Self-Assembled Nanoreactors - Cluster for Molecular Chemistry

1484 Chemical Reviews, 2005, Vol. 105, No. 4 Vriezema et al.
Figure 40. Strategies for the specific chemical modification of viral particles to generate virus-based scaffolds for
nanomaterials. (Reproduced with permission from ref 306. Copyright 2004 Elsevier.)
material. CCMV undergoes a reversible pH-dependent
swelling, which results in a 10% increase in virus
dimensions. Structural analysis has revealed that
CCMV swelling leads to the formation of 60 separate
openings (2 nm in diameter). Douglas and Young
have used the pH-dependent gating of the CCMV to
control the entrapment of different guest compounds.
Inspired by ferritin, CCMV has been used as a host
system for the encapsulation of guest compounds that
have a complementary size and shape. 303,360 The
host-guest relationship between the viral cage and
the encapsulated material is primarily based on a
complementary electrostatic interaction. In the native
viral protein cage, the cationic interior of the
virus interacts with the polyanion of RNA. This
electrostatic host-guest interaction can be used to
bind other (poly)anions.
Douglas and Young have reported on the encapsulation
of two polyoxometalate species (paratungstate
and decavanadate) and an anionic polymer
inside the CCMV cage, controlled by the pH-dependent
gating of the virion’s pores. For example, empty
virions were incubated with inorganic molecular
tungstate (WO4 2+ ) at pH 6.5, concentrated, and
subsequently washed at pH 5, at which point the
pores in the protein shell closed and the tungstate
ions oligomerized (Figure 41). The latter process is a
crystallization that yielded uniform nanocrystals
with a diameter of 6.7 nm, which were characterized
by TEM.
To further investigate the electrostatic aspects of
this host-guest interaction and the influence of
gating on the entrapment, the authors studied the
pH-dependent encapsulation of an anionic organic
polymer. The polymer was incubated with empty
virions at pH 7.5, after which the pH was lowered to
well below the gating threshold (pH 4.5), resulting
Figure 41. Schematic illustration of the approach used
to achieve mineralization within the CCMV virus particle.
(Reproduced with permission from ref 360. Copyright 1998
Nature.)
in the selective encapsulation of the polymer. Evidence
for this encapsulation came from TEM experiments,
which showed that no staining dyes were
included in the interior of the virus particles.
The electrostatic properties of the CCMV protein
cage interior surface could be altered by replacing
nine basic residues at the N-terminus of every protein
subunit with glutamic acid residues. The mutant
CCMV was formed to assemble readily into a cagelike
architecture similar to that of the wild-type. The
modified protein cages were treated with Fe(II) salts
at pH 6.5 and allowed to oxidize in air, yielding iron
oxide cores as was shown by EM. 361 Genetic modification
of the exterior surface of CCMV has also been
reported, the objective being to form self-assembled
arrays of CCMV viral cages on gold surfaces. 362