jo025856w_si_001.pdf (323.4 kB)
Photochemistry of N-Isopropoxy-Substituted 2(1H)-Pyridone and 4-p-Tolylthiazole-2(3H)-thione: Alkoxyl-Radical Release (Spin-Trapping, EPR, and Transient Spectroscopy) and Its Significance in the Photooxidative Induction of DNA Strand Breaks
journal contribution
posted on 2002-07-19, 00:00 authored by Waldemar Adam, Jens Hartung, Hideki Okamoto, Stefan Marquardt, Werner M. Nau, Uwe Pischel, Chantu R. Saha-Möller, Kristina ŠpeharUVA-irradiation of the photo-Fenton reagents N-isopropoxypyridone 2b and N-isopropoxythiazole-2(3H)-thione 3b releases radicals which induce strand breaks. Transient spectroscopy establishes
N−O bond scission [ΦN-O = (75 ± 8)% for 2b and (65 ± 7)% 3b] as the dominating primary
photochemical process to afford the DNA-damaging radicals. Product studies and laser-flash
experiments reveal that the thiazolethione 3b leads primarily to the disulfide 5, from which through
C−S bond breakage, the bithiazyl 6, the thiazole 7, and the isothiocyanate 8 are derived. Upon
irradiation of pyridone 2b (300 nm) in aqueous media, a mixture of isopropoxyl and 2-hydroxyprop-2-yl radicals is formed, as confirmed by trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO)
and EPR spectroscopy. In contrast, the photolysis of the thiazolethione 3b (350 nm) affords
exclusively the DMPO adducts of the isopropoxyl radicals. Control experiments disclose that the
thiazolethione-derived photoproduct disulfide 5, or the intermediary thiyl radicals B, scavenge the
carbon-centered 2-hydroxyprop-2-yl radicals, which are generated from the isopropoxyl radicals
by hydrogen shift. With supercoiled pBR 322 DNA in a 60:40 mixture of H2O−MeCN, the pyridone
2b and the thiazolethione 3b display moderate strand-break activity (17% open-circular DNA for
2b and 12% for 3b). In pure water, however, the pyridone 2b photoinduces substantially more
DNA cleavage (32% open-circular DNA), which is attributed to the peroxyl radicals generated from
the 2-hydroxyprop-2-yl radicals by oxygen trapping. The lower strand-break activity of the
thiazolethione 3b derives presumably from isopropoxyl radicals, because only these are detected
in the photolysis of this photo-Fenton reagent.