The interaction of L-(+)-histidine methyl ester with metal ions. Part II. The metal-ion catalysed base hydrolysis of L-(+)-histidine methyl ester

Abstract

The base hydrolysis of L-(+)-histidine methyl ester (histOMe) has been studied in aqueous solution at l= 0·1M and three temperatures (25, 37, and 50 °C). Specific rate constants have been obtained for the hydrolysis of the unprotonated ester (E) and for the monoprotonated (α-amino-group) species (EH⁺), and the corresponding activation parameters determined.

Rate constants for the base hydrolysis of the ester function in the bis-complexes of copper(II) and nickel(II) with L-(+)-histidine methyl ester have been obtained at 25 °C and l= 0·1M. The hydrolytic behaviour of the biscomplexes is consistent with the kinetic scheme shown in equations (i) and (ii). The metal complexes undergo, ME₂²⁺+ OH^–→ MEA⁺+ MeOH (i), MEA⁺+ OH^–→ MA₂+ MeOH (ii) base hydrolysis considerably more quickly than the free ligand E. Thermodynamic parameters for the hydrolysis of the ester and the copper(II) complexes are compared. Specific rate constants for the base hydrolysis of the mono-complexes CuE²⁺, NiE²⁺, and the hydroxy-complex CuEOH⁺ have also been obtained at 25 °C and l= 0·1M. Kinetic stereoselective effects have been observed in the hydrolysis of the mixed ligand complexes NiEA⁺; thus [Ni(L-histOMe)(D-hist)]⁺ hydrolyses some 33% more quickly than [Ni(L-histOMe)(L-hist)]⁺. Such effects do not occur with the corresponding copper(II) complexes and the reasons for this behaviour are discussed.