Amine-catalyzed biomimetic hydrolysis and condensation of organosilicate
Biogenic silica production occurs at mild conditions with greater control of pore size, shape, and micropatterning than is possible with typical industrial sol−gel methods, providing inspiration for potential alternative routes to silica synthesis. Researchers have implicated the amine moieties, histidine and polylysine, on proteins isolated from sponges and diatoms as catalysts for biogenic silica precipitation. Different mechanistic roles have been ascribed to the amines, but few systematic, quantitative studies isolating one effect from another have been conducted. In the present study, we use 29Si NMR spectroscopy to systematically examine the different possible mechanistic roles of mono- and polyamines in catalyzing silica synthesis at mildly acidic pH (5) from an organosilicate starting compound, trimethylethoxysilane (TMES). TMES has a single organosilicate bond, so there are no competing reactions and the reaction progress can be followed with little ambiguity. Hydrolysis and condensation (dimerization) of TMES lead to the products trimethylsilanol (TMSiOH) and hexamethyldisiloxane (HMD). The Refocused Insensitive Nuclei Enhanced by Polarization Transfer pulse sequence (RINEPT+) provides unambiguous, quantitative 29Si NMR spectra from which the hydrolysis and condensation rates in the presence of each amine can be obtained. For both mono- and polyamines, the catalytic efficiency scales with the concentration of conjugate base form and inversely with pKa. Thus, catalysis is most efficient with more acidic monoamines, such as pyridine and imidazole, as well as for the longer polyamines, where the most acidic protonation constant is lower than the experimental pH (5). We postulate a nucleophile-catalyzed hydrolysis mechanism where the conjugate base of the amine attacks Si to form a pentacoordinate intermediate with TMES. Condensation is interpreted as an acid-catalyzed SN2 mechanism. Our findings potentially explain the evolutionary selection of histidine-containing proteins for biogenic silica synthesis by sponges and address the chemical mechanisms at work for the precipitation of silica by polylysine-containing proteins in diatoms. Along with the physical mechanisms suggested by other research groups, the systematic results from the present study indicate that amines may be employed in more than one type of mechanistic strategy for catalyzing biogenic and biomimetic silica polymerization.