UV-induced photoreactions of DNA-protein complexes and protein-based repair of photolesions
Final Report Abstract
During the course of the duration of this stipend, a UV-visible pump-probe spectrometer has been cet up and optimized. The instrumet allows for simultaneous detection in the wavelength range from 325 nm to 720 nm with a time resolution of 150 to 200 fs and a maximum delay time of 3.2 ns and in the wavelength range from 375 nm to 720 nm with a time resolution of 500 ps and a maximum delay time near 100 µs while maintaining an identical sample geometry. The widely used structural isomer of adenine, 2-aminopurine(2AP), has been found to exhibit quenching of the singlet state by intersystem crossing (ISC) to the triplet manifold (by as much as a factor of three in acetonitrile). This new aspect has been investigated in the three solvents acetonitrile, ethanol and aqueous phosphate buffer. The lifetimes of the ISC and fluorescence have been found to correlate well with older data by Wierzchowski et al. in all three solvents. The successful and unambiguous interpretation of experiments utilizing 2AP as a fluorescent probe obviously requires a better understanding of the importance of fluorescence quenching by ISC in this system, to which we could contribute a valuable first step. Two other nucleobase analogs have been characterized with regard to their photochemical energy relaxation pathways. Both 4-thiothymidine (4TT, the thio analog of thymidine) and 6-thioguanosine (6TG, thio substituted guaonsine) show a very high triplet yield for the monomer with sub-ps ISC while showing normal base pairing properties. The red-shifted absorption bands of these molecules with absorption maxima > 330 nm allow for a selective electronic excitation even when incorporated into DNA strands. This property, paired with the rapid and effective population of triplet states, makes these two monomers ideal tools to gain valuable insights into the role of triplet energy in photo-crosslinking and photooxidation reactions. The hardly measurable triplet yield in the natural nucleobases had always impeded an investigation of this facet of DNA photochemistry, the importance of which might have been largely underestimated. In the investigation of solvent effects on the photochemical energy pathways we could also show that ionic liquids can be used effectively to control the solvent environment properties for DNA research.
Publications
- Room-temperature phosphorescence of the DNA monomer analogue 4-thiothymidine after UVA excitation. J. Phys. Chem. Lett. 2010, 1: 2239-2243
Reichardt, C. and Crespo-Hernandez, C. E.
- Ultrafast spin crossover in 4-thiothymidine in an ionic liquid. Chem. Commun. 2010, 46: 5963-5965
Reichardt, C. and Crespo-Hernandez, C. E.
- Excited-state dynamics in 6-thioguanosine from the femtosecond to microsecond time scale. J. Phys. Chem. B 2011, 115(12): 3263-3270
Reichardt, C., Guo, C. and Crespo-Hernandez, C. E.
