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

Abstract

Polarographic, potentiometric, and spectrophotometric measurements show that copper(II) and N-dansylglycine (5-dimethylaminonaphthalene-1-sulphonylglycine) 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 the ligand co-ordination from O-co-ordination through the carboxylate group to N,O-bidentate co-ordination 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.