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Abstract
Reactions of pulse radiolytically generated hydroxyl (()OH) radicals and one-electron
specific oxidants, Br(2)(-) radicals with bergenin, a polyphenolic tannin derivative,
were studied and the transients detected by absorption spectrometry. The transient
absorption spectrum produced during the reaction of ()OH radicals with bergenin was
broad, and pH dependent. Different modes of reactions of ()OH radicals with bergenin,
viz., addition to the aromatic ring adduct and hydrogen abstraction was established
by time resolved (5-400micros) transient absorption studies and also by the reaction
of Br(2)(-) radicals. Comparing the transient spectra with ()OH radicals and Br(2)(-)
radicals at pH 4.5 and 8.5, the absorption maximum of the phenoxyl radical was found
to be at 440nm at pH 4.5 and 480nm at pH 8.5. Phenoxyl radicals are produced during
()OH radical reaction through the formation of ()OH radical adduct followed by water/OH(-)
elimination. While the phenoxyl radicals of bergenin are oxidizing in nature, the
hydroxyl radical adducts and the radicals produced from hydrogen abstraction are of
reducing nature. The yield of the oxidizing radicals produced from the ()OH radical
reaction with bergenin was determined to be 26.2% by secondary electron transfer reaction
from TMPD. On the other hand the yield of reducing radicals produced from the ()OH
radical reaction with bergenin was determined to be 74.1% by secondary electron transfer
reaction to MV(2+). ()OH radical reactions with bergenin under oxygenated conditions
and reaction with trichloro methyl peroxyl radicals with bergenin produced a new transient
absorbing at 400nm, which is attributed to peroxyl type of radicals. The one-electron
reduction potential for the formation of phenoxyl radical from bergenin was determined
to be 0.938V versus NHE at pH 7, by electron transfer equilibrium between bergenin
and chlorpromazine. The above results confirmed that reaction of ()OH radicals with
bergenin, mainly produced radical adducts and one-electron oxidation accounts to only
a minor process. The radical adducts may be converted to peroxyl radicals in presence
of oxygen. Based on these results it can be concluded that although bergenin is a
polyphenol, it may not act as a potent antioxidant, but may be act as pro-oxidant.