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Biological Sciences
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May 2005

Study of rare tautomers featured on journal cover

Work performed under the U.S. Department of Energy's Low-Dose Radiation Research Program is featured on the cover of the May 21, 2005 issue of Physical Chemistry Chemical Physics.

The graphic, which shows a possible mechanism of destruction of the six-member ring structure of uracil, accompanies a research paper entitled "Stabilization of very rare tautomers of uracil by an excess electron." The authors are PNNL chemist Maciek Gutowski and Polish scientists Rafal Bachorz, and Janusz Rak from the University of Gdansk and Adam Mickiewicz University, respectively.

A tautomer is one of two or more structural isomers—a compound having the same molecular formula but different structures—that exist in equilibrium and are readily converted from one isomeric form to another. Rare tautomers of nucleic acid bases are involved in mispairing of nucleic acid bases in DNA and thus in the development of point mutations. PNNL scientists are uncovering a molecular pathway of destruction of the six-member ring structure of pyrimidine bases by low-energy electrons. In the current work, the scientists focused on chemical transformations of nucleic acid bases induced by low-energy electrons.

The Low-Dose Radiation Research Program supports fundamental science to determine health risks from exposures to low levels of radiation. DNA damage from radiation-induced mutation is potentially a critical pathway to adverse health effects. For example, single- and double-strand breaks in DNA are induced by lesions formed by low-energy electrons, which are produced in copious amounts by high-energy radiation. The molecular mechanism forming these lesions is not yet known.

The principal finding is that low-energy electrons favor very unusual tautomers of nucleic acid bases. These are not canonical or conventional rare tautomers, in which proton transfer takes place between electronegative atoms N or O. Instead, the most stable anionic tautomers result from enamine-imine transformation—where a proton is transferred from a nitrogen atom to a carbon atom. These new tautomers might affect the structure and properties of DNA and RNA. In particular, they undergo a barrier-free decomposition of the ring structure of a nucleic acid base upon an excess electron detachment (this is what is shown on the cover). These decompositions might be viewed as lesions to DNA or RNA. The new anionic tautomers might contribute to the chemistry of RNA and DNA exposed to low-energy electrons in condensed-phase environments. The researchers will continue to explore this problem.

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