Transport and reactions of radical cations in DNA: Steric and energetic control of reactivity

A series of DNA oligomers was prepared that contain an anthraquinone group (AQ) linked to a 5'-terminus and have 8-methylguanine (_MG) or 8-bromoguainine (_BG) substituted for G at various positions. Irradiation of the AQ injects a radical cation into the oligonucleotide, which migrates through the DNA and reacts primarily at G_n sites where n = 2 or 3. Substitution with _MG which has an oxidation potential (E _OX) slightly below G, traps the migrating radical cation, substitution with _BG, which has an E_OX above G, does not. However, both _MG and _BG affect the relative reactivity of the guanines in G_n steps. Moreover, a (G)₃ sequence has a much smaller effect on the efficiency of radical cation migration than does _MG, which demonstrates that the E_OX of G in the (G) ₃ sequence is above that of _MG. These findings show that the E_OX of the base controls the efficiency of radical cation transport and that steric effects influence the relative reactivity of G in G_n sequences.