Equilibrium and structural studies on proton and copper(II) complexes of N-D-gluconylglycine

Abstract

N-D-Gluconylglycine, a pseudopeptide derivative of glucono-1,5-lactone and glycine, was prepared and the equilibrium constants of its protonation and copper(II) co-ordination and the structures of the copper complexes formed were studied in aqueous solution by potentiometry, spectrophotometry, CD, EPR and ¹³C NMR relaxation. The parent complexes formed in an acidic medium have low stabilities, characteristic of carboxylate co-ordination. In the range pH 5–9 the amide group and the 2-OH group of the ligand undergo deprotonation. In parallel with these processes, one ligand is replaced from the copper(II) co-ordination sphere. For the species MLH_–2, 3O,1N co-ordination in the equatorial plane is proposed. The EPR measurements indicate that dimeric species are also formed. At pH > 9, further base-consuming processes start as an indication of the deprotonation of other alcoholic hydroxy groups of the sugar moiety or the formation of mixed hydroxo complexes.