UV-Strahlung und DNA-Schäden

Ultraviolet radiation can lead to photo-lesions and mutations in
DNA, which can cause several diseases, most notably skin
carcinomas. The most abundant UV-induced photo-lesions result from
the formation of a cyclobutane ring between adjacent thymine bases
(CPD-lesions). Although the formation of these thymine dimers has
been reported in the early 1960s, until now neither the time scale
of dimer formation nor the reaction mechanism has been resolved.
The focus of this work is on the investigation of photophysical and
photochemical processes in nucleic acids on the picosecond time
scale. Pump-probe spectroscopy allows the investigation of
ultrafast, photo-induced processes. The applied spectrometer is
based on a central femtosecond laser system. The emitted short
light pulses (800 nm, 90 fs) are converted via nonlinear processes
into the required spectral regions (ultraviolet pump: ~ 270 nm, mid
infrared probe: 3 - 10 µm). In this way the high-structure
sensitivity of vibrational spectroscopy can be combined with a time
resolution in the sub picosecond regime. For the investigation of
thymine dimer formation, two thymine derivatives were chosen: the
18-mer all-thymine single strand (dT)18 and the mononucleotide
thymidine-5'-monophosphate. Additional experiments were performed
on the all-adenine single strand poly(A) and the mononucleotide
adenine-5'-monophosphate, as adenine is the complementary base of
thymine in the DNA double helix. While in AMP virtually all excited
adenine bases return to the vibrationally excited ground state via
a fast, internal conversion (< 1 ps), there is an additional
population of long-lived, electronic states in poly(A), with
lifetimes in the 100 ps- and ns-regime. The population of these
states correlates with the amount of stacked bases in poly(A). This
can be explained by the formation of excimer states. The IR
absorption of these states could be deduced in this paper for the
first time. To resolve the UV-induced formation of CPD-lesions, the
characteristic IR absorption of thymine dimers was determined from
stationary irradiation experiments on (dT)18. Afterward the
formation of thymine dimers in (dT)18 was shown to occur within one
picosecond (10^-12 s) by comparing the time resolved measurements
on TMP and (dT)18. Long-lived electronic states (100 ps - 1 ns) do
not lead to dimer formation. Therefore, the photoreaction can only
take place if the conformation of two bases at the moment of UV
absorption is already suitable for dimerization. This
interpretation can be transferred to the DNA double helix, in which
deviations from the ideal helix structure are necessary for dimer
formation. In this work a fundamental question of photochemical
reactions in DNA is resolved, which is of central importance to the
understanding of the frequency of damage and mutation patterns in
the genome.