SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS–XII. SPIN TRAPPING STUDY OF THE FREE RADICALS GENERATED DURING THE PHOTOLYSIS OF PHOTO ALLERGENS BITHIONOL and FENTICHLOR

Abstract

Free radicals were trapped and observed by ESR when photoallergens bithionol and fentichlor were irradiated in the presence of spin traps N‐t‐butyl‐α‐phenylnitrone (PBN) and 5,5‐dimethyl‐pyrroline‐N‐oxide (DMPO). In the absence of air, both PBN and DMPO trapped a carbon‐centered radical. The carbon‐centered radical, which was capable of abstracting a hydrogen atom from cysteine, glutathione, ethanol and formate, was identified as an aryl radical derived from the homolytic cleavage of the carbon‐chlorine bond. In the presence of air, both carbon‐centered radicals and hydroxyl radicals were trapped by DMPO. Under similar conditions, the yield of the hydroxyl radicals was greater from bithionol than from fentichlor. The presence of the hydroxyl radical was confirmed by kinetic experiments employing hydroxyl radical scavengers (ethanol, formate). Superoxide and H2O2 were not involved. Experiments with oxygen‐17O indicated that the hydroxyl radicals came exclusively from dissolved oxygen. The precursor of the hydroxyl radical is postulated to be a peroxy intermediate (ArOO*) derived from the reaction of an aryl radical (Ar*) with molecular oxygen. Both bithionol and fentichlor photoionized only when excited in the UVC (<270 nm) region. Free radicals have long been postulated in the photodechlorination of bithionol and fentichlor and the present study provides supporting evidence for such a mechanism. Aryl and hydroxyl radicals are reactive chemical species which may trigger a series of events that culminate in photoallergy.

The Photo‐oxidation of N, N‐diethylhydroxylamine by Rose Bengal in Acetonitrile and Water

“…Electron spin resonance spectra were acquired at room temperature with a Varian E-109 spectrometer equipped with a TM,," cavity. Irradiation was conducted in situ with a 1000 W Xe arc lamp and monochromator combination (bandpass 10 nm) as described previously (Li and Chignell, 1987).…”

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS‐XIII. pH DEPENDENCE OF THE PHOTOCHEMISTRY OF PHOTO ALLERGENS BITHIONOL AND FENTICHLOR: AN ELECTRON spIN RESONANCE STUDY OF THE FREE RADICAL PHOTOPRODUCTS

“…A hand-help pump (Pipet-aid) was used as the source of air. Electron spin resonance instrumentation, optical measurements and irradiation conditions have been previously described (Li and Chignell, 1987a). The g-values were measured using a fieldlfrequency lock accessory (Varian Assoc., Palo Alto, CA) by comparison with Fremy's salt ( g = 2.0057; Wertz rl d.. 1961).…”

“…Omura and Matsuura (1971) ,have proposed that a transient intermediate, may be involved. However, our previous studies (Li and Chignell, 1987a) with the spin trap DMPO have suggested that the chlorine atoms of BT and FT are lost via a homolytic process that results in the generation of the corresponding aryl radicals (see also Grabowski, 1961). It is not possible at present to determine what role this free radical mechanism may play in the photoconversion of 4-chlorophenols to hydroquinones.…”

“…Although the mechanism behind photoallergic reactions is still not clear, photo-induced free radicals have been implicated in previous studies of BT and FT (Barratt and Brown, 1985;Rickwood and Barratt, 1984;O'Quinn et al, 1967). We have previously employed ESR spectroscopy, coupled with the spin trapping technique, to establish that both BT and FT generate reactive free radicals upon UV excitation (Li and Chignell, 1987a). However, during the course of that study, we also observed that when UV-irradiated in 0.1 N NaOH, both BT and FT generate strong ESR signals that are observable by conventional ESR in the absence of spin traps.…”