Nonequilibrium ultrafast excited state dynamics in DNA

Abstract

The results of simulations for linear and two-dimensional electronic spectroscopy of DNA nucleobases have been presented in this work. How nucleic acids respond to radiation is relevant to human health because UV radiation can be the starting point of damaging photochemical reactions leading to permanent damage of DNA. Moreover it is also important for our understanding of how life on earth developed. According to the popular reductionist approach, the study of deexcitation processes of DNA double strands should start with the investigation of nucleobases, the main chromophores in DNA. The possible photochemical paths following UV excitation in DNA monomers are in general prevented by ultrafast decay processes, through which the deexcitation of photoreactive states is allowed to take place. Ultrafast internal conversion is responsible for this relaxation process, which can be investigated by identifying the conical intersections (CIs) between ground and excited states involved in the radiationless decay. As a first step in the understanding of such nonradiative processes, excited state properties and linear absorption spectra have been simulated for the four DNA nucleobases in their microsolvated structures, by combining time-dependent density functional theory calculations and the semiclassical nuclear ensemble method. This approach includes explicitly vibrational broadening, which seems essential for a reliable comparison of simulated photoabsorption spectra with experimental data. The second part of the project was devoted to the determination of optical properties of the DNA nucleobase isomer 9H-adenine in terms of the third-order response function, with a direct connection of the theoretical modelings to experimental results of 4-wave-mixing time-resolved optical spectroscopies, in particular to 2D-UV Fourier and pump probe spectroscopy. A minimal kinetic model derived from experimental results was proposed to underline the decay behaviour observed in adenine, where after excitation to the bright pi-pi* state, the deexcitation can be direct to the ground state or via a dark n-pi* state. Two excited state absorptions from the pi-pi* and n-pi* are also proposed to take place. Time Nonlocal, and Hierarchy Equations of Motion approachs have been used to simulate the Non-Markovian quantum dynamics of 9H-adenine. A good agreement between theoretical and experimental 2DES and pump probe spectra has been reached in terms of size and energy range characterizing the peaks forming the spectra. The present study will serve as a basis for future simulations and experimental investigations, for instance further linear and 2D electronic spectrocopy simulations where different methods can be applied, or the extension of the model from single nucleobases to nucleotides (and nucleosides) polymers.