chimeras to generate a diversity of size of condensed silica particles.
Thus, a potential mechanism to control the size of silk–silica
particles at the nano- and micro-scale can be pursued by the sizes
of the silk domains as well as the charges of silica-binding domains
generated by sequence chemistry.
In the solution–solid interface system, silica condensation was
carried out on the surface of the films of the recombinant 6mer
and 6mer-SiBP fusion proteins. Before conducting the silica condensation
reaction, the films were treated by water vapor annealing
at 50 C and 70 C as well as by 70% methanol. As expected, the
lowest b-sheet content formed in the films treated at 50 C and
highest b-sheet content in the films treated by 70% methanol
(Fig. 5A) [24]. Intriguingly, the films treated by water vapor annealing
at 50 C generated highest while those treated 70% methanol
generated lowest density of condensed silica particles on the either
the 6mer or each of the chimeras (Fig. 6). This phenomenon may be
correlated to b-sheet content in the films, which form due to the
hydrophobic polyalanine domains [33], perhaps shielding the silica-
binding domains from the condensation reactions.
The 6mer-A1/-R5 films produced a higher density of condensed
silica particles than the 6mer or the 6mer-A3 films which were
treated with the same conditions (Fig. 6). However, the 6mer-A3
showed the greater rate of inducing silica precipitation than
6mer-A1 and -R5 in the solution–solution interface system
(Fig. 4). This phenomenon may be related to crosstalk between
the silk domain and the SiBP domains. In the process of annealing
the films, the silk protein self-assembled into energy-stable conformations,
b-sheet crystals [24,25]. The silica binding peptides A1
and R5 are able to precipitate silica efficiently because of the presence
of the motif ‘‘RRIL’’, which confers active silica precipitating
assemblies [17]. It appeared that the introduction of A1 and R5,
rather than A3, to the C-terminus of silk domain affected b-sheet
formation in the chimera proteins (Fig. 5A). In the solution, random
coil was the major structure and no b-sheet formed from the silk
domains [9]. Therefore, assembly of A1 and R5, rather than A3,
may antagonize b-sheet formation driven by the silk domain in
the films, accounting for the fact that that 6mer-A3 showed the
highest rate of silica precipitation at the solution–solution interface
with the lowest density of silica deposition at the solution–
solid interface.