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

A series of anthraquinone-linked DNA oligonucleotides was prepared to exami ne the migration and reactions of radical cations in duplex and single-stra nded regions. There are fewer nucleotides in the strand complementary to th e AQ-containing Strand, which creates a. duplex with an overhang. This comp lementary strand contains three GG steps, which serve as traps for the radi cal cation. One GG step is in the duplex region, a second is adjacent to th e duplex, and the third is in the single-strand overhang. Irradiation at 35 0 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 pat tern of reactivity at the three GG steps was examined for various oligonucl eotide 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 cat ion can migrate from the duplex region onto the single strand. Radical cati on transport in the single-strand overhang is proposed to proceed from guan ine to guanine by means of direct contact in GG mispairs.