Revisiting the peroxidase oxidation of 2,4,6-trihalophenols: ESR detection of radical intermediates.

The peroxidase oxidation of 2,4,6-trichlorophenol (TCP) has been clearly shown to result in 2,6-dichloro-1,4-benzoquinone (DCQ). DCQ is a 2-electron oxidation product of TCP that has undergone para dechlorination. Many peroxidases show similar oxidation of the substrate, TCP, to yield the quinone, DCQ. Depending on the substrate, peroxidases are thought to carry out both 1- and 2-electron oxidations; the mechanism can be confirmed by the detection of both enzyme and substrate intermediates. This article presents ESR evidence for the transient 2,4,6-trichlorophenoxyl radical intermediate (TCP•), which exists free in solution, i.e., is not enzyme associated. These data are best explained as a 1-electron peroxidase oxidation of TCP to form TCP•, followed by enzyme-independent radical reactions leading to the 2-electron oxidized product. Also presented are data for the peroxidase oxidation of 2,4,6-trifluorophenol and 2,6-dichloro-4-fluorophenol.