4 research outputs found
DNA Intercalated Psoralen Undergoes Efficient Photoinduced Electron Transfer
The interaction of psoralens with
DNA has been used for therapeutic
and research purposes for decades. Still the photoinduced behavior
of psoralens in DNA has never been observed directly. Femtosecond
transient absorption spectroscopy is used here to gain direct insight
into the photophysics of a DNA-intercalated psoralen (4′-aminomethyl-4,5′,8-trimethyl-psoralen
(AMT)). Intercalation reduces the excited singlet lifetime of AMT
to 4 ps compared with 1400 ps for AMT in water. This singlet quenching
prohibits the population of the triplet state that is accessed in
free AMT. Instead, a DNA to AMT electron transfer takes place. The
resulting radical pair decays primarily via charge recombination with
a time constant of 30 ps. The efficient electron transfer observed
here reveals a completely new aspect of the psoralen–DNA interaction
Femtosecond Spectroscopy of Calcium Dipicolinateî—¸A Major Component of Bacterial Spores
Bacterial spores are rich in calcium
dipicolinate (CaDPA). The
role of this compound in the high UV resistance of spore DNA and their
unique DNA photochemistry is not yet clarified. Here, the photophysical
properties of CaDPA dissolved in water are studied by means of steady-state
and time-resolved spectroscopy as well as quantum chemistry. Upon
255 nm excitation, a fluorescence emission with a yield of 1.7 ×
10<sup>–5</sup> is detected. This low yield is in line with
a measured fluorescence lifetime of 110 fs. Transient absorption experiments
point to further transitions with time constants of 92 ps and 6.8
μs. The microsecond time constant is assigned to the decay of
a triplet state. The yield of this state is close to unity. With the
aid of quantum chemistry (TD-DFT, DFT-MRCI), the following transitions
are identified. The primarily excited <sup>1</sup><i>ππ</i>* state depletes within 110 fs. The depletion results in the population
of an energetically close lying <sup>1</sup><i>nπ</i>* state. An El-Sayed allowed intersystem crossing process with a
time constant of 92 ps ensues. Implications of these findings on the
interaction between photoexcited CaDPA and spore DNA are discussed
Mechanism of the Decay of Thymine Triplets in DNA Single Strands
The
decay of triplet states and the formation of cyclobutane pyrimidine
dimers (CPDs) after UV excitation of the all-thymine oligomer (dT)<sub>18</sub> and the locked dinucleotide T<sub>L</sub>pT<sub>L</sub> were
studied by nanosecond IR spectroscopy. IR marker bands characteristic
for the CPD lesion and the triplet state were observed from ∼1
ns (time resolution of the setup) onward. The amplitudes of the CPD
marker bands remain constant throughout the time range covered (up
to 10 μs). The triplet decays with a time constant of ∼10
ns presumably via a biradical intermediate (lifetime ∼60 ns).
This biradical has often been invoked as an intermediate for CPD formation
via the triplet channel. The present results lend strong support to
the existence of this intermediate, yet there is no indication that
its decay contributes significantly to CPD formation
Decay Pathways of Thymine Revisited
The
decay of electronically excited states of thymine (Thy) and
thymidine 5′-monophosphate (TMP) was studied by time-resolved
UV/vis and IR spectroscopy. In addition to the well-established ultrafast
internal conversion to the ground state, a so far unidentified UV-induced
species is observed. In D<sub>2</sub>O, this species decays with a
time constant of 300 ps for thymine and of 1 ns for TMP. The species
coexists with the lowest triplet state and is formed with a comparably
high quantum yield of about 10% independent of the solvent. The experimentally
determined spectral signatures are discussed in the light of quantum
chemical calculations of the singlet and triplet excited states of
thymine