Initial excited-state structural dynamics of thymine derivatives.

Thymine is one of the pyrimidine nucleobases found in DNA. Upon absorption of UV light, thymine forms a number of photoproducts, including the cyclobutyl photodimer, the pyrimidine pyrimidinone [6-4] photoproduct and the photohydrate. Here, we use UV resonance Raman spectroscopy to measure the initial excited-state structural dynamics of the N(1)-substituted thymine derivatives N(1)-methylthymine, thymidine, and thymidine 5'-monophosphate in an effort to understand the role of the N1 substituent in determining the excited-state structural dynamics and the subsequent photochemistry. The UV resonance Raman spectrum of thymidine and thymidine 5'-monophosphate are similar to that of thymine, suggesting that large masses at N(1) effectively isolate the vibrations of the nucleobase. However, the UV resonance Raman spectrum of N(1)-methylthymine is significantly different, suggesting that the methyl group couples into the thymine ring vibrations. The resulting resonance Raman intensities and absorption spectra are self-consistently simulated with a time-dependent expression to quantitatively extract the initial excited-state slopes, homogeneous and inhomogeneous linewidths, and electronic parameters. These results are discussed in the context of the known photochemistry of thymine and its derivatives.