The dependence of thymine and thymidine Raman spectra on solvent

Abstract

Recent work has focused on developing Raman spectroscopy as a noninvasive probe of DNA interactions with solvents, intercalants, proteins, and other ligands. Here, we report the Raman spectra of thymine in eight solvents and thymidine in nine solvents obtained with visible excitation. Raman spectra under acidic, neutral, and basic conditions were also obtained of both thymine and thymidine. Changes in both the frequencies and intensities of several of the vibrational bands in the 800–1800 cm–1 region are observed. No evidence of deprotonation in the different solvents is observed for either thymine or thymidine. Correlations of the observed frequency shifts of specific vibrational modes with characteristic properties of the solvent for both thymine and thymidine show a significant correlation with acceptor and donor numbers, measures of the hydrogen-bonding ability of the solvent, in both thymine and thymidine. These results are interpreted in terms of hydrogen-bonding interactions between the N-H protons of the thymine base and lone pairs of electrons on the solvent molecules and between the solvent hydrogens and lone pairs on C=O sites. The solvent-dependent intensity in vibrational bands of thymine between 1500 and 1800 cm–1 indicates a strong interaction between thymine and solvent at the C=O and N-H sites that leads to separation of the C=O stretches from the C=C stretch. The intensity variations with solvent were much smaller for thymidine than for thymine, perhaps as a result of replacing the N1 proton by the sugar. These results suggest that Raman spectroscopy is uniquely sensitive to specific interactions of thymine and thymidine with their environment.Key words: Raman spectroscopy, thymine, thymidine, solvent effects, hydrogen bonding.