The mononucleoside, 5’-(tert-butyldimethylsilyl)-2’,3’-O-isopropylidene isoguanosine (isoG) 2, has a strong affinity for alkali metal cations. Previously, it was shown that isoG 2 self-assembles via hydrogen bonds to give a stable tetramer, (isoG)(4) 4, in organic solvents (Davis et al. J. Org. Chem. 1995, 60, 4167-4176). The isoG tetramer 4 can then coordinate metal cations. In this present Study, vapor phase osmometry and further H-1 NMR experiments confirmed that isoG 2 self-assembles via complementary hydrogen bonds to form tetramer 4. Molecular models obtained from molecular dynamics and MM2 energy minimization indicate that the isoG tetramer 4 is bowl-shaped, with four C2 oxygens located on the tetramer’s convex surface. It is likely that these four oxygens on the tetramer’s convex face coordinate cations. Potassium picrate was used to determine the stoichiometry of the isoG-K+ complex and its K+ binding affinity. Both H-1 NMR and UV-vis spectroscopic analysis demonstrated that isoG 2 forms an octamer, (isoG)(8)-K+ (5), in the presence of potassium picrate in CDCl3 and CD3CN. The octamer (isoG)(8)-K+ (5) has a single set of H-1 NMR resonances, even at -90 degrees C, consistent with a D-4-symmetric head-to-head stacking of two tetramers around the central K+ cation. Analysis of the picrate’s optical spectra indicated that the picrate salt of (isoG)(8)-K+ (5) is a separated ion pair in CDCl3, consistent with the K+ being sandwiched between two isoG tetramers. Picrate extraction experiments revealed that (isoG)(8)K+ (5) is an impressive ionophore, with a K+ association constant (log K-a 8.2 M(-1)) approaching that of 18-crown-6 ether derivatives. Indeed, NMR competition experiments for K+ binding between dicyclohexano-18-crown-6 and isoG 2 confirm that the K+ binding constants in CDCl3 for the crown ether and for the self-assembled ionophore are of the same magnitude (K-a=10(8) M(-1)).