Further Studies on the Buffer-Catalyzed Cleavage and Isomerization of Uridyluridine. Medium and Ionic Strength Effects on Catalysis by Morpholine, Imidazole, and Acetate Buffers Help Clarify the Mechanisms Involved and Their Relationship to the Mechanism Used by the Enzyme Ribonuclease and by a Ribonuclease Mimic

The cleavage and isomerization of 3‘,5‘-uridyluridine catalyzed by morpholine buffers and by imidazole buffers has been reinvestigated. The key evidence for a previously proposed partitioning mechanismin which the buffer acid BH+ converts the substrate to a phosphorane monoanion intermediate which then partitions either to the 2‘,5‘ isomer of the substrate or (with buffer base catalysis) to the cleavage productis confirmed. The negative catalytic effect of the buffer base on the isomerization reaction is not due to a medium effect. Indeed the medium effect on this reaction is in the opposite direction, strengthening the catalytic evidence. However, this branching mechanism with sequential bifunctional catalysis of the cleavage reaction is accompanied by an additional cleavage path using the buffer base only. This additional path, for which several alternative mechanisms are possible, is required by the results of improved studies on the imidazole catalysis. These show that the previously reported decrease...