Vibrational Raman optical activity of pyrimidine nucleosides

Abstract

The backscattered Raman and Raman optical activity (ROA) spectra of 5-methyluridine, α- and β-thymidine, uridine, 2′-deoxyuridine, cytidine and 2′-deoxycytidine between 600 and 1500 cm ^-1 have been reported and discussed. The spectral region between ca. 800 and 1200 cm ^-1 was found to be dominated by normal modes localised in the sugar ring. ROA sign patterns characteristic of ribo- and deoxyribo-sugar rings are identified and assignments proposed. Between ca. 1200 and 1500 cm ^-1 , the measured ROA spectra provide experimental evidence of coupling between the base and sugar rings. In particular, ROA signals are observed at ca. 1200, 1230, 1380 and 1400 cm ^-1 that have opposite signs for α- and β-thymidine, and which are therefore assigned to normal modes with dominant contributions from the stretching coordinate of the C(1′)–N(1) glycosidic bond. This spectral region is also found to have specific sign patterns for the cytosine, uracil and thymine derivatives studied here. No ROA signals are observed in the region above 1550 cm ^-1 because the normal modes involved are localised in the planar base rings and therefore do not sample the chirality of the nucleosides.