The Alkaline and Metal Ion Catalysed Hydrolysis of Amino Acid Esters

Abstract

The thermodynamic proton dissociation constants (pKaT values) of the following amino acid (hydrochlorides), and their methyl ester (di)hydrochlorides, have been measured at 25°C and an ionic strength. I, of 0.1M:- norleucine, norvaline valine, isoleucine, 2,4-diamino-n-butyric acid.HCl, ornithine.HCl (Orn.HCl).α-alanine, β-alanine, α-amine-n-butyric acid, γ-amine-n-butyric acid, serine, histidine.HCl (Hist.HCl), 2,3-diaminopropionic acid.HCl (Dap.HCl) and lysine.HCl (Lys.HCl). The pKaT values of the last mentioned 8 amino acid (hydrochlorides) and their methyl ester (di)hydrochlorides were also determined at 37 and 50˚C. The pKaT values of the α-amino acid ester hydrochlorides were about 2 units lower than their parent acids. The differences, ΔpKaT, in the 2 pKaT values of the diamine acid hydrochlorides decreased as the separation of the 2 amine groups was increased from 2.3 (Dap.HCl) to 2.6 (Lys.HCl). The same was true of their methyl ester dihydrochlorides except that ΔpKaT increased in going from 2.5 (Orn Me.HCl) to 2.6 (Lys Me.2HCl). The concentration stability constants (Kc values) of the Cu(II) complexes of Dap, Hist and their methyl esters, and ethylene diamine were measured at 25˚C and I – 0.1M. The Kc values, of the Ni(II) complexes of Dap, Dap Me, Hist Me and ethylenediamine, and of the Hg(II) complexes of Dap. Dap Me and ethylenediamine, were similarly measured. All of these diamine ligands were found to form very stable Cu(II) complexes. The Kc values of the Cu(II) complexes of Dap Me and Hist Me were also measured at 37 and 50˚C. The alkaline hydrolysis rates of all of the esters listed above were measured, at 25˚C and I = 0.1M, using a pH stat. The temperature dependence (37 and 50˚C) of the hydrolysis rates of the mentioned 8 esters was also determined. Enthalpies and entropies of activation were hence calculated. There was found to be a reasonable correlation between the hydrolysis rates and Taft’s polar substituent constant, σ*. except for valine and isoleucine methyl esters. Here, chain branching occurs near the reaction centre and hence steric effects are important. The enthalpies and entropies of activation of the selected 8 esters obeyed a poor isokinetic relationship. The Cu(II) and Hg(II) ion catalysed hydrolysis of Dap Me, and the Cu(II) and Ni(II) ion catalysed hydrolysis of Hist Me, were studied at 25˚C, I = 0.1M and M(II): ester ratios of 1:1 and 1:2. The temperature dependence (37 and 50˚C) of the Cu(II) catalysed reactions was also investigated. The results of the metal ion catalysed reactions were interpretable in terms of charge, inductive and statistical effects.