Siloxane bonds enzyme-catalyzed formation

In order to better imderstand the role of various proteins in the biosilicification process, a carefully chosen model study was performed to test the ability of homologous enzymes to catalyze the formation of siloxane bonds during the in vitro hydrolysis and condensation of alkoxysilanes under mild conditions (Scheme 1). Given the complications of silicic acid analogues (2), mono-functional silanes were chosen to focus on the formation of molecules with a single siloxane bond. It was understood that the in vitro biocatalyzed reactions might not be con5)arable to the natural in vivo reactions. 

Scheme 1. Enzyme-catalyzed siloxane bond formation.

Our model study (23-25) is believed to be the first rigorous study to demonstrate biocatalysis at silicon. This data suggests that homologous lipase and protease enzymes catalyze the formation of siloxane bonds under mild conditions. 

Followed by these observations, Bassindale et al. [ 19,20] studied the use of various homologous lipase and protease enzymes to catalyze the formation of molecules with a single siloxane bond during the in vitro hydrolysis and condensation of alkoxysilanes under mild reaction conditions. They found that non-specific interactions with trypsin promoted the hydrolysis of alkoxysilanes, while the active site was determined to selectively catalyze the condensation of silan-ols. One interesting observation was that when trypsin from various sources was employed different extents of conversion were observed. Comparatively, the activity of trypsin from a bovine pancreas was greater than the alternate sources of trypsin. Although various sources (e.g., mammalian, fish) of trypsin are similar (e.g., tertiary structure), their selectivity and activity was found to be different due to different optimum pH ranges and/or levels of calcium (an additive). 

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