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Jesse Stombaugh, Craig L. Zirbel, Eric Westhof and Neocles B. Leontis, Nucleic Acids Research 37(7), 2294–2312 (2009). The paper introduces the IsoDiscrepancy Index and compares RNA base-pair geometry with structural and sequence alignments. Results, Figure 2 and Table 3, calibrate isosteric/near-isosteric thresholds and distinguish differently oriented wobble pairs.

Additional cited passages: The IsoDiscrepancy Index, Definition, explains the C1′/attachment-nitrogen geometry, separate in-plane and flipped alignment cases, and the length interpretation of the rotation coefficients. IDI has units of ångströms; the in-plane rotation angle is multiplied by a 2-ångström length scale and is expressed in radians. Methods selects an observed base-pair exemplar by minimizing the row sum of pairwise discrepancies. Validation of the IDI, Figure 2 and Table 3, gives the study-specific isosteric and near-isosteric cutoffs.

Primary source: williams.chemistry.gatech.edu/course_Information/6572/papers/leontis_2002_isostericity.pdf

Verified locator: Neocles B. Leontis, Jesse Stombaugh and Eric Westhof, Nucleic Acids Research 30(16), 3497–3531 (2002), DOI 10.1093/nar/gkf481. Figure 1: base edges; Results, printed p. 3499: sugar 2′-OH and structural hydrogen limits; Isostericity, printed pp. 3499 and 3525, and Tables 3–5: geometric compatibility; Table 3/Family 1: canonical A–U, U–A, G–C and C–G isosteric subgroup. Observed versus modeled contacts are explicitly distinguished.