The mechanism of the metal ion promoted cleavage of RNA phosphodiester bonds involves a general acid catalysis by the metal aquo ion on the departureof the leaving group

A series of uridine 3'-alkyl phosphates and 3'-aryl phosphates were synthes ised and their cleavage was studied in the presence of Zn2+ aquo ions. A be ta(lg) value was determined for the Zn2+ promoted cleavage of both types of compounds. Comparison of the results obtained to those reported previously for the cleavage of the same substrates in the absence of metal ion cataly sts suggests that the alkyl leaving group departs as an alcohol in the pres ence of metal ion catalysts. Furthermore, metal ion catalysts seem to enhan ce the departure. The aryl leaving group, in contrast, departs as an oxyani on.