Anthraquinone photonucleases: Mechanisms for GG-selective and nonselective cleavage of double-stranded DNA

Anthraquinone derivatives 2AQA2(HEt(2)) and 2AQC2(HEt(2)) were examined as light-activated agents that initiate DNA cleavage. The substituents control the electronic configuration of the lowest excited state of the anthraquinone. 2AQC2(HEt(2)) has a lowest n pi* excited state and can react by electron transfer or hydrogen atom abstraction. 2AQC2(HEt(2)) has a lowest excited state of pi pi* or intramolecular charge-transfer character and reacts only by electron transfer. Spectroscopic evidence indicates that both quinones bind to double-stranded DNA by intercalation with essentially the same affinity. Picosecond time-resolved laser spectroscopy shows that single electron transfer from the DNA bases to either bound quinone occurs rapidly and to the same extent. Irradiation of either intercalated 2AQA2(HEt(2)) or 2AQC2(HEt(2)) followed by treatment with hot piperdine leads to equally effective cleavage of DNA at the 5'-G of GG steps. These findings indicate that electron transfer from a DNA base to the excited quinone is the dominant path for the GG-selective DNA cleavage. At high concentrations, where some quinone is free in solution, irradiation of 2AQC2(HEt(2)), but not 2AQA2(HEt(2)), leads to nonselective spontaneous cleavage of DNA. This second path to DNA cleavage is identified as direct hydrogen atom abstraction from the deoxyribose backbone by excited, nonintercalated 2AQC2(HEt(2)).