The pH Dependence for the Hydrolysis of Uridine 5′-Phosphoimidazolates

The pH-rate profile for the hydrolysis of uridine 5′-phosphoimidazolate (UMPIm) is reported together with the profiles for the corresponding methyl imidazolates. The profile of log kobs versus pH for UMPIm exhibits three breaks and consists of ascending limbs in the acidic and basic regions, a plateau between pH 2 and pH 5, a descending leg between pH 5 and pH 10, and a minimum at pH 10. The pH-rate profile for the hydrolysis of UMP-2-MeIm is similar between pH 3.5 and pH 13, and the profiles for the 4- and 5-methylimidazolates are similar between pH 3.5 and pH 8.5. The pH-rate profile for uridine 5′-phospho-N-methylimidazolate (UMP-N-MeIm) between pH 3.5 and pH 11 differs in that the hydrolysis rate is independent of pH between pH 3.5 and pH 10 and then increases at higher pH values. The rate law for hydrolysis of UMPIm is v = k1[UMPImH+] + k2[UMPIm] + (k3 + k3OHaOH)[UMPIm−]. Rate constants at 25°C for the hydrolysis of UMPIm are k2 = 0.34 ± 0.01 h−1, k3 = (2.1 ± 0.4) × l0−4 h−1, and k3OH = 0.15 ± 0.01M−1 h−1. The rate law and mechanism differ from those reported for guanosine 5′-phospho-2-methylimidazolate (A. Kanavarioti, C. F. Bernasconi, D. L. Doodokyan. and D. J. Alberas (1989) J. Am. Chem. Soc.111, 7247)in that thereis no term corresponding to the nucleophilic attack of hydroxide ion on the zwitterionic form of UMPIm. The value of βlg for the hydrolysis of UMP-imidazolates is −1.52, which implies that bond cleavage to the leaving group is important in the transition state. Thermodynamically measured values of pK2 and pK3 for UMPIm are 5.75 ± 0.02 and 9.69 ± 0.12, respectively. The kinetically determined value of pK2 for UMPIm is 5.68 ± 0.04, and for UMP-2-, 4-, and 5-MeIm they are 6.82 ± 0.07, 6.49 ± 0.04, and 6.40 ± 0.09, respectively. Synthesis, characterization, and kinetic parameters for the hydrolysis at 25°C of UMPIm, UMP-2- MeIm, UMP-4-MeIm, UMP-5-MeIm. and UMP-N-MeIm are reported.