The methyl ester of α-aminophenylacetic acid: pH-dependence and phosphate catalysis of hydrolysis

Abstract

The dependence of the rate of spontaneous (non-enzymic) hydrolysis of α-aminophenylacetic acid methyl ester on the acidity of a medium was studied over the pH range 0.95–11.6. The mono- and di-anion of phosphate was found to have a catalytic effect on this reaction, according to the mechanism of general base catalysis. Catalysis of the protonated substrate form hydrolysis by different phosphate ions, the second molecule of water, and the hydroxide ion follows the Brönsted catalysis law with the slope 0.60. At a strong alkaline pH, phosphate slows down the ester hydrolysis, probably due to the formation of an ester–phosphate complex; the calculated dissociation constant is 4.2 × 10^–3M, while the ratio of the hydrolysis rate constants for free ester and its phosphate complex is 7.7.