Mass-Tuned Initial Excited-State Structural Dynamics of DNA Nucleobases from UV Resonance Raman Spectroscopy: 5-Deuterouracil

Abstract

5-Deuterouracil (5-d-U) is an analogue of uracil (RNA) and thymine (DNA), which differ only in the mass of their 5-substituent. Previous work has shown that the initial excited-state structural dynamics primarily lie along C5═C6 bond lengthening in thymine, while these dynamics primarily lie along C5 and C6 pyramidalization in uracil. To test whether the 5-substituent mass determines these dynamics, the resonance Raman spectra of 5-d-U, with a 5-substituent mass intermediate between uracil and thymine, is measured at wavelengths throughout its 262 nm absorption band. The resulting spectrum of 5-d-U is intermediate between that of thymine and uracil, indicating both bond-lengthening and pyramidalization motions in the excited state. Self-consistent analysis of the resonance Raman excitation profiles and absorption spectra show that the initial excited-state structural dynamics of 5-d-U are indeed intermediate between those of thymine and uracil, supporting the mass-tuning model of initial excited-state structural dynamics in DNA and RNA pyrimidine nucleobases.