Principles of ion recognition in RNA: insights from the group II intron structures

K19 binds the S-turn motif and the AA parallel interaction in D3. (Left) The ion-binding site and the intron residues are colored as described in Figure 2. Inner-sphere coordination and hydrogen bonds are shown as black dashed lines for K19 and for the A₂₉₉A₃₁₉ parallel interaction. The corresponding distances are indicated in angstroms. Anomalous difference Fourier electron density map at the K19 site is shown as a green mesh at 9.0 σ for Tl⁺ (PDB id. 4E8Q). (Right) Electrostatic surface potential around K19. The color scale is indicated at the bottom in units of kT/e.