Diatom mimics: directing the formation of biosilica nanoparticles by controlled folding of lysine-leucine peptides

Abstract

Silaffins, long chain polyamines, and other biomolecules found in diatoms are involved in the assembly of a large number of silica nanostructures under mild, ambient conditions. Nanofabrication researchers have sought to mimic the diatom's biosilica production capabilities by engineering proteins to resemble aspects of naturally occurring biomolecules. Such mimics can produce monodisperse biosilica nanospheres, but in vitro production of the variety of intricate biosilica nanostructures that compose the diatom frustule is not yet possible. In this study we demonstrate how LK peptides, composed solely of lysine (K) and leucine (L) amino acids arranged with varying hydrophobic periodicities, initiate the formation of different biosilica nanostructures in vitro. When L and K residues are arranged with a periodicity of 3.5 the α-helical form of the LK peptide produces monodisperse biosilica nanospheres. However, when the LK periodicity is changed to 3.0, corresponding to a 310 helix, the morphology of the nanoparticles changes to elongated rod-like structures. β-strand LK peptides with a periodicity of 2.0 induce wire-like silica morphologies. This study illustrates how the morphology of biosilica can be changed simply by varying the periodicity of polar and nonpolar amino acids.

Figure 1

SEM images of the precipitates assembled by the LK peptide mimics. LKα-14, when introduced into a buffer-silicic acid solution, self-assembles silica nanospheres (a). LKβ-15 produces wire-like silica particles with no specific geometry (b), while LK3₁₀-13 induces the formation of monodisperse nanorods (c).

Figure 2

SFG experimental setup. During each experiment peptides (LKα-14, LKβ-15, or LK3₁₀-13) were adsorbed onto a thin layer of SiO₂ (~10 nm) coated onto the backside of a equilateral CaF₂ prism. In situ SFG spectra collected at the amide I band were used to identify the secondary structure of LKα-14, LKβ-15, and LK3₁₀-13 while interacting with SiO₂.

Figure 3

SFG amide I spectra of LK3₁₀-13 (top trace) LKα-14 (middle trace), and LKβ-15 (bottom trace) adsorbed onto SiO₂. Within the LK3₁₀-13 spectrum is single amide I mode present at 1660 cm⁻¹, which is characteristic of ordered 3₁₀ helices. A single peak is also present in the LKα-14 spectrum but is shifted by 5–1655 cm⁻¹, which is attributed to ordered α-helices. The LKβ-15 spectrum contains two modes at 1640 and 1670 cm⁻¹, which are characteristic of ordered β-sheet structures.

Figure 4

Renderings of low-energy structures from simulations. Figures are not to scale with each other. Vector and bead representations are colored per peptide. Beads represent backbone particles. Vectors are drawn between the ends to guide the eye. Surface representations are colored with hydrophobic leucines in yellow and charged lysines in red.