= RNA base-pair edge
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An <RNA base-pair edge> is a region around a <nucleobase> presenting groups that can participate in an edge-to-edge contact with another <nucleobase>. It is a chemical contact surface in <RNA base-pair geometry>, rather than an edge of a drawn secondary-structure graph. The arrangement of <hydrogen bond donors> and <hydrogen bond acceptors> constrains possible partners.
The Leontis–Westhof classification distinguishes the <Watson-Crick base-pair edge>, <Hoogsteen base-pair edge> and <sugar base-pair edge>. The last category includes the ribose $2'$-hydroxyl contribution and is also called the shallow-groove edge in the original classification.<Geometric nomenclature and classification of RNA base pairs by N. B. Leontis and E. Westhof>[Abstract, three contact edges and sugar-edge definition.]{ref} Thus an edge label does not necessarily refer only to atoms in the aromatic ring.
An edge belongs to one partner. A complete pair description identifies the edge used by each <nucleobase> and the relative orientation of their <glycosidic bonds>. For example, specifying that one <adenine> uses its <Hoogsteen base-pair edge> still leaves the second partner's identity, contacting edge and orientation to be determined. It does not by itself specify a unique <noncanonical base pair>.
For an original classification exercise, label two partners A and B. The record “A uses Watson–Crick; B uses sugar; orientation trans” contains more geometric information than “A contacts B”. Exchanging the names A and B reverses the written edge order but does not create a second physical contact. In contrast, moving B to a different contact surface changes the proposed structure.
Structural coordinates or solution restraints are needed to establish the contact. A plausible <hydrogen bond donor> near an appropriate <hydrogen bond acceptor> is a necessary geometric clue, not proof that a proposed <RNA> structure is populated under every condition. Edge labels make comparisons precise while retaining the need for experimental evidence.
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