Nucleoside conformations. 15. Flexibility of natural pyrimidine nucleosides around the glycosidic bond.

The flexibility of pyrimidine nucleosides has been investigated by measuring their circular dichroism in hydroalcoholic solutions over a large temperature range (-100 degrees to +40 degrees C). It was observed that ss-Uridine (Urd) and ss-Cytidine (Cyd) showed a decrease of the main dichroic band of about 40 percent, while sterically hindered nucleosides (alphaUrd, ara-Uracile, 02-2' anhy-dro-Urd, 2'3'-0-isopropylidene-Urd) showed only small decreases. It is concluded that the flexibility of the glycosidic linkage in conjunction with the pseudo-rotation of the sugar residue is responsible for these changes; the thermodynamic values which can be deduced from these decreases are compatible with an oscillatory motion around the glycosidic bond, but exclude anti-syn transformations in pyrimidine nucleosides.