The Photoaddition of a Psoralen to DNA Proceeds via the Triplet State.

Psoralens are natural compounds that serve in the light dependent treatment of certain skin diseases (PUVA therapy). They are DNA intercalators that upon photoexcitation form adducts with thymine bases. For one psoralen derivative, 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), the photoreactions are characterized here by nanosecond UV-vis and IR absorption spectroscopy. The triplet state of AMT is identified as the reactive one. On the 1-10 µs time scale this local triplet state transforms into a triplet biradical bearing one single bond between the addends. Within ∼50 µs this biradical forms the final adduct featuring a cyclobutane ring. This kinetic behavior is in stark contrast to the closely related photoaddition of two thymine moieties within the DNA. Origins of the differences 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 ∼10 000 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 ππ* 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 ∼10 ps (cyclohexane) to ∼3 ns (water). This is attributed to a varying energetic accessibility of nπ* excitations.

Impact of Mono-Fluorination on the Photophysics of the Flavin Chromophore.

Three mono-fluorinated derivatives of the flavin core system 10-methyl-isoalloxazine (MIA) were synthesized. Aqueous solutions of these compounds were characterized by steady-state and time-resolved spectroscopy. The positions for the fluorination (6, 7 and 8) were motivated by the nodal structure of the frontier orbitals of MIA. In comparison with MIA, the fluorination results in bathochromic (6F- and 7F-MIA) and hypsochromic (8F-MIA) shifts of the adiabatic excitation energy of the lowest allowed transition. Shifts of up to ~500 cm-1 were observed. These spectroscopic shifts go along with changes in fluorescence quantum yields and lifetimes. In addition, triplet yields are affected. For 7F-MIA, a 50% increase in the fluorescence quantum yield as well as a 50% decrease in triplet yield is observed rendering the compound interesting for fluorescence applications. The measured effects are discussed in terms of qualitative perturbation theory.

Triplet Harvesting with a Simple Aromatic Carbonyl.

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive 1 ππ* excitation may rapidly equilibrate with an upper triplet state (3 nπ*). This equilibrium may persist for nanoseconds. Population of the 3 nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light.

Femtosecond Spectroscopy of Calcium Dipicolinate-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(-5) 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 (1)ππ* state depletes within 110 fs. The depletion results in the population of an energetically close lying (1)nπ* 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.

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.

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 ∼10(4) M(-1) s(-1) for the hydrogen abstraction. The process also occurs intramolecularly as experiments on zebularine (1-(ß-d-ribofuranosyl)-2(1H)-pyrimidinone) show.