Sol–gel glass is not necessarily an inert matrix for enzyme encapsulation. Catalysis of sulfoxidation by silica

In sulfoxidation of thioanisole (PhSMe) by hydrogen peroxide, horseradish peroxidase (HRP) encapsulated in sol–gel silica glass exhibits the same yield as dissolved HRP does but lower excess of the (S)-enantiomer (68% versus 91% at 22°C and 43% versus 86% at 40°C). Unexpectedly, the yield of methyl phenyl sulfoxide, PhS(O)Me, is the same in the presence of undoped and HRP-doped glasses. All of these findings result from the ability of the silica matrix itself to catalyze the sulfoxidation reaction. The drop in the enantiomeric excess is due to two simultaneous processes, namely enantioselective HRP catalysis and non-enantioselective silica catalysis. This side effect of the silica matrix makes possible an efficient method for the sulfoxidation of aryl methyl sulfides by hydrogen peroxide in water. In the presence of silica gel, the corresponding sulfoxides are obtained in the 76–90% yield. Evidently, sol–gel glass may not be an inert matrix for the encapsulated enzyme. The possible catalysis by silica should be taken into consideration in the design of biosensors, supported catalysts, and other composite materials.