E.s.r. studies on the mechanism of hydroxyl radical-induced strand breakage of polyuridylic acid.

Reaction of photolytically produced .OH radicals with polyuridylic acid [poly(U)] in neutral solutions resulted in the electron spin resonance (e.s.r.) spectrum of the C(5)-OH-6-yl (= 6-yl) radical 1b of the nucleobase. At pH less than or equal to 4 the spectrum of the base radical had disappeared and instead the cyclic 2'-oxo-3'-yl sugar radical 2a was observed. The assignment of the sugar radical was supported by model reactions with SO4-. as the radical inducing agent. Time-resolved e.s.r. measurements showed that the rate of decay of the 6-yl base radical at neutral pH is virtually the same as that of the strand break (sb) formation. These results prove that: (i) in agreement with an earlier proposal the C(2') mechanism contributes to sb formation of poly(U), and (ii) the decay of the 6-yl radical is the rate-determining step in the reaction sequence leading to strand breakage. The change in the e.s.r. spectra at pH 4 is due to an increase in the rate of sb formation with increasing proton concentration. This effect is explained by generation of the radical cation, 4, from the 6-yl radical and/or by rearrangement of the 6-yl radical into the C(6)-OH-5-yl (= 5-yl) radical in proton-induced reactions and subsequent rapid H abstraction from the sugar moieties by species 4 and/or 5.