Generation of thiyl radicals by the photolysis of 5-iodo-4-thiouridine.

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.