Proton, copper(II) and nickel(II) complexes of some amadori rearrangement products of D-glucose and amino acids

Abstract

The formation of the copper(II) and nickel(II) complexes of six Amadori rearrangement products of D-glucose with amino acids (‘fructose-amino acids’) was investigated by potentiometry, CD and EPR spectroscopy. The − I effect of the fructose residue in the ligands was reflected by the protonation constants of their amino groups. In the complexes ML formed with the copper(II) ion, besides the amino acid-type coordination a weak interaction with non-deprotonated alcoholic hydroxy groups was demonstrated by CD studies. Increase of the pH may lead to transformation of the species ML to give complexes ML 2 and/or MLH −1. The former contains two nitrogen donor atoms in the copper(II) coordination sphere, while in MLH −1 the ligands are coordinated through their carboxyl, amino and deprotonated alcoholic OH groups, as revealed by equilibrium, CD and EPR studies. Above pH ∼7, a redox reaction takes place between copper(II) and the ligand. The nickel(II) ion forms amino acid- type parent complexes ML and ML 2, while deprotonated species predominate in solution above pH ∼9.