2′‐5′‐Isomerically Linked Thrombin‐Binding Aptamer (isoTBA) Forms a Stable Unimolecular Parallel G‐Quadruplex in the Presence of Sr 2+ Ions

The ability of the isomeri 2′‐5′‐phosphodiester‐linked thrombin‐binding DNA aptamer pentadecamer G 2 T 2 G 2 TGTG 2 T 2 G 2 (isoTBA), to fold into G‐quadruplex structures in the presence of different mono‐ and divalent cations is reported. Strikingly, Sr 2+ ions cause isoTBA to fold into a stable parallel unimolecular G‐quadruplex, in contrast to the antiparallel unimolecular G‐quadruplex fold observed in the presence of K + . IsoTBA being advantageously more stable than TBA to nuclease degradation, may thus be useful in the selective detection of Sr 2+ ions, which it can sense with a detection limit of ∼ 55 μM.