Internal Sodium Ions and Water Molecules in Guanine Quadruplexes: Magnetic Relaxation Dispersion Studies of [d(G3T4G3)]2 and [d(G4T4G4)]2

The structural stability of guanine quadruplexes depends critically on an unusual configuration of dehydrated Na+ or K+ ions, closely spaced along the central axis of the quadruplex. Crystallography and NMR spectroscopy indicate that these internal ions can be located between the G-quartet planes as well as in the thymine loops, but the precise ion coordination has been firmly established in only a few cases. Here, we examine the bimolecular diagonal-looped foldback quadruplexes [d(G(3)T(4)G(3))](2) (Q3) and [d(G(4)T(4)G(4))](2) (Q4) by H-2, O-17, and Na-23 magnetic relaxation dispersion (MRD). The MRD data indicate that both quadruplexes contain Na+ ions between the T-4 loops and the terminal G-quartets and that these ions have one water ligand. These ions exchange with external ions on a time scale of 10-60, mu s at 27 degrees C, while their highly ordered water ligands have residence times in the range 10(-8)-10(-6) s. The MRD data indicate that Q4 contains three Na+ ions in the stem sites, in agreement with previous solid-state 23Na NMR findings but contrary to the only crystal structure of this quadruplex. For Q3, the MRD data suggest a less symmetric coordination of the two stem ions. In both quadruplexes, the stem ions have residence times of 0.6-1.0 ms at 27 degrees C. The equilibrium constant for Na+ -> K+ exchange is similar to 4 for both loop and stem sites in Q3, in agreement with previous H-1 NMR findings.