Figure 1. Driving forces for TMV assembly. TMV CP experiences various interactions, including hydrophobic interactions, RNA initiation, electrostatic interactions in the Caspar carboxylate cluster (CCC), hydrogen bonding, and RNA-protein interaction at different stages of assembly.
Metal nanoparticles are frequently synthesized spontaneously on viral biotemplates using aqueous metal solutions. The metal precursor ions adsorb and are chemically reduced on the viral particle surfaces at many adsorption and nucleation sites to form a metallic nanomaterial [38, 39]. The chemical interactions that drive metal precursor adsorption and reduction are not well understood. However, the adsorption process is frequently described by a single-step Langmuir isotherm that is solely driven by covalent interactions, e.g. palladium on TMV [38]. As metal ion precursor adsorption and reduction are the fundamental processes that drives metal coating formation, the oxidation potential of surface accessible residues must be sufficient to drive metal reduction (Table 2). Amino acid residues that are easily oxidized, such as cysteine, tyrosine, and lysine, more readily interact with metals driving deposition [40–42]. Metals with higher positive reduction potentials, including gold, silver, and platinum, can be reduced by the various functional groups present on the CP of TMV (Table 2) [39]. This deposition is frequently enhanced by engineering the amino acids residues that are presented on the virus/VLP [30, 39]. Other metals such as nickel, iron, and cobalt cannot be reduced this way as they have negative reduction potentials. Instead, a different metal that is more readily reduced such as palladium is mineralized first onto the CP, which then serves as a catalyst to reduce target metal ions to metal atoms [43, 44]. For example, nickel and cobalt are deposited in the inner channel of TMV after mineralization of TMV with Pd and Pt [44]. Fundamentally, appropriate pairing of amino acid side chain that can chemically reduce and interact with metals intrinsically sets the metal adsorption capacity and controls the rate of reaction as nanoparticle synthesis proceeds spontaneously under ambient conditions.
Plant-produced BSMV has been demonstrated to be a viable biotemplate for mineralization of palladium nanowires, however, biomineralization with BSMV differs from that of TMV [16]. The surface of BSMV allows metal ion precursor deposition to proceed via a multi-step Langmuir isotherm that incorporates both electrostatic and covalent adsorbent-adsorbate interactions. This difference may arise in part due to the larger amount of BSMV surface-exposed residues, compared to TMV, in an unstructured insertion loop containing 10 amino acids that protrudes from the particle surface [33]. These stronger interactions increase the adsorption capacity for Pd on BSMV two-fold compared to that on TMV [16]. Similarly, the rate of adsorption is increased compared to TMV, suggesting that BSMV can be fully coated in fewer processing cycles saving both time and expensive precursor material. Furthermore, BSMV biotemplates produce more uniformly sized nanoparticles relative to TMV. The additional opportunities to engineer metal deposition via the insertion loop and superior adsorption and metal nanoparticle synthesis characteristics make BSMV an attractive alternative to TMV that may generate more uniform metal nanostructures more economically.