FURTHER-STUDIES ON THE BUFFER-CATALYZED CLEAVAGE AND ISOMERIZATION OFURIDYLURIDINE - 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 mor pholine buffers and by imidazole buffers has been reinvestigated, The key evidence for a previously proposed partitioning mechanism-in 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 product-is c onfirmed. The negative catalytic effect of the buffer base on the isom erization reaction is not due to a medium effect. Indeed the medium ef fect 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 a dditional cleavage path using. the buffer base only, This additional p ath, for which several alternative mechanisms are possible, is require d by the results of improved studies on the imidazole catalysis. These show that the previously reported decrease in rate at a low BH+/B rat io is due to ionic strength effects, The relative importance of these two pathways-one with kinetic dependence on the buffer acid and one wi thout such dependence-depends on the buffer basicity/acidity. With the acidic buffer acetate/acetic acid, a buffer-acid-catalyzed mechanism for the cleavage and the isomerization is dominant, A bifunctional mec hanism, in which one step involves simultaneous acid-base catalysis, s eems most likely, The medium effects of added dioxane on all these rea ctions are sensible in terms of the detailed mechanisms proposed, The relationship of these results to the mechanisms of catalysis by ribonu clease and by an enzyme mimic is discussed.