Fig 1. Structure of the natural C:G and unnatural Z:P base pairs.

The natural C:G and unnatural Z:P base pairs all fit the Watson-Crick geometry, with large purines (or purine analogs, both indicated by “pu”) pairing with small pyrimidines (or pyrimidine analogs, both indicated by “py”) joined by hydrogen bonds. The hydrogen-bonding donor (D) and acceptor (A) groups are listed from the major to the minor groove. The arrow indicates the hydrogen bond between the donor and acceptor. Unshared pairs of electrons (or “electron density”) presented to the minor groove are shown by the shaded lobes.