Proton ionisation constants and kinetics of base hydrolysis of some α-amino-acid esters in aqueous solution. Part III. Hydrolysis and intramolecular aminolysis of αω-diamino-acid methyl esters

Abstract

The proton ionisation constants, pKa1T and pKa2T, for the ammonium ionisation of the αω-diamino-acids, NH3[CH2]nCH(NH3)CO2– have been determined at 25 °C and I= 0·1M for 2,3-diaminopropionic acid (n= 1), 2,4-diaminobutyric acid (n= 2), ornithine (n= 3), and lysine (n= 4). The corresponding constants for the methyl esters have also been obtained at 25 °C and I= 0·1M. The temperature dependence of the ionisation constants has been studied for 2,3-diaminopropionic acid, lysine, and their methyl esters. The thermodynamic parameters ΔH0 and ΔS0298 for the acids and esters are compared.The base hydrolysis of the methyl esters of 4-aminobutyric acid, 2,3-diaminopropionic acid, 2,4-diaminobutyric acid, ornithine, and lysine have been studied at 25 °C and I= 0·1M. Intramolecular aminolysis (lactamisation) is an important reaction pathway for the methyl esters of 4-aminobutyric acid, 2,4-diaminobutyric acid, and ornithine in basic solution. Simple base hydrolysis occurs with the methyl esters of 2,3-diaminopropionic acid and lysine.