An evolved fluorescent ribonucleoside alphabet comprising isomorphic purine ((tz)A, (tz)G) and pyrimidine (U-tz, C-tz) 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 (tz)A as effectively as adenosine, the native substrate.