THE allowed conformations of nucleic acids depend on the flexibility of the structure of the nucleotides from which they are constructed. Many X-ray crystallographic studies of the degree of flexibility of these structures have shown that nucleosides exhibit a larger number of preferred conformations than nucleotides, and this led Sundaralingam1 to suggest that a nucleotide is more ‘rigid’ than a nucleoside. Berthod and Pullman2, however, have drawn attention to the fact that considerations of conformational energy do not imply such a rigid structure.