Neighboring base sequence effect on DNA damage.

Guanine is the most strongly oxidized base in DNA; generation of a guanine radical cation as an intermediate in an oxidation reaction leads to migration through a resulting cationic hole in the DNA π-stack until it is trapped by irreversible reaction with water or other free radicals. In the case of normal sequences, the primary position of Guanine oxidations by one-electron oxidants such as carbonate radical anions, BPT(7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene), and riboflavin are 5'-G in GG doublets and the central G in a GGG triplet. According to results, the properties of guanine oxidation on abasic site containing sequences are independent from the position of AP(apurinic/apyrimidinic) site in the presence of carbonate radical anions under a short irradiation time, although this radical is exposed to solvent by the existence of an abasic site. The lack of abasic site effect on guanine oxidative damage by the carbonate radical may be due to a sequence-independent property of the initial electron transfer rate in the hole injection step, or may relate to an electron transfer mechanism with large reorganization energy dependency. Consequently, the carbonate radical anions may easily migrate to another single G in the charge re-distribution step. Meanwhile, there is a strong dependency on the presence of an AP(apurinic/apyrimidinic) site in the cleavage patterns of guanine oxidations by physically large oxidizing agents, such as BPT(7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene) and riboflavin. These radicals show strong AP(apurinic/apyrimidinic) site dependency and clear G-site selectivity.Communicated by Ramaswamy H. Sarma.