Silicatein filaments and subunits from a marine sponge direct the polymerization of silica and silicones in vitro

Abstract

Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators, optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis experiments. The catalytic activity of the "silicatein" (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.

Figure 1

Scanning electron micrographs of the products of the reaction between silicon alkoxides and silicatein or cellulose filaments. (A) Silicatein filaments before the reaction. (B) Silicatein filaments after a 12-h reaction with TEOS (1.0 ml; 4.5 mmol) plus Tris⋅HCl buffer. (C) Air-dried silicatein filaments incubated with TEOS as in B, but with no additional water. (D) Silicatein filaments after an 8-h reaction with phenyltriethoxysilane (1.0 ml; 4.1 mmol) plus Tris⋅HCl buffer. (E) Cellulose fiber. (F) Cellulose fiber after a 12-h reaction with TEOS as in B.

Figure 2

²⁹Si magic-angle spinning NMR spectra of silica and silsesquioxane products on silicatein filaments were acquired. Samples were prepared as described for Fig. 1 B and D. (A) A single-pulse ²⁹Si magic-angle spinning spectrum of the reaction product of silicatein filaments and TEOS. (B and C) Crosspolarization magic-angle spinning spectra of the reaction products of silicatein filaments and phenyltriethoxysilane (B) and TEOS (C).

Figure 3

Proposed reaction mechanism of silicon ethoxide condensation catalyzed by silicatein α, based on the well characterized mechanism of catalysis by the Ser/His and Cys/His active-site proteases (22). R = phenyl- or methyl- for the silicon triethoxide substrates, and R = CH₃CH₂—O— (= EtO—) for TEOS. Hydrogen-bonding between the imidazole nitrogen of the conserved histidine and the hydroxyl of the active-site serine is proposed to increase the nucleophilicity of the serine oxygen, potentiating its attack on the silicon atom of the substrate. Nucleophilic attack on the silicon displaces ethanol, forming a covalent protein—O—Si intermediate (potentially stabilized as the pentavalent silicon adduct via donor bond formation with the imidazole nitrogen). The addition of water completes hydrolysis of the first alkoxide bond. Condensation initiated by nucleophilic attack of the released Si—O⁻ on the silicon of the second substrate molecule then forms the disiloxane product.