Figure 2.

Ca²⁺-binding site: the short arm comprising C₁₁ to G₁₄ of the noncrossing strand (blue) base-paired with C₇ to G₁₀ of the other crossing strand (red) is shown. The (2F _o − F _c) Fourier map (light grey) and (F _o − F _c) difference Fourier map (blue) are contoured at the 1σ and 3σ levels, respectively. (a) A water molecule (red sphere) shows positive density in the difference Fourier map at 3.5σ. (b) An Na⁺ ion (blue sphere) shows positive density in the difference Fourier map at 4.2σ. (c) A Ca²⁺ ion (green sphere) shows no positive density in the difference Fourier map and is bound to sugar hydroxyl O4′ of G₁₃ (represented by the black dashed line) on the minor-groove side.