Reactivity of Damaged Pyrimidines: DNA Cleavage via Hemiaminal Formation at the C4 Positions of the Saturated Thymine of Spore Photoproduct and Dihydrouridine

摘要

Describedhere are mechanistic details of the chemical reactivitiesof two modified/saturated pyrimidine residues that represent naturallyoccurring forms of DNA damage: 5-thyminyl-5,6-dihydrothymine, commonlyreferred to as the “spore photoproduct” (SP), and 5,6-dihydro-2′-deoxyuridine(dHdU), formed via ionizing radiation damage to cytosine under anoxicconditions and also serving as a general model of saturated pyrimidineresidues. It is shown that due to the loss of the pyrimidine C5–C6double bond and consequent loss of ring aromaticity, the C4 positionof both these saturated pyrimidines is prone to the formation of ahemiaminal intermediate via water addition. Water addition is facilitatedby basic conditions; however, it also occurs at physiological pH ata slower rate. The hemiaminal species so-formed subsequently convertsto a ring-opened hydrolysis product through cleavage of the pyrimidineN3–C4 bond. Further decomposition of this ring-opened productabove physiological pH leads to DNA strand break formation. Takentogether, these results suggest that once the aromaticity of a pyrimidineresidue is lost, the C4 position becomes a “hot spot”for the formation of a tetrahedral intermediate, the decay of whichtriggers a cascade of elimination reactions that can under certainconditions convert a simple nucleobase modification into a DNA strandbreak.