Initial excited-state structural dynamics of 2′-deoxyadenosine

Purine nucleobases (adenosine and guanosine) are prone to spontaneous breaking of the nucleosidic bond to form abasic sites in both DNA and RNA. However, the purines also undergo photochemical reactions, including oxidation and cycloaddition, to form damage sites, albeit at lower photochemical quantum yields than the pyrimidines. In this study, we use ultraviolet resonance Raman spectroscopy to measure the initial excited-state structural dynamics in the nucleoside, 2'-deoxyadenosine. The resonance Raman-derived initial excited-state structural dynamics throughout the 260 nm L-a excited electronic state of adenine are found to be smaller in the nucleoside than in the previously reported 9-methyladenine nucleobase derivative, consistent with what is found for the pyrimidines thymine and uracil. Interestingly, resonance-enhanced vibrational modes in this electronic state also contain internal coordinates localized on the sugar, which may represent a different energy dissipation mechanism than in the pyrimidine nucleosides. The results will be discussed in terms of the initial excited-state photophysics and photochemistry of DNA and RNA.