Nucleobases like adenine possess a remarkable UV photostability due to ultrafast internal conversion to the ground state in a few hundred femtoseconds. However, adenine oligonucleotides exhibit poorly understood and much more complex excited-state dynamics. In particular, an excited-state lifetime on the order of hundreds of picoseconds has been linked to exciton and excimer interactions between stacked adjacent bases. Here, we report on femtosecond and picosecond-to-nanosecond time-resolved spectroscopic investigations of the two adenine model dimers Ade-C3-Ade and Ade-C4-Ade, covalently linked by short propyl or butyl chains. Following femtosecond fluorescence up-conversion spectroscopy, broadband femtosecond transient electronic absorption spectroscopy (TEAS) and time-correlated single photon counting (TCSPC), our results reveal remarkable deviations from the excited-state dynamics of canonical adenine oligonucleotides. Most notably, we observe much longer (and linker-dependent) dynamics dominated by nanosecond decay components of τ = 2.2 ns (Ade-C3-Ade) and τ = 1.2 ns (Ade-C4-Ade) in both fluorescence and absorption. Ab initio RI-SCS-ADC(2) calculations confirm that the initially prepared exciton states can undergo direct and barrierless transition towards closely bonded excimer states previously associated with nanosecond lifetimes. Our findings demonstrate that the linker moiety substantially influences the excited-state dynamics in stacked nucleobases and suggest that the ribose-phosphate backbone itself plays a key role in mitigation of photodamage in DNA.

Observation of excimer states with nanosecond lifetimes in stacked adenine model dimers connected by alkyl linkers

256th National Meeting (2018)

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