New insights into the photophysics of the DNA base thymine

DNA stores our genetic code. Solar UV radiation is sufficiently high energy to break bonds in the DNA and thus cause DNA damage. However, even though DNA (eg in our skin cells) is exposed to intense UV rays from the sun daily, the DNA turns out to be astonishingly light-stable. It has long been known that this can be explained by mechanisms that highly efficiently transform electronic energy into other forms of energy, especially heat. Interfaces of the multidimensional potential energy surfaces, so-called conical intersections, play an important role between the electronically excited states and the electronic ground state. These conical penetrations are associated with structural changes in the molecules. The exact ways back to the electronic ground state are the subject of intensive research.

Although the DNA is a macromolecule with several billion atoms (in the case of human DNA), it can be divided into only a few different structural (and functional) elements: four DNA bases, a sugar residue and a phosphate group. The absorption of UV light takes place exclusively in the DNA bases. Therefore, it is a common research approach to first study only the reaction of the DNA bases to UV absorption.

Fig. 1 After UV excitation, the photoreaction in thymine occurs along two different paths: 1. the twist of the aromatic ring and 2. a movement of the carbonyl group from the ring plane. In contradiction to the existing literature, the nπ state seems to play no role.

A team of scientists from MBI and the universities of Hokkaido and Hirosaki in Japan has now for the first time investigated the DNA base thymine in aqueous solution with the help of time-resolved photoelectron spectroscopy and questioned existing ideas for the relaxation process in this base. So far it has been suggested that a significant portion of the excited state initially remains in a dark nπ* state and does not immediately return to the ground state via conical interpenetration. This dark state can not be directly observed by optical spectroscopy (such as time-resolved fluorescence or time-resolved absorption). However, there are no corresponding restrictions for photoelectron spectroscopy.

In the interplay between experiment and simulation, two different reaction pathways could be identified for the first time. Both paths are in the first excited (ππ*) state. The faster reaction pathway is associated with a drive of the aromatic ring and returns to the ground state in about 100 fs. The second path is about 400 fs back to the ground state via the movement of the carbonyl group from the ring plane. Evidence that the next higher excited nπ* state plays an important role in the relaxation dynamics of thymine, the scientists did not find, from which they concluded that this state is not involved in the relaxation process.

Original publication

Excited-state relaxation of hydrated thymine and thymidine measured by liquid-jet photoelectron spectroscopy: experiment and simulation

F. Buchner, A. Nakayama, S. Yamazaki, H.-H. Ritze, A. Lübcke

Journal of American Chemical Society 137 (2015) 2931-2938