2. Viral particle self-assembly and metal nanoparticle
synthesis
Viral biotemplates such as TMV and BSMV spontaneously self-assemble from
identical CP subunits due to several covalent, hydrogen, and
electrostatic interactions encoded within the CP primary sequence and
its associated nucleic acids (Figure 1) [34]. These interactions
must be strong enough to withstand the pHs, temperatures, and ionic
strengths required for successful nanoparticle synthesis. CP is first
translated and folded before ultimately forming flat disks via
hydrophobic interactions between residues on the CP surface [34]. An
RNA sequence naturally found in the TMV viral genome, known as the
origin of assembly sequence or OAS, assumes a hairpin secondary
structure that then serves as a nucleus for nanotube formation. CP disks
are threaded by OAS-containing RNA, allowing for electrostatic
interactions between the OAS and CP [35]. This interaction forces
the CP disk to assume a helical conformation, reorienting themselves by
induced proton adsorption and subsequent hydrogen-bonding, which then
rapidly polymerizes with other CP molecules into a nanotube [34].
Strengthening the final assembly are electrostatic interactions between
adjacent CPs, which are mediated by a handful of negatively charged
residues in a motif known as the Caspar carboxylate cluster [36].
Calcium ions typically neutralize the repulsive negative charges of
adjacent CPs and drive viral polymerization. Control of these
interactions via genetic engineering of the CP and/or the OAS offers
tremendous potential to modify nanotube architecture and its stability
in subsequent biotemplating [23, 37].