Selective 226Ra2+ Ionophores Provided by Self-Assembly of Guanosine and Isoguanosine Derivatives

The self-assembled guanosine (G 1)-based hexadecamers and isoguanosine (isoG 2)-based decamers are excellent 226Ra2+ 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 226Ra2+ cations without any such additives. Both G 1−picrate and isoG 2 assemblies show 226Ra2+ extraction even at a 0.35 × 106 fold excess of Na+, K+, Rb+, Cs+, Mg2+, or Ca2+ (10-2 M) to 226Ra2+ (2.9 × 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 226Ra2+ ceased at an M2+/226Ra2+ ratio of 106 and 104, respectively. With the isoG 2 assembly, 226Ra2+ extraction also occurred at a Sr2+/226Ra2+ ratio of 106 but ceased at a 106 excess of Ba2+. The results clearly demonstrate the power of molecular self-assembly for the construction of highly selective ionophores.