4RKV:
reference: Structural Variations and Solvent Structure of r(UGGGGU) Quadruplexes Stabilized by Sr(2+) Ions., Fyfe AC, Dunten PW, Martick MM, Scott WG, J Mol Biol. 2015 Jun 19;427(12):2205-19. doi: 10.1016/j.jmb.2015.03.022. Epub, 2015 Apr 8. PMID: 25861762

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