Properties of phenoxyl radical of dehydrozingerone, a probable antioxidant

Free radical reactions of dehydrozingerone (DZ), a methoxy phenol, were studied at dfferent pHs with a variety of oxidants using nanosecond pulse radiolysis technique. Hydroxyl radical (OH) reaction with the phenolic form at pH 6 led mainly to the formation of an OH-adduct absorbing at 460 nm in addition to a minor oxidation product. On the other hand, at pH 10 with the deprotonated phenoxide ion, the only reaction observable was oxidation generating a phenoxyl radical absorbing at 360 nm. HPLC analysis indicated formation of two different products at pH 6 from addition and oxidation reactions, whereas at pH 10, only the oxidation product was detectable. Reactions of more specific secondary oxidizing radicals, N3(.), Br-., Br-(.)(2) and Tl(II) with DZ gave rise to the phenoxyl radical over the entire pH range. DZ in the phenoxide ion form reacted with nitrogen dioxide and trichloromethyl peroxyl radicals with rate constants 6 x 10(8) and 8.8 x 10(8) dm(3) mol(-1) s(-1) respectively leading to the phenoxyl radicals. The DZ phenoxyl radical reacted with trolox C (an analogue of alpha-tocopherol) with a rate constant of 8.3 x 10(7) dm(3) mol(-1) s(-1). One electron reduction potential of the DZ phenoxyl radical at pH 6 was determined to be +1.1 V vs NHE using N3(.)/N-3(-) as the standard couple. (C) 1999 Elsevier Science Ltd. All rights reserved.