The effect of a dansyl group on the coordinative ability of N-protected amino acids. Part 1. Behaviour of the copper(II) ion-N-dansylglycinate system in aqueous and methanolic solution

Polarographic, potentiometric and spectrophotometric measurements show that copper(II) and N-dansylglycine form the same complexes in aqueous and methanolic solution. Mixed-hydroxy complexes are the prevailing species. In aqueous (methanolic) solution up to pH 5 (5) no formation of complexes is observed; in the pH range 7-8.5 (8.5-11) a change in ligand coordination from O-coordination through the carboxylate group to N,O-bidentate coordination through the carboxylate oxygen and the deprotonated amide nitrogen is revealed. There is a low tendency for two N-dansylglycine ligand molecules to coordinate to the Cu(II) ion, probably due to the steric hindrance of the dansyl group. The equilibria in which the prevailing complexes are involved and the reduction processes at the electrode are proposed. The nucleophilic and electrophilic properties of the two solvents are used to explain the solvent effect on the polarographic parameters and on the stability constants of the complexes.