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Title: Biocatalysis in organosiloxane chemistry using silicateins
Author: Tabatabaei Dakhili, Seyed Yasin
ISNI:       0000 0004 6496 8387
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2017
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Organosiloxanes are compounds with a C-Si-O-C bond. Over the past few decades, they have provided appealing possibilities in organic chemistry and biotechnology, for example protecting group chemistry. However, their production, particularly the hydrolysis and condensation of siloxanes, involves energy intensive and environmentally adverse catalysts, solvents, reagents and reaction conditions, such as chiral transition-metal catalysts. Enzymes are biocatalysts that accelerate the rate of a biochemical transformation in mild reaction conditions, including ambient temperature and neutral pH. In addition, they are regio- and sterioselective catalysts. The principal aim of this thesis is to take advantage of green and selective biocatalysts for hydrolysis and condensation of organosiloxanes. To achieve this, silicatein-alpha (the dominant serine hydrolase involved in deposition and decomposition of silica in sponges) is employed as a model biocatalyst. First, the overproduction of soluble recombinant enzyme was optimised in a bacterial system and subsequently a reproducible procedure developed for the isolation of pure silicateins. To facilitate the enzymological studies and primarily to estimate the kinetic parameters of this enzyme; an efficient high-throughput screening method was established. The results indicated that silicatein-alpha is effective in hydrolysing silyl ether bonds-kcat/KM within the range of 2-50 min-1 μM-1-in buffer medium. Using this high-throughput technique, the pH-activity profile, esterase and protease activity of silicatein-alpha were investigated. Hydrolase enzymes such as silicatein-alpha can catalyse the reverse of the hydrolysis reaction (i.e. in the silyl ether condensation direction) in organic solvents. After confirming the hydrolytic activity of silicatein-alpha in an aqueous buffer, the next goal was to investigate the catalytic activity of silicatein-alpha in non-aqueous medium (i.e. condensation of siloxanes). To accomplish this, the silyl etherification and trans-etherification of a mono-hydroxy primary alcohol (octan-1-ol) with model silanols (R3Si-OH) and ethoxy silanes (R3Si-OEt) was examined in n-octane (organic solvent), respectively. After improving the reaction conditions, the product conversion (R3Si-OOc) of the etherification reaction reached ~99%. The product conversion (R3Si-OOc) of the trans-etherification process with ethoxy silane and octan-1-ol reached ~50%.The substrate selectivity of enzymes has several valuable applications in organic chemistry, for example in production of enantiopure pharmaceuticals through the kinetic resolution of racemic compounds. To explore the catalytic ability of silicatein-alpha in selective silyl etherification reactions, a particular di-hydroxy alcohol (possessing a primary aliphatic and a phenolic alcohol) was selected as the starting substrate. The product conversion suggested that silicatein-alpha prefers to catalyse the silyl etherification of a primary aliphatic alcohol (by ~85%) over a phenolic hydroxy group. In brief, this work has discovered that unlike the conventional hydrolyses, such as serine/cysteine proteases, recombinant silicateins are competent, selective and thermostable catalyst for the hydrolysis and condensation of silyl ether bonds in aqueous and non-aqueous solvents, respectively.
Supervisor: Wong, Lu Shin ; Micklefield, Jason Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available