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.