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specialized and multifunctional devices. Many applications in nanotech-
nology deal with the immobilization of nanomaterials on solid devices. The
bifunctionality of rod-shaped particles can be exploited to utilize the end
structures for selective binding onto surfaces. This has been demonstrated
using TMV. Researchers have made use of the polar nature of TMV; the helical
encapsulation of the RNA molecule results in sequence-definable 5
′
and 3
′
ends. A mild disassembly protocol was used to partially disassemble the
protein coat and expose the RNA at the 5
end. Spatially oriented assembly of
TMV on solid supports was achieved in a controlled manner via nucleic acid
hybridization using complementary oligonucleotides (Yi
′
et al.
, 2005, 2007)
(see also Chapter 7).
4.3.5 Breaking the Symmetry of Icosahedral Paricles
Icosahedral VNPs are highly polyvalent and symmetrical particles. For
many applications, polyvalency and high degree of symmetry are desired.
However, for some applications it may be desired to break the symmetry
of the VNPs. For example, it can be challenging to control immobilization
of VNPs directionally and orientationally because of multivalency. A range
of Cys-added mutant VNPs have been utilized for immobilization of gold
surfaces (see also Chapter 7). Because of the multivalent display of Cys
residues on the exterior surface and the propensity of thiols to undergo
formation of disulfide linkages, binding of Cys-added VNPs onto solid
supports can result in the formation of interparticle aggregates. Efforts to
break the symmetry of VNPs have been made utilizing the platform CCMV.
A solid-state synthetic approach has been utilized to break the symmetry of
Cys-containing CCMV mutants (Fig. 4.14). The method utilizes a three-step
protocol: in step 1, the Cys-mutant of CCMV was immobilized on a Sepharose
resin containing an activated thiol. Unbound particles were removed by
extensive washing. In step 2, all remaining free thiols on the virus surface
were chemically passivated using iodoacetic acid (IAA) that readily reacts
with free thiols. The passivated CCMV particles were then eluted from
the resin in step 3 by treatment with 2-mercaptoethanol, a chemical that
reduces the disulfide bonds that are holding the VNPs attached to the resin.
After elution, only the Cys side chains that were engaged in binding to the
resin are free and remain reactive. Immobilization of symmetry-broken
particles was tested in comparison with wild-type particles (that do not
contain any solvent-exposed Cys side chains), passivated mutant particles
(in which all the Cys side chains had been passivated), and non-treated Cys-
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