Synthesis of guanosine 3'-phosphodiesters of 1, N6-ethenoadenosine-2-sulfonate or 1, N6-etheno-2-azaadenosine can be done readily with ribonuclease N1. There is a 10-fold difference in quantum yield between the phosphodiesters and the starting fluorescent nucleosides. This difference provides the basis for the development of a fluorescent assay for nuclease.