Figure 1.

One example of an ‘artifically expanded genetic information system’ (AEGIS). Nucleobase pairing in this system conforms to 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’). The hydrogen-bonding acceptor (A) and donor (D) groups are listed from the major to the minor groove as indicated. The heterocycles shown are current implementations of the indicated hydrogen-bonding patterns; others are conceivable. Unshared pairs of electrons (or ‘electron density’) presented to the minor groove are shown by the shaded lobes. The nucleotides implementing the pyDDA:puAAD hydrogen-bonding pattern, the subject of this article, is at the bottom right.