Long-range guanine damage in single-stranded DNA: Charge transport through a duplex bridge and in a single-stranded overhang

A series of anthraquinone-linked DNA oligonucleotides was prepared to examine the migration and reactions of radical cations in duplex and single-stranded regions. There are fewer nucleotides in the strand complementary to the AQ-containing Strand, which creates a. duplex with an overhang. This complementary strand contains three GG steps, which serve as traps for the radical cation. One GG step is in the duplex region, a second is adjacent to the duplex, and the third is in the single-strand overhang. Irradiation at 350 nm excites the anthraquinone and results in radical cation injection into the duplex DNA region. Treatment of the irradiated sample with piperidine or with formamidopyrimidine glycosylase results in strand cleavage that is detected by polyacrylamide gel electrophoresis and autoradiography. The pattern of reactivity at the three GG steps was examined for various oligonucleotide sequences. These studies confirm that the products of radical cation reaction differ in single-strand and duplex regions with 7,8-dihydro-8-oxo-guanine predominating in the latter. The results show that the radical cation can migrate from the duplex region onto the single strand. Radical cation transport in the single-strand overhang is proposed to proceed from guanine to guanine by means of direct contact in GG mispairs.