Excited-state structural dynamics of cytosine from resonance Raman spectroscopy.

Cytosine, a nucleobase found in both DNA and RNA, is known to form photoproducts upon UV irradiation, damaging the nucleic acids and leading to cancer and other diseases. To determine the molecular mechanism by which these photoproducts occur, we have measured the resonance Raman spectra of cytosine at wavelengths throughout its 267 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism yields both the excited-state structural changes and electronic parameters. From this analysis, we have been able to determine that, at most, 31% of the reorganization energy upon excitation is directed along photochemically relevant modes.