The Sodium Ions Inside a Lipophilic G-Quadruplex Channel as Probed by Solid-State ²³Na NMR

We report solid-state 23Na NMR and X-ray crystallographic results for a self-assembled G-quadruplex channel formed by a guanine nucleoside, 5‘-tert-butyl-dimethylsilyl-2‘,3‘-O-isopropylidene guanosine (G 1). The study provides an unambiguous 23Na NMR identification for the Na+ ions inside a lipophilic G-quadruplex channel. The crystalline nature of the sample yields a remarkably high resolution in the 23Na multiple-quantum magic-angle spinning (MQMAS) spectrum, making it possible to extract very accurate 23Na NMR parameters for each of the three crystallographically distinct Na sites. The observation of a single Na+ ion from a 9-kDa system demonstrates the potential of solid-state 23Na NMR as a complementary technique to X-ray for detecting Na+ ions in biological structures.