Kurt Brandstadt

Enzyme-catalysed siloxane bond formation

Biosilicification occurs on a globally vast scale under mild conditions. Although research has progressed in the area of silica biosynthesis, the molecular mechanisms of these interactions are effectively unknown. The natural production of silica in the Tethya aurantia marine sponge, Cylindrotheca fusiformis diatom, and Equisetum telmateia plant appear to be similar. However, the studies were complicated mechanistic queries due to the use of silicic acid analogues. Given these complications, a carefully chosen model study was carried out to test the ability of enzymes to catalyse the formation of molecules with a single siloxane bond during the in vitro hydrolysis and condensation of alkoxysilanes. Our data suggest that homologous lipase and protease enzymes catalyse the formation of siloxane bonds under mild conditions. Non-specific interactions with trypsin promoted the in vitro hydrolysis of alkoxysilanes, while the active site was determined to selectively catalyse the condensation of silanols.

Simple and mild preparation of silica-enzyme composites from silicic acid solution

Silica-enzyme composite materials including nanoparticles are formed readily from silicic acid and some hydrolase enzymes under mild conditions when the enzyme pI is greater than about 10.

Inclusion complexes between cyclic siloxanes and cyclodextrins: synthesis and characterization

Molecular inclusion complexes between cyclodextrins and cyclic siloxanes were prepared and characterized via a combination of liquid and solid state NMR, FT-IR, TGA, powder X-ray diffraction, SEM–EDS and elemental analyses. The crystalline complexes adopted the channel-type conformation. Depending from the size of both the cyclic sugar cavity and the silicon guest, various yields (between 0 and 41%) and host–guest molar ratios (between 1:1 and 4:1) were obtained. α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD) were observed to form crystalline inclusion complexes only with D3 (cyclic dimethyltrisiloxane) due to steric effects, whereas the larger γ-cyclodextrin (γ-CD) formed inclusion complexes both with D3, D4 (cyclic dimethyltetrasiloxane) and D5 (cyclic dimethylpentasiloxane). This study is believed to be the first step towards the selective removal of cyclic siloxanes impurities from commercial PDMS preparations.

An artificial organosilicon receptor

A biomimicking approach for the selective capture of dimethylcyclosiloxanes was developed. Inclusion complexes between cyclodextrins (CDs) and cyclosiloxanes were isolated and subsequently treated with toluene-2,4-diisocyanate (TDI) in DMSO to afford molecularly imprinted cyclodextrin (MICD) polymers. Following removal of the siloxane-based templates, the imprinted biomimetic polymers were characterized via scanning electron microscopy (SEM), cross-polarization magic angle spinning (CP-MAS) NMR and elemental analysis. Substrate affinity and selectivity were evaluated via equilibrium batch-rebinding assays and quantitative gas-chromatographic analysis. The imprinting effect was assessed by comparing the binding of the synthetic receptors with blank (non-imprinted) polymers. Adsorption isotherms were measured and data fitted using several mathematical models and the dissociation constants (Kd) and the binding site densities (Bmax) were calculated. The study is believed to have delivered the first case of an artificial receptor for an organosilicon substrate, opening a new way for separation and purification in silicon chemistry.