Enzyme-Catalysed Siloxane Bond Formation

The intricate siliceous architectures of diatom species have inspired our exploration of biosilicification. 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. Previous in vitro studies of natural systems within the area of silica biosynthesis were complicated by other factors. These earlier mechanistic queries including biomimetic approaches often failed to recognize the chemistry of silicic acid and its analogues. In order to better understand the role of various proteins in the biosilicification process, a carefully chosen model study was performed to test the ability of homologous enzymes to catalyse the formation of molecules with a single siloxane bond during the in vitro hydrolysis and condensation of alkoxysilanes. This model study is believed to be the first rigorous study to demonstrate biocatalysis at silicon. Our data suggests that homologous lipase and protease enzymes catalyze the formation of siloxane bonds under mild conditions. In particular, 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. Comparatively, the rate of hydrolysis was one order of magnitude faster than condensation. The rate of condensation appeared to be proportional to both the concentration of trimethylsilanol and trypsin. The trypsin-catalysed condensation of non-saturated solutions of trimethylsilanol is thought to fit the Michaelis-Menten model, where the formation of the trypsin-silanol intermediate is the rate-limiting step. This is consistent with the fact that organosilicon molecules are larger than analogous hydrocarbon tryptic substrates. Conversely, although trypsin would theoretically catalyse the hydrolysis of a siloxane bond due to the law of microscopic reversibility, the reverse reaction was not favored. Given the selectivity and mild reaction conditions of enzymes, the opportunity to strategically use biocatalysts to synthesise novel hybrid materials with structural control and spatial order is promising