VNPs as Templates for Materials Synthesis - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 145

Bozeman, MT, USA). Empty (CCMV) VLPs

were employed as size-constrained reaction vessels for the mineralization
of the polyoxometalate species paratungstate and decavanadate (Douglas
& Young, 1998). The pH-dependent structural transitions of CCMV were
coupled to a pH-dependent inorganic oligomerization reaction to load,
crystallize, and entrap mineral particles of well-defined size within the VLPs.
In brief, aqueous molecular precursors (WO
4

Cowpea chlorotic mottle virus

) were added to CCMV
in the swollen conformation. Next, the pH was lowered to <6.5. By lowering
the pH, two effects are induced: (i) a pH-dependent oligomerization of the
inorganic species occurs to form large polyoxometalate species (H

2−

and VO

−

3

W O

42

10−

2 12

and V ), which were readily crystallized as ammonium salts; and (ii) the

structural transition of the CCMV cage from the swollen form to the non-
swollen form takes place, trapping the formed material within the viral cage
(Fig. 6.2) (Douglas & Young, 1998).
Crystallization occurred only within the capsids; bulk mineralization
was not observed. The mineralization reaction is electrostatically driven and
induced at the interior surface of the capsid. The exterior and interior capsid
surface can be regarded as two chemically distinct environments. Although
the exterior of CCMV is not highly charged, the interior is highly positively
charged and thus provides an interface for inorganic crystal nucleation. The
negatively charged WO

O

6−

10 28

anions interact with the interior capsid
surface via electrostatic interactions; aggregation of the anions then leads

2−

and VO

−

4 3

Figure Transmission electron micrographs of paratungstate-mineralized

CCMV particles after isolation by centrifugation on a sucrose gradient. (a) An
unstained sample showing discrete electron dense cores; (b) a negatively stained
sample of (a) showing the mineral core surrounded by the intact virus protein cage.
Reproduced with permission from Douglas, T., and Young, M. (1998) Host-guest
encapsulation of materials by assembled virus protein cages,

6.2

Nature , 393 , 152-155.