We report a solid-state Na-23 NMR study of the bound sodium cations in a G-quadruplex formed by Oxytricha nova telomere DNA repeat, d(G(4)T(4)G(4)) (Oxy-1.5). Using a 2D multiple-quantum magic-angle spinning Na-23 NMR method, we observed three sodium cations residing inside the quadruplex channel of the Na+ form of Oxy-1.5. Each of these sodium cations is sandwiched between two G-quartets. We found no evidence for sodium cations in the T-4 loop region. For comparison, solid-state N-15 MAS NMR spectra were also obtained for the (NH4+)-N-15 form of Oxy-1.5. The insufficient resolution in the N-15 MAS NMR spectra did not permit determination of the number of NH4+ ions inside the quadruplex channel. The solid-state Na-23 and N-15 NMR spectra for Oxy-1.5 were also compared with those obtained for guanosine 5'-monophosphate. (C) 2004 Elsevier Inc. All rights reserved.