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].