FIGURE 7.

K⁺ binding to O_b atoms in the major groove of cis-WC G•U pairs. (A) K⁺ binding to a guanine Hoogsteen edge; coordination number: 8. (B) K⁺ binding to the major groove of a G•U pair; coordination number: 8. (C) K⁺ binding to the major groove of a G•U pair. To highlight the 5.0 σ level K⁺ density, the radius of the K⁺ sphere (magenta) has been reduced. (D) Mg²⁺ modeled in the major groove of a G•U pair that could correspond to a monovalent cation. The electron density pattern is too imprecise for an unambiguous assignment. (B,C,D) (Green) G•U carbons; (cyan) G•U H-bonds; (yellow) other carbon atoms.