The Effect of N-Substitution on the Rates of Saponification of Some Amino Acid Esters

The saponification rates (measured at 25 ° by a titrimetric method) of the unprotonated forms of the methyl esters of glycine, alanine, leucine, valine, and phenylalanine were compared with those of the N-methyl, the N-acetyl, and the N-acetyl, N-methylamino acid analogues. N-Acetylation slightly increased or decreased the rate but N-methylation caused a reduction by as much as a factor of ten, depending on the complexity of the side-chain. The esters of the N-acetyl, N-methylamino acids, which exist as cis and trans isomers, were saponified at rates intermediate between those of the esters of the N-acetylamino acids and N-methylamino acids. Activation parameters were obtained for the phenylalanine and leucine derivatives. N-Methylation resulted in an increase in ΔH≠ and ΔS≠ which was attributed in part to solvation effects. The hydrolysis of the cationic esters of glycine and alanine was still evident at pH 11.0. N-Methylation had little effect on the rates of saponification of the charged forms.