Resonance Raman Intensities Demonstrate that C5 Substituents Affect the Initial Excited‐State Structural Dynamics of Uracil More than C6 Substituents

Abstract

Resonance Raman derived initial excited-state structural dynamics provide insight into the photochemical mechanisms of pyrimidine nucleobases, in which the photochemistry appears to be dictated by the C5 and C6 substituents. The absorption and resonance Raman spectra and excitation profiles of 5,6-dideuterouracil were measured to further test this photochemical dependence on the C5 and C6 substituents. The resulting set of excited-state reorganization energies of the observed internal coordinates were calculated and compared to those of other 5- and 6-substituted uracils. The results show that the initial excited-state dynamics along the C5C6 stretch responds to changes in mass at C5 and C6 in the same manner but that the in-plane bends at C5 and C6 are more sensitive to substituents at the C5 position than at the C6 position. In addition, the presence of two deuterium substituents at C5 and C6 decreases the initial excited-state structural dynamics along these in-plane bends, in contrast to what is observed in the presence of two CH3 groups on C5 and C6. The results are discussed in the context of DNA nucleobase photochemistry.

Theoretical simulation of surface‐enhanced resonance Raman spectroscopy of cytosine and its tautomers

“…Furthermore, this technique can be used to assign and distinguish the characteristic spectra of the low‐energy tautomeric and ionic compositions in the mixture . In this subject, the extensive studies by G. R. Loppnow and coworkers provide a valuable source of excited state structural information for different nucleobases. In most of these studies, theoretical calculations as the necessary and complementary investigations were carried out to analyze and interpret the experimental data.…”