Chemical Mutagenesis of an Emissive RNA Alphabet

An evolved fluorescent ribonucleoside alphabet comprising isomorphic purine (tzA, tzG) and pyrimidine (tzU, tzC) analogues, all derived from isothiazolo­[4,3-d]­pyrimidine as a common heterocyclic core, is described. Structural and biochemical analyses illustrate that the nucleosides, particularly the C-nucleosidic purine analogues, are faithful isomorphic and isofunctional surrogates of their natural counterparts and show improved features when compared to an RNA alphabet derived from thieno­[3,4-d]-pyrimidine. The restoration of the nitrogen in a position equivalent to the purines’ N7 leads to “isofunctional” behavior, as illustrated by the ability of adenosine deaminase to deaminate tzA as effectively as adenosine, the native substrate.