Femtosecond Dynamics of Double Proton Transfer in a Model DNA Base Pair: 7-Azaindole Dimers in the Condensed Phase

Abstract

The dynamics of double proton transfer in 7-azaindole (7-AI) dimers, a model DNA base pair, are investigated in real time using femtosecond transient absorption and fluorescence upconversion techniques. In nonpolar solvents we examine the isotope effect, the excitation energy dependence, and the structure analogue of the tautomer (7-MeAI). A detailed molecular picture of the nuclear dynamics in the condensed phase emerges with the relationship to the dynamics observed in molecular beams: Following the femtosecond excitation there are three distinct time scales for structural relaxation in the initial pair, proton (hydrogen) transfers, and vibrational relaxation or cooling of the tautomer. The molecular basis of tunneling and concertedness are elucidated by careful examination of the isotope effect and the time resolution. Comparison with the results in the isolated pair indicates the critical role of the N−H and N···N nuclear motions in determining the effective potential, and the thermal excitation in solution. Because the barrier is small, ∼1.3 kcal/mol, both are important factors and experiments at much higher energies will be unable to test either tunneling or concertedness. Finally, we compare the experimental results and the dynamical picture with detailed ab initio and molecular dynamics simulations.