pubs.acs.org/JPCL
Ultrafast Intersystem Crossing of 4-Thiothymidine in Aqueous Solution Yosuke Harada,†,‡ Chie Okabe,‡ Takashi Kobayashi,† Tadashi Suzuki,*,† Teijiro Ichimura,† Nobuyuki Nishi,‡ and Yao-Zhong Xu§ †
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152-8551, Japan, Institute of Molecular Science, Nishigo-naka, Myodaiji, Okazaki 444-8585, Japan, and §Department of Chemistry and Analytical Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom ‡
ABSTRACT Ultrafast intersystem crossing of UVA-sensitive 4-thiothymidine in aqueous solution was investigated by pump-probe transient absorption. 4-Thiothymidine in the excited singlet state exhibited a distinctively different photophysical pathway from that of thymidine, with intersystem crossing to the triplet manifold being dominant and the triplet formation being completed within approximately 10 ps. This was further supported by quantum chemical calculations. We propose that the unique behavior of photoexcited 4-thiothymidine results from its distinctive molecular and electronic structures and that the triplet form is the initial toxic source to DNA and other biomolecules. SECTION Dynamics, Clusters, Excited States
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readily incorporated into the cellular DNA and that the incorporated 4-thiothymine, like its parent base (thymine), can form a base pair very well with adenine. On the other hand, low doses of UVA light can easily inflict lethal damage on the DNA containing S4-TdR, causing cell death.9 Recently, we reported that photoexcitation of S4-TdR gave rise to its triplet with the quantum yield of unity and effectively generated reactive singlet oxygen O2(1Δg) that is highly cytotoxic.10 These results clearly indicates that S4-TdR can be used as a promising agent for PDT. However, an understanding of the dynamic behavior of the excited state of S4-TdR would be required to further the exploitation of S4-TdR-related PDT. In this Letter, we report our findings on ultrafast excited-state dynamics of S4-TdR by use of pump-probe transient absorption measurements and quantum chemical calculations. Figure 1a shows a time profile of transient absorption of thymidine (TdR) in aqueous solution (pH 7.4) monitored at 570 nm after femtosecond laser excitation (at 263 nm). The transient absorption decayed quickly to the initial level. The time profile can be well fitted by a Gaussian function with a time constant of 0.50 ps. As the time constant is equal to the present instrumental response time, the lifetime of the transient would be equal to or shorter than 0.50 ps. This result is in good agreement with the lifetime of the bright 1ππ* state reported by Pecourt et al. (540 fs).11 It has been proposed that the S2(1ππ*)-S1(1nπ*) internal conversion (in the cases of uracil and thymine) is followed by remarkably rapid S1(1nπ*)-S0 relaxation.12,13 Therefore, it is highly likely that TdR in the initially excited S2(1ππ*) state would relax into the
NA is the most fundamental molecule. Although generally stable, DNA is susceptible to internal and external factors, including chemical and physical forces. It is well-known that DNA can be damaged by its exposure to UVB (280-320 nm) and UVC (
