Divalent metal ion catalyzed reactions of acyl phosphates

Article abstract of DOI:10.1021/ja00170a020

The hydrolysis of the acyl phosphate, 1,10-phenanthroline-2-carbonyl phosphate, proceeds with a hydroxide ion catalyzed reaction of the dianionic species at high pH, a pH-independent reaction of that species in the pH range 7-10, and a reaction of the zwitterionic species (protonated phenanthroline nitrogen) at pH < 6 (pk_app = 5.6). The divalent metal ions Cu²⁺, Ni²⁺, Co²⁺, and Zn²⁺ have a large effect on the rate of hydrolysis. At saturating concentrations of Ni²⁺, Co²⁺, and Zn²⁺ (0.003 M, 50-fold excess over the acyl phosphate), hydroxide ion catalyzed reactions occur that are > 10⁷-fold more favorable at 30°C than in the absence of the metal ions. Likewise, Mg²⁺ exerts a sizable catalytic effect (> 10⁴), although binding is considerably weaker than with the other metal ions (saturation only occurs at Mg²⁺ concentrations greater than 0.1 M). The cupric ion promoted OH^--catalyzed reaction at 30 °C (0.002 M Cu²⁺) is (5 × 10¹⁰)-fold more favorable than OH^- catalysis in the absence of the metal ion. At pH < 4 a Cu(II)-promoted pH-independent reaction takes place that has a ΔS* of -34.3 eu. Incorporation of ¹⁸O into the carboxylic acid product when the hydrolytic reactions were carried out in ¹⁸O-enriched water showed conclusively that C-O bond breaking occurs in the metal ion promoted hydroxide ion and pH-independent processes. Both imidazole and pyridine are catalysts in the hydrolysis of the acyl phosphate, and the imidazole-catalyzed reaction is markedly enhanced by a saturating concentration of Ni²⁺. The effect of the metal ion is much smaller in the reaction with pyridine; the second-order rate constant for the pyridine reaction is only increased 3-fold by the presence of a saturating concentration of Ni²⁺. Thus, the strongly chelated metal ions greatly facilitate nucleophilic reactions that occur at the carbonyl carbon of the acyl phosphate rather than metaphosphate elimination or nucleophilic attack at phosphorus.