Dibenzothiophene oxidation by horseradish peroxidase in organic media: Effect of the DBT:H 2O 2 molar ratio and H 2O 2 addition mode

Abstract

Its is well known that in the biodesulfurization (BDS) process the low water solubility of sulfur compounds hinders its transference from the oil phase to the cells being the rate-limiting step in the metabolism of dibenzothiophenes (DBT). Thus sulfur compounds derivatives with high water solubility could be more easily transported increasing the BDS efficiency. The present work performed a stepwise evaluation of the enzymatic oxidation of DBT by horseradish peroxidase (HRP). Reactions were carried out in monophasic organic media containing 25% (v/v) acetonitrile. The following parameters were evaluated: DBT:H 2O 2 molar ratio (1:1–1:20); H 2O 2 addition mode (single or stepwise); pH (6.0–8.0) and temperature (37–50 °C). Best results were observed in a reaction medium at pH 8.0 presenting HRP 0.06 IU ml −1, DBT 0.267 mM, DBT:H 2O 2 molar ratio of 1:20 (stepwise hydrogen peroxide addition) and incubated at 45 °C for 60 min. Under these conditions 60% of DBT was converted into dibenzothiophene sulfoxide (12%) and dibenzothiophene sulfone (46%). The DBT oxidation rate observed in this work, of 5 mmol min −1 g −1 of HRP, was 250-fold higher than the BDS rate, 20 μmol min −1 g −1 of catalyst. As such a combined enzyme–microbial desulfurization process could be envisaged. Products were determined by HPLC RP C-18.