Oxidation of ABTS by hydrogen peroxide catalyzed by horseradish peroxidase encapsulated into sol-gel glass. Effects of glass matrix on reactivity

Encapsulation of horseradish peroxidase (HRP) by the sol-gel method into silica (SiO2) or alkylated silica (RSiO2, in which R is Me or Pr) yields biocatalytic glasses designated HRP@SiO2 and HRP@RSiO2. These new enzyme composite materials catalyze one-electron oxidation by H2O2 of the dye 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) into the radical cation ABTS(.+). In the presence of excess H2O2, ABTS(.+) is converted into a mixture of at least five compounds. This undesirable side-reaction can be suppressed by using stoichiometric amount of H2O2. The effects of ABTS concentration and ionic strength of the buffer on the ABTS(.+) yield and the apparent rate constant were investigated. The catalyst HRP@MeSiO2 achieves higher ABTS(.+) yields (70-85%) than HRP@SiO2 and HRP@PrSiO2 (30-40%) do. The apparent rate constants for HRP@MeSiO2 are 7-10 times higher than those for HRP@SiO2 and HRP@PrSiO2. When the ionic strength of the buffer is raised, the rate constants increase for HRP@SiO2 and HRP@PrSiO2, and do not change for HRP@MeSiO2. The diminished electrostatic interaction between the negatively-charged substrate, ABTS, and the surface of methylated silica is the main cause for the increased catalytic efficiency. These results are important for understanding of encapsulated and otherwise immobilized enzymes. The possible electrostatic effects should be taken into consideration in the choice of the matrix materials used in the design of biosensors, supported catalysts, and other composite materials. (C) 2002 Elsevier Science B.V. All rights reserved.