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Cyclization/cleavage of the 2-(p-nitrophenyl) phosphate ester of propylene glycol is catalyzed by imidazole and, much more effectively, by Zn2+ with imidazole. In the latter case, the mechanism involves simultaneous Lewis acid/base catalysis. Similar Zn2+ and imidazole catalysis of cyclization/cleavage is seen with the dinucleotide 3',5'-UpU (uridylyluridine). Again, the zinc system is much more effective than is catalysis by imidazole alone, and in this case simultaneous Lewis acid/base catalysis substitutes for the sequential proton acid/base catalysis seen with polynucleotides or dinucleotides and imidazole buffer catalysts. A mechanism is proposed for catalysis of RNA cleavage by the enzyme ribonuclease A, and the relationship of that mechanism to the action of the enzyme model systems is discussed.

On the mechanism of action of ribonucleases: dinucleotide cleavage catalyzed by imidazole and Zn2+.