DEMETHYLATION KINETICS OF ASPARTAME AND L-PHENYLALANINE METHYL-ESTER IN AQUEOUS-SOLUTION

The kinetics of demethylation of aspartame and L-phenylalanine methyl ester were studied in aqueous solution at 25-degrees-C over the pH ran ge 0.27-11.5. The pseudo-first-order rate constant for aspartame was r esolved into individual contributions from methyl ester hydrolysis and diketopiperazine formation. pH-rate profiles were quantitatively desc ribed by chemically reasonable kinetic schemes. Aspartame is maximally stable at pH 4 (t90 = 53 days at 25-degrees-C); phenylalanine methyl ester, at pH 3. The potentiometrically measured pK(a) values were pK(a 1) 3.19 and pK(a2) 7.87 for aspartame and pK(a) 7.11 for phenylalanine methyl ester.