Why does DNA prove to be photostable?
MBI scientists get new insights into the photophysics of the DNA base thymine
DNA stores our genetic code. Solar UV radiation has sufficiently high energy to basically break bonds of the DNA and thus cause DNA damage. Although DNA (e. g. in our skin cells) is exposed to intense UV light irradiation, DNA proves to be surprising photostable. It is well established that this is due to efficient mechanisms that convert electronic energy into other forms of energy, in particular heat. An important role is played by so-called conical intersections between electronic excited potential energy surfaces and the ground state potential energy surface. These conical intersections are associated with structural changes of the molecules. The exact pathways back into the electronic ground state however are topic of intense research.
Although DNA is a macro molecule with billions of atoms (in case of human DNA), it can still be divided into only a few structural (and functional) elements: four DNA bases, a sugar moiety and a phosphate group. The absorption of UV light exclusively takes place in the DNA bases. For this reason it is a common scientific approach to investigate the UV response of DNA bases, first.
A team of scientists from MBI and universities of Hokkaido and Hirosaki in Japan have for the first time investigated the DNA base thymine in aqueous solution by time-resolved photoelectron spectroscopy and questioned existing ideas about the excited-state relaxation process in this base. So far was supposed that a significant fraction of the excited-state population remains in a dark nπ* state instead of immediately returning to the ground state via a conical intersection. This dark state cannot be observed by optical spectroscopy (e. g. transient absorption or fluorescence upconversion), directly. Corresponding limitations however do not exist for photoelectron spectroscopy.
By combining experiment and simulation, for the first time two different relaxation pathways were identified. Both pathes evolve in the first excited state (ππ*). The faster reaction path is associated with a twist of the aromatic ring and leads to repopulation of the electronic ground state within 100 fs. The second path involves an out-of-plane motion of the carbonyl group, and the molecule returns to the ground state within 400 fs. The scientists did not find any indication for an important role of the second excited nπ* state and conclude that this state is not involved in the relaxation process.
Original publication:
Franziska Buchner, Akira Nakayama, Shohei Yamazaki, Hans-Hermann Ritze, Andrea Lübcke
Excited-State relaxation of hydrated thymine and thymidine measured by liquid-jet photoelectron spectroscopy: experiment and simulation, JACS,
JACS, DOI: 10.1021/ja511108u
Contact
Dr. Andrea Lübcke
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI)
Max-Born-Str. 2A, 12489 Berlin
Tel: 030 6392-1207