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Pyrimidinone: Versatile Trojan horse in DNA photodamage?
(6-4) Photolesions between adjacent pyrimidine DNA bases are prone to secondary photochemistry. It has been shown that singlet excited (6-4) moieties form Dewar valence isomers as well as triplet excitations. We here report on the triplet state of a minimal model for the (6-4) photolesion, 1-methyl-2(1H)-pyrimidinone. Emphasis is laid on its ability to abstract hydrogen atoms from alcohols and carbohydrates. Steady-state and time-resolved experiments consistently yield bimolecular rate constants of ∼104 M−1 s−1 for the hydrogen abstraction. The process also occurs intramolecularly as experiments on zebularine (1-(β-D-ribofuranosyl)-2(1H)-pyrimidinone) show
Effiziente blaue OLED Emitter durch reverse Interkombination in aromatischen Carbonylverbindungen?
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
On the large apparent Stokes shift of phthalimides
The photophysics of N-methylphthalimide (MP) in solution (cyclohexane, ethanol, acetonitrile, and water) was characterized by steady state as well as time resolved fluorescence and absorption spectroscopy. In all solvents the compound exhibits an unusually large Stokes shift of approximate to 10000 cm(-1). It is attributed to an ultrafast (<100 fs) depletion of the initially excited state, which results in the population of a weakly emitting state. Quantum chemical computations (DFT-MRCI) support this. They identify two energetically low-lying singlet * excitations of different oscillator strength. Whereas the Stokes shift and thereby the ultrafast depletion of the initial excitation are hardly affected by the solvent later processes respond strongly. The fluorescence lifetime varies from approximate to 10 ps (cyclohexane) to approximate to 3 ns (water). This is attributed to a varying energetic accessibility of n* excitations