title: Generation of thiyl radicals by the photolysis of 5-iodo-4-thiouridine.
abstract: The photochemistry of 2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine (3) in deoxygenated 1:1 CH(3)CN-H(2)O pH 5.8 (phosphate buffer) solution has been studied by means of steady-state and nanosecond laser flash photolysis methods. Under steady-state irradiation (lambda > or = 334 nm), the stable photoproducts were iodide ion, 2',3',5'-tri-O-acetyl-4-thiouridine (4), and two disulfides. The disulfides were the symmetrical bis-(2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine) (5) and unsymmetrical 6, which contains both 4-thiouridine and 5-iodo-4-thiouridine residues. The formation of the dehalogenated photoproduct suggests that C(5)-I bond cleavage is a primary photochemical step. Attempts to scavenge the resulting C(5)-centered radical by suitable addends, bis-(N-alpha-acetyl)cystine-bis-N-ethylamide or benzene, were unsuccessful. Analysis of the photoproducts formed under these conditions showed that the S-atom is the reactive center. The photoproduct 4, obtained by irradiation of 3 in CD(3)CN-H(2)O, followed by reversed-phase HPLC isolation using nonlabeled eluents, did not contain deuterium. An analogous experiment performed in CH(3)CN-D(2)O gave deuterated product 4-d with 88% of the deuterium incorporated at C(5). Transient absorption observed upon laser excitation (lambda= 308 nm) of 3 was assigned to the 4-uridinylthiyl radical on the basis of the similarity of this spectrum with that obtained upon laser photolysis of the disulfide: bis-(2',3',5'-tri-O-acetyl-4-thiouridine) 14. On the basis of the results of steady-state and laser photolysis studies, a mechanism of the photochemical reaction of 3 is proposed. The key mechanistic step is a transformation of the C(5)-centered radical formed initially by C(5)-I bond cleavage into a long-lived S-centered radical via a 1,3-hydrogen shift. Theoretical calculations confirmed that the long-lived S-centered radical is the most stable radical derived from the 4-thiouracil residue.
authors.affiliation: Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland. gwenska@amu.edu.pl
authors.forename: Grazyna
authors.initials: G
authors.lastname: Wenska
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Taras-Goślińska
authors.forename: Bohdan
authors.initials: B
authors.lastname: Skalski
authors.forename: Gordon L
authors.initials: GL
authors.lastname: Hug
authors.forename: Ian
authors.initials: I
authors.lastname: Carmichael
authors.forename: Bronislaw
authors.initials: B
authors.lastname: Marciniak
mesh_terms: D000345:Affinity Labels / Q000378:metabolism; D005609:Free Radicals / Q000378:metabolism*; D010782:Photolysis; D012995:Solubility; D013891:Thiouridine / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism* / Q000528:radiation effects*
chemical_list: C500682:5-iodo-4-thiouridine; D000345:Affinity Labels; D005609:Free Radicals; D013891:Thiouridine