Encapsulating Materials within VNPs - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 122

Chapter

enCAPSulAtIng MAterIAlS WIthIn VnPs

Viral nanoparticles (VNPs) contain three surfaces that can be exploited
for functionalization: the exterior capsid surface, the interior, and the
interface between adjacent coat proteins. The previous chapter dealt with
the functionalization of either the exterior or interior surfaces using covalent
bioconjugation strategies (see Chapter 4). This chapter will summarize
different approaches that allow the encapsulation of materials within the
interior cavity of the VNP.
In order to achieve encapsulation, a range of techniques may be applied:

1. Infusion of small molecules

Although they often appear to be closed shells in structural
representations, virus capsids contain pores on their surfaces of varying
sizes. Not only do the capsids of VNPs contain pores, but they are also
highly dynamic: structural transitions of the pentamers and hexamers
occur as a result of thermal fluctuation (Bothner ., 1998; Gibbons

& Klug, 2007; Witz & Brown, 2001). Small molecules can diffuse freely
between the bulk medium and the capsid interior through these pores
and openings. Once inside the interior cavity, the compound of interest
can be covalently attached to addressable amino acids on the interior
surface of the capsid. Covalent interior modification has been shown
for

et al

Cowpea mosaic virus (CPMV), MS2, and Tobacco mosaic virus (TMV)

(discussed in Section 4.3.7).

Retention can also be achieved based on interactions with the
encapsidated nucleic acids; these are typically non-covalent interactions.
Small positively charged molecules or molecules with natural affinity to
nucleic acids can be stably entrapped (see Section 5.1).

An alternative strategy makes use of the so-called gating mechanisms;
recall the pH- and metal ion-dependent structural transitions of, for