Exposure to UV light, particularly from the sun, is the primary controllable risk factor for the development of skin cancer. The damaging effects of UV photons results from their ability to induced photochemistry in DNA bases. While the many possible photoproducts of DNA are well known, the formation mechanisms for these photoproducts are not. In order to better understand these processes, we seek to better understand the events that occur between photon absorption and photoproduct formation - the photophysics of DNA.
Femtosecond UV pump/UV probe transient absorption spectroscopy was used to study the ground-state vibrational cooling of the DNA base derivative 9-methyladenine (9MA) in solution. Photoexcitation of 9MA to the lowest bright electronic excited state at 267 nm is followed by rapid (τ ≈ 0.4 ps) internal conversion to the electronic ground state, generating more than 30 000 1/cm of excess vibrational energy. Transfer of this excess vibrational energy to the solvent was monitored via changes in the ground-state electronic absorption band at 250 and 285 nm. The vibrational cooling time increases in H2O (2.4 ps), D2O (4.2 ps), methanol (4.5 ps) and acetonitrile (13.1 ps) solvents. The studies show that the initial vibrational energy transfer from the hot solute molecule to the first solvent shell determined the thermalization rate. The studies also suggest that energy transfer between high-frequency solute and solvent modes play a more important role in vibrational cooling than expected.
While the majority of excitation lead to ultrafast internal conversion, in pyrimidine bases additional decay pathways exist involving long-lived, intermediate, 1nπ* and 3ππ* states. The 1nπ*, 1ππ*, 3ππ* and S0 states of single pyrimidine bases have strongly-overlapping electronic absorption spectra which complicates study of their dynamics with conventional UV and visible techniques. A UV-pump/mid-IR-probe femtosecond transient absorption spectrometer was constructed for the purpose of studying, with high resolution and specificity, the excited-state dynamics of DNA bases.
Unique marker bands for the 1nπ* and 3ππ* were determined for 1-cyclohexyluracil and thymine, respectively. A marker band for the 1nπ* was observed at 1760 1/cm in acetonitrile and methanol-d1. Marker bands at 1603 and ~1714 1/cm were assigned to the 3ππ* state of thymine. This assignment is consistent with previous nanosecond TRIR measurements and theoretical calculations. The triplet state is fully formed on a timescale significantly faster than the 1nπ* lifetime, suggesting that intersystem crossing from the 1nπ* state to the 3ππ* state occurs before the excited population reaches the minimum on the 1nπ* potential surface, supporting previous conclusions from UV-visible transient absorption measurements.
The excited-state dynamics of poly(A) were studied with UV and mid-IR transient absorption. Comparison with mid-IR measurements of AMP allow the quantum yield for ultrafast internal conversion in poly(A) to be estimated. Comparison of transient spectra with ground-state absorption spectrum of poly(A) provides insight into the location of absorption bands of the long-lived excimer-like state.DNA photophysics; femtosecond spectroscopy; time-resolved spectroscopy; time-resolved infrared; vibrational cooling; vibrational relaxation; excited state dynamics