Figure 2.

Structures of various non-standard nucleobases used in various efforts to support synthetic biology that alters the biopolymers at the center of life.
(Top panel) Three examples of hydrophobic pairs that approximate the geometry of standard Watson-Crick pairs but lack inter-nucleotide hydrogen bonding.
(Middle panel) Eight-letter genetic system that retains inter-nucleotide hydrogen bonding complementarity, but with geometrically larger pairs.
(Bottom panel) AEGIS nucleobases examined here. Those with superscript “2” are second-generation AEGIS components that implement the indicated hydrogen bonding pattern (red H-bond acceptors; blue = hydrogen bond donors). Those with superscript “3” are third-generation AEGIS species with properties better suited for intracellular performance.