VIBRATIONAL RAMAN OPTICAL-ACTIVITY OF PYRIMIDINE NUCLEOSIDES

The backscattered Raman and Raman optical activity (ROA) spectra of 5- methyluridine, alpha- and beta-thymidine, uridine, 2'-deoxyuridine, cy tidine and 2'-deoxycytidine between 600 and 1500 cm(-1) have been repo rted and discussed. The spectral region between ca. 800 and 1200 cm(-1 ) was found to be dominated by normal modes localised in the sugar rin g. ROA sign patterns characteristic of ribo- and deoxyribo-sugar rings are identified and assignments proposed. Between cn. 1200 and 1500 cm (-1), the measured ROA spectra provide experimental evidence of coupli ng between the base and sugar rings. In particular, ROA signals are ob served at cn. 1200, 1230, 1380 and 1400 cm(-1) that have opposite sign s for alpha- and beta-thymidine, and which are therefore assigned to n ormal 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 d erivatives studied here. No ROA signals are observed in the region abo ve 1550 cm(-1) because the normal modes involved are localised in the planar base rings and therefore do not sample the chirality of the nuc leosides.