Selective <sup>226</sup>Ra<sup>2+</sup> Ionophores Provided by Self-Assembly of Guanosine and Isoguanosine Derivatives

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