5-thyminyl-5,6-dihydrothymine, also called spore photoproduct or SP, is the exclusive DNA photo-damage product found in bacterial endospores. Thymidine solid state photoreaction (in dry film or ice) results in two dinucleoside SP stereoisomers, adopting an R and S configuration at the C5 chiral center respectively. After UV irradiation of the d3-thymidine with a -CD3 moiety, a deuterium is found to be transferred to the H6proS and H6proR position in the resulting 5R and 5S SP respectively, confirming that SP is formed via an H atom migration pathway, and this reaction obeys a strict stereoselectivity which is demanded by the ultrafast photoreaction rate. Surprisingly, the trace amount of protiated thymidine impurities in d3-thymidine were drastically enriched in the formed SPs. The unlabeled thymidine is favored over its d3-counterpart in the ice environment; such a striking discrimination against deuterium is not due to the kinetic isotope effect, but due to the slightly stronger methyl-methyl/methyl-nucleobase interaction between the protiated thymidine species. Our results highlight the importance of the methyl-methyl interaction to the stability of thymine stacking conformation, which subsequently alters the DNA photo-reactivity.

Mechanistic studies of spore photoproduct formation using deuterium labeled thymidine

246th National Meeting (2013)

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