The Art of Bioconjugation: Functionalization of VNPs - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 97

4.3.7 Labeling the Interior Capsid Surface Via Bioconjugaion

Besides utilizing the exterior solvent-exposed surface of VNPs, the interior
surface can also be utilized for bioconjugation. This has been shown for
CPMV, MS2, and TMV (Endo et al. , 2007; Hooker et al. , 2004, 2007; Ma et al. ,

2008; Miller , 2002b,c).

Small chemical modifiers can, in general, diffuse freely between the
bulk medium and the interior cavity of VNPs, and thus be attached to
available targets on the interior capsid surface. For example, interior
surface modifications can be achieved using CPMV, which displays reactive
Cys side chains on the interior capsid surface. As CPMV does not display
any Cys residues on the exterior surface, thiol-selective chemistries are
specific for the interior (Wang

et al. , 2007; Schlick et al. , 2005; Wang et al.

, 2002b,c).
Modification of the interior surface has been extensively studied and
utilized using the bacteriophage MS2 (Hooker

et al.

et al. , 2004, 2007; Kovacs

et al. , 2007). Modification of the interior cavity of MS2 is more feasible

compared with modification of the interior of CPMV. The particles of CPMV
have 12 pores located at the fivefold axis with a diameter of about 0.7 nm
(Lin

, 1999). MS2 offers 32 pores at the five- and threefold axes with a
diameter of 1.8 nm (Golmohammadi

et al.

, 1996), making the interior more
accessible. MS2 offers a highly reactive Tyr residue on the interior capsid
surface that has been utilized for various bioconjugation strategies, and a
selection of imaging molecules including fluorescent dyes and gadolinium
complexes used in MRI have been successfully introduced (Hooker

et al.

et al. ,

2004, 2007; Kovacs , 2007). The possibility of modifying the interior

of VNPs in addition to attaching functionalities to the exterior surface
is beneficial, especially when developing multifunctional devices. For
example, probes for tissue-specific imaging can be designed in such a way
that the imaging modality is inside the VNP and tissue-specific targeting
ligands could be attached to the exterior surface (discussed in detail in
Chapter 8).
The interior of the rod TMV can also be utilized; small organic dyes can
be covalently attached at two interior Glu residues (Schlick

et al.

, 2005).
Besides using intact TMV particles, interior labeling can also be achieved by
disassembling the particles into coat protein monomers, followed by labeling
and reassembly. This technique has been used for the development of light-
harvesting systems (Endo

et al.

et al. , 2007; Ma et al. , 2008; Miller et al. , 2007)

(Section 4.4.3).

.. non-Covalent Coupling Methods

Biospecific interactions, such as biotin-streptavidin or hexa-His-nickel-
nitrilotriacetic acid (Ni-NTA) have also been used for interlinking VNPs with