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The coordination geometries of the metal ions and the spermine molecule. (a) The Sr2+ ion (purple) is coordinated to eight guanine O6 atoms. (b) Three Ca2+ ions are coordinated to U6 (Ca1), U7 (Ca2), and U12 (Ca3) residues, respectively. (c) Coordination of the spermine molecule (blue) in stereoview. Hydrogen bonds are shown in white dashed lines. Individual RNA strands are depicted in different colors.
