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.