Torsion Angles about Glycosidic Bonds Have only 1 or 2 Stable Positions

1. Purine residues have 2 sterically allowed orientations relative to the ribose group, syn- and anti-
2. For pyrimidines only the anti conformation is allowed due to steric hindrance between the sugar and the C2 of the pyrimidine.
3. Most double helical nucleic acids are in the anti- conformation
4. Exception is Z-DNA which has alternating anti and syn- pyrimidine and purine residues.

Figure 4.4: The sterically allowed orientations (syn- vs. anti-)of purine and pyrimidine bases with respect to their attached ribose units.

Figure 4.5: The Structure of B-DN (Ball and stick) and corresponding space-filling model viewed down the helix axis.

Figure 4.6: The Structure of Z-DNA (Ball and stick) and corresponding space-filling model viewed down the helix axis.

Figure 4.7: The Conversion of B-DNA to Z-DNA.