Barrier-Free Intermolecular Proton Transfer - A Step towards Understanding of Mutagenic Effects of Low-Energy Electrons
Thymine, lower left, bonds to glycine in DNA
Low-energy electrons (with energy below 20 eV), such as those released during radiolysis of water, are known to induce single- and double-strand breaks in DNA. A newly discovered phenomenon - barrier-free proton transfer (BFPT) induced by an excess electron - might explain how the breaks occur. In parallel theoretical calculations and photoelectron spectroscopic experiments, we studied anionic binary complexes, made up of nucleic acid bases (NB) and various weak acids (HA), generally represented as [(NB)(HA)]-. Attachment of an excess electron triggers a barrier-free, intermolecular proton transfer from many acids to the base with the new product forms being a neutral hydrogenated base radical, (NBH.) and the deprotonated acid, A-.
(NB)(HA) + e- → (NBH·) + (A-)
An example of this is shown in the figure. Thymine, lower left, bonds to glycine in DNA. An electron attaches to the thymine, giving it a negative charge. A proton moves from the glycine moiety to the thymine in a barrier free proton transfer, giving the glycine a negative charge.
The driving force for proton transfer is the stabilization of the excess electron onto a π* orbital of the base. Thus, one may envision the process as one in which the excess electron first attaches itself to the nucleic acid base moiety of the dimer, forming a very strongly basic anion, which then extracts a proton from the HA portion of the dimer to yield the complex, (NBH·)(A-). The radical, NBH·, may then initiate further transformations, leading to the rupture of the sugar-phosphate bond. We computed the barrier for NBH radical-induced, sugar-phosphate rupture to be less than 7 kcal/mol.