The Art of Bioconjugation: Functionalization of VNPs - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 92

2005a) and as tracers for biosensors (Sapsford et al. , 2006; Soto et al. , 2006),

and have been developed for light-harvesting systems (Endo et al. , 2007; Ma

et al. , 2007) (as summarized in Table 4.1 and Section 4.4).

There is high demand for generating multifunctional devices, that is a
VNP comprising different functionalities. For example, for the development
of “smart” devices for drug delivery, one has to combine at least two
functional groups into a single formulation, that is a targeting ligand
(to give tissue specificity) and a therapeutic molecule (to treat the diseased
tissue). Generation of Cys-added mutants is of importance; with the
introduction of the Cys residues, not only additional attachment sites are
introduced but also a different type of target, thus allowing sequential
labeling of different sites exploiting different site-selective chemistries.

, 2008; Miller et al.

...1 Introducing thiols—a chemical approach

Thiols are highly reactive toward a broad collection of chemicals and thus
are useful for bioconjugation. As Cys mutants of VNPs can be challenging
to work with and “smart” design can be time-consuming, alternative
strategies have been employed allowing the introduction of reactive thiols
by means of chemistry. Chemically, engineered thiolated VNPs can be
generated by making use of the coupling reagent -

acetylpropionate (SATP), a compound containing a protected acetylated
thiol. Covalent attachment is achieved via coupling Lys side chains. To
facilitate thiol-selective chemistry, the protecting group can be removed by
deacetylation. This method was found to be a useful addition to generating
Cys mutants. Thiolated VNPs were found to be stable, did not aggregate over
a monitored time frame of 3 months, and were reactive toward maleimide-
specific chemistries (Steinmetz

N -succinimidyl- S

et al. , 2007).

4.3.4 Spaially Controlled Labeling of Rod-Shaped VNPs

Rod-shaped particles can potentially be labeled with sequential and
spatial control at the ends versus the virus body. End-specific labeling has
been demonstrated for the rods SIRV2, TMV, and M13 (Nam

et al. , 2004;

Steinmetz et al. , 2008a; Yi et al. , 2005, 2007). End labeling of M13 is

discussed in Section 4.3.8.
The capsids of the rod-shaped archaeal virus SIRV2 are composed of
two different coat proteins, a major coat protein that forms the virus body
and a minor coat protein that forms the end structures of the virions. It
has been shown that different chemistries can be applied to the different