The interaction of L-(+)-histidine methyl ester with metal ions. Part I. Formation constants

Abstract

The thermodynamic proton ionisation constants of L-histidine methyl ester have been determined at 25, 37, and 50 °C and I= 0·1M(KCl). Values of ΔH° and ΔS°₂₉₈ for the various ionisation equilibria have been calculated. Similar measurements have been made for the ammonium ionisations at the pyridine nitrogen and α-amino-group of L-histidine. The concentration formation constants for the copper(II), nickel(II), and mercury(II) complexes of L-histidine methyl ester and for the copper(II) complexes of L-histidine have been determined by potentiometric titration at 25 °C and I= 0·1M(KCl). Formation constants have also been obtained for the mixed L-amino-acid-L-amino-acid ester complex with copper(II) at 25 °C and I= 0·1M. Values of ΔG°, ΔH°, and ΔS°₂₉₈ are reported for the formation of the mono- and bis-complexes of L-histidine methyl ester with copper(II).