FIGURE 2.
The noncanonical base pairs. (A) C and U interact via their Watson–Crick edges; the distances are shown in Å. The alignment of functional groups of the Watson–Crick edges of the C–U pairs corresponds with relatively high values of the λ angles (green), compared with the typical value of 55°. This indicates that the residues are inclined toward the major groove. The distance between the C1′ atoms is also shown. (B,C) Fo–Fc omit maps calculated for the nucleobases of pairs C–U and U–U in the shorter duplex. Contours mark the 3σ level. Red sphere represents a water molecule in the major groove. (D) 2Fo–Fc map of the U–U pair in the longer duplex.