Photoreaction of iodouracil in DNA duplex; C–I bond is cleaved via two different pathways ‘homolysis and heterolysis’

We recently found that a selective photoreaction of 5-iodouracil (IU) occurs in 5′-(G/C)AAIUIU-3′ and 5′-(G/C)AIUIU-3′ sequences in IU-substituted duplex DNA. In this study, the photoreactivity of the 5′-G(A)nIUT-3′ sequence was examined using various IU-containing oligonucleotides. HPLC analysis revealed that their photoreactivity largely depends on the number of As between G and IU. The most efficient reactivity was observed when the number of As was two and this decreased with increasing numbers from three to five, as observed for the 5′-G(A)nIUT-3′ sequence. These results indicate that the G located 5′ from IU acts as an electron donor for IU, as in the photoreaction of BrU. In sharp contrast to the BrU photoreaction, IU was photoreactive when the number of As was zero or more than five. These results indicate that both homolytic and heterolytic pathways operate in the formation of the uracil-5-yl radical in the photoreaction of IU in duplex DNA. In addition, the ratio of these pathways is highly dependent on DNA sequence.