Exploring photochemistry of uranyl(VI)

Photoexcited uranyl(VI) is a mild oxidant and can decompose various biological substances including large molecule like DNA. This reaction constitutes another environmental risk of uranium in addition to its radiotoxicity and chemotoxicity. There are several different mechanisms which may lead to decomposition of organic substances by photoexcited uranyl(VI). This includes hydrogen abstraction by “yl”-oxygen, decarboxylation of uranyl-bound carboxylic group, and ligand-to-metal charge transfer. These mechanisms may also compete with each other and makes mechanistic understanding far from being straightforward. In my talk, I will focus on our recent experimental and theoretical development on DNA photocleavage study. Although hydrogen abstraction is widely believed to be the key reaction in uranyl(VI)–mediated photocleavage of DNA, density functional theory calculations show that direct charge transfer from DNA to photoexcited uranyl(VI) can be an alternative pathway that leads to DNA strand break. In the oxidized state of DNA, electron deficiency is centered mainly on guanine as well as on uranyl–free phosphate, and lesser extent is distributed on adenine and thymine. Presumably there is no unique “hot spot” in DNA and upon irradiation local oxidation occurs in nucleobase or in uranyl–free phosphate in the vicinity of uranium. Oxidation of phosphate eventually leads to DNA strand break. Experiments using circular dichroism (CD) and X-ray absorption spectroscopy (XAS) are in progress.