Temperature effects on the internal conversion of excited adenine and adenosine

This work aims to elucidate the dependence of the excited-state lifetime of adenine and adenosine on the temperature. So far, it has been experimentally shown that while adenine’s lifetime is unaffected by temperature, adenosine’s lifetime strongly depends on it. However, the non-Arrhenius temperature dependence has posed a challenge in explaining this phenomenon. We used surface hopping to simulate the dynamics of adenine and adenosine in the gas phase at 0 and 400 K. Temperature effects were incorporated in the initial conditions via Wigner sampling with thermal corrections. Our results confirm that adenine’s excited-state lifetime does not depend on temperature, while adenosine’s lifetime does. Adenosine’s dependency is due to intramolecular vibrational energy transfer from adenine to the ribose group. At 0 K, this transfer reduced the mean kinetic energy of adenine’s moiety so much that internal conversion is inhibited, and the lifetime elongated by a 2.3 factor compared to that at 400 K. The modeling also allowed definitively discarding the influence of viscosity, an alternative previously proposed explanation.