III-F-1 Excited-State Dynamics of 4- Thiothymidine with UVA Light Irradiation

Thionucleobases and thionucleosides have received renewed attention because of their distinct property, namely, high sensitivity to UVA light (320–400 nm) in which region normal DNA constituents are transparent. 4-Thiothymidine (4-TT), an analogue of the naturally occurring nucleoside thymidine, has strong absorption in the UVA region. Recently it was reported that 4-TT can be readily incorporated into cellular DNA and that low doses of UVA light can easily inflict lethal damage on the DNA containing 4-TT, causing cell death. The synergistic use of 4-TT and UVA light offers a novel approach to cancer treatment. Apparently, electronically excited state of 4-TT is at the initial and crucial stage of the UVA-induced cell killing, and thus photophysical and photochemical studies of 4-TT would be of great significance. We have measured transient absorption spectrum of 4-TT in deaerated acetonitrile with the nanosecond 355 nm laser. Immediately after the laser shot, an intense bleaching at 335 nm and a broad absorption band at 380–600 nm (Abs. Max. 530 nm) were observed. The bleaching is ascribed to the depletion of the ground state molecules while the absorption band is assigned to the absorption of the lowest excited triplet (T1) state of 4TT (T–T absorption). The T–T absorption decayed with the rate constantof 1 × 106 s–1. In triplet quenching experiments by KI, the absorption intensity of the photoproducts decreased correspondingly to shortening of triplet lifetime of 4-TT. Our experimental results suggest that photochemical process would take place via the T1 state of 4-TT and other several intermediate states. A good understanding of the photophysics and photochemistry of 4-TT could offer valuable insights to the mechanisms by which cancer cells are killed. III-F-2 Photochemical Reaction Dynamics of oQuinones