METAL-ION-PROMOTED HYDROLYSIS OF URIDYLYL(3',5')URIDINE - INTERNAL VSEXTERNAL GENERAL BASE CATALYSIS

The hydrolysis of uridylyl(3',5')uridine promoted by Mg2+, Zn2+ and Zn 2+([12]aneN(3) = 1,5,9-triazacyclododecane) has been studied in imidaz ole, HEPES and triethanolamine dagger buffers and the origin of marked electrolyte effects observed with Zn2+[12]aneN(3) has been examined. The results obtained suggest that the basic buffer constituent does no t serve as an external general base, but the catalytic activity of the metal ion species is influenced by the coordination to the buffer bas e and other Lewis bases present in the solution. The data lend additio nal support to a bifunctional mechanism, which consist of coordination of the metal ion to the anionic phosphodiester and intracomplex gener al base (or nucleophilic) catalysis by its hydroxo ligand.