= Hoogsteen base-pair edge
{c}
The \b[Hoogsteen base-pair edge] is a contact surface of a <nucleobase> distinct from its <Watson-Crick base-pair edge> and <sugar base-pair edge>. For a <purine>, it is the side containing N7 and the exocyclic group at position 6; for example, <adenine> presents N7 and its N6 amino group on this side. The structural classification extends the corresponding category to the CH edge of <pyrimidines>.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Figure 1, identification of base edges; Abstract, Hoogsteen/CH terminology]{ref}
The word describes \b[one partner's contacting edge], not a complete pair or a mandatory rotation of the base. A complete <RNA base-pair geometry> identifies the second partner's edge and the cis/trans orientation of the <glycosidic bonds>. The primary matrices include Hoogsteen–Watson–Crick, Hoogsteen–Hoogsteen and Hoogsteen–sugar contacts.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Table 2 and Figures 4–5, 8–11]{ref}
An original counting example makes this useful. Fix one <adenine> as the labeled first partner and stipulate that it uses its Hoogsteen edge. The second partner still has three edge categories available; each can appear with a cis or trans relationship. There are therefore $3\times2=6$ labeled edge/orientation descriptions before specifying the second base's identity. This counts descriptions, not six experimentally established partners for that particular adenine.
Edge identity must also be distinguished from chemical role. N7 of ordinary neutral <adenine> is a <hydrogen bond acceptor>, while its amino group can supply a <hydrogen bond donor>. A favorable contact requires an appropriately positioned complementary group, not merely two bases facing sides with the same category name. Structural conditions and chemical state matter.
When annotating an experimental <RNA> structure, record the interacting atoms and the two <glycosidic bonds>, rather than classifying from the sequence alone. An A–U pair can use a different geometry from the canonical Watson–Crick arrangement. Conversely, an edge category can recur with different base identities. This is why <RNA base-pair isostericity> is evaluated within explicit geometric families rather than inferred only from the letters A, C, G and U.
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