the precise spatial localisation in which
it is formed has made biomineralisation processes a focal point for nanotechnology. Understanding the interac-
tions between hard (inorganic) and soft (organic) materials has stimulated the design and application of synthetic
biomimetic systems. Ferritin appears as an apparently simple system with only a single protein component, which
directs biomineralisation of iron oxide at the protein solution interface closed shell. Ferritin itself catalyses the

transformation of the substrate, possesses a nucleation site for the biomineral, and its architecture defines and
imposes the overall morphology of the final product. Add to this that the protein shell maintains the final bio-
mineral product both soluble and mobile, while at the same time being biochemically inert, and one can
understand the drive to push ferritin biomineralisation towards biomimetic synthesis.
The potential of some synthetic ferritins in which a non-native inorganic material has been introduced into the
cavity or the external shell has been modified, has become a very active area of research. In early studies, the
apoferritin protein shell was simply as a 'nano reactor' for the formation of a variety of non-native, unusual,
mineralised nanoparticles. For example, under conditions of high pH and limited oxygen, it is possible to produce
ferritin cores corresponding to the magnetic cores of mixed valence minerals. Thus, magnetite (Fe 3 O 4 ) and/or
maghaemite (

e

-Fe 2 O 3 ) can be generated in vitro which may have potential interest as MRI contrast agents. Cores
of amorphous iron sulfide have been produced, containing either 500 or 3000 iron atoms with the iron mostly in
the

g

3 state in FeS 4 tetrahedra with connecting FeS 2 Fe bridges. Cores of manganese oxyhydroxide (MnOOH)
have been synthesised in both H and L chain homopolymers, and in both mammalian ferritins and L. innocua Dps
protein, a mineral core of cobalt and oxygen can be generated by the protein-catalysed oxidation of Co 2 þ to Co 3 þ .
Ferritins without their iron core can be used to photocatalyse the formation of Cu(0) colloids from aqueous Cu 2 þ
within the protein cavity. Ferritin cores containing Cu and CuFe Prussian Blue derivative nanoparticles
( Figure 19.9 ) have been prepared.

þ

FIGURE 19.9 Cu and CuFe Prussian Blue nanoparticles have been prepared using a Cu(II) loaded apoferritin as a chemically and spatially
confined environment for their construction. (from Ga ´ lvez et al., 2005. Copyright 2005 with permission from the Royal Society of Chemistry.)

Material scientists have exploited a range of ferritin superfamily proteins as supramolecular templates to
encapsulate nanoparticles and/or as well-defined building blocks for fabrication of higher order assembly. For
example, the organometallic Rh(nbd) (nbd norbornadiene) can be immobilised at specific sites within the

apoferritin molecule where it can catalyse the polymerisation of phenylacetylene within the protein shell
( Figure 19.10 ) . This is but one example of the quest to develop highly effective “artificial metalloenzymes” by
rational design of metal coordination sites within the ferritin molecule.

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