= Sugar base-pair edge
The \b[sugar base-pair edge] is the side of a <nucleobase> adjoining its ribose attachment region, including the ribose $2'$-<hydroxyl group> when it participates in the contact. It is one of the three <RNA base-pair edges> used to classify <RNA base-pair geometry>. The original nomenclature also calls it the shallow-groove edge.<Geometric nomenclature and classification of RNA base pairs by N. B. Leontis and E. Westhof>[Abstract, Sugar edge and inclusion of the 2′-OH]{ref}
This definition deliberately extends beyond the base ring. In the usual anti orientation about the <glycosidic bond>, the ribose $2'$-hydroxyl can act as a <hydrogen bond donor> or <hydrogen bond acceptor> in the pairing interaction. Some sugar-edge contacts therefore connect a base to its partner's ribose rather than consisting solely of direct base-to-base <hydrogen bonds>.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Results, printed p. 3499, sugar-edge interactions and the 2′-OH; Figures12–13]{ref}
For an original annotation example, suppose residue 1 contributes its $2'$-hydroxyl to an inter-residue <hydrogen bond>. Recording only the two aromatic base rings would omit that contact. A full record must identify the ribose atom, its partner and the geometry. Calling the interaction sugar-edge does not mean that an additional sugar molecule has joined the <RNA>; the group belongs to an existing <nucleotide>.
A complete pair description still needs the other partner's edge and the cis/trans relationship of the two <glycosidic bonds>. Sugar–sugar and Watson–Crick–sugar contacts are different families. The words “sugar edge” alone do not specify a unique pair of bases or a unique three-dimensional arrangement.
There is also an experimental limitation. The primary structural survey did not infer the $2'$-hydroxyl hydrogen's exact position from the available <X-ray crystallography> coordinates. It represented the group as a unit and used compatible heavy-atom geometry to discuss possible <hydrogen bonds>. A close oxygen contact is evidence to interpret with donor/acceptor chemistry, not direct observation of the hydrogen's orientation.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Results, printed p. 3499, handling of hydroxyl and water hydrogens]{ref}
This distinction is useful when comparing <RNA> with a modified backbone: removing or replacing a ribose group can change a contact opportunity without changing the attached <nucleobase>'s letter. Whether the whole fold survives that alteration remains a structural and experimental question.
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