Structures of non-canonical tandem base pairs in RNA helices: review.

The structures of tandem non-canonical base pairs, a frequently recurring motif in RNA molecules, are reviewed and analysed. The tandem non-canonical base pair motifs can be roughly divided in three groups, containing seven subgroups based on their base pairing patterns and local geometries. Structural details and helical parameters that can be used to numerically distinguish between the subgroups are tabulated. Remarkably, while the individual helical twists of the tandem and adjacent base pair steps can be substantially smaller or larger than the typical A-form value of 32.7 degrees, the average value is close to A-form. This and other striking regularities resulting from compensating geometrical adjustments, important for understanding and predicting the configurations of non-canonical base pairs geometries are discussed.