Selectivity in the peroxidase catalyzed oxidation of phenolic sulfides

The catalytic oxidation of ortho- and para-alkylthiophenols, carrying methyl or ethyl substituents at sulfur, by lactoperoxidase (LPO), horseradish peroxidase (HRP) or chloroperoxidase (CPO), in the presence of hydrogen peroxide, has been investigated. HPP and CPO are active toward these substrates, whereas LPO is only active with the ortho-substituted compounds. The enzymatic solutions containing ortho-alkylthiophenols develop an intense blue color (with optical absorptions near 400 and 600nm) that is attributed to the formation of relatively stable dimeric three-electron bonded complexes resulting from the association of enzyme-derived radical cations with the phenolic sulfide. The products of the enzymatic reactions by HRP and LPO are oligomers resulting from phenol oxidative coupling reactions and the sulfoxide, with minor amounts of oligomers containing mono or disulfoxide functionalities. With CPO the major product is always the sulfoxide, while phenol coupling products are formed in minor amounts. The selectivity exhibited by LPO toward 2- and 4-methylthiophenol has been investigated through binding experiments, NMR relaxation measurements of LPO-substrate complexes and docking calculations. The para-isomer binds much more strongly than the ortho-isomer to the enzymes. The stronger binding depends on the establishment of hydrogen bonding interactions with protein residues in the active site. (C) 2003 Elsevier Science B.V. All rights reserved.