| Photochemical reactions resulting from the absorption of ultraviolet light are one of the main causes of DNA damage. For any excited-state photochemical reaction, it is the structural changes in the excited state after the absorption of the photon that ultimately decide the photochemical fate of the molecule. In this thesis, I have explored the initial excited-state structural dynamics of nucleic acid derivatives and a rhodopsin analogue to understand the structural distortions upon photon absorption and the correlation between the observed structural dynamics and the known photochemistry.;The resonance Raman-derived initial excited-state structural changes of homopentamer oligonucleotides lie along similar modes as in the corresponding nucleobases or nucleotides, but with smaller distortions. The smaller excited-state distortions suggest that the initial excited-state structural dynamics are restricted by the polymeric structure. The observed homopentamer homogeneous broadening is consistent with this model.;The sequence dependence of UV-induced miRNA damage was also studied on a microarray platform. The results suggest that guanine provides a protective effect and sequences with cytosine and uracil are more susceptible to damage, although the errors are large.;The visible resonance Raman spectroscopic studies on a rhodopsin analogue show similar initial molecular distortions along the C=C bond during the isomerization as in rhodopsin. The hydrogen out-of-plane (HOOP) mode is absent in N-alkyalated indanylidene-Pyrroline (NAIP), as expected due to the absence of C-H modes. The computed excited-state trajectories are consistent with the experimentally observed initial distortions along the C=C bond.;Resonance Raman spectroscopy was used to probe the initial excited-state structural dynamics of 5,6-dimethyluracil, to understand the effect of mass changes at the C5 and C6 positions, and found that the observed initial excited-state structural dynamics are similar to those of thymine. This study showed that the methyl groups at the C5 and C6 positions are a major factor in determining how the initial excited-state structural dynamics are partitioned between the CH bending and C5=C6 stretching modes, which is directly related to the difference in the photochemistry of uracil and thymine. |
