| Abstract
| - Hydrolysis and condensation of trimethylethoxysilane, a model organosilicate, are catalyzed by weakly acidic amines at pH ∼ 5. Rates are proportional to the concentration of the amine conjugate base in a proposed nucleophile-promoted mechanism, This indicates that amines in silica biomineralization proteins are necessary for chemical catalysis of silica precipitation.
- Biogenic silica production occurs at mild conditions with greater control of pore size, shape, andmicropatterning than is possible with typical industrial sol−gel methods, providing inspiration for potentialalternative routes to silica synthesis. Researchers have implicated the amine moieties, histidine andpolylysine, 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 studiesisolating one effect from another have been conducted. In the present study, we use 29Si NMR spectroscopyto systematically examine the different possible mechanistic roles of mono- and polyamines in catalyzingsilica 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 reactionprogress can be followed with little ambiguity. Hydrolysis and condensation (dimerization) of TMESlead to the products trimethylsilanol (TMSiOH) and hexamethyldisiloxane (HMD). The RefocusedInsensitive 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 eachamine can be obtained. For both mono- and polyamines, the catalytic efficiency scales with theconcentration of conjugate base form and inversely with pKa. Thus, catalysis is most efficient with moreacidic monoamines, such as pyridine and imidazole, as well as for the longer polyamines, where themost 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 pentacoordinateintermediate with TMES. Condensation is interpreted as an acid-catalyzed SN2 mechanism. Our findingspotentially explain the evolutionary selection of histidine-containing proteins for biogenic silica synthesisby 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 onetype of mechanistic strategy for catalyzing biogenic and biomimetic silica polymerization.
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