Localization of 23Na+ in a DNA quadruplex by high-field solid-state NMR

Monovalent cations such as Na+, K+, and NH4+ are known to stabilize DNA quadruplexes formed of guanine-quartets. Such structures readily form from the guanine-rich repeat sequences found in telomeres, the physical ends of eukaryotic chromosomes. We present a solid-state NMR approach for studying ions associated with G-quartets based on the direct NMR observation of Na-23(+) ion resonances and report new methods for studying quadrupolar nuclei in biological solids. In the tetraplex forming oligonucleotide d(TG(4)T), high-field (17.6 T) NMR spectra cleanly resolve three distinct classes of sodium ions. A high-resolution 2D-MQMAS spectrum established the assignment of an amorphously broadened signal at -19 ppm (relative to 0.1 M NaCl) to surface-bound Na+ ions. Two-dimensional nutation spectroscopy was used to indicate the relative size of the quadrupole coupling for each line, while a standard exchange experiment established a correlation between surface (-19 ppm) and channel-bound (6.8 ppm) Na+ ions. Finally, free sodium ions are observed at 0 ppm. This work demonstrates the utility of high field spectroscopy in combination with a suite of 2D solid-state NMR experiments for resolving and assigning multiple Na+ sites in [d(TG(4)T)](4).