Formation of pyrimidine dimers in DNA is a major initiating event in the induction of skin cancer. Model experiments suggest that, upon absorption of UVA, one type of dimers induced by UVB, the pyrimidine (6‐4) pyrimidone photoproducts, photosensitizes the formation of mutagenic cyclobutane pyrimidine dimers by triplet ‐triplet energy transfer (TTET). We investigated whether this photoreaction actually took place when 64PP were located within a DNA duplex rather than added as external sensitizers like in available data. Our results show that this process is not detectable in DNA and double‐stranded oligonucleotides exposed to a combination of UVB and UVA. TTET could only be observed, as a very minor photoreaction, in a short single‐stranded oligonucleotide bearing a 64PP. It may be concluded that 64PP‐mediated TTET does not significantly contribute to UV‐induced DNA damage. In contrast, the photoisomerization of 64PP into their Dewar valence isomers is very efficient.

中文翻译：

---

UVA辐射的DNA中的嘧啶（6-4）嘧啶酮光产品：光敏化还是光异构化？

DNA中嘧啶二聚体的形成是诱导皮肤癌的主要起始事件。模型实验表明，吸收UVA后，由UVB诱导的一种二聚体，即嘧啶（6-4）嘧啶酮光产物，通过三重态-三重态能量转移（TTET）光敏化诱变的环丁烷嘧啶二聚体。我们研究了当64PP位于DNA双链体中而不是像现有数据中那样作为外部敏化剂添加时，这种光反应是否确实发生了。我们的结果表明，在暴露于UVB和UVA组合的DNA和双链寡核苷酸中无法检测到此过程。仅在带有64PP的短单链寡核苷酸中观察到TTET，这是非常小的光反应。可以得出结论，64PP介导的TTET对UV诱导的DNA损伤没有显着贡献。相反，将64PP的光异构化成其杜瓦价异构体非常有效。