Relative potential energies for various conformations of model compounds 3 (A) and 4 (B) – nucleosides of adenine and cytosine, respectively. Six conformations for each model compound were considered to probe energetics of the A-to-B DNA transition on the nucleoside level. Conformations 2 and 5 are those sampled in canonical A and B form DNA, respectively. Conformations 1 and 4 differ from 2 and 5, respectively, by values of the glycosidic torsion χ (B-like for 1, and A-like for 4). 3 and 6 are non-canonical conformations with the sugar moiety being in the O4’-endo and planar conformation, respectively. See Methods section for details.