The self-assembled guanosine (G 1)-based hexadecamers and isoguanosine (isoG 2)-based decamers are excellent Ra-226(2+) selective ionophores even in the presence of excess alkali (Na+, K+, Rb+, and Cs+) and alkaline earth (Mg2+, Ca2+, Sr2+, and Ba2+) cations over the pH range 3-11. G 1 requires additional picrate anions to provide a neutral assembly, whereas the isoG 2 assembly extracts Ra-226(2+) cations without any such additives. Both G 1-picrate and isoG 2 assemblies show Ra-226(2+) extraction even at a 0.35 x 10(6) fold excess of Na+, K+, Rb+, Cs-,(+) Mg2+, or Ca2+ (10(-2) M) to Ra-226(2+) (2.9 x 10(-8) M) and at a 100-fold salt to ionophore excess. In the case of the G 1-picrate assembly, more competition was observed from Sr2+ and Ba2+, as extraction of Ra-226(2+) ceased at an M2+/(226) Ra2+ ratio of 10(6) and 10(4), respectively. With the isoG 2 assembly, Ra-226(2+) extraction also occurred at a Sr2+/(226) Ra2+ ratio of 10(6) but ceased at a 10(6) excess of Ba2+. The results clearly demonstrate the power of molecular self-assembly for the construction of highly selective ionophores.